Progress on NSTX towards steady state at low aspect ratio

1. Supported by A = 1.4,  = 2.3, L = 0.75, li = 0.49 Old PF1A Coil T = 20% N = 5.2%m•T/MA E = 52ms H89L = 2.0 Modified PF1A Coil Progress on NSTX towards steady state at low aspect ratio D. A. Gates, Princeton Plasma Physics Laboratory on behalf of the NSTX Research Team * Work supported by US DOE Contract No. DE-AC02-76CH03073 ST Requires high bootstrap fraction, fbs, simultaneous with high t New inboard divertor coils increase accessibility of high-triangularity, high-elongation shapes  = 1.8,  = 0.6, S ~ 22  = 2.75, = 0.8, S ~ 37  = 2.0,  = 0.8, S ~ 23  = 3, = 0.8, S ~ 41  = 2.3,  = 0.6, S ~ 27 • High fbs and high t competing requirements (at fixed shape and N) • Progress for ST and advanced tokamak given by sus fbst ~ SN2[S = q95(Ip/(aBt))] • If Nmax = CTroyon, then only shape improves sus • sus increases linearly with increasing S [S = q95(Ip/(aBt))] • Component Test Facility requires t ~ 20% and fbs ~ 50%, sus = 10% • ARIES-ST requires t ~ 40% and fbs ~ 90% • Optimized shaping with new PF coils for high triangularity and elongation • NSTX has achieved record values of elongation and shape factor • Leads directly to record values of the sus for the ST • For NSTX 100% non-inductive operation with N ~ 7 only with strong shaping Highest elongation k=3.0 (transient) Sustained elongation k=2.7 (0.1s) 2001 2006 2005 2002-3 2004 • Highest know obtained at highest d≈0.8  S  q95 IP/aBT = 41 MA/m·T • Small (Type V) ELM regime recovered at high k > 2.5 with new coils • Previously observed onset of large ELM-like events when k > 2.2 D. Gates , J. Menard Record pulse-lengths achieved at high current by operating with sustained H-mode Expected performance improvements observed as shaping has increased Pulse averaged t versus pulse length Data are sorted by year and by S Pulse averaged approximate sus versus S (S is averaged over the same time window as sus) • H-mode with small ELMs lower flux consumption, slow density rise Long duration discharges reach ~70% non-inductive current NH89 vs. pulse/E • TRANSP model agrees with measured neutron rate during high-b phase • Model includes anomalous fast ion diffusion during later phase when low-m MHD activity is present • 85% of non-inductive current is p-driven • Bootstrap + Diamagnetic + Pfirsch-Schlüter • 1/1 mode onset causes  drop, fast ion diffusion (Menard, PRL) 2001-2004 2005-2006 External kink mode ultimate limit on t -long pulse discharges above no-wall limit Plasma shaping enhancements Implementation of rtEFIT improves shape control reproducibility Upgrade of PF1A coil enables simultaneous achievement of high  and  PEST eigenfunction Shot 117707 Troyon diagram showing tmax vs. Ip/aBt Boundary overlay time window ~3cm includes MHD perturbations Control points Time history of t and n=1 (no-wall) kink mode growth rate, , for Shot 117707, calculated by PEST using LRDFIT equilibrium including MSE Overlay of EFIT boundaries from shots 117707 and 117814 between 0.3 and 0.9s Effect of modifications provide clear increase in NSTX operating space Reduction in control system latency increases elongation (2004)  % Shot 121241 (record ) All shots 2001-2003 All shots 2004-2006 Alfven Each point in the above plots represents an EFIT equilibrium reconstruction Data span entire NSTX database and are filtered against rapid plasma motion

