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Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 1/181
Thomson scattering
Roberto Pasqualotto
11 February 2009
European Joint Ph.D Programme on Fusion Science and Engineering2 Advanced Course in Lisboa, February 2009,
On Diagnostics and Data Acquisition
roberto.pasqualotto@igi.cnr.it
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 2/181
OUTLINE
Theory: TS from single electronTS from plasma Te & ne
TS measurement: experimental issues
TS diagnostic: main components
Examples of existing TS systems:RFXTCVTextorHRTSLIDAR JET
ITER core LIDAR issues & design
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 3/181
LASER-AIDED PLASMA DIAGNOSTICS
A. J. H. DONN, C. J. BARTH, H. WEISEN - FUSION SCIENCE AND TECHNOLOGY VOL. 53 FEB. 2008, p.397
Laser-aided diagnostics are widely applied in the field of high-temperature plasma diagnostics for a large variety of measurements.
Various types of laser-aided plasma diagnostics exist, all based on different physical interactions between the electromagnetic wave from the laser and the plasma.
In general one can distinguish interaction based on:
(a) absorption and/or reemission, (b) changes in the refractive index, (c) changes in the polarization ellipse,(d) scattering.
Incoherent Thomson scattering is used for highly localized measurements of the electron temperature and density in the plasma.
Coherent Thomson scattering yields information on the fast ion population in the plasma and/or depending on the geometry and wavelength chosen electron density fluctuations.
Interferometry and polarimetry are often combined in a single diagnostics setup to measure theelectron density and the component of the magnetic field parallel to the laser chord.
Density fluctuations can be measured by means of phase contrast imaging, scattering, and various other laser-aided techniques.
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 4/181
Active diagnostics with lasers as the probing source have a number of distinct merits:
(a) the laser beam can be focused in the plasma, resulting in good spatial resolution; (b) the measurements do not perturb the plasma because of the relatively small
interaction cross sections;(c) lasers have a high spectral brightness good signals @ t,x,;(d) both with pulsed and continuous wave laser systems a good temporal resolution
can be obtained;(e) the lasers (and in many applications also the detectors) can be positioned far
from the plasma, where they can be more easily maintained.
LASER-AIDED PLASMA DIAGNOSTICS
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 5/181
Why Thomson Scattering? Why Thomson Scattering?
What is it? Laser beam scatters off of
electrons in the plasma doppler effect gives wavelength
shift
Straightforward stand alone measurement (direct method: no models, assumptions,..)
Electron velocity distribution directly observed (ne, Te)
Accurate spatial location viaimaging or time of flight
Lase
r bea
m
Detec
tor
ve
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 6/181
Thomson Scattering
Scattering of electromagnetic radiation by a charged particle. The electric and magnetic components of the incident wave
accelerate the particle, which in turn emits radiation in all directions.
Phenomenon was first explained by J.J.Thomson. It can be split into coherent and incoherent scattering (more later). The experimental application of TS as a diagnostic tool had to wait
for the development of high power light sources, e.g., the Q-switched ruby laser in the early 1960s. Since then, various plasma parameters have been measured by means of this technique.
The first demonstration of TS as a suitable diagnostic tool for hot plasmas was given by Peacock et al. in 1969 when they measured the electron temperature and density in the Russian T3 tokamak.
Further developments: Te and ne along the full plasma diameter, resolving up to ~ 100 spatial elements with time separations of ~10 s to 10 ms.
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 7/181
Role of Thomson scattering in
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 8/181
John Sheffield, Plasma scattering of electromagnetic radiation, Academic Press 1975
S.E. Segre, Thomson scattering from a plasmaCourse on Plasma diagnostics and data acquisition systems, Varenna 1975,
P Nielsen, Thomson scattering in high temperature devices , Varenna 1986,
Some PhD Thesis: Rory Scannel (MAST), Alberto Alfier (RFX), R. Pasqualotto (RFX)
In this lesson the focus will be on:
Logic of steps to derive TS spectrum (less on math)
What can be measured
Under which conditions
Thomson scattering spectrum
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 9/181
Thomson scattering from a single electron(classical limit of the Compton scattering)
- scattering of an incident photon by a moving electron (=v/c)
- electron energy is constant (Ee>>)
The scattered radiation is frequency shifted as a double Doppler effect takes place, one in the reception, one in the emission of radiation by the electron:
1. the photon approaching the moving electron
2. the photon leaving the moving electron
iEr i
sObserver
Incident electric field
Propagation&
scattering directions
scattering angle
kIncident photon
electron
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 10/181
TS as limit of Compton scattering
Ignoring the term i/mec2 we gets i = = (ks - ki) ve = k ve
Conservation of energy and momentum
i + mic2 = s + msc2ki + mivi = ks + msvs
where: mi,s = m0/(1-i,s2)
The solution to these equations is:
s = i (1-i i) / (1-i s + (1-cos())i/mic2)
When incident wave has frequency such that
i
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 11/181
Seen by the electron, initially stationary (vi =0):
With simple algebra:
In the TS limit
Transforming back to the lab reference system:
TS as limit of Compton scattering(some math)
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 12/181
i
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 13/181
Incident wave electric field:
and associated magnetic field:
TS from single electron
Force on the electron by the e.m wave:
with
Acceleration produced by this force
withnegligible if v
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 14/181
TS from single electron
unit vector in propagation directionquantities in bracket evaluated at retarded time
distance electron point of observation
retarded time: delay between the photon emission and the moment at which it reaches the observer
Phase of scattered field = phase of incident field (evaluated at ret- time)
if v = const (influence of e.m. wave on electron is ignored and no static B field):
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 15/181
The time dependent part of the phase indicates that the scattered wave is monochromatic, with a frequency s:
Scattered radiation is still monochromaticDisplacement in frequency proportional to the component of the e velocity in the k directionThis expression is valid also at relativistic velocity.If we want to observe the drift velocity of a plasma, scattering geometry must be such that k vd 0
TS from single electron
When
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 16/181
Incident wave electric field:
and associated magnetic field:
TS from single electronOnly a flavour of full math formulation
Equation of motion of the electron accelerated by the e.m wave:
with
Acceleration produced by this force
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 17/181
TS from single electronOnly a flavour of full math formulation
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 18/181
Intensity of scattered wave does not depend on
and it is zero in the direction of the polarization (=0) of the incident wave (not true if finite)
Max intensity when = /2 (s Ei)
ss the classical radius of electron
TS from single electron
Low electron velocity: non-relativistic approximation
Standard geometry: 90 scattering
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 19/181
Measured quantity is scattered power per unit solid angle:
Is the Poynting vector
Averaging over many periods, and using
Incident intensity
Is the Thomson scattering cross section
Scattered power 1/m2 in a plasma contribution from the ions is negligible : m_e = 10-32 kgm_p = 10-27 kg
