Exploration of High Harmonic Fast Wave Heating on NSTX J. R. Wilson 2002 APS Division of Plasma Physics Meeting November 11-15, 2002 Orlando, Florida

APS-DPP R.E, Bell, S. Bernabei, M. Bitter, R. Dumont, D. Gates, J. Hosea, B. LeBlanc, S. Medley, J. Menard, D. Mueller, M. Ono, C. K. Phillips, A. Rosenberg Princeton Plasma Physics Lab P. Ryan, D. Swain, J. Wilgen, ORNL P. Bonoli, MIT T.K. Mau, UCSD R. I. Pinsker, GA R. Raman, U. of Washington S. Sabbagh, Columbia U. D. Stutman, Johns Hopkins U. Y. Takase, U of Tokyo and the NSTX Team Supported by With contributions from

APS-DPP OUTLINE Role and Characteristics of High Harmonic Fast Wave (HHFW) heating Technical Application on NSTX Electron Heating and Confinement Results Interaction with Fast Ions Current Drive Experiments

APS-DPP HHFW HEATING HAS BEEN IMPLEMENTED ON NSTX TO PROVIDE A TOOL FOR ELECTRON HEATING AND CURRENT DRIVE ST’s need auxiliary current drive High beta plasma makes Lower Hybrid and Conventional Electron cyclotron CD impossible HHFW in high beta plasmas has strong single pass absorption on electrons  Can allow off-axis deposition

APS-DPP NSTX UTILIZES THE TFTR ICRF SYSTEM 30 MHz Frequency corresponds to  D = MW from 6 Transmitters for up to 5 s 12 Element antenna with active phase control allows wide variety of wave spectra  k T = ± 3-14 m -1

APS-DPP Antenna takes up almost 90° toroidally Provides high power capability with good spectral selectivity HHFW 12 ELEMENT ANTENNA ARRAY

APS-DPP Antennas   D1 D2 D3 D4 D5 D6 P1P1 P2P2 P3P3 P4P4 P5P5 P6P6 V1V1 V3V3 V4V4 V5V5 V6V6 V2V2 RF Power Sources 5 Port Cubes Cube Voltages Decoupler Elements I1I1 I7I7  V21 Digital based phase feedback control is used to set the phase between the voltages of antenna elements 1 through 6 Decouplers compensate for large mutual coupling between elements and facilitate phase control PHASE FEEDBACK CONTROL CONFIGURATION

APS-DPP ELECTRON HEATING AND CONFINEMENT For typical NSTX plasmas HHFW deposits all its power into electrons Energy Confinement on NSTX follows L- mode and H-mode scaling when heat is applied via the electron channel Improved electron energy confinement discharges have been observed

APS-DPP HHHFW PROVIDES STRONG ELECTRON HEATING T e > T i He Discharge NOTE LOW INTIAL T e

APS-DPP Both Ions and Electrons Exceed Neoclassical Transport in HHFW L-mode Plasmas  i NC <  i,  e A. Rosenberg Increasing  e with radius RF power deposition from HPRT ray tracing code t = 0.24 s

APS-DPP HEATING WITH HHFW IS IN QUANTITATIVE AGREEMENT WITH CONVENTIONAL SCALING

APS-DPP Deuterium, 0.8 MA, 4.5 kG LARGE T e INCREASE SMALL n e DECREASE SOME DISCHARGES DISPLAY BEHAVIOR OF INTERNAL TRANSPORT BARRIER

APS-DPP Increase in T e Corresponds to Decrease in  e Power deposition from ray tracing T io (t) obtained from X-ray crystal spectrometer  e progressively decreases in the central region

APS-DPP EXPERIMENTS WITH DIRECTED SPECTRA HAVE EXHIBITED CHARACETIRISTICS OF CURRENT DRIVE Compare discharges with wave phased ±3-7 m -1 Adjust power levels and fueling to match density and temperature profiles Loop voltage differences seen in the absence of central MHD (sawteeth, m=1) Amount of driven current inferred from circuit analysis of magnetic signals is comparable to theoretical predictions

I P = 500 kA, B T = 4.5 kG ELECTRON PARAMETERS MADE COMPARABLE BY ADJUSTING POWER AND FUELING

APS-DPP Less loop voltage is required to maintain I P constant when driving HHFW current in the co direction Internal inductance is similar for the two cases and  V is not caused by d l i /dt SIGNIFICANT DIFFERENCE IN LOOP VOLTAGE OBSERVED BETWEEN CO AND CTR PHASING

