Thompson Scattering

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

[email protected]


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


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.


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


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


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.


Role of Thomson scattering in


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


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


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


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)


i


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



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):


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


When


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


TS from single electronOnly a flavour of full math formulation


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


Low electron velocity: non-relativistic approximation

Standard geometry: 90 scattering


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




non relativistic

relativisticfrom Sheffield


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


In (a) the phases do not add up while in (b) the opposite is true

The scattering parameter is = 1/kd

>> 1 coherent scattering


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


Thermal equilibrium: Maxwel distributionAssuming relativistic effects negligible

Scattered spectrum is a gaussian centred on input frequency

for a ruby laser

Incoherent TS


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


Incoherent TS: relativistic effects - depolarization


Incoherent TS: relativistic effects blue-shift


Incoherent TS: relativistic spectrum


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


TS attractive as diagnostic tool for plasmas:



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



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


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(


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)


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


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


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


Standard technique: CW light source + monochromator + calibrated energymonitor

Te: relative calibration

Measure the relative transmission function of spectralchannels in each spectrometer


Te: relative calibration


ne: absolute calibration

Rotational Raman Signal

induced by the laser in N2 gas

Torus filled with 50-500 mbar


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


B 1/Te

Calculation of Te & neYi = measured signal of channel-iyi = theoretical signal of channel-i,

given Te, nei = experimental errors


only depends on Te

non linear minimization

Calculation of Te & ne


Realising a TS System

Spectral Calibration Plasma Measurement Density Calibration

Results

Data Analysis

Data Acquisition

Spectral Analysis

Collection of Light

Scattering

Lasers


Well look now at main components of a TS diagnostic:LaserCollection opticsSpectrometerDetectorData acquisitionAnalysis

Then well see examples of working systems

TS diagnostic: main components


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


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)


laserlaser

Modern TS systems use multiple lasers (typically up to 8)

These can be bunched to tackle different physics and provide redundancy


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.


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%).


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


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.


Spectral analysis


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)


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


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.


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


delay

Plasma light onlyPlasma light +TS pulse + Stray pulse

Gated acquisition

delay

Plasma light Fluctuation only

Plasma light fluct. +TS pulse + Stray pulse


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-


R&D

Photocathodes


Examples of working systems:

Details of experimental setup

Measurements

Physics studies


main TS at RFX


84 scatt. volumes on equatorial diameter (-0.95


main TS at RFX: filter polychromators & APDs


Polychromator

Z-posadjustable

Objects

Imaged on detectorsImaged on filtersField


raw signals0.5GHz

84 points1cm

10 profile per discharge (


-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


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


- 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


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)


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


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


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


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


4. Project schedule

2001 2002 2003 2004 2005 2006 2007 2008


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


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



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



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)


Te and ne profiles in March 2007

Tene

Nucl. Fusion 48 (2008) 115006


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


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


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


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


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


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


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)


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


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.


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


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


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


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


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


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


- 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


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


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


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


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


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


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


plasma light image


Plasma light and TS spectra


TS spectrum


Temperature profiles Density profiles

Sequence of profiles in a burst


Temperature profiles through 2 phases of an m=2 island


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


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)


LIDARThomson Scattering

Principle


Scattered signals at different times

Gives Te and ne atdifferent positions


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


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


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


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


JET LIDAR laserJET LIDAR laser


JET LIDAR polychromatorsJET LIDAR polychromators


JET LIDAR detectorsJET LIDAR detectors


JET LIDAR raw dataJET LIDAR raw data


Pedestals as measured with ECE, Li beam, LIDAR and CXS

JET LIDAR profilesJET LIDAR profiles


Divertor LIDAR at JETDivertor LIDAR at JET


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


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


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


ITER requirements for Te & ne


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.


ITER LIDAR


Beam

dump

Mirrors

Large mirrors collect suitable amount of light

Laser


The Relay Mirror

A possible solution for ITER LIDAR 2007, ~92inch


The Relay Mirror


Birds eye view

Laser diagnosis unit

New proposed laser beam test area


The Neutron/Radiation Challenge

Influence of optical labyrinthHigh level of detail obtainable


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


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


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


Scattered Spectrum

Note getting to /0~0.35 gets past the peak for 40keV

For 532nm laser, this means getting to 186nm



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)


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


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


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


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


Dielectric mirrors

Protected Aluminium

to compare

Broadband Dielectric

Max size now


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


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


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)


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.


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?


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 (


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


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


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


EU-Core TS (LIDAR)Spatialrange

Parameter range Timeresolution

Spaceresolution

Accuracy

ElectronTemperature

r/a


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


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


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)


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


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


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


NIR detectors


Signal simulation with NIR detectors


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


ITER divertor TSs


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

Documents

Thompson Scattering