TS from single electron
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 20/181
TS from single electron
non relativistic
relativisticfrom Sheffield
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 21/181
Total E given by contribution from each electron:
Average scattered power
First term: sum of power scattered by each electron independently of othersSecond term: contribution due to correlation between electron positions. = 0 if electrons randomly distributed
For a plasma, typical correlation length is
Phase very large and changes rapidly from electron to an other, when summing over distances of order d
2 term = 0, incoherent scattering
TS from a plasma
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 22/181
In (a) the phases do not add up while in (b) the opposite is true
The scattering parameter is = 1/kd
>> 1 coherent scattering
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 23/181
Incoherent TS
Electron contained in
Electron velocity distribution function f(v)
Contribute to total scattered power per unit volume, in frequency range
If we define differential scattering cross section
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 24/181
Thermal equilibrium: Maxwel distributionAssuming relativistic effects negligible
Scattered spectrum is a gaussian centred on input frequency
for a ruby laser
Incoherent TS
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 25/181
The total power, integrated over frequency collected from volume
whereA area of beam cross sectionl length of scattering volume observedW0 = A: Power of laser beam
If the laser is pulsed, we consider the energy of the pulseand the total collected energy Es
Incoherent TS
-10 -5 0 5 100
1
2
3
Laser-
- electrons are in thermal equilibrium (Maxwellian distribution) - relativistic effects are negligible
ne
Te
the scattered spectrum has a Gaussian shape
Visible or IR laser ~ 90 geometry
Ps_plasma = Ps_e n V
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 26/181
Incoherent TS: relativistic effects - depolarization
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 27/181
Incoherent TS: relativistic effects blue-shift
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 28/181
Incoherent TS: relativistic effects blue-shift
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 29/181
Incoherent TS: relativistic spectrum
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 30/181
From a maxwellian plasma:
Spectrum scattered from single electron: series of lines centred on the line at frequency and separated by c
Modulation distinguishable
Incoherent TS: effect of B
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 31/181
TS attractive as diagnostic tool for plasmas:
TS measurement: experimental issues
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 32/181
Scattered Spectra
0
1
2
3
4
5
6
7
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Normalised Wavelength
Spec
tral
Inte
nsity
0.5keV5keV10keV20keV40keV
Selden-Matoba, =180oElectron density, neScattering angle, scatLaser wavelength, 0Electron temperature, Te
Incoherent TS: spectrum depends on
TS measurement: experimental issues
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 33/181
Imaging Thomson ScatteringImaging Thomson Scattering
The scattered light is
imaged from the plasma
A spectrometer
disperses the light
A set of detectors
collects the light
The data is digitised and
analysed
PlasmaLaser (pulse)
Collection optics
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 34/181
Experimental issues
Critical aspects:- low cross section - low collection angle - detection of the scattered radiation
Background noise:- plasma light: broadband radiation
- stray light from baffles and dumps: monochromatic radiation (at the laser wavelength)
1310=incphotons
TSphotons
NN
One of the most fundamental but critical diagnostics in fusion experiments
Defines the time resolution (50Hz)
Define the spatial resolution(
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 35/181
800 850 900 950 1000 1050 1100 11500
0.005
0.01
0.015
0.02
0.025
wavelength (nm)
a.u.
0.1 keV0.5 keV1 keV2 keV10 keV
Detected signal
Detected power, over entire spectrum:
Photons entering the spectrometer
Fraction of spectrum detected by i-th spectral channel: 10-20 %
# photoelectrons detected/channel:
102 5x103
l = 10 mm
Balanced # of spectral channels: low to maximise signal, high to maximise resolution (min 2)
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 36/181
Signal errors
Poisson statistic of photoelectrons (p.e)
Background plasma light (most dangerous at high frequency: plasma fluctuations, ELMs):It can be 100-1000 x Bremmstrahlung contribution:
Stray light: monochromatic ifrom diffusion from inner wall, windows and opticsrejection R = 104-5 usually required in spectrometers to sufficiently reduce ithowever it is quite reproducible
Detector noise: dark noise noise equivalent number of p.e.multiplication noise noise factor F equivalent number of p.e. N* = N /F
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 37/181
Npe
ch.1ch.2
ch.3
ch.4
EDGE CORE
keV keV
ch.4 3 2 1
(%) (%)
Npe
TeTe
ne ne
Signal simulation
800 850 900 950 1000 1050 1100 11500
0.005
0.01
0.015
0.02
0.025
wavelength (nm)
a.u.
0.1 keV0.5 keV1 keV2 keV10 keV
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 38/181
Te and ne from relative & absolute calibration
Te: from the relative sensitivity of the 4 spectral
channels
relative spectral response of a spectrometer
1000 eV
50 eV
Nd:YLF1053nm
Ch 1
Ch 4
Ch 3Ch 2
ne: from the absolute sensitivity of the 4 spectral
channels
Rotational Raman Scattering in N2
TS spectrum
Nor
mal
ized
tran
smis
sion
3.51019m-3
31019m-3
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 39/181
Standard technique: CW light source + monochromator + calibrated energymonitor
Te: relative calibration
Measure the relative transmission function of spectralchannels in each spectrometer
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 40/181
Te: relative calibration
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 41/181
ne: absolute calibration
Rotational Raman Signal
induced by the laser in N2 gas
Torus filled with 50-500 mbar
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 42/181
ne: absolute calibration
Two main issues affect itsvalidity and make it verydifficult:
- Laser misalignment
- plasma deposition on collection window (mayinfluence also relative calibration)
The dependenceof signal on the pressure gives the abs. cal.
Rayleighscattering
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 43/181
B 1/Te
Calculation of Te & neYi = measured signal of channel-iyi = theoretical signal of channel-i,
given Te, nei = experimental errors
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 44/181
only depends on Te
non linear minimization
Calculation of Te & ne
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 45/181
Realising a TS System
Spectral Calibration Plasma Measurement Density Calibration
Results
Data Analysis
Data Acquisition
Spectral Analysis
Collection of Light
Scattering
Lasers
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 46/181
Well look now at main components of a TS diagnostic:LaserCollection opticsSpectrometerDetectorData acquisitionAnalysis
Then well see examples of working systems
TS diagnostic: main components
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 47/181
Most present TS experiments employ Q-switched ruby or Nd:YAG lasers as the source. The ruby laser operating at 694.3 nm produces outputs up to 25 J in 15 ns However, their repetition rate is usually rather low: 5 Hz (1 J / pulse). When more than several pulses per minute are required, an intracavity ruby laser can produce a burst of high-energy pulses (~15 J/pulse, t ~ s) with a repetition rate of ~10 kHz (see Textor). Ruby lasers are usually employed in systems where good spatial resolution is preferred above a high time resolution.
The most frequently used system for periodic TS measurements is based on the applicationof Nd:YAG lasers operating at 1064 nm, with outputs of ~1 J, 15 ns and a repetition rate of 20 to 50 Hz. Combining a set of lasers the repetition rate can be increased.