2. Changes in X-point balance affect ELM characteristics • Very small changes in the plasma boundary reproducibly lead to large differences in ELM behavior • ELMs have a major impact on plasma performance, controlling them is crucial • Precise plasma control provides an important tool for controlling ELMs - highly ITER relevant 117407 LSN Shot 117425 Shot 117425 Shot 117424 117432 DN 117407 LSN 117432 DN 117424 high- DN rsep (mm) for 117424 (black) and 117425 (red) 117424 high- DN 1.0 0.0 Increased triangularity actually reduces peak heat flux to divertor target rsep is the radial separation of the flux surfaces which pass through the x-points measured at the outboard midplane Shot 117424 • Flux expansion decreases peak heat flux despite reduced major radius • Compare single-null & double-null configurations with triangularity  ≈ 0.4 at X-point and high triangularity  = 0.8 double-null plasmas • Measure heat flux with IR thermography of carbon divertor tiles • Peak heat flux decreases as 1 : 0.5 : 0.2 • ELM character changes: Type I  Mixed  Type V R. Maingi   Analysis of current profile information Full complement of kinetic profile data enables analysis of current profile composition Full neoclassical calculation of ohmic, and pressure driven currents Shot 120001 • Black total predicted current • Gray total reconstructed current (MSE) • Orange - Ohmic current • Red - Bootsrap • Blue - Beam driven current (TRANSP) • Data averaged over 0.7 - 0.8s • Loop voltage profile calculated from equilibria constrained with MSE data • Neoclassical resistivity and bootstrap current from Sauter, et al., Phys. Plasmas 6 (1999) 2834 2 Long pulse discharges have elevated q(0) without low frequency MHD modes 1) 2) Neoclassical 3) Measured • A spectrogram of magnetic fluctuations as measured by a Mirnov coil for shot 120001. • The colors represent toroidal mode numbers as indicated in the legend. • Notice the period of time after 0.6s where there are only small amplitude high-n MHD modes present. 4) The time history of shot 120001 showing: 1) the components of non-inductive current as indicated in the plot legend, 2) The plasma current (black) and the surface voltage (red), 3) the reconstructed q(0) (black) and qmin (red) and q(0) as determined by a TRANSP magnetic diffusion calculation (blue) 4) t (black) and the neutral beam power (red). The profiles in preceding figure are calculated over the time interval indicated in green. Steady state scenario predicted with 100% non-inductive current using only NBICD and pressure driven currents Encouraging results from both EBW emission and HHFW current drive experiments Measured EBW emission angle matches theoretical predictions • EBW uses efficient Ohkhawa current drive • Data on efficiency of EBW emission from identical plasmas for which the EBW antenna pointing angle is varied. • The colors represent measured efficiencies of • 81-90% (red) • 71-80% (orange), • 61-70% (green) • 51-60% (blue) • 41-50% (purple), • 31-40% (black). • The ellipses are contours of theoretically predicted emission efficiency Plasma shape and profiles for predicted 100% non-inductive scenario • Need 60% increase in T, 25% decrease in ne • Lithium for higher tE & density control? • 20% increase in thermal confinement • 30% increase in HH98 • Core HHFW heating • Want q0 qmin 2.4  higher with-wall limit Target Experiment (116313) • Higher k for higher q, bP, fBS • High d for improved kink stability • (shape parameters already achieved in other discharges) HHFW heats efficiently in current drive phasing • Achieved high Te= 3.6keV in current drive phasing for first time using high BT = 5.5kG • Improvement consistent with reduced PDI and surface waves expected at higher BT • Expect similar improvements from higher k|| • Useful for HHFW-CD during ramp-up • Useful for HHFW heating at high-b Target scenario: Present high-fNI long-pulse H-modes: IP = 750kA bN < 5.6, bP < 1.5, bT < 17% li = 0.6, qmin=1.3, BT=4.5kG k = 2.3, dX-L = 0.75, q*=3.9 IP = 700kA bN = 6.7, bP = 2.7, bT =15% li = 0.5, qmin = 2.4, BT=5.2kG k = 2.6, dX-L = 0.85, q*=5.6 Inductive current drive is replaced by: Higher JNBI from higher Te Higher JBS from higher bP-thermal