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Operation of ICRF antennas in a full tungsten environment in ASDEX Upgrade

Operation of ICRF antennas in a full tungsten environment in ASDEX Upgrade. V. Bobkov , F. Braun, R. Dux, L. Giannone, A. Herrmann, A. Kallenbach, H.-W. Müller, R. Neu, J.-M. Noterdaeme, Th. Pütterich, V. Rohde and ASDEX Upgrade team.

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Operation of ICRF antennas in a full tungsten environment in ASDEX Upgrade

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  1. Operation of ICRF antennas in a full tungsten environment in ASDEX Upgrade V. Bobkov, F. Braun, R. Dux, L. Giannone, A. Herrmann, A. Kallenbach, H.-W. Müller, R. Neu, J.-M. Noterdaeme, Th. Pütterich, V. Rohde and ASDEX Upgrade team 18th International Conference on Plasma-Surface Interactions, Toledo, Spain, May 26-30, 2008

  2. Outline Introduction ICRF system and diagnostics on ASDEX Upgrade Operational possibilities to reduce W source Experiment and near-fields calculations Conclusions

  3. Introduction ASDEX Upgrade: full-W machine, no boronozation 2007 and first half of 2008 ICRF in full-metal machine is problematic tolerable concentration of highZ is low (< 510-5) high sputtering rates at PFCs during ICRF (up to 10-3) RF E|| (|| to magnetic field) fields are involved far-fields: mainly due to bad fast wave central absorption near-fields: antenna and structures connected to antenna along field lines [S. Wukitch et al., PSI-17]

  4. Introduction Simplified picture of sputtering due to E|| electrons react fast and follow E||fields along field lines electrons are lost  plasma charges positively, sheaths form V||= E|| dl along field lines defines sheath potential drop [Perkins F., Nucl. Fusion 29 4 (1989), 583] Potential of a magnetic field line: V|| in vacuum plasma (simple, for pi>0) plasma time-averaged rectified sheath voltages accelerate light impurity ions W sputtering [R. Dux, I-6]

  5. ICRF system and diagnostics on ASDEX Upgrade Local diagnostics: Antenna 4 shunts on limiters, DC current Antenna 3 Langmuir probes, floating potential Antenna 1 Spectroscopy, sputtering yield YW=W /D, a measure of V|| Antenna 2 4 antennas with 2 straps each, usually operated in pairs (12) and (34)

  6. ICRF system and diagnostics on ASDEX Upgrade Characterization of the problem using diagnostics in AUG: 2 PICRF[MW] P12P34 #22795 RF power, E|| fields 1 PNBI = 5 MW 0 IDC[A] 2 loss of electrons shunt on limiter 1 0

  7. ICRF system and diagnostics on ASDEX Upgrade Characterization of the problem using diagnostics in AUG: 2 PICRF[MW] P12P34 #22795 RF power, E|| fields 1 PNBI = 5 MW 0 IDC[A] 2 loss of electrons shunt on limiter 1 0 200 high rectified voltages on field lines connected to antenna Vfl[V] Langmuir probe 100 0

  8. ICRF system and diagnostics on ASDEX Upgrade Characterization of the problem using diagnostics in AUG: 2 PICRF[MW] P12P34 #22795 RF power, E|| fields 1 PNBI = 5 MW 0 IDC[A] 2 loss of electrons shunt on limiter 1 0 200 high rectified voltages on field lines connected to antenna Vfl[V] Langmuir probe 100 0 1 stronger erosion on PFCs (limiters) spectroscopy (local signal) YW[10-4] 0.1 0.01

  9. ICRF system and diagnostics on ASDEX Upgrade Characterization of the problem using diagnostics in AUG: 2 PICRF[MW] P12P34 #22795 RF power, E|| fields 1 PNBI = 5 MW 0 IDC[A] 2 loss of electrons shunt on limiter 1 0 200 high rectified voltages on field lines connected to antenna Vfl[V] Langmuir probe 100 0 1 stronger erosion on PFCs (limiters) spectroscopy (local signal) YW[10-4] 0.1 0.01 5 CW at Te=1 keV [10-5] W spectroscopy (global signal) increase of W concentration 1 0.5

  10. ICRF system and diagnostics on ASDEX Upgrade Characterization of the problem using diagnostics in AUG: 2 PICRF[MW] P12P34 #22795 RF power, E|| fields 1 PNBI = 5 MW 0 IDC[A] 2 loss of electrons shunt on limiter 1 0 200 high rectified voltages on field lines connected to antennas Vfl[V] Langmuir probe 100 0 1 stronger erosion on PFCs (limiters) spectroscopy (local signal) YW[10-4] 0.1 0.01 5 CW at Te=1 keV [10-5] W spectroscopy (global signal) increase of W concentration 1 0.5 4.5 increase of radiation Prad close to PICRF bolometer 3.5 Prad[MW] 2.5

  11. Introduction ICRF system and diagnostics on ASDEX Upgrade Operational possibilities to reduce W source Experiment and near-fields calculations Conclusions

  12. Operational possibilities to reduce W source during ICRF Shifting plasma away from antenna: # 22100 Rout [m] 2.16 # 22098 PICRF=2.0 MW 2.12 100 YW[10-4] Constant gas puff rate 10 Rout 1 10 edge CW[10-5] at 1 keV 1 2.2 2.4 2.6 2.8 Time [s] At antennas and PFCs connected to antennas along field lines: ne  E|| Te