The beam divergence of both types of lasers is ~0.3 to 1.0 mrad. The polarization of the laser beam is chosen perpendicular to the scattering plane. The high laser powers require special precautions for the used optics. Laser beam diameters should be kept large enough such that for 15-ns pulses the energy density is below the damage threshold of ~5-10 J/cm2.Transmitting surfaces need to be coated and tilted with respect to the beam propagation to
Prevent losses and back-reflected light entering the laser system again. Furthermore, curved transmission optics should have concave entrance surfaces, to prevent focusing of the back-reflected beams (which might lead locally to very high power densities).Other types of pulsed lasers (e.g., Ti:Sapphire and Alexandrite lasers) have been proposed for TS (e.g., for ITER). Nevertheless, there are not yet applications of these sources to present devices.
laser
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 48/181
Ruby laser ( = 694.3 nm) TEM00 oscillator (35 mJ, 25 ns, single pulse) 3 amplifiers (15 J, 25 ns, 0.4 mrad )
Q-switched ruby laser in RFX
7 mW HeNespatial filter
F = 67 cm lens F = 30 cm lens
300 m pinhole
R = 5 m rear mirror
Pockels cell Brewster angle polarizer
mode selection aperture
etalon output coupler R =
E = 35 mJ (2 x 20 mJ)
45 steering mirror
45 steering mirrorE = 15 J (2 x 12 J)
Amp. 1 8 x 3/8" ruby 4 flashlamps
Amp. 2 8 x 5/8" ruby 4 flashlamps
Amp. 3 4" x 22.5 mm ruby
4 flashlamps
TEM00 oscillator 8 x 1/4" ruby 2 flashlamps
E = 1 J (2 x 0.75 J)
E = 10 J (2 x 7 J)
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 49/181
laserlaser
Modern TS systems use multiple lasers (typically up to 8)
These can be bunched to tackle different physics and provide redundancy
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 50/181
Stray light reduction
The laser beam enters and leaves the plasma vessel through vacuum windows. Passing these windows especially the entrance onegenerates stray light, which can reach the collection optics. Without precautions this stray light level can be six to eight orders of magnitude larger than the TS light. Reduction of the vessel stray light can be achieved: by tilting the windows (placing them under the Brewster angle isvery effective), by positioning them relatively far from the plasma, by using baffles in both entrance and exit ducts, and by mounting a viewing dump on the vessel wall opposite to the collection optics. A very effective light trap (reduction up to 100 times) can be made from a stack of knife-edge blades. Carbon tiles on the inner wall of the plasma vessel can give a reduction by a factor of ~ 20. Finally, the laser beam is dumped on e.g. a piece of absorbing glass placed under the Brewster angle.
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 51/181
Collection optics
Scattered light is collected after passing a vacuum window and subsequently relayed to a spectrometer. Because of the low scattering yield, the transmission of the collection and relay optics should be of course as high as possible. In devices with hot plasmas, a shutter is required between the plasma and the window to reduce deposition of all kinds of materials on the inner window surface during the times the diagnostic is not in use. Various kinds of optics are used to collect the scattered light. These systems are used to guide the TS light to the spectrometer. Basically, there are two possible ways to guide the scattered light from the plasma to the detection system: via flexible fibers and via conventional optics (lenses and mirrors).The main advantage of fibers above conventional optics is that the linear etendue of the source can be matched to that of the detector, albeit at the cost of reduced spectral resolution. For this purpose the fiber array is rearranged such that the slit height is reduced and the slit width increased, for example, by a factor of 2. As a result, the usable solid angle of the collection lens increases by a factor of 4. However, for TS diagnostics at small-sized plasma devices where the detection system can be positioned relatively close to the plasma (10m), conventional optics gives a better transmission (up to a factor of 3) than fiber optics. For systems with a comparable length of the optical path from collection lens to spectrometer, e.g., the multiposition TS systems of JFT-2M and RTP, the overall transmission is larger for systems using conventional relay optics (RTP: 25%) than for those using fiber optics (JFT-2M: 7%).
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 52/181
The major contributions to the losses in fiber-optic systems are the core-cladding ratio, the packing fraction, the attenuation, input and output reflection losses, and an increase of the exit cone. For fiber-optic arrays, transmissions of ~55% and even higher have been reported. Despite the lower transmission, fiber-optic systems have to be preferred when the scattered light needs to be relayed over longer distances (e.g., to get outside the biological shield of the plasma device). To bridge long distances with conventional optics would require many large-sized lenses and mirrors, resulting in a low transmission as well.
NA = 0.37
fibres
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 53/181
Spectral analysis
Mainly two different techniques to disperse the scattered light are in use for TS systems: filter and grating spectrometers. In filter spectrometers, the scattered light is separated into different wavelength bands by means of a cascade of interference filters. The number of separate wavelength channels in these systems is usually rather limited (three to eight channels), and therefore, the interpretation of the data relies on the assumption of a Maxwellian electron velocity distribution in the plasma. In grating spectrometers a grating is used for dispersing the scattered light. Both mechanicallyand holographically ruled gratings are used for this purpose. In this case, the number of independent spectral channels can be quite large: up to 80 for the TVTS system on TEXTOR. In case of good photon statistics, this enables one to determine the shape of the Maxwellian distribution.
To prevent the residual of the vessel stray light from disturbing the TS spectrum, the laser wavelength should be carefully filtered out after dispersion has taken place. This can be done by blocking the laser wavelength, by reflecting light at this wavelength away from the detector, or by focusing it onto a special detector. Both filter and grating spectrometers have typical stray light rejection ratios of 10-4 to 10-5.
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 54/181
Spectral analysis
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 55/181
grating Littrow spectrometer in RFX
input: 10 bundles of optical fibres Concave holographic grating with flat field (F/3.4) Interferential notch filters at = 694. 3 nm (R= 4x 10-3) Stops on the focal plane at = 656.3 nm (H) and = 694.3 nm. Rejection: RH= 2x10-4 , RRuby = 10-6, T = 30%
7600
5400
holographic grating
( = 120 mm, f/3.4)
flat spectral
plane
entrance slit
input fiber optic bundle
achromatic doublet a.r. @ 694.3 nm
interference notch filter (30 incidence)
interference notch filter (normal incidence)
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 56/181
Scattered light is collected by an F/19 achromatic doublet (item 3) and guided to a Littrow polychromatorwhere the light is detected after dispersion. A field lens (item 4) and a spherical mirror (item 8) serve for pupil imaging.The Littrow lens ~F/12.5 (item 6) collimates the incoming light beams and focuses the dispersed light at the two-partmirror (item 8), giving a two dimensional image (, z). This image is projected onto the GaAsP cathode ~18% tubeefficiency of a 25 mm image intensifier by means of a Canon 50 mm, F/0.95 TV objective. Finally, the phosphor screen of the intensifier is imaged at the cathode of two ICCD cameras (item 13) by a coupling lens system that consistsof three F/1.2 Rodenstock objectives (item 11) and a beam splitter (item 12). Double pulse operation is feasible with this system. Light emitted by the phosphor screen of the GaAsP image intensifier (item 10) and generated by the firstlaser pulse is recorded by one ICCD camera by gating it open during 20 s. The second ICCD camera is gated openat the moment of the second laser pulse, again during 20 s. Time separation is typically 100800 s. The ICCD integration time is made 20 s to keep the overlap of the two pulses as small as possibel (~ 7% for 100 s separation).
grating Littrow spectrometer in RTP & Textor
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 57/181
Detection and data acquisition
In general, two different types of detection systems can be distinguished: time-resolving single-element or multielement detectors, and signal-integrating multielement detectors. The first category includes avalanche photodiodes (APDs) employing the high quantum efficiency of Sibetween 500 and 1000 nm, photomultiplier tubes (PMTs), photodiode arrays, and multianodePMTs.These systems enable time resolutions of the order of the laser pulse duration of 15 ns. As a result plasma light can easily be sampled just before or after the laser pulse. TS systems using periodic Nd:YAGlasers mostly apply APDs for detection of scattered and plasma light. The signals of photodiode detectors are recorded with charge-integrating analogue-to-digital convertors (ADCs) or by means of fast transient recorders.Time-integrating detectors have a lower time resolution and are called TV systems because the detection principles are similar to those of a television camera able to receive a two-dimensional image.Vidicon, charge coupled device (CCD), complementary metal oxide semiconductor CMOS, and streak cameras belong to this category. These cameras have large numbers of image pixels, e.g., 106 to 107. The low readout time can vary for different types of cameras. For a 16-bit CCD camera, the readout time can be ~1 s, while ultrafast CMOS cameras sample with frame rates of 104 images/s at a 12-bit dynamic Range. The scattered light of the short laser pulse is captured with a gated image intensifier coupled to the TV-like recording system using a lens system. Data detected with TV systems are usually stored in internal memories and after termination of the plasma discharge are sent to a computer for analysis.
