Initial Exploration of HHFW Current Drive on NSTX J. Hosea, M. Bell, S. Bernabei, S. Kaye, B. LeBlanc, J. Menard, M. Ono C.K. Phillips, A. Rosenberg, J.R. Wilson Princeton Plasma Physics Laboratory M. Carter, P. Ryan, D. Swain Oak Ridge National Laboratory R. Pinsker General Atomic P. Bonoli Massachusetts Institute of Technology T.K. Mau University of California at San Diego NSTX Team APS DPP Meeting November 2002 Orlando, Florida

Goal: Develop HHFW to support non-inductive operation of the ST concept Outline: HHFW antenna arrangement Phase feedback control configuration for selecting antenna spectra Selected case for evaluating co/counter current drive effects –-/+ 90° phasing for k  = 7.6 m -1 –Closely match plasma parameters [T e (r), n e (r)] –Measure change in loop voltage Modeling of current drive effects Conclusions and future directions

HHFW 12-strap antenna array on NSTX NSTX antennas installed in the vacuum vessel Antenna takes up almost 90° toroidally Provides high power capability with good spectral selectivity B

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 Phase Feedback Control Configuration 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

Spectra Launched for Co and Counter Current Drive with k  = 7.6 m -1 Large pitch angle of the magnetic field results in asymmetric spectra Loading is larger for co-CD and heating efficiency is larger for counter-CD To compare V Loop between co and counter cases we use different RF powers to produce very similar electron parameters k  (m -1 ) Co  = - 90 ∞ Counter  = + 90 ∞ GLOSI/RANT3D calculations of power spectra Dipole  Spectral Power (au)

Electron Parameters Made Similar for Co and Counter CD By adjusting P RF and Gas Feed : Co-CD P HHFW = 2.1MW ( solid lines) : Counter-CD P HHFW = 1.2MW ( dotted lines) n e L (cm -2 ) T e 0 (keV) time (s) I P = 500 kA, B T = 4.5 kG, D 2

Electron Temperature and Density Profiles are Very Similar for Co and Counter Cases Radius (m) T e n e Co Counter T e (keV) n e (10 19 m -3 )

A Significant Difference in Loop Voltage is Observed Between Co and Counter Current Drive 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 RF on Counter-CD Co-CD  V ≈.23V Loop Voltage (V) time (sec)

I CD from Circuit Analysis is Bracketed by Current Drive Modeling Predictions 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) See posters -P.M. Ryan et al., GP1.121; T.K. Mau et al., GP1.124; and C.K. Phillips et al., GP Tuesday afternoon

C. Petty et al., Plasma Physics and Controlled Fusion 43 (2001) 1747 DIII-D (With NBI) HHFW Current Drive on NSTX is Consistent with DIII-D Results Current drive figure of merit,  fw, falls in range of DIII-D data at lower T e (0) Dimensionless CD efficiency,  fw =  fw *  3.27/T e (0) (keV), is comparable to that for DIII-D at lower temperatures RF power losses are important in reducing the HHFW current - - trapped electrons are predicted to reduce  fw significantly for NSTX (HHFW only)

Calculated Driven Current Density Profiles J RF (A/m 2 /W inc ) TORIC Code Predicts a Large Reduction in Current Drive Due to Trapped Electrons The “no trapping” profile is indicative of the power deposition profile

Summary and Future Directions Digital phase feedback control has been used successfully to compare co and counter current drive with HHFW on NSTX A significant reduction in V LOOP is observed for co-CD relative to counter- CD for comparable discharge parameters –Heating effects on V LOOP are mitigated by closely matching T e (r) and n e (r) –Ohmic Poynting flux reduction of ~ 30% is observed (  V LOOP ~ 0.23V) Modeling gives values of dimensionless current drive efficiency comparable with high harmonic DIII-D results Future directions for extending the study of HHFW CD on NSTX include: –Increasing HHFW CD by pushing RF power toward 6 MW and increasing T e –Bringing the MSE system on-line to afford direct measurement of the HHFW CD effects