Oscillating Field Current Drive on MST

Oscillating Field Current Drive on MST John Sarff A. Blair, K. McCollam, P. Nonn, J. Anderson, D. Brower 1, D. Craig, B. Deng 1, D. Den Hartog, W. Ding 1, F. Ebrahimi, D. Ennis, G. Fiksel, S. Gangadhara, S. Prager, and the MST Team 1 UCLA Innovative Confinement Concepts Workshop University of Texas-Austin Feb 13-16, 2006

Current sustainment: The RFP!s second biggest challenge. Current sustainment is difficult in the RFP Must have a large plasma current (poloidal field dominant) Pressure-driven current is relatively small Usual inductive formation is pulsed, not steady-state (not RFP specific) If dynamo required, must be compatible with confinement requirements Options? Low-BT pulsed reactor likely better than PULSAR (last major pulsed tokamak system study) Inductive Oscillating Field Current Drive (aka AC helicity injection)

Current sustainment: The RFP!s second biggest challenge. Current sustainment is difficult in the RFP Must have a large plasma current (poloidal field dominant) Pressure-driven current is relatively small Usual inductive formation is pulsed, not steady-state (not RFP specific) If dynamo required, must be compatible with confinement requirements Options? this talk Low-BT pulsed reactor likely better than PULSAR (last major pulsed tokamak system study) Inductive Oscillating Field Current Drive (aka AC helicity injection)

Outline. OFCD basics Partial current drive results from MST Scaling of equilibrium modulation induced by oscillating loop voltages OFCD application to RFP reactor: As envisioned in TITAN system study Possible hybrid scenario, with self-similar current ramp-down

DC helicity injection using AC loop voltages. Magnetic helicity balance evolution: "K "t = 2V #$ % 2' E & BdV (K = ' A & BdV ) apply oscillating V! &! : V " = ˆ V " sin#t & "2V # $% = ˆ V # ˆ V & 2' sin( V " = ˆ # $ sin$t + " dc " = Phase[V #,V $ ] Bevir & Gray, 1981 (Conventional induction maintains helicity balance with constant V! &! )

Oscillating mean-field pinch beats with oscillating field to produce parallel induction. Separate AC & DC parts: E = ˆ E B = ˆ B + B! AC amp.! cycle avg. Cycle-average Ohm!s law: V ˆ r " B ˆ =! mean-field pinch 0 E+ V " B = #J # E ˆ " B& # % $ B 2 (" B ˆ E = ˆ ) B ˆ & % ' $ B 2 ( B '! parallel induction

Nonlinear resistive MHD computation demonstrates OFCD current drive. S=5"10 5 V ˆ r " B ˆ ( ) v " b Ebrahimi et al, 2003 Fluctuation-induced emf (dynamo) from MHD tearing at amplitudes similar to steady induction. " Is OFCD compatible with confinement requirements?

OFCD on MST produces 10% increase in plasma current. Installed oscillators: 280 Hz, 1 MVA (insufficient power for 100% drive). V tor V loop V dc V pol OFCD On t (ms) OFCD current drive efficiency measured the same as for steady induction (" 0.1 A/W)

Maximum current does not occur at maximum helicity injection. Fully relaxed plasma with constant resistivity would have "I p ~ sin(#) (" = relative oscillator phase) # Instead, peak current with "! " / 8 # Magnetic fluctuations & heating minimum at maximum "I p

Maximum current does not occur at maximum helicity injection. Fully relaxed plasma with constant resistivity would have "I p ~ sin(#) (" = relative oscillator phase) # Instead, peak current with "! " / 8 Possible explanations: # Magnetic fluctuations & heating minimum at maximum "I p Helicity dissipation from cyclic J(r) modulation or transport effects (changing resistivity). Resistivity varies with phase # current change from background induction

Other effects, described in poster. Entrainment of the sawtooth cycle to the oscillator frequency. Cyclic modulation of tearing fluctuation amplitudes. m = 0 tearing modes smaller than standard RFP (at max. OFCD current drive). Modulated ion heating (anomalous), probably due to tearing modulation. Current profile modulation, measured using Faraday rotation and MSE. FIR Faraday rotation B # (r, t)

Energy balance with rigid current profile allows estimate of evolution for inductive scenarios at high S = $ R / $ A. " "t # 1 B 2 dv = I 2µ 0 $ V $ + I % V 142 43 % input power & # ' J 2 dv! prescribe loop voltages for OFCD, self-similar ramp-down, etc. Evolve 1D equilibrium: " # B = $(r,t)b + B # "p/b 2 "(r,t) = " o (t)#(r) fixed shape (marginal tearing-stable)

Energy balance with rigid current profile allows estimate of evolution for inductive scenarios at high S = $ R / $ A. " "t # 1 B 2 dv = I 2µ 0 $ V $ + I % V 142 43 % input power & # ' J 2 dv! prescribe loop voltages for OFCD, self-similar ramp-down, etc. 100% OFCD, MST-like plasma Evolve 1D equilibrium: " # B = $(r,t)b + B # "p/b 2 "(r,t) = " o (t)#(r) fixed shape (marginal tearing-stable)

Lundquist number scaling projects small equilibrium modulation for reactor-like parameters. "I # I # S "1/4 100% OFCD Toroidal current modulation scales as ~ S 1/4 from reactive impedance. (see Ebrahimi et al, PoP!03) "F (~ "I tf ) $negative extrema # standard range # PPCD range Large, cyclic equilibrium modulation for S < 10 6. ZT40 MST Reactor S = " R /" A

The L p /R p time for saturated OFCD current addition is comparable to MST!s pulse length. I p (ka) " 15% addition predicted for MST oscillators (" = % / 5) Time (ms)

TITAN system study used OFCD for steady-state. TITAN ITER I p = 18 MA ; & % = 20% R / a = 3.8 m / 0.6 m OFCD parameters: f = 25 Hz &I p / I p = ±4% &F = ±0.07 P CD = 57 MW I p / P CD = 0.3 A/W

Possible hybrid inductive reactor scenario: self-similar ramp-down for pulsed burn, OFCD for ramp-up. I p 0.3 " ssrd # 0.3 (0.1" R ) " R # 2000a 2 [s] # OFCD ramp-up Features: Fully inductive current drive More consistent with established RFP confinement physics Reduced mechanical stress w/t conventional pulsed (?) Periodic ash removal, during OFCD ramp (?) Time (hybrid cycles) Requires modest confinement during the OFCD ramp-up to minimize recirculating power inefficiency (T e0 > 2 kev?) Self-similar ramp-down: Nebel et al., PoP!02

Summary. OFCD increases current by 10% in MST Ohmic current drive efficiency Maximum current drive not at maximum helicity injection MHD tearing amplitudes depend on relative oscillator phase OFCD equilibrium modulation expected small at high Lundquist number. Pulsed or hybrid inductive scenarios need to be assessed for reactor COE penalty. Near term emphasis for MST experiments: Confinement and profile measurements Increase OFCD power and/or pulse length Identify optimum loop voltage waveforms (possibly non-sinusoidal)