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 58/181
Both PMTs and TV detectors employ different kinds of photocathode materials to improve the photon to electron conversion process. PMT and image intensifiers are equipped with GaAsP, S25, extended S20 cathodes to reach high conversion efficiencies in the visible and near infrared wavelength ranges. The signal-noise S/N ratio of a detector directly depends on this conversion (quantum) efficiency:
where Npe denotes the number of photoelectrons generated by the incoming photons (Nph) and _conversion denotes the efficiency of the conversion from photons to electrons. However, the S/N ratio of the complete detector will be lower because of the noise added by the amplification and readout processes. More useful for evaluation of a detector is the effective detector efficiency, which includes the noise factor: _det = _conversion /noise factor. The noise factor refers to noise increase in thedetector caused by the amplification process.The S/N ratio of a complete detection system is determined by the statistical noise, the dark current of the detector and background signals, as plasma light and stray light. Plasma light due to bremsstrahlung and line radiation can be easily corrected for when photodiode detectors are used. Using TV systems in combination with fiber optics for light relay offers the ability to sample plasma light from an area just next to the laser beam and guide this to the same detector for simultaneous recording.Alternatively, one can measure the plasma light just before and after each laser pulse. The contribution ofplasma light can be kept negligibly low when the laser energy is high (>10 J) and the sampling interval is almost as short as the laser pulse (40 ns, for a pulse with 15 ns full width at half-maximum (FWHM)) . Grating spectrometers combined with TV systems result in a large number of spectral channels, which enables line radiation to be suppressed..
Detection and data acquisition
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 59/181
delay
Plasma light onlyPlasma light +TS pulse + Stray pulse
Gated acquisition
delay
Plasma light Fluctuation only
Plasma light fluct. +TS pulse + Stray pulse
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 60/181
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 61/181
Multianod MCP photomultipliers in RFX
40 mm S20 photocathode (Q.E. = 7 % @700 nm). V-stack MCP (G= 105 @ 1800 V, js = 230 A/cm2,
recovery time = 10 s). Array of 10 x 10 anodi Insensitive to B (3mT in spectrometer). 100 parallel channels in one detector.
spectral channel (110)
posi
tion
(1-1
0)
VBias VMCP Vanode
Photochathode MCP Anode
Photons
e-
e-
e-
e-e-
e-e-
e-e-
e-e-
e-
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 62/181
R&D
Photocathodes
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 63/181
Examples of working systems:
Details of experimental setup
Measurements
Physics studies
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 64/181
main TS at RFX
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 65/181
84 scatt. volumes on equatorial diameter (-0.95
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 66/181
main TS at RFX: filter polychromators & APDs
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 67/181
Polychromator
Z-posadjustable
Objects
Imaged on detectorsImaged on filtersField
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raw signals0.5GHz
84 points1cm
10 profile per discharge (
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-400 -200 0 200 4000
200
400
600
800
Radius (mm)
Te (eV) 19532 @ 95ms19532 @ 45ms
Te profile during different plasma states and in various scenarios:
stochastic plasma core
partially ordered
plasma core
Tomographicreconstruction of SXR emissivity
(pressure) profile in a poloidal section
Results obtained on RFX-mod
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 72/181
RFX: Te from TS & double filter
1. the entire profile is
pumped up for all the QSH
cycle;#22159 @ 199ms
Double-foil Te in 2D
Double-foil Te
Double foil and TS Te profiles
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 73/181
- Te and ne profiles along external radius (R=2.9-3.9m);
- maximum spatial resolution of 15 mm in 63 positions on 21 spectrometers with optical delay lines;
- time resolution of 20Hz (Nd:YAG laser - 5J );
- partially share the laser path of the other TS system (red path)
- interference filters spectrometers and digitizers
Similar to the MainTS on RFX-mod
HRTS at JET
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2. HRTS layout in torus hall
HRTS system currently operational:- outer radius covered- 61 points, 1.5 cm sampling resolution- 20 Hz repetition rate, full JET pulse
Laser beam
Scattering volume
lens
Vacuumwindow
Paraboloidalmirrors & fibres (126)
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fibres
first mirror
lenssecond mirror
double vacuum window, 192 mm diam, F/25
imaging lens and 2 motorised mirrors the lens images scattering volumes to West Wall
5m optical bench on West Wall holds 126 paraboloidal mirrors (3x4 cm)
each mirror images the lens onto one fiber optic one fibre corresponds to 8 mm scattering volume
Vacuum window192 mm diam
2. Imaging optics
Fiber
Paraboloidal mirror
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2. Laser
Nd:YAG (=1064 nm) laser from Quantel (France):2 beams vertically displacedE = 2.5 J / beamRepetition rate 20 HzFull remote control
Beams profileBurnspot on paper
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7 fibers bundleAPD + amplif
Lens +interf filter
650 750 850 950 1050 nm
800 900 1000 1100 nm
21 filter polychromators with avalanche photodiodes (APD)from GA / PPPL 4 spectral channels Two sets of filters: 7 for the edge (Te = 30 eV - 3 keV)
14 for the core (Te = 0.2-15 keV)
core
edge
Amplifiers from PPPL: AC output for TS signal: - lower frequency cutoff ( = 5 s)
filters plasma background light, - upper frequency limit allows to separate
3 time-delayed signals, 50 ns apart DC signal for plasma background light
2. Polychromators
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Waveform recorders (oscilloscopes):AC output (TS) into 1 GS/s, 150 MHz, 8 bit.DC output (plasma background) recorded at 1kHz, 12 bits.Data acquired between laser shots : real time acquisition
3 positions / polychromator with optical delay lines:2 fibres/position (15-20 mm spatial resolution)
2. Acquisition system
20 m20 + 30 m (150 ns)
20 + 60 m (300 ns)
Fiber bundle into each polychromator
TS pulses from 3 positions into same channelns
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4. Project schedule
2001 2002 2003 2004 2005 2006 2007 2008
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JPN 63863 (dry run) & 63865
ch1
ch2
ch3
ch4
White stray from inner wall (dump)
TS signals
Spectrometer 16 (core) Spectrometer 7 (edge)
TS signals
ch1
ch2
ch3
ch4
Raman in air
First TS measurements on 4th October 2005 (JPN 63804)
Green: with plasma; blu: without plasma
4. Project evolution: 2005
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Improvements in 2006
End 2005: good sensitivity demonstrated, but spurious pulses pollute TS signal
During 2006, general upgrade of the system (operation restarted in October)
- Broadband stray light nearly cancelled by enlarging the laser beam on the dump
- Monochromatic stray light nearly cancelled by tilting last filter in spectrometers
- Edge spectrometers realigned and all recalibrated with more accurate procedure
- Fiber optic delay lines installed with long delays to avoid problems with spurious pulses, but with smaller number of positions (37 instead of 60)
- Raman calibration improved
- Analysis programs finalised
- Protection system for laser beam risking to damage optics: burst max duration 20/10 pulses
4. Project evolution: 2006
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TS improved measurements in November 2006 (1)
blu: with plasma; green: without plasmaMain improvement: both monochromatic and broadband stray light nearly cancelledSpectrometer 7
ch1
ch2
ch3
ch4
Spectrometer 5
ch1
ch2
ch3
ch4
20 m20 + 30 m (150 ns delay)
20 m20 + 30 m (150 ns delay)
20 + 60 m (300 ns delay)Delay lines configurations
4. Project evolution: 2006
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The new HRTS system compared to existing electron diagnostics at JET- The systems are in fair agreement- All systems show single profiles, but core LIDAR averaged over 5 profiles (1 s)
4. Project evolution: 2007
Te and ne profiles in March 2007
Tene
Nucl. Fusion 48 (2008) 115006
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4. Damaged optics
Laser
Laser
NORMAL
FAULTY
If last amplifier doesnt work:beam divergence is changed and beam focuses on 2nd lens
because each amplifier has a thermal lensing effect on the beam
Laser produces2 beams, vertically displaced.Burnspots:
2 lens, damaged
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Scaled ruler positioned along laser beam path by remote handling
Each fibre is back-illuminated: an image of collection mirror is produced
Circular spot with diameter: average separation=8 mm
5. Position calibration
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Single profiles are now of good quality
1.5 cm sampling resolution
across full profile Temperature
Density
LIDAR
HRTS
LIDAR HRTS
Spatial profile
Pedestals are very steep!
