lower hybrid antenna coupling and power handling experiments...

APS DPP00 Meeting October 23-27, 2000

Lower Hybrid AntennaCoupling and Power Handling

Experiments on MST †

M.A. THOMAS, P.K. CHATTOPADHYAY, G. FIKSEL, C.B. FOREST, University of Wisconsin--Madison, M.D. CARTER, P.M. RYAN, F.W.BAITY, Oak Ridge National Laboratory, R.I. PINSKER, GeneralAtomics, E. UCHIMOTO, University of Montana--Missoula

† Work Supported by U.S.D.O.E.

1

thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

Abstract

A system for launching slow lower hybrid waves into

MST at 800 MHz and n||=7.5 has been implemented. The

antenna is an enclosed interdigital line using 1/4 wave-

length resonators with an opening in the cavity through

which the wave is coupled to the plasma. Results of low

power (10 W) experiments indicate that antenna loading

is density dependent, and that a directional traveling wave

is launched. Results of higher power experiments will be

presented. Diagnostics include antenna impedance

measurements and probes to measure wave fields within

the plasma. The power handling capabilities of the present

antenna design will be investigated. The results of antenna

loading and RF probe measurements will be compared with

the results of numerical modeling.

2

thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

Introduction and Status

• LHCD Antenna has been installed in MST

• Up to 20 kW RF power applied at 800 MHz

• Plasma loading of inboard and outboard antenna ports is asymmetric

• Scalar reflection coefficient at inboard port decreases with applied power

• RF Probes and diagnostics still under development

• Computer modeling of antenna proceeding

3

thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

LHCD Antenna Construction

Interdigital Line Detail Full Assembly

The antenna consists of an interdigital slow wave structure, enclosed in a cavity, with an aperture for electrostatic coupling to the plasma. Phase between rodsis π /2. One port is powered and the other terminated. Wave launch direction is reversed by interchanging driven and terminated ports.

4

thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

LHCD Antenna installed in MST

• Plasma facing surface and rods are Molybdenum, edge limiter is Boron Nitride

• Antenna still intact after 1 year in vessel(photo taken at installation 10/99)

5

thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

Video Images

During initial high power operation, some localized antenna-plasma interaction was observed:

Inboard Port Outboard Port

• After several shots the light emission decreased to zero, suggesting gas desorption

• Presently no light emission is seen for input power as high as 20 kW

• A CCD Spectrometer viewing the antenna directly shows no indication of Cu or Mo impurity lines

6

thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

Instrumentation

RF transmission system schematic:

• Directional couplers with diode detectors used to monitor RF power in antenna feed and termination

• |S11| and |S12| calculated

• Improved electronics under development for vector measurements of antenna impedance

7

thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

Typical Inboard Port Data

Ip = 425 kA ne = 6x1012 cm-3 Prf = 10 kW

Date = 2-OCT-2000 Shot =13 Inboard Port�

0

5.0•1031.0•1041.5•1042.0•104

For

war

d (W

)

0

200

400

600

800

Ref

lect

ed (

W)

�

0

20

40

60

80

Thr

ough

(W

)

�

20� 22� 24� 26� 28� 30�t (ms)

0.00.20.40.60.81.0

|s11|,

|s12|

• Input reflection coefficient decreases smoothly as input power rises and impedance match improves

• Less than 0.5% of input power exits the outboard port to the load (see next slide)

8

thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

Typical Outboard Port Data

Ip = 425 kA ne = 6x1012 cm-3 Prf = 10 kW

Date = 2-OCT-2000 Shot =12 Outboard Port�

0

5.0•1031.0•1041.5•1042.0•104

For

war

d (W

)

0

2000

4000

6000

8000

Ref

lect

ed (

W)

�

0

10

20

30

40

Thr

ough

(W

)

�

20� 22� 24� 26� 28� 30�t (ms)

0.00.20.40.60.81.0

|s11|,

|s12|

• Input reflection coefficient remains high at all input power levels, impedance match is poor in the presence of plasma

• Outward equilibrium shift may contribute to asymmetry in plasma loading of antenna cavity; inboard BN limiter does not fully shadow outboard end of antenna aperture

