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

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 This would be an interesting experiment in the RFP ConfigurationHeatingCurrent Drive Stellarator   Tokamak   ST  RFP  Genray/CQL3D case, zero diffusion EBW is efficiently damped at cyclotron resonance; coupling power to the EBW is key issue

6. 14th IEA RFP Workshop RFP geometry is challenging for RF heating/ CD Goal: heat and drive current at cyclotron resonance Overdense:  p >>  c 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.

7. 14th IEA RFP Workshop Coupling to EBW in MST: X-mode launch  ~ 1mm vac ~ 8 cm 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 Simulation data

8. 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 simulation BN cover Correct interguide phase (grill antenna) is critical for coupling to EBW 14th IEA RFP Workshop

9. BN cover steepens local density gradient 14th IEA RFP Workshop 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.

10. Measurement of wave E in plasma Crossed dipole RF probe measures E r, E  within plasma (few cm) –Probe position scanned for fixed n e –Probe position fixed for evolving n e 14th IEA RFP Workshop For x-mode launch At upper hybrid resonance Electric field becomes longitudinal Cold plasma dispersion:

11. 14th IEA RFP Workshop Wave E field within plasma consistent with EBW Vacuum: E r / E  ~ 0: TEM Discharge reaches state where |B|, n e (edge), and antenna phase (scanned) are optimal. Probe at Xuh: E r / E  > 1 Recall cold plasma dispersion:

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

13. SXR enhancement requires good confinement 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 Net Power in 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

15. 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  quartz vacuum window –Circular choke joint –Cylindrical molybdenum antenna

16. Lower Hybrid Current Drive 14th IEA RFP Workshop 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

17. Meticulously designed antenna successful to klystron power limit 14th IEA RFP Workshop 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

18. Large HXR Flux Generated During LHCD 14th IEA RFP Workshop Viewing chords look across MST toward antenna. Large flux up to 40keV, intensity follows electric field strength.

19. Strong near-field E accelerates electrons 14th IEA RFP Workshop 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

20. Launch Direction, Toroidal HXR Asymmetries 14th IEA RFP Workshop Stronger flux to lower toroidal angle - consistent with drift of trapped particle orbit Higher flux for Co- launch than Counter- - 2 nd pass through antenna more likely.

21. 14th IEA RFP Workshop 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.

22. 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 2 nd tube, 2 nd antenna. Broader impact than just MST/ RFP: –EBW, LH waves are of general interest in high  plasmas –Ongoing modeling: Fokker-Planck and ray tracing validation in unique parameter space (RFP)

23. Second pass of inboard-going trapped e- 14th IEA RFP Workshop Test particle initial distribution: inboard-travelling trapped electrons from first pass calculation. Co- current direction now has higher density of 30-50 keV e-

24. EBW current drive efficiency in MST: TBD 14th IEA RFP Workshop Fisch-Boozer and Ohkawa effects both factors in MST Example EBW CD  @ MST T e, n e COMPASS-D 0.2 A/W0.09 A/W W7-AS 0.005 A/W0.05 A/W CQL3D for MST with:Zero diffusion0.15 A/W D  v ll -0.02 A/W EBW resonance Fisch-Boozer Ohkawa

25. 14th IEA RFP Workshop Four waveguide grill, heating experiments Optimum phasing of 4-guide antenna qualitatively similar to that of 2-guide grill Sustained good coupling at > 100kW

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 Pericoli et al Nuc Fusion 2005 This may give insight to: How much power can we get through the antenna?

28. 14th IEA RFP Workshop Empirical power handling: Waveguide grills 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 X X Pericoli et al Nuc Fusion 2005 This may give insight to: How much power can we get through the antenna?

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

31. Raw SXR vs input power level

32. 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

33. 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

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

35. 14th IEA RFP Workshop Coupling to EBW in MST Cutoff ( R ) Upper hybrid resonance Cutoff ( L ) Launched EM wave couples to Bernstein mode at upper hybrid resonance In near field of antenna Reflection occurs from each cutoff; Distance between layers determined by n e and B profiles. Interference of reflected waves leads to optimized transmission ~ 2 cm  ~ 1mm vac ~ 8 cm

36. Coupling Improvements available at 5.5 GHz Insertion to steeper L n possible; Partial field error mitigation Antenna cover acts as limiter due to field error. S-band antenna in 4.5” portC-band antenna (~2” OD) in 4.5” port 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 14th IEA RFP Workshop Delta phi 6-15 cm Antenna aperture ~3cm Delta_phi short way = 0- 5cm

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

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)