Only one point in barrier
5. HRTS single profile measurements
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Pulse #73634
Time evolution of Te and ne by KE11 at fixed position (R = 3.2m), compared with core LIDAR (ne) and ECE (Te)
5. Time evolution
HRTSLIDARECE
Electron Temperature
Electron Density
HRTSLIDAR
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ELM mitigation through impurity seeding:
the pedestal pressure stays about constant
during the type I ELM phase and then
collapses
after Frad ~ 50 % during the type III ELMs
5. Profile comparison
Nucl. Fusion 48 (2008) 095004, M.Beurskens et al.
Average over 3-4 measured profiles
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1.5 cm ROG sweep
ROG sweep and pre-ELM data selection Increases data sampling:
Pedestal width analysis is possible
5. ROG sweep to improve spatial sampling
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Te: HRTS vs ECE
KK3 pre-ELM (shifted 8 cm)HRTS pre-ELM
1.7MA/1.8T
r/a
Agreement with ECE is often very good
However a shift of the ECE profile is required
5. Diagnostics comparison
ne: HRTS vs reflectometer
Agreement with preliminary data from KG8A
is also good
(see movie)
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5. RLCFS from HRTS
EFIT and HRTS agree in LCFS within +/- 0.5 cm
At start of campaign the position was not right and the profile was shifted to match LCFS. But technique was validated when we had absolute calibration in 2007
55 56 57 58 59 60 61 62 63 64 65
R_L
CFS
(m)
disc
repa
ncy
(cm
)
Time (s)
ROG sweep
EFIT
HRTS (single profile fits)
3.8
3.81
3.82
3.83
3.84
-1.0
-0.5
0
0.5
1.0
73344
3.7 3.8 3.90
10
20
P e(k
Pa)
Rlcfs= C+1/2
Pedestal fit: RLCF=C+1/2
A problem we encountered is that the absolute position calibration got lost(presumably the position stepper motors moved during shutdown)
Before position loss a good agreement was found between EFIT LCFSand HRTS pedestal foot in 2007. (Nucl. Fusion 48 (2008) 115006, A.Alfier et al.)
Use EFIT LCF position as reference for absolute calibration
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Filaments in the edge
during the
ELMy H mode phase.
Evidence confirmed by the
fast visible camera
5. Edge filaments
paper IAEA-CN- EX/P3-4 (2008), M.Beurskens et al.
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Main TS (blue):- 25 spatial channels (3 cm)- 40Hz time resolution (2 x 20Hz 1J Nd:YAG)
Edge TS on loan from Consorzio RFX:- 9 spatial channels (1cm) - sharing the same laser
The TS diagnostic(s) on TCV
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H-mode (high confinement regime): a transport barrier develops at the edge, called pedestal
enhanced confinement properties
ELMs: MHD instabilities appear at the edge when the edge pedestal gradient overcome a stability threshold (H lines)
Released energy and particle may damage plasma facing components
their control is crucial
TS laser
H-mode and ELMs
tELM
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XX
X
eeeXaaXaaXF
+
= )4()1(tanh)1()5()(
where X is the normalized coord.:
)3()2(
aaRX =
a(1)+a(5) : height
2a(3) : width a(1)/a(3) : slope
Pedestal parameters
The edge profiles is fitted with a modified tanh fit (5 parameters)
Analitical fit of the edge profile
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In the narrow time window 150s around the ELM peak :- Relaxed : monotonic slope, no clear sign of a pedestal a significantlysmaller gradient than normal profiles;- Bumpy : bumps at the LCFS;- Normal.
bumpyrelaxed
normal
Type-III ELM on TCV
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Time evolution of pedestal height and width during an ELM phase:- drop of Te(15%) and ne(35%) Te drop 500s after the ELM peak- sub-ms recovery time scale- transient region not available, profile deviates from tanh fit
Transient phase of the ELM
Results from single profile fit method
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Time evolution during ELM cycle from coherent averaging
Results obtained withthe single fit methodare confirmed
1. Single profiles are grouped in bins with respect totheir tELM (quasi-stationari condition);
2. the tanh-fit is then avereged out
From two bins
From
seve
ralb
ins
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- Time resolution: 1 shot per pulse with a Ruby laser (7J @ 694nm, 30ns at FWHM);
- Spatial resolution: 1 cm resolution on 12 measuring points;
- Dispersion system: Intensified CCD spectrometer measuring from few eV to 500eV;
- Novelty: the input system and collection window are on the same mechanical structure: easier alignment
Main TS
Edge TS
Why an edge TS on RFX-mod?
Outer edge region:
- edge physics is influenced by the active MHD control system
- not covered by the main TS
RFX: edge TS
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Main TS- Profiles @ 50Hz;- 84 spatial points @ 10mm;- Te = 20 1500eV and ne > 1019m-3.
RFX-mod: TS systems (1/5)
Edge TS (is being commissioned)- Single profile;- 12 spatial points @ 10mm resolution;- Te = 3 300eV and ne > 0.31019m-3.
RFX-mod
RFX
Plasma
Vessel wall
Ruby laser
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Input system
A ruby laser is focused on a 3mm pin-hole in vacuum.
Pin-hole
Vacuum
chamber
Sapphire prism deflects the beam by 30; a sapphire lens images the
pin-hole in the vacuum vessel.
The entrance port hosts the input system & the collection window stable alignment.
