Profile Measurement of HSX Plasma Using Thomson Scattering

K. Zhai, F.S.B. Anderson, J. Canik, K. Likin , K. J. Willis, D.T. Anderson, HSX Plasma Laboratory, U. of Wisconsin, Madison

1. Abstract 3. The HSX TS System CalibrationAt the HSX plasma laboratory, a 10 channel Thomson scattering system has been established and is now operational. The system has been absolutely calibrated for density measurement using Raman scattering with nitrogen gas. It is found that for the QHS configuration the electron temperature gradually decreases when we increase the density at a fixed ECRH power and that the whole temperature profile increases with the heating power at a fixed plasma density. The central temperature increases from about 500eV to 950eV while the launched heating power increases from 37KW to 150KW for the QHS configuration plasma with a density of 1.51012cm-3. At the present density and heating power, the difference between the QHS and Mirror mode is not pronounced. Detailed results will be presented at the conference.

*Work supported by US DoE under grant DE-FG02-93ER54222

• The image spot is less than 100 microns for all the ten channels.

• Transmission efficiency of the fiber bundle is around 60%.

• At plasma density of 1012/cm3, the collection lens collects ~20000 photons for each of the ten fibers, which then couple the photons to the ten polychromators through fiber bundles.

3.a Spectrum Dispersion Calibration

4. Experiment Results

4.b QHS Mode vs Mirror Mode

4.e ECH Off-axis Heating

• Ten-Channel HSX TS system for electron temperature and density measurement has been calibrated and is now operational for daily routine service.

• Peaked electron temperature and density profile has been observed for QHS mode. • Temperature increases with ECH heating power at fixed density and decreases as density increase at fixed heating power.• Off-axis ECH heating affects plasma profiles for both QHS and Mirror mode• Signal in different spectral channels indicates the existence of the superthermal electrons.

2.The HSX Device and HSX TS System

Major Radius 1.2 m

Average Plasma Minor Radius 0.15 m

Plasma Volume ~.44 m3

Rotational Transform

Axis1.05

Edge 1.12

Number of Coils/period 12

Magnetic Field Strength (max) 1.25 T

Magnet Pulse Length (full field) 0.2 s

Auxiliary Coils (total) 48

Heating source 28GHz

200 kW

HSX (the Helically Symmetric eXperiment) is a new concept in toroidal stellarators that is operational at the HSX Plasma Laboratory of the University of Wisconsin - Madison. It is the only device in the world that will have a magnetic field structure that has been termed Quasi-Helically Symmetric (QHS)

2.a HSX Parameters

2.b HSX TS System Introduction

The Thomson scattering system installed on Helical Symmetry Experiment (HSX) is a polychromator-type system, which covers the complete plasma cross section, providing a 10-point plasma parameter profile measurement during a single plasma shot. The system consists of a commercial 1J-10nsYAG laser, 10 polychromators from GA, the specially designed collection optics, a CAMAC data acquisition unit, and a controlling computer.

The spectral calibration determines the response of the detection system to a radiation source of constant spectral emissivity. The result of spectral calibration will be used to build a look-up-table for electron temperature measurement.

3.b Absolute Signal Calibration – Raman Scattering

Calibration stable over time.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

200

400

600

800

1000

Ele

ctr

on

Tem

pera

ture

(eV

)

Radius r/a

37KW 50KW 100KW 150KW

This shows that the whole temperature profile increase with the heating power at the fixed density of 1.51012/cm3. The central temperature increases from about 500eV to 950eV while the heating power increases form 37KW to 150KW.

0.0 0.2 0.4 0.6 0.80

200

400

600

800

0.5x1012

/cm3

1.0x1012

/cm3

1.6x1012

/cm3

2.0x1012

/cm3

Ele

ctr

on

Tem

pera

ture

(eV

)

Radius (r/a)

• QHS mode, 50 kW ECRH heating power

• This result shows that the temperature gradually decreases when we increase the density at a fixed ECRH power.

1.2 1.6 2.0

1.2

1.6

2.0

QHS Mode

Inte

rfe

rem

ete

r d

en

sity

(1

012/c

m3 )

TS density (1012

/cm3)

0.0 0.2 0.4 0.6 0.8 1.00

200

400

600

800

QHS Mirror

Ele

ctro

n te

mpe

ratu

re (

eV)

r/a0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Ele

ctro

n D

ensi

ty(1

012/c

m3 )

r/a

QHS, Interferometer QHS, TS Mirror, Interferometer mirror, TS

0.0 0.2 0.4 0.6 0.80

200

400

600

800

Ele

ctro

n T

empe

ratu

re (

eV)

r/a

TS time at 820 ms TS time at 820.5 ms TS time at 821 ms

0.79 0.8 0.81 0.82 0.83 0.84 0.85 0.86 0.870

0.5

1

1.510/22/04 HSX Mode = QHS AntiClockwise

0.79 0.8 0.81 0.82 0.83 0.84 0.85 0.86 0.87

0

20

40

0.79 0.8 0.81 0.82 0.83 0.84 0.85 0.86 0.870

1

2

Line Average Density (1012

cm-3

)

