RF Heating and Current Drive
Experiments on MST
Jay Anderson for the MST team
14th IEA RFP Workshop
Summary
• Two rf experimental approaches are underway,
complementary strengths
– Lower hybrid: established physics, technically
challenging antenna
– Electron Bernstein wave: simple antenna, complicated
wave coupling in RFP edge plasma
• Modest power (~100kW) shows rf-plasma interaction
– Power levels too low for significant current drive,
observing and understanding any effect is encouraging.
– EBW: localized SXR increase
– LHCD: localized HXR emission
• Power upgrades under development for each
experiment
14th IEA RFP Workshop
Outline
• Motivation
• EBW
– Coupling between edge EM and EBW waves occurs
• Blackbody-level emission measured: EBE
• Reflected power ratio, wave electric fields from grill
antenna understood
– SXR enhanced during select launch conditions
– Field error induced by port hole has substantial effect
– Upgrade to MW level source power underway
• LHCD
– Generation of large HXR flux with injection of ~100kW
• Toroidal localization, asymmetries understood
• Particle trapping and guiding center drifts important
– Upgrade to 400+kW system underway
14th IEA RFP Workshop
Motivation
14th IEA RFP Workshop
EBW Heating and Current Drive
Configuration
Heating
Current Drive
Stellarator


Tokamak


ST

RFP

EBW is efficiently damped at
cyclotron resonance; coupling
power to the EBW is key issue
Genray/CQL3D case, zero diffusion
This would be an interesting
experiment in the RFP
14th IEA RFP Workshop
RFP geometry is challenging for RF heating/ CD
Goal: heat and drive current at
cyclotron resonance
Overdense: wp >> wc
EM waves not accessible to ECR
There is no high field side.
Mode conversion at UHR, in
antenna near-field, is critical
Edge density fluctuations hinder
coupling, particularly O-mode
Field error caused by hole in conducting shell (MST) has deleterious effect on coupling,
Limits maximum size of antenna. 14th IEA RFP Workshop
Coupling to EBW in MST: X-mode launch
EBW is an electrostatic wave
carried by gyromotion of electrons.
Blackbody levels of EBE
demonstrate coupling
R/F from waveguide grill understood
in terms of local density
lvac ~ 8 cm
l B ~ 1mm
Simulation
data
14th IEA RFP Workshop
Correct interguide phase (grill antenna) is
critical for coupling to EBW
• Interguide phasing critical parameter
in optimization of coupling.
• BN Antenna cover improves coupling
– Affects local electron density gradient
– Blocks plasma from entering antenna
(source of arcing at high power)
14th IEA RFP Workshop
No cover, PPCD
BN cover
simulation
BN cover steepens local density gradient
Effect of field error:
field lines protrude into
antenna in port
No cover, PPCD
Antenna cover acts as
limiter due to field error.
14th IEA RFP Workshop
BN cover
Measurement of wave E in plasma
• Crossed dipole RF probe measures Er,
Ef within plasma (few cm)
– Probe position scanned for fixed ne
– Probe position fixed for evolving ne
Cold plasma dispersion:
tan 2   
 P  n 2  L  n 2  R 
  90
 Sn2  RL  n2  P 
For x-mode launch
2
w
RL
p
2
S

1

n 
w 2  wc2
S
iD


E   Er 
E0 Ef  E0 E  0 
S


At upper hybrid resonance
wUH  w p2  wc2
S 0
Electric field becomes longitudinal
14th IEA RFP Workshop
Wave E field within plasma consistent with EBW
Discharge reaches state
where |B|, ne(edge), and
antenna phase (scanned) are
optimal.
Vacuum: Er/ Ef ~ 0: TEM
Probe at Xuh: Er/ Ef > 1
Recall cold plasma dispersion:
iD