  13. Operational possibilities to reduce W source during ICRF Increasing gas puff: # 22099 Gas puff rate [1021 s-1] 8 PICRF=2.0 MW 4 Rout = 2.12 m 100 YW[10-4] 10 1 10 edge CW[10-5] at 1 keV 1 2.6 2.8 3.0 3.2 Time [s] At antennas and PFCs connected to antennas along field lines: is likely the important player ne  E|| Te

  14. Operational possibilities to reduce W source during ICRF 50  ~ ~ Standard 3dB hybrids connections: antenna 3 matching 0° 50  3dB splitter RF transmitters 3dB coupler 90° antenna 4 Work with paired antennas, good load tolerance, but with fixed  = 90°

  15. Operational possibilities to reduce W source during ICRF 50  ~ ~ Bypassing 3dB hybrid connections: 50  0° 3dB splitter 3dB coupler  To operate at any , bad load tolerance  L-mode discharges Used also for the experiments with one antenna powered

  16. Operational possibilities to reduce W source during ICRF 3 2 1 3 2 1 Optimizing phase between antennas: 400 [°] Minimum in CW close to 270° (-90 °) is likely due to changes in V|| #22925 270° 200 90° 90° 0 CW[10-5] at 1 keV 20 CW correlates with YW at antenna 3 8 6 YW[10-4] antenna 3 at Z=0.2 At positive netto effect there are locations with high YW (antenna 4) YW[10-4] antenna 4 at Z=0.2 More experiments with more flexible 3dB-hybrid configuration needed 1.5 2.0 2.5 3.0 3.5 Time [s]

  17. Operational possibilities to reduce W source during ICRF Limits and drawbacks: 1) Shifting plasma away from antenna low antenna resistance, voltage stand-off issues, E|| penetrate further away from antennas [V. Bobkov et al., AIP Conference Proc. AIP Press Melville NY 933 (2007) 83] 2) Increasing gas puff rate high density and worse confinement 3) Optimizing phase between antennas visible only in low density discharges All methods are limited and limit operation themselves: antenna design with reduced near-fields needed! Validation of computational tools for E|| needed

  18. Introduction ICRF system and diagnostics on ASDEX Upgrade Operational possibilities to reduce W source Experiment and near-fields calculations Conclusions

  19. Experiment and near-field calculations “Simple approach” on considering V|| : V||= E|| dl IRF IRF  0 V|| due to RF flux from straps, uncompensated contributions in the corners

  20. Experiment and near-field calculations >6 [kV/m] <-6 -3.6 -1.2 1.2 3.6 Approach based on recent code calculations for E||: “Simple approach” on considering V|| : Re E|| V||= E|| dl HFSS code IRF IRF IRF IRF 1 MW matched  0  0 Contribution from box currents to V|| can be significantly larger V|| due to RF flux from straps, uncompensated contributions in the corners first by: [L. Colas et al., PPCF 49 (2007) B35]

  21. Experiment and near-field calculations Antenna 4: Antenna 3: shields to cover corners spectroscopic observations From “simple approach” reduction of YW at shields is expected

  22. Experiment and near-field calculations Antenna 4: Antenna 3: shields to cover corners spectroscopic observations Antenna 3, HFSS: Antenna 4, HFSS: Re E|| Re E|| B at 11° B at 11° Reduction of YW at shields is expected from “simple approach” HFSS code shows no significant difference

  23. Experiment and near-field calculations Antenna 4: Z Antenna 3: Antenna comparison: 1.0 YW/YWmax antenna 3, only antenna 3 on normalized to YWmax to compensate small toroidal asymmetry 0.8 0.6 antenna 4, only antenna 4 on 0.4 #22926 0.2 antenna 0.0 -0.4 -0.2 0.0 0.2 0.4 vertical position Z in AUG [m] No large difference at shields + high YW on antenna 3 edge  contributions of box currents important

  24. Experiment and near-field calculations Z Antenna 3: Spectroscopy LOS spot is broad: various field lines need accounting V||= E|| dl Only antenna 3 on varying connection lengths and limiter shape Relative contribution to V|| varies along Z ( )

  25. Experiment and near-field calculations Z Antenna 3: Spectroscopy LOS Spot is broad: various field lines need accounting V||= E|| dl YW[10-4] #22926 2.0 1.0 experiment Only antenna 3 on antenna 3 0.0 V||= E|| dl [V] 600 400 HFSS calculations 200 0 -0.4 -0.2 0.0 0.2 0.4 vertical position Z in AUG [m] varying field line connection length and limiter shape Relative contribution to V|| varies along Z ( ) No conclusive statement possible

  26. Experiment and near-field calculations Antenna 4: Antenna 3: Only antenna 4 on Antenna 3 diagnostics used to characterize antenna 4 Fields line types have similar connection lengths

  27. Experiment and near-field calculations Antenna 4: Z Antenna 3: YW[10-4] #22926 2.0 1.0 experiment Only antenna 4 on antenna 4 0.0 V||= E|| dl [V] 600 Antenna 3 diagnostics used to characterize antenna 4 400 HFSS calculations 200 0 -0.4 -0.2 0.0 0.2 0.4 vertical position Z in AUG [m] Fields line types have similar connection lengths and V|| profiles Reasonable agreement between the shapes of YW and V||

  28. Conclusions During ICRF operation W source can be reduced by: Shifting plasma away from PFCs at low field side Increasing gas puff Optimizing the phase between antennas Improvements can be useful, but better antenna design needed Calculations (HFSS code) were validated: Dominant influence of box currents on E|| confirmed for AUG Reasonable agreement achieved between shapes of YW profile in experiment and calculated V|| profile Antenna designs with reduced E||are in progress [EPS 2008]

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