Stored Energy (J)

ECH Power

0

200

400

600

Te(

eV)

QHS center heating QHS offaxis heating at 0.4

0.0 0.2 0.4 0.6 0.8 1.00.0

0.5

1.0

1.5

2.0

2.5

Ne(

1012

/cm

3 )

r/a

100

200

300

Te

(eV

)

Mirror center heating Mirror offaxis heating at r/a=0.4

0.2 0.4 0.6 0.8 1.00.4

0.8

1.2

1.6

2.0

r/a

Ne(

1012

/cm

3 )

Ratio of signals in different spectral channels

Electron Temperature (eV)

0 500 1000 1500 20000

5

10

15

20

ratio21 ratio31 ratio41 ratio32 ratio42 ratio43

Since the stray light contribution to the first wavelength channel makes the Rayleigh scattering troublesome, we use rotational Raman scattering for density calibration. Nitrogen gas is chosen For safety reason and the second spectral channel extending from 1050 to 1060 nm is used for the calibration, which cover the most Raman scattering power.

The TS signal from spectral channel i, after allowance for channel sensitivity, will be

For the Raman scattering, the scattered light corresponding to the rotational transition J>J-2 is given by

in which

Electron density is then given by

dTa

rLPnS ie

Te

Ti )(),(

20

0

Raman signal from channel i after allowance for channel sensitivity

Principles of Raman Scattering

850 900 950 1000 1050 1100-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

ch1 ch2 ch3 ch4 Te=50eV Te=100eV Te=200eV Te=500eV Te=1000eV te=1500eV te=2000eV

Wavelength (nm)

Spectral response function and TS scattering spectrum for different

electron temperature.

Raman Scattering Calibration Results

1045 1050 1055 1060 1065

0.0

0.2

0.4

0.6

0.8

1.0

0.00

0.61

1.22

1.84

2.45

3.06

Ram

an S

catt

erin

g C

ross

Sec

tio

n (

10-3

6 cm2 )

Ch

ann

el 2

sp

ectr

al r

esp

on

se a

nd

R

aman

Sca

tter

ing

lin

e cr

oss

sec

tio

n (

a.u

.)

Wavelength (nm)

Most Raman scattering power collected in second spectral channel

0 1 2 3 4 5 6 7 8 9 100.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

10_21_04/10_12_04 10_28_04/10_12_04 11_04_04/10_12-04

Raman scattering signal changes linearly with gas pressure.

0 20 40 60 80 100 120

0

50

100

150

200

250

300

Dig

itiz

er c

ou

nt

Gas pressure (torr)

polychromator 1 polychromator 2 Linear Fit of Linear Fit of

The calibration factor is defined by the slope of the fitted line

TS results compared with interferometer results

4.a

Pla

sma

Den

sity

Sca

n an

d

EC

H P

ower

Sca

n fo

r Q

HS

Mod

e

• Electron temperature is a little higher in QHS mode than Mirror mode.

• Density profile is more peaked in QHS mode and flat in Mirror mode.

• TS measurement gives similar density profile compared with the inverted density profile from interferometer measurement.

4.c ECH Heating Power Modulation

• Energy confinement time from diamagnetic flux measurement: ~0.8 ms

• Electron temperature drops during ECH modulation of 1 ms.

4.d Superthermal Electron Tail

Mirror mode * Electron temperature increases at heating location.

* Density profile changes to a center-peaked shape compared with the flat shape of the normal central heating mirror mode.

QHS mode * Electron temperature peaks at the heating location and decreases dramatically at center compared with central heating.

* Shape of plasma density profile doesn’t change with off-axis heating.

Channel 4 have unusual high signal level compared with channel 2 and 3.

Fitting the Thomson Scattering signal with bi-Maxwellian electron distributionftotal=(1-p) f(Tb)+p f (Ttail),we can separate the superthermal electrons’ contribution to the scattering signal.

•Significant tail in center region at ECH central heating.

•Tail temperature around 5-8 KeV

•Increasing ECH heating power increases the tail fraction.

0 0.2 0.4 0.6 0.8 10

200

400

600Central heating power 70kWCentral heating power 40kW0ffaxis heating, 40kW

0 0.2 0.4 0.6 0.8 14000

6000

8000

0 0.2 0.4 0.6 0.8 10

0.25

0.5

r/a

Bulk Electron Temperature (eV)

Tail Electron temperature (eV)

Tail fraction 0

20

40

60

80

Sto

red

Ene

rgy

(J)

E=Te*Ne

E from Diamagnetic loop E=Te_bulk*Ne_bulk+Te_tail*Ne_tail

Integrated stored energy compared with the diamagnetic

flux measurement.

5. Summary

Superthermal electron at different ECH heating power and location at QHS mode of 1.51012/cm3.

70KW 40KW 40KW off-axis

Spe

ctra

l Res

pons

e (a

.u.)

Rat

io

Spatial channel number