APS-DPP CURRENT DRIVE MODELING ANALYSIS AND CODE PREDICTIONS IN ROUGH AGREEMENT Circuit analysis (0D): I P = (V- 0.5*I P *dL i /dt)/R P + I BS + I CD (Assumes steady state, R P and I BS (pressure profiles) independent of array phasing, I CD  P RF /n e )   I CO ≈ 110 kA (0.053 A/W) Codes - Calculated electron power absorption profiles are coupled to Ehst- Karney adjoint solution for current drive efficiency to obtain current density profiles TORIC: Full wave ICRF field solver [(k   i ) 2 << 1, B  = 0 for electric field polarization]  I CO ≈ 96 kA (0.046 A/W) CURRAY: Ray tracing code (damping is linear on Maxwellian species, all orders in k   i, k  determined locally)  I CO ≈ 162 kA (0.077 A/W)

Calculated Driven Current Density Profiles The “no trapping” profile is indicative of the power deposition profile Trapped electrons are predicted to reduce  fw from 0.19 to TRAPPING PLAYS SIGNIFICANT ROLE IN REDUCING DRIVEN CURRENT TORIC CALCULATION FOR NSTX CO CD SHOT

APS-DPP Current drive figure of merit,  fw, falls in range of DIII-D data at lower T e (0) Dimensionless current drive efficiency,  fw =  fw *  3.27/T e (0) (keV), is comparable to that for DIII-D at lower temperatures HHFW CURRENT DRIVE CONSISTENT WITH D-IIID AND TFTR CD EXPERIMENTS OPERATION AT INCREASED T e WILL BE NEEDED TO ACHIEVE NSTX GOALS

APS-DPP HHFW INTERACTION WITH FAST IONS Even at high harmonic numbers (n ≥ 9) ion damping can be important due to large k perp  i effects  On NSTX k perp  i ~ x for 80 keV beam ion  Damping important if  k perp  i ) 2 /2 ≥ n Damping on beam ions may reduce current drive efficiency

APS-DPP D + tail extends to 130 keV Tail saturates in time during HHFW beam injection energy at RF end +10 ms +20 ms noise level n e L(0) T e (0) P NB P RF IpIp Tail decays on collisional time scale NPA SHOWS FAST ION TAIL BUILD-UP AND DECAY

APS-DPP HPRT computes hot plasma absorption over cold ion/hot electron ray path 25 rays used TRANSP output used as input for fast ion temp and density distribution Total power evenly split Fast ions dominate central absorption Electrons dominate further off- axis n e0 = 3.2  cm -3 n b0 = 2.5  cm -3 T e0 = 638 eV T b0 = 17.3 keV P e =54.5% P b =45.5% D beam e-e- Shot Time 195 ms RAY TRACING PREDICTS SIGNIFICANT FAST ION ABSORPTION

APS-DPP B 0 =4.5 kG B 0 =4.0 kG B 0 =3.5 kG B 0 =4.5 kG B 0 =4.0 kG B 0 =3.5 kG No RF (B 0 =4.5 kG) RF turn on RF full power RF turnoff As B 0 increases, the NPA sees a larger tail Enhanced neutron rate with higher B 0 until MHD ~240 ms Likely due to larger  with lower B, which promotes greater off-axis absorption where fast ion population is small STRONGER TAIL AT HIGHER B T

APS-DPP NBI Source A, 80kV 1.6 MW NBI Sources B & C, 60 kV,.8 MW each (1.6 MW total) I p = 1 MA I p = 750 kA No RF (I p =750 kA) Larger tail observed with greater total beam energy, fixing power Tail of low vs. high I p similar, but neutron rate greater enhanced Greater ion absorption predicted with lower k ||, but surprisingly little variation in tail, small neutron enhancement with higher k || SCANS IN BEAM ENERGY, PLASMA CURRENT AND WAVE PHASE VELOCITY PERFORMED

APS-DPP NPA scan, k || = 7 m -1, B 0 =4.5 kG NPA at 90 cm NPA at 70 cm NPA at 50 cm Depletion in particle flux with NPA R tan further off-axis Tail extends to same energy range MEASUREMENTS SHOULD ALLOW DETAILED COMPARISON TO DEPOSITION MODELING

APS-DPP HHFW PROVIDES MEANS OF ELECTRON HEATING ON NSTX  CONFINEMENT CONSISTENT WITH STANDARD SCALINGS  IMPROVED CONFINEMENT REGIME OBSERVED INITIAL EXPERIMENTS WITH DIRECTED PHASING PROVIDE EVIDENCE FOR CURRENT DRIVE  DRIVEN CURRENT APPEARS CONSISTENT WITH MODELING AND PREVIOUS FWCD EXPERIEMENTS  TO ACHIEVE NSTX GOALS INCREASED T e NEEDED INTERACTION BETWEEN HHFW AND NBI IONS OBESERVED SUMMARY