2
3
4Ruby laser
beam1
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Collection system
12 points for measuringTS spectrum (1-12)
4 points for measuring BKG (13-16)
Image of the back illuminated fibers with the extracted structure during the alignement
process
Schematic top view
He-Ne laser totrace the path
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The spectrometer
A: 4x4 fiber patternB G K : camera lensesC: four square lenses f = 400mm
I.I: Image Intensifier, 25mm CCD : 578x385 pixels, 22m x 22m pixel size, 1.5ms frame transfer
Expected accuracy of Te and ne measurements
D: three square spherical mirrors f = 200mmF: one square lens f = 400mmJ: energy monitor fiber
2eV
500eV
Transmissionfunctions
CCD controller
I.I.controller
B
DG
C
EKF
8
Fiberbundle
Energymonitor fiber
I.I.
A CCD
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10 kHz Repetitive High-Resolution TV Thomson Scattering System for TEXTOR
* Intracavity laser with three bursts of 50 100 pulses with 15 J each
* Ultra fast detector with CMOS camera* expected performance: errors on Te ~ 8%
and ne ~ 4% @ 0.05 Te 2 keV & ne = 2.51019 m-3
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Double-pass system Achieved Number of bursts 1 Number of pulses 10-12 Number of back and
forth passes per pulse 10 Lens-spher. mirror space 8.5 m Effective cavity length 18 m Pulse probing energy 12-23 J Pulse probing power 6-12 MW Total probing energy 150-220 J Pulse duration 0.002 ms Pulse interval 0.200 ms Beam divergence 0.7 mrad Beam chord in plasma 900 mm Probing region diameter 2-5 mm
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plasma light image
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Plasma light and TS spectra
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TS spectrum
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Temperature profiles Density profiles
Sequence of profiles in a burst
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Temperature profiles through 2 phases of an m=2 island
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LIDARLIDAR
LIght Detection And Ranging
Point and shoot method
So required access minimised
Short pulse of light transmitted to the JET - ITER plasma.
The back-scattered light is collected and analysed.
Note the spatial extent is recovered by the time delay.
Plasma
Laser
(short pulse)
Mirror labyrinth
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LIDARLIDAR Spatial resolution means short pulse lasers and fast detectors are
required e.g. ITER requires ~7cm
Note that the profile length in time is dt=2L/C.
Effectively 15cm/ns!
Detector and laser response defines spatial resolution
Plasma,Length L
LaserPulse
ScatteredLight
ScatteredLight
7cm is equivalent to ~460ps combined laser and detector response time
(so det/laser response ~300ps FWHM)
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Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 122/181
LIDARThomson Scattering
Principle
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Scattered signals at different times
Gives Te and ne atdifferent positions
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Advantages over more conventional 90 degree scattering geometry
One set of (6) detectors for all spatial positions easy calibration
180 degree geometry makes alignment simple easy to maintain
All sensitive components can be outside biological shield easy access
Because of time localisation, stray laser light pulses can be traced to particular objects easy (ish) to remove
Fast detectors means background plasma light level is low (cant think of anything easy about that easy to subtract perhaps?)
BUT
Spatial resolution? not so easy
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Tene
x= c.t
Laser
Collection optics
Space resolution:
x2= c2.( tlaser2+ tdet2+ tdaq2)/4tlaser=300ps tdet= 600pstdaq= 400psx= 12 cm
x=c.t/2
LIDARLIDAR
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KE1 was the first TS on JET, ready nearly from the start of operation in 1983.
The temperature measurement on JET were based mostly on ECE. TS was required to keep it honest.
The KE1 system was designed with this in mind and the fact it was only single point improved S/N in any case as the laser could be more focussed.
First time that all essential components were outside Torus Hall. Limited access. Long distances.
LIDAR at JETLIDAR at JET
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The idea of LIDAR was around at the Varenne meeting i 1982
Fortuitously it required only minor changes of KE1 to implement the system
LIDAR improves S/N by a factor >100 from the shorter integration time. This factor is reduced by ~ 10 due to a larger etenduerequired.
Weakness: limited resolution
LIDAR at JETLIDAR at JET
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JET LIDAR laserJET LIDAR laser
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JET LIDAR polychromatorsJET LIDAR polychromators
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JET LIDAR detectorsJET LIDAR detectors
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JET LIDAR raw dataJET LIDAR raw data
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Pedestals as measured with ECE, Li beam, LIDAR and CXS
JET LIDAR profilesJET LIDAR profiles
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Divertor LIDAR at JETDivertor LIDAR at JET
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If we can assume that Te and ne are constant on a flux surface
and
If we can align our LIDAR system so that the angle its line of sight makes with the flux surfaces, instead of being perpendicular, is much closer to tangential
then
Although the spatial resolution is still 12 cm along the line of sight,perpendicular to the flux surfaces it can be ~4 - 5 times better giving
L = 2 3 cm
However, the radial extent over which this resolution is achieved is limited
Mapping on flux surfacesMapping on flux surfaces
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4 channel filter spectrometer Optical path lengths to detectors are the same. Three filters at 12 degree incidence (F1 F3) are shown A fourth filter limits the channel nearest the laser line.
Divertor LIDAR at JET: polychromatorDivertor LIDAR at JET: polychromator
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KE9D L
aser
line-of-
sight
Rmid
Flux surfaces
n e(x
1019
m-3
)
0
10
JET boundaryne(x
1019
m-3)
0
10
Mapping increases spatial
resolution
Divertor LIDAR at JET: raw data & profilesDivertor LIDAR at JET: raw data & profiles
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ITER requirements for Te & ne
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ITER requirements for Te & ne
Electron temperatures in ITER of up to 40keV and densities of up to several times 1020 m-3 are expected. Thomson Scattering is a proven technique for making these measurements. Successful deployment of such a system requires that all components maintain adequate performance throughout the lifetime of the experiment. The parameters accessed by ITER lead to very different operating conditions from existing devices. These range from a high dose neutron environment to in-vacuum mirrors and the extremely long plasma discharges.
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ITER LIDAR
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Beam
dump
Mirrors
Large mirrors collect suitable amount of light
Laser
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The Relay Mirror
A possible solution for ITER LIDAR 2007, ~92inch
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The Relay Mirror
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Birds eye view
Laser diagnosis unit
New proposed laser beam test area
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The Neutron/Radiation Challenge
Influence of optical labyrinthHigh level of detail obtainable
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Laser Options/Requirements
Needs reasonable energy and short pulse simultaneously
Options to chose from:
Nd:YAG (1064nm)
Nd:YAG second harmonic (532nm)
Ruby(694nm)
Ti:Sapphire (~800nm)
Nd:YLF (1056nm)
Wide temperature range
Time repetition expected from laser(s) 100Hz
Also need to consider
Space envelope/ Maintainability/ Power consumption/ Data quality
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Laser System Laser specifications
wavelength~ ~1.06microns (1 +2 +cal ) laser energy ~5J/pulse laser pulse ~250-300ps
Proposing 7 lasers at ~15Hz More achievable technology Compact footprint Measurement capability maintained even if 1,2,3... lasers
malfunction Burst mode available to exploit plasma physics e.g. very fast MHD
events
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Options to combine lasers
Above shows a 7 laser hexagon pack at machine vacuum boundary
In this option beams are expanded as they go to the machine area to minimise risk of damage to windows
Hexagonally pack lasers no moving parts
Use a scanning mirror all beams can overlap
Rotating wheel with encoder all beams can overlap
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Scattered Spectrum
Note getting to /0~0.35 gets past the peak for 40keV
For 532nm laser, this means getting to 186nm
For 800nm laser, this means getting to 280nm
For 1064nm laser, this means getting to 370nm
Scattered Spectra
0
1
2
3
4
5
6
7
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Normalised Wavelength
Spec
tral
Den
sity
0.5keV5keV10keV20keV40keV
Selden-Matoba, =180o
0
0.1
0.2
0.3
0.4
0.5
200 400 600 800 1000 1200
Qua
ntum
Eff
icie
ncy
Wavelength [nm]
GaAsP
GaAs
Nd:YAG
S-25
NIR region( > 850 nm)
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0
0.1
0.2
0.3
0.4
0.5
200 400 600 800 1000 1200
Qua
ntum
Eff
icie
ncy
Wavelength [nm]
GaAsP
GaAs
Nd:YAG
S-25
NIR region( > 850 nm)
0
2
4
6
8
10
12
200 400 600 800 1000 1200Nd:YAGP
hoto
n sp
ectr
al d
ensi
ty (a
.u.)
