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RF Heating and Current Drive Experiments on MST

RF Heating and Current Drive Experiments on MST. Jay Anderson for the MST team. Summary. Two rf experimental approaches are underway, complementary strengths Lower hybrid: established physics, technically challenging antenna

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RF Heating and Current Drive Experiments on MST

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  1. RF Heating and Current Drive Experiments on MST Jay Anderson for the MST team 14th IEA RFP Workshop

  2. 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

  3. 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

  4. Motivation 14th IEA RFP Workshop

  5. EBW Heating and Current Drive 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

  6. 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

  7. 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

  8. 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) No cover, PPCD BN cover simulation 14th IEA RFP Workshop

  9. BN cover steepens local density gradient Effect of field error: field lines protrude into antenna in port No cover, PPCD BN cover Antenna cover acts as limiter due to field error. 14th IEA RFP Workshop

  10. 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: For x-mode launch At upper hybrid resonance Electric field becomes longitudinal 14th IEA RFP Workshop

  11. 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: 14th IEA RFP Workshop

  12. Localized SXR measured with EBW injection Genray predicted ray trajectory RFX SXR camera, Measuring 4-7 keV PPCD discharge PPCD + rf 14th IEA RFP Workshop

  13. 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 14th IEA RFP Workshop

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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 14th IEA RFP Workshop

  20. 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. 14th IEA RFP Workshop

  21. 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

  22. 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

  23. 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 e- 14th IEA RFP Workshop

  24. EBW current drive efficiency in MST: TBD Fisch-Boozer and Ohkawa effects both factors in MST Ohkawa Fisch-Boozer EBW resonance 14th IEA RFP Workshop

  25. 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

  26. 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

  27. Empirical power handling: Waveguide grills This may give insight to: How much power can we get through the antenna? Pericoli et al Nuc Fusion 2005 14th IEA RFP Workshop

  28. 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 EBW 3.6 GHz proposed X 5.5 GHz: 1 MW, 4.5” port 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. X X Pericoli et al Nuc Fusion 2005 14th IEA RFP Workshop

  29. 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 For x-mode launch At upper hybrid resonance Electric field becomes predominantly longitudinal 14th IEA RFP Workshop

  30. 14th IEA RFP Workshop

  31. Raw SXR vs input power level 14th IEA RFP Workshop

  32. 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

  33. 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

  34. EBE verifies mode conversion Conversion efficiency  ~TEBE/T • X mode >  O mode 14th IEA RFP Workshop

  35. Coupling to EBW in MST Launched EM wave couples to Bernstein mode at upper hybrid resonance In near field of antenna Cutoff ( L ) Cutoff ( R ) Upper hybrid resonance ~ 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

  36. Coupling Improvements available at 5.5 GHz S-band antenna in 4.5” port C-band antenna (~2” OD) in 4.5” port Insertion to steeper Ln possible; Partial field error mitigation Antenna cover acts as limiter due to field error. Field error reduction by use of smaller port: C-band antenna in 2” port 14th IEA RFP Workshop

  37. 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

  38. 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

  39. 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

  40. Particle trapping, toroidal drift explain asymmetries Delta phi 6-15 cm Antenna aperture ~3cm Delta_phi short way = 0-5cm 14th IEA RFP Workshop

  41. Inference: C-band coupling via emission Still needs to be measured in launch mode; different geometry Conversion efficiency  ~Tebe/T • @ 5.5 GHz >  @ 4. GHz • X mode >  O mode 14th IEA RFP Workshop

  42. 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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