Wave-Plasma Interactions in Ion Cyclotron Range: NSTX Research

1. Supported by Wave - plasma interactions in the ion cyclotron range of frequencies: theory and experiment in NSTX Cynthia K. Phillips Princeton Plasma Physics Laboratory for the NSTX Team APS meeting April 2003

2. ~ ~ B E NSTX IS STUDYING HIGH HARMONIC FAST WAVES IN HIGH BETA PLASMAS B0 • HHFW are compressional fast Alfvén waves • ~ k^ VA ~ N WD ~ (6 - 12) WD • plasma beta ~ 4% - 38% • choosew / k// ~ electron thermal speed • strong electron absorption via transit time magnetic pumping and Landau damping : • ion cyclotron damping generally weak, except for energetic ions and low k// • For NSTX, find ki ≥ 1 and  / a << 1 • Since BP ~ BT, 2D magnetic equilibrium effects important x k^

3. RADIO FREQUENCY (RF) WAVES USED FOR HEATING AND CURRENT DRIVE • ST’s need auxiliary heating (H) and current drive (CD) • choose wave spectra for H or CD • absorption profile depends on plasma parameters • HHFW are absorbed by electrons in high beta plasmas • competitive absorption by ions degrades CD efficiency

4. POWER SPECTRUM OF ANTENNA IS PROGRAMMABLE OVER A WIDE RANGE OF k|| (~ 2 to ~ 14 m-1) Antenna phasing controls heating or current drive 12 element antenna for 6 MW @ 30 MHz co-CD B counter-CD Heating -20 0 20 antenna extends almost 90° toroidally kz (m-1) 4

5. 4 1.5 1.2 PRF (MW) 0.6 0 0 0 0.2 1.6 0 0.5 Time (s) HHFW STRONGLY HEATS ELECTRONS • B = 0.44 T • kT = 14 m-1 • ne0 = 4.0 x 1019 m-3 4He Discharge Te > Ti keV Major Radius (m) Note decay of Te after RF turn-off

6. AORSA-2D full wave code 0.4 0 0.8 CENTRAL ELECTRON HEATING PREDICTED BY THEORETICAL MODELS Full wave code WKB ray tracing codes 3 3 ray tracing codes AORSA - 2D HPRT Pabs ( MW / m3 ) Pabs ( MW / m3 ) CURRAY 0 0 0.4 0.8 0 normalized minor radius normalized minor radius

7. HHFW HEATS FAST DEUTERIUM IONS INJECTED WITH NEUTRAL BEAMS ne 0 0.2 0.4 0.6 Neutral particle analyzer 2 12 12 108251 2 10 Te0(keV) beam injection energy 8 ln (flux / Energy1/2) 8 1 ln ( flux / energy 1/ 2 ) ne ( 1019 m-3) Ip (MA) at RF end +10 ms +20 ms 6 4 4 RF 2.4 noise level 2 NBI 1.6 MW 0 0 0 40 80 120 Time (s) Energy (keV) • fastD+tail builds up during and decays after HHFW

8. THEORY PREDICTS SIGNIFICANT ABSORPTION BY FAST IONS Shot 105908 Time 195 ms Pelec = 55% Pbeam= 45% Pabs (MW / m3) 0.4 0.8 0 normalized minor radius • fast ion and electron absorption comparable • no thermal ion absorption for this k// range • fast ion absorption degrades CD efficiency BUT • fast ion absorption decreases at lower BT , higher b • observed and predicted 1 0.6 beam e- 0.2

9. CURRENT DRIVE INFERRED FROM DIFFERENCES IN LOOP VOLTAGE WITH PHASED WAVES 1.0 0.8 0.6 0.4 0.2 0 0.2 0.3 0.4 0.5 0.6 • 2 discharges with similar ne(r),Te(r) • inductance similar • V not caused by dli/dt • less loop voltage required for constant IP with co-CD phasing Counter - CD DV ≈ .2V Loop Voltage (V) Co - CD RF on Time (s) • circuit analysis (0D): IP = (V- 0.5*IP*dLi/dt)/RP + IBS + ICD • ICD ~ 110 kA inferredvs 96 – 160 kA predicted by codes • driven current consistent with previous tokamak experience

10. WAVE-PLASMA INTERACTIONS PLAY A CRITICAL ROLE IN NSTX RESEARCH • HHFW provides means of electron heating • Interaction between HHFW and fast ions observed • ion interaction decreases with increasing electron beta • Initial evidence found for HHFW current drive • driven current consistent with modeling and previous FWCD experiments • higher Te needed to achieve NSTX research goals

11. HHFW CURRENT DRIVE CONSISTENT WITH D-IIID AND TFTR CD EXPERIMENTS C. Petty et al., Plasma Physics and Controlled Fusion 43 (2001) 1747 • Operation at higher Te required to meet NSTX goals • Increased power and improved confinement should allow this