0.2 keV
1 keV
5 keV10 keV40 keV
Upper: The Thomson scattering spectrum for an input wavelength of 1064 nm and a scattering angle q = 180, calculated at 5 different plasma temperatures.
Lower: examples of the spectral quantum efficiency of visible photo-cathodes available for LIDAR TS.
Long wavelength laser (e.g. NdYAG)Wide spectral range Shorter wavelengths efficient fast
detectors exist Recently proven at JET
(GaAsP)Modest improvement required
Detectors in the >850nm requiredTernary alloy InxGa1-xAs could
produce a QE of the order of 5% up to a cut-off wavelength of l~ 1000 nm.
Transferred electron (TE) detector. Externally biased, InGaAsP/InP photocathode with a possible QE in excess of 25% up to =1.33 m
Core detectors
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Attenuation in LIDAR Windows on ITER, due to ionising dose
Light Collection Window (Double total thickness 3 cm)Over ITER lifetime, equivalent ionising dose is 1.7 x 10-2 MGy
(nm) Absorption (%)400 800 0.5
350 0.9300 2.9 250 8.3
Laser Input Window (Double total thickness 3 cm)Over ITER lifetime, equivalent ionising dose is 1.0 MGy
(nm) Absorption (%)400 800 20
350 35300 76 250 98.5
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The value for the radio-luminescence intensity from the LIDAR light-collection window, falling within the tendue of the detection optics, is around seven orders of magnitude lower than that due to plasma bremsstrahlung collected within the same tendue. Consequently, the radio-luminescence signal can be ignored in the assessment of the parasitic light that will be collected along with the laser light scattered from the ITER plasma.
Variation of Luminescence with Wavelength for Various Glasses
Radio-luminescence in KU1 Silica Windows
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500 1000 1500 200030
40
50
60
70
80
90
100
SS
W
MoRh
Cu
Ref
lect
ivity
(%)
Wavelength (nm)
Motivation
1Handbook of optical constants of solids, ed. E.D. Palik, Acad. Press, 1985 and 1991
Calculated with (n, k) from [1]
Rhodium is a very attractive
option for first mirror material:
Good reflectivity
High melting point (1966 C)
Low sputtering yield (high Z)
High price of the raw material calls for developing thin film technology:
Magnetron sputtering (Vacuum deposition technique)
First mirror
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Dielectric mirrors
Protected Aluminium
to compare
Broadband Dielectric
Max size now
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LasersLasers
Study of short pulse high rep rate Nd:YAG lasers for scattering needs to be carried out (both 1st and 2nd harmonic)
2nd Harmonic will generally have half the energy and half the photons!
Ruby has been demonstrated to work but the repetition rate is a problem
Now to have a brief look at the TiS option
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Use of TiS lasersUse of TiS lasers TiS lasers have never been used for LIDAR TS in fusion
experiments
Potential issues need to be studied and analysed: bandwidth, ASE(amplified stimulated emission), maintenance, stability, functionality.
May be desirable to set up a test experiment (perhaps look at scattering off a gas) using an existing TiS facility after a feasibility study
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LIDAR Detector sensitivity?LIDAR Detector sensitivity?
For LIDAR, presently use gated 20 mm photocathode dual
chevron-MCP photomultipliers (10-12mm may be acceptable)
Photocathodes such as S20, Gallium Arsenide phosphide,
Gallium Arsenide, etc can cover the region to ~850nm
What about a detector in the 850-1060nm region? (sensitive
detectors in this region would beneficial)
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Detector Response time?Detector Response time?
Spatial resolution in the LIDAR system is directly related to the system response with 400ps (complete response when convoluted with detector, digitiser and laser) corresponding to 6 cm (ITER specification)
Laser pulse can be ~200ps
Detectors currently in use have about 650ps response time (800ps when response of complete system is included)
But very recent detector developments are encouraging: Detector ~10mm diameter(between 10 and 20mm required):
response time between 110 and 133ps.
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Laser coupling efficiencyLaser coupling efficiency
To optimise laser energy coupling efficiency, one should make use of high reflectivity mirrors where possible.
This is not possible if broadband metal mirrors are used for simultaneously transporting-in and collection-of the scattered light. For example, if 5 rhodium mirrors were used in the duct area, then immediately a transmission of 20% could be the result (mirrors and windows)
Can we get around this?
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Can use separate laser and collection
F/18 F/12 F/6
14.8m
Schematic straight throughoptical path shown for clarity
Detector 18mm
Vacuum window 110mm
Separate Laser path(Fold not shown)
Bio ShieldPort Plug
11.2m8.2m6.2m4.2m
Using this approach, one can optimise the mirrors for the laser and collection separately.
This would require a small hole in the back of the First mirror (
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Reliable electron temperature and density diagnostic for modern tokamaks
All across the operating range Current devices have temperatures exceeding 10 keV But up to 40 keV predicted for ITER. Then electron velocities are a substantial fraction of the
velocity of light Large blue shift in the scattered spectrum Change in the polarisation of the scattered light
The incoherent Thomson scattered power per unit solid angle per unit angular frequency can be written
( ) ( ) ( ) 322
111 df
s
i
vk( )( )( )
2232
2
11cos11dSr
ddPd
si
eie
ss
= r
High Temperature Thomson Scattering Theory Review
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Depolarisation term important at high temperatures Theory is solid but experiments challenged this due to fact TeTS can be up to 15-20%
lower than TeECE in some experiments However the presence of high energy electron tail would cause TS to overestimate Urgency to investigate the cause of the temperature discrepancy
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For the TS systems, reliability is of paramount importance and redundancy in design must be incorporated where possible.
Typical operation of a multi-laser system on the MAST device has shown that out of 4 lasers, at least 1 laser was available almost 100% of the time while all 4 lasers were available >70% of the time (this corresponds to an individual laser availability of about 92% per plasma shot).
Translating this simply to ITER for a 7 laser system would give 5 or more lasers available more than 98% of the time, and all 7 lasers > 56% of the time.