S
E 0  Er 
E0
S

Ef  E0

E  0 

14th IEA RFP Workshop
Localized SXR measured with EBW injection
Genray predicted
ray trajectory
PPCD discharge
PPCD + rf
RFX SXR camera,
Measuring 4-7 keV
14th IEA RFP Workshop
SXR enhancement requires good confinement
Net Power in
• 4 arm antenna, ~130 kW
forward power.
• PPCD discharge.
• SXR, outboard edge.
• Signal <0 during
confinement loss; real effect
of rf pickup
• m=0 indicator of PPCD
quality
• Qualitatively in agreement
with CQL3D: Diffusion
reduces emission.
• Boron injected into plasma
during rf; emission enhanced
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EBW experiment upgrading to MW level
• Move from 3.6 GHz to 5.5 GHz
system (tube availability)
– Target discharge higher Ip
– Shorter wavelength, smaller
antenna, smaller porthole
• Goal: Demonstrate feasibility
of MW level EBW experiment
– Optimize launch through 11cm
port
– Test power capability in 5cm
port
– Test OXB scheme; very
simple with cylindrical
antenna.
1 MW generated in bench test,
20 April 2010
14th IEA RFP Workshop
EBW experiment upgrading to MW level
• Move from 3.6 GHz to 5.5 GHz
system (tube availability)
• Prototypes being tested:
– 1/4 l quartz vacuum window
– Circular choke joint
– Cylindrical molybdenum antenna
14th IEA RFP Workshop
Lower Hybrid Current Drive
•
Fokker-Planck modeling predicts
efficient current drive
– 0.5 A/W at 250 MHz
•
Experiments ongoing at 800 MHz
– Efficiency still quite high: ~0.3 A/W
– Physical size of antenna more tenable
– Make use of existing klystrons
14th IEA RFP Workshop
Meticulously designed antenna successful
to klystron power limit
•
•
800 MHz launcher
– Interdigital line antenna.
– Power (up to 220kW) fed in one port,
then along structure
– co-, counter- CD by choice of port
Clear RF/ plasma interaction:
– Hard x-rays generated
14th IEA RFP Workshop
Large HXR Flux Generated During LHCD
Viewing chords look across MST toward antenna.
Large flux up to 40keV, intensity follows electric field strength.
14th IEA RFP Workshop
Strong near-field E accelerates electrons
• Test particle computation: e- initially 40eV Maxwellian
• Single pass through antenna electric field (COMSOL) shows
acceleration to ~50 keV, mostly perpendicular
• Particle trapping.
• Directionality in parallel velocity, consistent with proposed wave
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Launch Direction, Toroidal HXR Asymmetries
Stronger flux to lower toroidal angle
- consistent with drift of trapped particle orbit
Higher flux for Co- launch than Counter- 2nd pass through antenna more likely.
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LH Summary
• Successful antenna designed for strict space constraints in MST
– Small port holes for coaxial power feeds
• Strong HXR flux in antenna near field is understood: Acceleration of
plasma electrons via Lorentz force
– COMSOL modeling of antenna E field
– Test particle calculation shows electrons are primarily heated in
perpendicular direction
• Explains existence of localized high energy x rays.
• Explains co-, counter- magnitude difference and toroidal
asymmetry
– Also shows directional current drive qualitatively consistent with
Fokker-Planck modeling: asymmetry in parallel speed near 0.2c
• Complete power accounting is required:
– Measured HXR flux does not consume full radiated power
– Near term plans are to double input power: 2 tubes.
14th IEA RFP Workshop
Summary
• Two rf current drive schemes are being tested on MST
– EBW: Simple antenna, coupling verified.
• Building MW level experiment, rf source tested short pulse.
– Lower Hybrid: Complex antenna, successful to 200+ kW
• HXR generation explained by large perpendicular E in
antenna near-field
• Computed near-field effect also shows parallel directionality
– Yet to be measured
• Next step: Double power with 2nd tube, 2nd antenna.
• Broader impact than just MST/ RFP:
– EBW, LH waves are of general interest in high b plasmas
– Ongoing modeling: Fokker-Planck and ray tracing
validation in unique parameter space (RFP)
14th IEA RFP Workshop
Second pass of inboard-going trapped e-
Test particle initial distribution: inboard-travelling trapped
electrons from first pass calculation.
Co- current direction now has higher density of 30-50 keV e14th IEA RFP Workshop
EBW current drive efficiency in MST: TBD
Fisch-Boozer and Ohkawa effects both factors in MST
Ohkawa
Fisch-Boozer
EBW resonance
Example
EBW CD h
@ MST Te, ne
COMPASS-D
0.2 A/W
0.09 A/W
W7-AS
0.005 A/W
0.05 A/W
Zero diffusion
0.15 A/W
D a vll
-0.02 A/W
CQL3D for MST with:
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Four waveguide grill, heating experiments
Optimum phasing of 4-guide
antenna qualitatively similar
to that of 2-guide grill
Sustained good coupling at > 100kW
14th IEA RFP Workshop
EBW Hardware Upgrades: Power Supply
• Require -80kV at
40A for 10-20ms
to run klystron
tube
• 0.3F at 1200V
capacitor bank
• 3 phase IGBT
inverter 1200V at
5000A
• Resonant
transformers
• Voltage doubling
rectifier
• Harmonic filtering
for low ripple
14th IEA RFP Workshop
Empirical power handling: Waveguide grills
This may give insight to:
How much power can we
get through the antenna?
14th IEA RFP Workshop
Pericoli et al Nuc Fusion 2005
X
Empirical power handling: Waveguide grills
This may give insight to:
How much power can we
get through the antenna?
X
X EBW 3.6 GHz achieved
X
X EBW 3.6 GHz proposed
X 5.5 GHz: 1 MW, 4.5” port
X
X 5.5 GHz: 1 MW, 2” port
EBW on MST is different than
LH grills on tokamaks:
X-mode launch: E perp. to B0
may enable higher power
density.
14th IEA RFP Workshop
Pericoli et al Nuc Fusion 2005
X-mode to EBW Conversion
•
•
•
•
•
•
•
•
•
•
Fast X-mode launched from RFP edge
Cold plasma approximation valid for Fast X-mode region
X-mode wave crosses R cutoff layer and begins to
evanescently decay
Steep edge density gradient leads to closely spaced R,
UH, and L layers leading to efficient coupling
Slow X-mode propagates between UH and L layers
Electric field becomes predominantly parallel to k near
UH layer
Slow X-mode reflected off of L
Interference between UH and L minimizes reflected
wave traveling past UH
Mode conversion to EBW between UH and L layers
EBW propagates past L layer into plasma
Cold plasma approximation
 P  n 2  L  n 2  R 
2
tan   
 Sn2  RL  n2  P 
For x-mode launch
w
RL
S  1 2 p 2
n 
w  wc
S
At upper hybrid resonance
2
2
wUH  w p2  wc2
S 0
Electric field becomes
predominantly longitudinal
iD

