Helicon Wave Current Drive in KSTAR Plasmas

Transcription

1 Daejeon Helicon Wave Current Drive in KSTAR Plasmas S. J. Wanga, H. J. Kima, Jeehyun Kima, V. Vdovinb, B. H. Parka, H. H. Wic, S. H. Kimd, and J. G. Kwaka anational Fusion Research Institute, Daejeon, Korea bkuruchatov Institute, Moscow, Russia ckwangwoon dkorea University, Seoul, Korea Atomic Energy Research Institute, Daejeon, Korea

2 High Frequency Fast Wave - Motivation Most of the plasma current in a steady-state tokamak fusion reactor is bootstrap Still need ~MA of current driven by other means Reactor study typically show that the current must be driven at midradius High frequency, ωci ω ωlh helicon wave current drive is suggested for efficient off-axis current drive The idea may be reactor relevant because the higher performance is expected for higher density and temperature The engineering is moderate compared to the other RF current drive technique Cost per power may also low compared to other current drive method due to matured technology at that frequency band

3 Collision-less RF Heating Collision-less, resonant energy transfer from RF to charged particles.! v k k k = n s In resonant, particle sees steady electric field. Acceleration or deceleration depending on the phase or polarization. n=1, cyclotron resonance damping (ICR Minority ion heating, ECH) - Perpendicular E field of R or L wave depending on the sign of charge. n>1, cyclotron harmonic damping (ICRH, ECH) - In addition to n=1 case, FLR affects damping provided ρsk+ >> 1. n=0, Landau damping (LHCD, Helicon Wave CD) - Parallel E field accelerates particles. Net energy gain depends on slope of particle energy distribution. - Driving structure (antenna) decides delivered power spectrum in k domain. Thus, asymmetric spectrum delivers net current.

4 Current Drive - Requirement Accessibility - wave must propagate to the desired location of damping without excessive damping Off-axis current drive - Electron damping at well defined radial zone Wave coupling - Excite the wave from an antenna in the vacuum region without excessive wall interaction

5 Helicon/Whistler Wave Fast wave branch below lower hybrid resonance At low frequency below ω Ωi, phase velocity approaches Alfven velocity VA As frequency approaching ωlh, phase velocity increases with frequency, whistler wave whistler/helicon

6 Field Line Alignment of Helicon Wave FW near ion cyclotron resonance travels in almost perpendicular direction. Helicon wave tends to be aligned with static magnetic field line in both toroidal and poloidal direction. whistler/helicon

7 Ray Tracing Ray tracing in rectangular coordinates ne0=6x10 19 /m 3 BT0=2 T

8 In Serious Geometry and Parameters 50MHz 100MHz 500MHz Profiles ne, Te and equilibrium from #10603 H-mode plasma

9 Lower Hybrid Wave In cold plasma, fast and slow branches of waves co-exist in the outer region of lower hybrid resonance At outer region, slow wave collapses with fast wave with Stix-Golant accessibility condition at LH resonance n < nc Evanescent layer of fast wave at the vacuum region is thicker than that of slow wave

10 Avoiding Confluence B 0 =2.0T, Using B T (R=2.2m) 1.5T 2.0T 1 n k Stix-Golant accessibility condition n 2 k > 1+(!2 pe/! ce) 2 res =n 2 c states minimum parallel refractive index to access certain level of density and magnetic field at resonant condition. At lower frequency, f < 0.6 GHz, B0 > 2 T, worrying about accessibility condition is not necessary at any density with n >2.

11 Helicon Wave - Damping High electron beta is essential for high damping Frequency plays dominant role in certain range Too much damping causes edge power deposition

12 Coupling, Cutoff n cutoff 0 e B!n2 k, or 0c 2 e B k2 k! Fast wave cutoff density decreases as frequency increases with fixed k, however optimum k increases 30.8 MHz KSTAR minority ion heating : k ~ 8.26/m 500 MHz Helicon near optimum : k ~ 36.6/m Variation of cutoff density is within 20 % R L exp( k k d) Factor of 4 increase of k may degrade coupling if same antenna system is used Increase of coupling through antenna technology is required Loading resistance of minority ion heating in KSTAR H-mode

13 Tools for Quantitative Prediction TORIC CURRAY PSTELION by V. Vdovin

14 High Beta Plasma Using equilibrium of #10528, NB heated H-mode plasma Additional NBI & ECH may enable high beta plasma

15 KSTAR Plasma in 2014 #10528, NB heated H-mode plasma, βn=1.55 High axially driven current

16 High Temperature #10528, NB heated H-mode plasma High axially driven current

17 High Density #10528, NB heated H-mode plasma Acceptable for advanced plasma research?

18 Summary and Future Works High frequency, ωci ω ωlh helicon wave current drive is suggested for efficient off-axis current drive Helicon/Whistler wave characteristic, field aligned propagation leads longer path from antenna to desired damping location In addition to the longer path, moderate damping in high beta or high density plasmas enables well defined damping zone in mid-radial region For the confirmed KSTAR plasmas and high temperature, low density plasmas, high driven current can be expected without off-axis advantage Wave coupling can be a problem like other RF heating system As antenna system is completely different from existing one, experimental verification of wave coupling characteristic is essential

19 KSTAR and Auxiliary Heating System

20 C ,1, eq#10548, 0.7MA, prof#10603_ keV, 4.6e19 H-mode prof, launcher at +/-40 deg

21 C ,3, eq#10548, 0.7MA, prof#10603_8500 7keV, 10e19 H-mode prof, launcher at +/-40 deg

22 Frequency 300 MHz 400 MHz 500 MHz

23 Temperature and density

24 Refractive index

25 Attenuation and Group Velocity Fastwave optical thickness High electron beta and low parallel refractive index is required. Helicon wave tends to be aligned with magnetic field longer wave path before reaching axis off-axis power deposition pitch, peak deposition position is dependent on density and can be controlled by frequency V. L. Vdovin, Current Generation by Helicons and Lower Hybrid Waves in Modern Tokamaks and Reactors ITER and DEMO Scenarios, Modeling and Antennae1, Plasma Physics Reports, 2013, Vol. 39, No. 2, pp

26 Helical path in TORIC Parallel refractive index in TORIC in ray path varies due to magnetic pitch and poloidal mode number. Refraction may be recovered with including large number of driving toroidal modes. Single mode TORIC shows helical path qualitatively. 3-D fullwave and ray tracing codes naturally include refraction effect.