Laser Reliability
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EU-Core TS (LIDAR)Spatialrange
Parameter range Timeresolution
Spaceresolution
Accuracy
ElectronTemperature
r/a
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High Importance Generic Topics to be Addressed
First/second Mirror surface recovery (MSR) techniques Deposition prevention First dielectric laser mirror Background light calculations (need to get much better modelling) Wide-band in-situ calibrations Detectors (previously discussed at ITPA-need more physics
assessment) Laser development Shutter/calibration combination specification/outline Alignment systems Beam Dumps (common issue) Reliability (should define what is expected) TS/ECE issue resolution Measurement requirements-still consistent- (detailed physics case) Diagnostic exploitation
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The ITER TS design is based principally on the design of the JET LIDAR.
JET has the only LIDAR systems in the world. It was generally acknowledges that LIDAR was the only way of introducing TS on ITER. To a large extent the ITER system is a copy of KE3.
The reliability is very high > 90%
Alignment is stable, no components on the Vacuum vessel
BUT, The Jet systems are not using one window but a cluster of 7 windows with the laser in the center Vignetting!
ITERCore LIDAR vignetting
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Signal from inside double cone has no vignetting Laser beam can be anywhere in this cone Simple calculation of solid angle if the two apertures are
relayed to the detectors
Ddet / Fdet = Dblanket / Fblanket = Dblanket x Dmirror/(L2 - L1)
Dmirror/Dblanket = L2/L1
Eliminate the central window, i.e. use the full aperture for collection.
Use the collection window/mirror for laser beam as well
ITER Core LIDAR vignetting (old design)
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The aim is to re-image the entrance pupil on the detector surface. The advantage of this idea is a field-independent image diameter. The entrance pupil size is given by the size of the first surface of the Collection Optics, M1. The diameter of M1 follows from the field definitions and F/#s.M1 and M4 are spherical mirrors, M2 and M3 are identical toroidals. M1 is imaged onto M4
1104200 F/17
502100
100 F/6
Field [mm] at field positionField position [m] + 2100 [m] before M1
For the relay-optics there is a balance between size-of-the-components and the total number-of-components. Task is to re-image M4 by a relay and keeping every link in the chain identical.
ITERCore LIDAR optical design
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ITERCore LIDAR detectors
The TS spectrum in ITER will range from NIR (low Te) down to UV (high Te).IR laser will be used only if IR detection is available this influences also
calibration techniques.
Long wavelength laser (e.g. NdYAG)Wide spectral range Shorter wavelengths
efficient fast detectors exist Recently proven at JET (GaAsP)Modest improvement required
Detectors in the >850nm requiredTernary alloy InxGa1-xAs could
produce a QE of the order of 5% up to a cut-off wavelength of ~ 1000 nm.
Transferred electron (TE) detector. Externally biased, InGaAsP/InPphotocathode with a possible QE in excess of 25% up to =1.33 m
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The main requirements for the ITER LIDAR TS detectors are :Active area diameter D 11 mmEquivalent quantum efficiency EQE 6%Pulse response time t 330 ps FWHM.[1]gating shutter ratio ~ 106.gating on-off time 5ns.
EQE = QE/kFkF is the excess noise factor that accounts for any additional noise introduced after the primary detection.
At present these specifications can be met only by photoemissivedetectors. The above specifications are at the limit of the present technology for the detectors operating in the visible. To extendthem to the NIR is a real challenge. Two types of detectors available for the above spectral range: the transferred electron (TE) hybrid photodiode and the InxGa1-xAs microchannel plate (MCP) image intensifier
ITERCore LIDAR detectors
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NIR detectors
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NIR detectors
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NIR detectors
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Signal simulation with NIR detectors
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Key ProjectMilestones1.1.1.0.0
Key ProjectDeliverables
1.1.2.0.0
Key ITERMilestones & IPL
1.1.3.0.0
OverallManagement
1.1.4.0.0
Safety & HPManagement
1.1.5.0.0
RiskManagement
1.1.6.0.0
QualityManagement
1.1.7.0.0
LIDAR ProjectManagement
1.1.0.0.0
Overall ClusterCo-ordination
1.2.1.0.0
PerformanceAnalysis1.2.2.0.0
LIDARNeutronics1.2.3.0.0
ScatteringTheory
1.2.4.0.0
R&DTasks
1.2.5.0.0
RadiationEffects Data
1.2.6.0.0
RemoteHandling1.2.7.0.0
ItemTest Unit1.2.8.0.0
EngineeringAnalysis1.2.9.0.0
LIDAR SystemConcepts1.2.0.0.0
Lasers1.3.1.0.0
LaserLayout
1.3.2.0.0
Laser BeamCombiner1.3.3.0.0
LaserSystems1.3.0.0.0
Collection Optical Design
1.4.1.0.0
CollectionWindows1.4.2.0.0
In-VacuumCollection Mirrors
1.4.3.0.0
Ex-VacuumCollection Optics
1.4.4.0.0
Collection OpticsMechanical Design
1.4.5.0.0
SpectrometerSystem
1.4.6.0.0
Detectors1.4.7.0.0
AlignmentSystem
1.4.8.0.0
CalibrationSystem
1.4.9.0.0
CollectionOptics
1.4.0.0.0
Laser PathOptical Design
1.5.1.0.0
LaserWindows1.5.2.0.0
Plasma FacingLaser Mirrors
1.5.3.0.0
Other LaserMirrors
1.5.4.0.0
Laser PathMechanical Design
1.5.5.0.0
BeamDump
1.5.6.0.0
AlignmentSystem
1.5.7.0.0
CalibrationSystem
1.5.8.0.0
Laser PathOptics
1.5.0.0.0
Control SystemInterface Definition
1.6.1.0.0
ControlSystem
1.6.2.0.0
AcquisitionSystem
1.6.3.0.0
LIDARInstrumentation
1.6.4.0.0
SafetyInterlocks1.6.5.0.0
SafetySystem
1.6.6.0.0
Control &Acquisition1.6.0.0.0
Shutters1.7.1.0.0
Labyrinth1.7.2.0.0
Extension Tubes &Mirror Mounting
1.7.3.0.0
External Port OpticsMounting1.7.4.0.0
Bioshield1.7.5.0.0
BSMPenetrations
1.7.6.0.0
EM Analysis forIn-Port Comp.
1.7.7.0.0
LIDARPort Engineering
1.7.0.0.0
WaterServices1.8.1.0.0
InterspaceVacuum1.8.2.0.0
LIDARPower
1.8.3.0.0
SpectrometerArea
1.8.4.0.0
LaserRoom
1.8.5.0.0
Port Cell/Interspace1.8.6.0.0
LIDARServices1.8.0.0.0
LIDARInterfaces1.9.1.0.0
Mock-upFacility
1.9.2.0.0
Basic Mock-upTests
1.9.3.0.0
TokamakTests
1.9.4.0.0
Final SystemTesting
1.9.5.0.0
System Assembly& Dis-assembly
1.9.6.0.0
Interfaces &Integrated Testing
1.9.0.0.0
Thomson ScatteringCore (LIDAR)5.5.C.1.0.0.0.0
ITER core LIDAR project Work Breakdown Structure
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ITER divertor TSs
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ITER divertor TSs
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The life-time of optical components is expected to be limited due to contamination with carbon and beryllium-based material eroded from the beryllium wall and carbon tiles.
As well as significantly reduced optical transmission, thin layers can dramatically change the slope of the spectral reflectivity of rather low reflectivity mirrors, especially like W or Mo.
ITER divertor TSs
Recommended