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HHFW Heating and Confinement Studies

HHFW Heating and Confinement Studies

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HHFW Heating and Confinement Studies

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  1. HHFW Heating and Confinement Studies NSTX Results Review 2004 Presented by J. R. Wilson with contributions from: S. Bernabei, M. Bell J. Hosea, C. K. Phillips, P. Ryan, D, Swain the whole NSTX crew. LOADING - COUPLING -----> HEATING HEATING SCALING MODULATION EXPERIMENTS -----> HEATING EFFICIENCY 4 SUMMARY & FUTURE EXPERIMENTS

  2. Global quantities indicate puzzles in HHFW heating Electron stored energy is smaller fraction of total than expected for pure electron heating Phase dependence of heating observed Overall confinement can be quite poor

  3. COUPLING & HEATING

  4. Edge phenomena show phase dependence • 180° has best heating • (@ t=0.22 there is MHD) • -90° has very high Da • -90° reflected power shows no • time variation • +90° has intermediate behaviour THESE FINDINGS POINT TO A DEGREE OF POWER COUPLED TO SURFACE WAVES, ESPECIALLY FOR -90° (CO-CD) HIGH LOADING DOES NOT NECESSARILY MEAN EFFICIENT COUPLING TO CORE PLASMA

  5. Heating efficiency depends on the plasma edge conditions May indicate importance of surface waves

  6. Incremental electron energy confinement time dWe/dPHHFW is ~4ms

  7. Confinement consistent with L-mode but We is small fraction of WMHD

  8. Confinement trends point toward moderate Ip and maximum BT • Each point represents a shot: spread is due to power. • only shots with power between 1 MW and 2 MW are plotted.

  9. MODULATION EXPERIMENTS Modulation experiments were performed in order to obtain the percentage of RF power absorbed in the plasma. Heating efficiency decreases with k|| 180° has a power absorption of ~ 80% +90° (counter CD) is more efficient than -90° Heating in Helium and Deuterium are approximately the same Power at -30° (theoretically the most efficient at driving current) is not absorbed at all.

  10. time to = 1 2 3 4 Fast EFIT used to allow fit to stored energy total stored energy Derivation of DE and t at four times (to) curve fit E ~ E +DE (1- exp(-t/t)) o o RF power % (pwr. abs.) = DE/(Pt) error: D%(pwr.abs) = DE + ExDt Pt Pt2

  11. 0.04 0.035 0.03 0.025 0.02 0.015 0.01 0.005 0 0.015 0.02 0.025 0.03 0.035 14m-1 shows higher incremental confinement tau t=0.2542 14m-1 -7 m-1 tau delta t

  12. 14 12 10 8 6 0.01 0.015 0.02 0.025 0.03 0.035 0.04 ±900 show same total DE but less than 1800 180° DE Deuterium +90° -90° dE delta t

  13. +7 m-1 shows higher absorption but lower tau

  14. Electron Response to Modulation __ Te __ Pe/4 __ ne/2 __ PRF/2  = 36 msec  = 15.4 msec Shot 112699, 8 ms delay, n|| = 14 Time (sec)

  15. Profile response complicated by MHD Shot 112705, 8 ms, 7 m-1 (co) Soft Xray Array Shows Considerable MHD/Sawtoothing Activity - especially during the laser time at the end of the second RF pulse

  16. Less MHD in some shots Shot 112699 Has Less MHD Activity (8 ms, 14 m-1)

  17. Use first pulse to obtain deposition profile 14 15 16 14 m-1 yields broad profile for electron response as expected 14 - 16 14 - 15 Te vs Radius for Shot 112699 for Times After First RF Pulse - Times 14, 15, 16

  18. Profile narrower for -7 m-1 Te vs Radius for Shot 112705 for Times After First RF Pulse - Times 14, 15, 16 Profile narrower

  19. Summary HHFW heats electrons: heating efficiency ranges from ~80% to ~0% and is strongly dependant on k||. Electron heating profiles consistent with theory (Broader for high k) Difference between -90° and +90° is a puzzle. MHD modes, either (1,1) or (2,1), can cause degradation of the heating efficiency. Phase dependence of surface phenomenon as well as core may indicate surface waves/sheaths playing important role Ions have more stored energy than expected (see Biewer and Diem) Parametric decay leading to edge ion heating

  20. Continued Experiments • Reverse the magnetic field to determine if the difference between-90° and +90° is due to geometrical constraint. • Use edge probes, surface reflectometer, Rogowski coils between plates and wall to determine if (and how much) RF power is coupled to surface waves and dissipated in sheaths. • Change edge conditions and configuration to understand coupling. • Complete modulation experiments (H-Mode), varying power Ip? B? ne?)