Bricette Cyrin et al- The HelCat Helicon-Cathode Device at UNM

8/3/2019 Bricette Cyrin et al- The HelCat Helicon-Cathode Device at UNM

1/23

The HelCat Helicon-CathodeDevice at UNM

Bricette Cyrin, with

Christopher Watts, Mark Gilmore, Tiffany Hayes,

Ralph Kelly, Christopher Leach, Andrew Sanchez, Alan Lynn,Jacek Osinski, Shuangwei Xie, Lincan Yan, Yue Zhang


2/23

Abstract The HelCat helicon-cathode device is a dual-source linear

plasma device for investigating a wide variety of basic

plasma phenomena. HelCat is 4 m long, 50 cm diameter,

with axial magnetic field < 2.2 kG. An RF helicon source is

at one end of the device, and a thermionic BaO-Ni cathode

is at the other end. Current research topics include the

relationship of turbulence to sheared plasma flows,

deterministic chaos, Alfvn wave propagation and damping,

and merging plasma interaction. We present an overview ofthe ongoing research, and focus on recent results of

merging helicon and cathode plasma. We will present some

really cool movies.


3/23

Summary Studies focus primarily on effects of shear flow covering

a variety of topics Turbulence and chaos generation and suppression

Generation of drift and Kelvin-Helmholtz instabilities

Relative effects of poloidal vs. axial flows

Shear flow effects on injected plasma bubble

New diagnostics: 4m spectrometer and emissive probe

Alfvn wave propagation Damping length studies

Association with bubble injection

Plasma bubble interaction with background plasma Relevant to astrophysical jet formation and coronal mass ejections

New diagnostics Flow and shear measurements


4/23

HelCat: Helicon-Cathode Device Dual source helicon-cathode

Best of both worlds: high density, steady state helicon with high(er)temperature, broader profile

Vary collisionality over wide parameter range Variable ionization fraction

Pre-ionization of helicon at low pressure Different ionization mechanisms

-> different turbulence Colliding plasmas: way cool!

Science Motivation Drift waves and flow shear Intermittent, convective blobs

Alfvn wave propagation Expanding plasma bubble

into background Helicon physics

1 10000.05 100Collisionality vi /ci

1000


5/23

HelCat [Helicon-Cathode] Parameters Physical device

4 m long, 50 cm diameter

13 cm diameter helicon; 15 cm diameter thermionic cathode 13 magnetic field coils, 2.2 kGauss max field

Diagnostics Electrostatic and Magnetic Probes Microwave Interferometers (40 GHz, 94 GHz) Visible Spectroscopy

High resolution and survey Fast framing camera

Laser InducedFluorescence

Helicon

Cathode

Magnets

GridBias Rings

Interferometers


6/23

Pretty plasma picturesCathode Source with Ar Plasma

Helicon Source with Ar Plasma [top]and He Plasma [bottom]


7/23

Comparison of helicon & cathode plasma

Helicon Density profiles peaked Temperature profiles Gaussian like (or hollow) Steady-state (typically pulsed 250 ms)

Cathode Density profile varies depending on current & time (note cathode

inhomogeneity) Temperature broad

Pulsed, 10 ms

0

5

10

15

20

1

1.5

2

2.5

3

3.5

0 2 4 6 8 10

Helicon density & temperature profilesCathode density profile

helcion ne

cathode ne

helicon Te

ne(

1018) T

e(eV)

distance (cm)

HeliconB: 400 G

gas fill: 3.00 mTorrpower: 1000.00 W

Cathode

B: 1 kGgas fill: 0.5 mTorr

icath: 500A

0

2 103

4 103

6 103

8 103

1 104

1.2 104

0 50 100 150 200 250 300 350

Helicon ion saturation current

isat

(bits)

time (s)

0

0.5

1

1.5

2

2.5

0 5 10 15 20 25 30 35 40

Cathode ion saturation current

isat

(arb)

time (ms)


8/23

Shear Flow Studies


9/23

Flow and Shear Modification with E

Attempt to actively control turbulence-driven particle transport

Use imposed E to modify Shear flow using bias rings

Changes ExB velocity Independently adjustable

Eventual goal of active feedback control

Manipulation of flow profiles provides control knob for manipulating the

transport

Cathode

AnodeRF Helicon

Source

Magnet CoilsBias Rings


10/23

0

0.2

0.4

0.6

0.8

1

1.2

-5 0 5 10 15 20 25

Fluctuations under Bias

2.2 mT

Bias Voltage (V)

