RF current drive experiments on MST

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RF current drive experiments on MST

• Jay K. Anderson
• University of Wisconsin Madison

M. Burke, M. Cengher, W. A. Cox, C. B. Forest, J.
A. Goetz, M. C. Kaufman, S. M. McMahon, V.
Svidzinski, M. A. Thomas
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Summary

• Two experiments online
• Lower Hybrid
• Electron Bernstein Wave
• 100 kW power level each
• RF - plasma interaction occurs
• No dramatic reduction of magnetic fluctuations
  observed yet
• Upgrades to each system in progress

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Outline

• Motivation
• Electron Bernstein wave 3.6 GHz experiment
• Why the EBW?
• Coupling to the EBW
• 120 kW experiment online
• RF sources tested to long pulse (10 msec) at full
  power (75 kW)
• Current system uses 2 tubes to antenna
• Suitable antenna identified conditioned to 130
  kW
• Windows, transmission lines worthy for power
  upgrade
• Lower Hybrid 800 MHz experiment
• 80 kW experiment
• X-rays generated during LH experiments
• Next antenna under construction, 300kW
  capability

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Motivation RF can drive localized current
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EBW Current Drive Experiment

• Twin-waveguide antenna installed on MST
• BN antenna cover (not shown) has improved
  coupling.
• Coupling to EBW with this antenna studied in
  detail
• M. Cengher et al, Nucl. Fusion 2005 (sub)
• Antenna, RF sources, transmission line and vacuum
  windows conditioned to 120kW

Simulation
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Electron cyclotron resonance not accessible to EM
wave
Goal is to heat and drive current at cyclotron
resonance 3.6 GHz experiment in MST r/a
0.3 - 0.9 by changing Ip RFP plasma is
overdense 3.6 GHz electromagnetic wave is
cutoff 2 cm of antenna
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Coupling to EBW depends sensitively on edge
density.
Launched EM wave couples to Bernstein mode at
upper hybrid resonance
Cutoff ( L )
Cutoff ( R )
Upper hybrid resonance
Reflection occurs from each cutoff Distance
between layers determined by ne and B
profiles. Interference of reflected waves leads
to optimized transmission
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Coupling to EBW inferred from measured reflection

• Predicted dependence with edge density is
  measured.
• Oblique launch enhances coupling to EBW
• BN Antenna cover improves coupling
• Affects local electron density gradient
• Blocks plasma from entering antenna (source of
  arcing)

Simulation
data
Coupling can be good R/F lt 10 Ln n/Ñn of
MST plasmas is near ideal value for this
coupling.
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Coupling to EBW inferred from measured reflection

• Predicted dependence with edge density is
  measured.
• Oblique launch enhances coupling to EBW
• BN Antenna cover improves coupling
• Affects local electron density gradient
• Blocks plasma from entering antenna (source of
  arcing)

No cover
BN antenna cover
Simulation (no cover) Not in agreement.
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120 kW experiment successful

• Demonstrates RF sources good for 10 msec
  pulse
• Antenna, line and windows conditioned to gt 130 kW
• Total injected power exceeds 100 kW in best case
  to date

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Boron level enhanced by EBW injection.
PPCD plasmas typically have a good target density
profile for coupling. Two nearly identical
(before EBW) PPCD discharges No EBW low
boron, burst free. With EBW BIV increases with
time, m0 burst occurs. Need more power, less
boron
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4-tube, 4 waveguide system under construction

• Can be run in parallel with existing 120 kW
  system 360 kW available
• 1 year timescale.

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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

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Antenna loading measured through damping length

• Loops along antenna measure power vs distance
• Power launch optimized for Ldamp 8-10 cm (nll
  7.5)
• Damping length depends on edge conditions
• electron density,
• magnetic field strength and pitch

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Hard X-ray flux generated during LH operation

• X-rays coincident with coupled LH power
• (power at center of antenna)
• Spectrum peaked near 20 keV
• Detectors directly viewing antenna power feed
  measure highest flux
• Measureable flux viewing off antenna (same
  toroidal angle)

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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)

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Summary

• Two RF current drive scenarios under development.
• 100 kW level now, 300 kW upgrades in progress
• Both promise localized current drive
• Improve stability to resistive tearing modes
• Improved understanding coupling with MST-specific
  antennas
• EBW edge density key parameter PPCD profile
  near ideal
• LH edge density, magnetic field strength and
  field line pitch
• Localized puffing affects loading
• Hard x-ray flux generated