9

thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

Vacuum vs. Plasma Loading

Date = 21-SEP-2000 Shot =11 Outboard Port�

0

500

1000

1500

2000

For

war

d (W

)

0

200

400

600

800

Ref

lect

ed (

W)

0

200

400

600

800

Thr

ough

(W

)

20� 22� 24 26� 28� 30�t (ms)

0.00.20.40.60.81.0

|s11|,

|s12|

Date = 21-SEP-2000 Shot =45 Inboard Port�

0

50

100

150

200

For

war

d (W

)

0

20

40

60

80

Ref

lect

ed (

W)

0

10

20

30

40

Thr

ough

(W

)

44 46� 48� 50� 52� 54 t (ms)

0.00.20.40.60.81.0

|s11|,

|s12|

Vacuum Plasma Termination at 50.7 ms

• Both ports are well matched in vacuum

• Vacuum impedance match is restored within ~0.5 ms of plasma termination at low power

• A distributed limiter is being designed to protect the antenna cavity from internal plasma loading and will consist of an array of narrow Boron Nitride strips across the antenna aperture

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thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

Computer Modeling

• The Interdigital Line (IDL) coupled to MST plasmas is being modeled at ORNL using RANT3D and GLOSI

• 90 degree phasing between elements gives desired spectrum

• Electric and magnetic fields are found to be appropriate for launching the lower hybrid wave

• Power absorption is consistent with a damping length of ~40cm, approximately twice the length of the present antenna

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thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

Simulation Geometry

Preliminary RANT3D model of Inter-Digital Line for MST800 MHz, truncated side-walls, fixed π/2 incremental phasingcurrent amplitudes linearly adjusted to balance power betweeninput, output, and launched into the plasma

Recessedfirst wall

Projectingmount

location

"Digits"exposed

to plasma

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thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

Parameters for Warm Plasma Slab Model

poloidal�

toroidal�

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thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

E Field and Absorbed Power vs. Minor Radius

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thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

Electric Field

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@A:;S<T B<:U@AV V<:;TW@ X<:;C<YI0Z�/<F = &)H�I0&)1E&)+E-0/<14JLK[M\O*&,5]I0&,R = R�%

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Poloidal electric field and phase are correct for lower hybrid~37% power to plasma, ~63% of power to the dump

15

thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

Magnetic Field

0.0e+00_

5.0e-06`

1.0e-05

modulus of RF B componenta

-0.1

0-0

.08

-0.0

6-0

.04

-0.0

2-0

.00

0.02

0.04

0.06

0.08

0.10

-0.0

40

-0.0

20

0.00

0

0.02

0

0.04

0

polo

idal

(m

)

toroidal (m)b

Poloidal RF component

-0.1

0-0

.08

-0.0

6-0

.04

-0.0

2-0

.00

0.02

0.04

0.06

0.08

0.10

-0.0

40

-0.0

20

0.00

0

0.02

0

0.04

0

polo

idal

(m

)

toroidal (m)b

Toroidal RF component

Approximately consistent magnetic field componentsfor IDL in MST using RANT3D model

16

thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

Wave Spectrum

cd�efff!gcfd

f efff!ghff

d efff gcfd

ijklnmkgop mqdd

rsttrutt

t

utt

stt

v)wyxzx x wWxzx-1

-1

Plasma response to main driving polarization

{ eff!g cfd

d�eff g cfd

d�e|}g cf~

� o�i�� g��j�gk�gklj� g

rsttrutt

t

utt

stt

v)wyxzx x wWx�x-1

-1

Dominantmodes matchscale of IDL

IDL launches power at desired angle

17

thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

Summary

• LHCD System is installed and functional, plasma loading problems are presently being addressed

• A limiter is being designed to control density along front of antenna

• 3D electromagnetic computer modeling of antenna coupled to plasma is ongoing

• Preliminary results are in good agreement with previous circuit simulation and measured fields of bench prototype, and also indicate desired plasma response

18

thomas@davros.physics.wisc.edu

APS DPP00 Meeting October 23-27, 2000

Reprints

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thomas@davros.physics.wisc.edu