5000

6000

7000

8000

9000

1 104

5000 6000 7000 8000 9000 1 104

2D phase plot (0V)

Isat (bits)

5000

6000

7000

8000

9000

1 104

1.1 104

1.2 104

5000 6000 7000 8000 9000 1 104

1.1 104

1.2 104

2D phase plot (40V)

Isat (bits)

4000

5000

6000

7000

8000

9000

1 104

1.1 104

4000 5000 6000 7000 8000 9000 1 104 1.1 104

2D phase plot (46V)

Isat (bits)

6400

6600

6800

7000

7200

6400 6600 6800 7000 7200

2D phase plot (50V)

Isat (bits)

4000

5000

6000

7000

8000

9000

1 104

1.1 104

100 1 05 1 10 1 15 120 1 25 1 30 1 35 140

Ion saturation current (0V)

time (ms)

4000

5000

6000

7000

8000

9000

1 104

1.1 104

1 00 10 5 1 10 115 12 0 1 25 130 13 5 1 40

Ion saturation current (50V)

time (ms)

Fluctuation Suppression by Radial Bias Density gradient region show large amplitude, drift wave type

fluctuations

Imposed radial electrical field reduces drift wave fluctuations to a verylow level

Period doubling is observed from phase plot of ion saturation signal

-80

-70

-60

-50

-40

-30

-20

-10

100 1000 104

105

Power spectrum (0V)

freqency (Hz)

Xie: This session GP8.00107


11/23

-150

-100

-50

0

50

100

150

0 10 20 30 40

Phase Difference between Probes

Bias Voltage (V)

60o

m1

50o

-300

-200

-100

0

100

200

300

400

0 0.02 0.04 0.06 0.08 0.1

Radial Electric Field

Er (0V)

Er (17V)

Er (27V)

Radial Scale

0

1

2

3

4

5

6

7

8

-2 0 2 4 6 8 10

Floating Potential under Different Bias

Vbias=0V

Vbias=17V

Vbias=27V

Radius (cm)

Drift Wave Frequency From power spectrum, it is clear the drift

wave frequency is 600 Hz

This can be verified by plasma movies[see left and laptop]

taken at 1200 frame/sec

rotation period is 2 frames

: so the frequency is 600 Hz

Corroborated by 2-probe correlationmeasurements

Simple theory confirms basic picture See poster GP8.00106 and 107 for more details


12/23

Helicon Source

Mesh GridBias Rings

Mach probeplacement

Controlling and Understanding Flows

(Sheared) flows appear in many plasmas Space Plasmas, such as Solar Wind Laboratory plasmas

(Sheared) flows affect turbulence and transport Create or stabilize instabilities

Using Helicon source, create and control flows Use system of grid (s) and bias ring (group of concentric rings) Bias grid from -40V to 40V

Hayes: This session GP8.00108


13/23

A

B

D

C

Shearflow yields varied behavior

Control flows with voltage (D) flow with no grid

Create instabilities (A)

Plasma is able to stabilize forlow voltages (B)

Plasma unstable for high voltage(C)


14/23

Cool Movies

Very Cool movies of the m=1 mode in the Ar helicon plasma Shot with a 1200 fps Canon digital camera

Check thenearby laptop


15/23

Model includes:

i) resistive drift modes, ii) Kelvin-Helmholtz modes, and iii) rotation-driven interchange modes

Drift Modes Kelvin-Helmholtz Modes

Radial region of max. instability close toplasma edge, where max. observed

Predicted real + ExB very close tomeasured frequency

m=1 predicted most unstable, asmeasured in experiment

Linear Stability Analysis of Plasmas Under Biasing

Instability predicted with v0z/robserved under biasing.

Radial region of max. instability at radiiof highest observed fluctuations.

Predicted real within 25% of measured

High m (m=8), high kz (z29 cm)

predicted

0/

~nn

Gilmore: This session GP8.00106


16/23

Interchange Modes

Unstable rotation-driven interchangemodes predicted in the edge gradientregion

- where rotation is highest

Predicted real + ExB varies widely acrossunstable region.