E   Er 
E0
S

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Ef  E0

E  0 

  90
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Raw SXR vs input power level
14th IEA RFP Workshop
X-Mode launch coupling to EBW
• No high field side in RFP;
fast X-mode launch.
• Evanescent layer
encountered at R cutoff
• Width of layer sensitive
to edge density profile,
typical value ~2cm
14th IEA RFP Workshop
OXB Conversion in MST
• Most other machines
use OXB conversion
scheme for heating
and current drive
(most others at higher
field)
• OXB efficiency on
MST is less than XB
efficiency
14th IEA RFP Workshop
EBE verifies mode conversion
Conversion efficiency h ~TEBE/T
h X mode > h O mode
14th IEA RFP Workshop
Coupling to EBW in MST
Launched EM wave couples to
Bernstein mode at
upper hybrid resonance
In near field of antenna
~ 2 cm
lvac ~ 8 cm
l B ~ 1mm
Reflection occurs from
each cutoff;
Distance between layers determined
by ne and B profiles.
Interference of reflected waves leads
to optimized transmission
14th IEA RFP Workshop
Coupling Improvements available at 5.5 GHz
S-band antenna in 4.5” port
Antenna cover acts as
limiter due to field error.
C-band antenna (~2” OD) in 4.5” port
Insertion to steeper Ln possible;
Partial field error mitigation
Field error reduction by
use of smaller port:
C-band antenna in 2” port
14th IEA RFP Workshop
EBW high voltage supply transformer
• Resonant secondary configuration
– Parallel LC resonator
– Large leakage inductance
• 20 turn primary, 160 turn secondary
(8:1)
• 50:1 voltage multiplication due to
resonance
• Microcrystalline iron core, low
hysteresis loss at high frequency
• 20kHz operation for low output ripple
• 3 phase Y configuration, center tap
connected to rectifier positive terminal
• Oil filled secondary
14th IEA RFP Workshop
EBW Hardware Upgrades: Waveguide
and Launcher
•
•
•
•
•
Previous copper rectangular waveguide arced in
vacuum with 3.6GHz at 150kW
Now injecting 5.5GHz at 1MW
Rectangular to circular transition
Circular fused silica RF window and choke joint
transition
Cylindrical molybdenum waveguide
– Cylindrical waveguides have lower electric
fields reducing arcing risk
– Molybdenum has high electron affinity and
good plasma damage resistance.
– Possible use without boron nitride limiter
– Capable of using smaller port on MST
14th IEA RFP Workshop
Lower Hybrid Current Drive Experiment
• 800 MHz launcher
– In MST vacuum vessel.
– Power fed through antenna
(more in than out)
– 80+ kW at present
• Antenna loading depends on
edge plasma conditions
– Localized puffing used for
density control
• Clear RF/ plasma interaction
observed:
– Hard x-rays generated
• Upgrade to 320+ kW in
progress
Particle trapping, toroidal drift explain
asymmetries
Delta phi 6-15 cm
Antenna aperture ~3cm
Delta_phi short way = 05cm
14th IEA RFP Workshop
Inference: C-band coupling via emission
Still needs to be measured in
launch mode; different geometry
Conversion efficiency h ~Tebe/T
h @ 5.5 GHz > h @ 4. GHz
h X mode > h O mode
14th IEA RFP Workshop
Next Step: ~300 kW Antenna
•
•
Larger coax feed through; expect 320 kW power handling capability
RF source development under way (need to run outside design parameters
for pulsed experiment)