lower hybrid antenna coupling and power handling experiments...
TRANSCRIPT
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.
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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.
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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
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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.
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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)
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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
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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
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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)
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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
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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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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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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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APS DPP00 Meeting October 23-27, 2000
Parameters for Warm Plasma Slab Model
poloidal�
toroidal�
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APS DPP00 Meeting October 23-27, 2000
E Field and Absorbed Power vs. Minor Radius
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APS DPP00 Meeting October 23-27, 2000
Electric Field
�������������������������������������������������������������
�������
������� ����
��������
������
��� ���! �"#$
%'&)(*&,+.-0/21436587
92:;9<9<9<=)>?929 @A:;9<9<92=)>?9<B C<:;9<9<92=)>?9<B58&)-0D�1ED�FG&)H�I0&)1E&)+E-0/<14JLKNMPOQ&)58I0&)R = R�%
@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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%'&)(*&)+E-0/<143^587
Poloidal electric field and phase are correct for lower hybrid~37% power to plasma, ~63% of power to the dump
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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
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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
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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
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