Full cylindrical geometry without WKBapprox. is underway.

Linear Stability Analysis cont. Seems to confirm our basic

understanding of the fluctuations But linear analysis cannot describe the

nonlinear physics (such as chaos)

Drift modes are most unstable atthe plasma edge Dominantly m=1 mode

real strongly Doppler downshifted

Kelvin-Helmholtz modes aredestabilized by axial velocity shear(v0z/r ) under biasing

Theory and experiment real agreewithin 25%.

A WKB slab model indicates thatinterchange modes are unstable inplasma edge region More complete analysis needed for

quantitative comparisons.

Conclusions:


17/23

Alfvn Wave Studies Induction coil used to wigglemagnetic field

Exciter circuit 14A @ 100kHz

Second coil measures Alfvn wavedownstream

Experiment Goals1. Do waves travel along field lines?2. Does the wavelength change as theory

predicts?

3. Effects of neutral fraction

5

Funtion Generator

100 kHz

Amp

Helicon plasmaAlfvn wave

exciter detector

Cross correlation

Kelly: Withdrawn


18/23

Black Probe and Emitter aligned verticallyRed -----Emitter rotated 90Blue -----Probe and Emitter both at 90Green ---probe rotated 90

Data near 120kHz with peaks slightly shiftedto emphasize the amplitude differences.

Frequency (kHz)

Power

Tantalizing evidence of Alfvn waves inconclusive

0 50 100 150 200 250

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1 Cross spectral density shows

detected signal at expectedfrequency

Changing probe orientationshould modify signal

However, though some amplitude

variation, no definitive change Conclusion: Variation in magnitude does not

indicate Alfvn waves detected

100 150

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1


19/23

Current Status Of Experiment Density calculated from slope of k vs.

n = 3.40x1019 A little high, but not unreasonable

Emitter works well

Receiver signal noisyWorking onbetter 10MHz filter

Preliminary data indicates Alfven wavesobserved

k vs.


20/23

Magnetic Bubble Expansion Experiments

Radio lobe structure is a fundamental aspect of extra-galactic evolution Coronal Mass Ejections (CMEs) interact with the solar wind, and lead to

severe geomagnetic storms Nonlinear plasma physics issues cannot at present be resolved from

numerical computation or astronomical observations alone Laboratory experiments are needed

Zhang: Thurs. PM; UP8.00133Lynn: Thurs. PM; UP8.00134


21/23

Coaxial Magnetized Plasma Gun

Plasma gun injects bubble intobackground HelCat plasma

Spheromak or other geometry Studies of propagation, evolution andinteraction with background flows

Fast camera verifies plasma injection

Langmuir probe and spectroscopy [next

slides] measure propagation speed.

75 80 85 90 95 100 105 110 115 120 125 130

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

IsatCurre

nt(A)

Time (us)

Tip 1&2Tip 9&10

Jet initiation

Detached jet


22/23

New optical diagnostics 4m spectrometer

Ion drift velocity and temperaturemeasurements

Survey escelette spectrograph Impurity content

Electron temperature from line ratios

Fast cameras

CCD camera with 600 Hz video Framing camera with 1.2ns gate time

Major new measurements: Azimuthal drift velocity

4m spectrometer used to detect Dopplershifts

Fast, accurate drift measurement via

two-lens/bifilar fiber optic cable setup Mach probe 300-500m/s drifts were not

detectable via our optics

Plasma bubble jet velocities and photos Mach probe 3,100m/s velocity confirmed

optically to be 2,900m/s via Doppler shift

1.2ns 4SpecE camera used to obtain

bubble jet images

Reference Line from

Helicon Plasma

Measured Line from

Plasma Bubble

Leach: This session GP8.00110


23/23

Emissive Probe Direct measurement of plasma

potential Tungsten filament heated to emissive

temps. Filament swept with high voltage Probe voltage compared to swept voltage

to determine Ion Saturation Current Electron Temperature

Entire circuit is floating Status

Simulations display positive results Real sweep circuit testing well Created 4th plasma source Beam used to map lines of force and

electric field Useful for lining up probes

Basic Emissive Probe Schematic

Floating

Ground

CurrentSupply

HelCat

Beam from emissive probe

Documents

Bricette Cyrin et al- The HelCat Helicon-Cathode Device at UNM