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M.G. Bell Princeton Plasma Physics Laboratory on behalf of the NSTX Research Team

Toroidal Magnetic Plasma Confinement at the Limit: Recent Results from the National Spherical Torus Experiment. M.G. Bell Princeton Plasma Physics Laboratory on behalf of the NSTX Research Team. “Spherical Torus” Extends Tokamak to Extreme Toroidicity.

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M.G. Bell Princeton Plasma Physics Laboratory on behalf of the NSTX Research Team

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  1. Toroidal Magnetic PlasmaConfinement at the Limit:Recent Results from the National Spherical Torus Experiment M.G. Bell Princeton Plasma Physics Laboratory on behalf of the NSTX Research Team

  2. “Spherical Torus” Extends Tokamak to Extreme Toroidicity • Motivated by potential for increased (Peng & Strickler, 1980s) max (= 20p/BT2) = C·Ip/aBT C·/Aq BT: toroidal magnetic field on axis; p: average plasma pressure; Ip: plasma current; a: minor radius; : elongation of cross-section; A: aspect ratio (= R/a); q: MHD “safety factor” (> 2) C: Constant ~3%·m·T/MA(Troyon, Sykes - early 1980s) • Confirmed by experiments • max ≈ 40%(START - UK, 1990s) Spherical Torus A ≈ 1.3 Conventional Tokamak A ≈ 3 Field lines MIT seminar / 040514 / MGB

  3. In Addition to High , New Physics Regimes Are Expected at Low Aspect Ratio • Intrinsic cross-section shaping (BP/BT ~1) • Large gyro-radius (a/ri ~ 30–50) • Large fraction of trapped particles √(r/R)) • Large bootstrap current (>50% of total) • Large plasma flow & flow shear (M ~ 0.5) • Supra-Alfvénic fast ions (vNBI/vAlfvén ~ 4) • High dielectric constant (e ~ 30–100) MIT seminar / 040514 / MGB

  4. NSTX Designed to Study High-Temperature Toroidal Plasmas at Low Aspect-Ratio Aspect ratio A 1.27 Elongation  2.5 Triangularity  0.8 Major radius R0 0.85m Plasma Current Ip 1.5MA Toroidal Field BT0 0.6T Pulse Length 1s Auxiliary heating: NBI (100kV) 7 MW RF (30MHz) 6 MW Central temperature 1 – 3 keV Experiments started in Sep. 99 MIT seminar / 040514 / MGB

  5. View of NSTX, Heating Systems and Diagnostics MIT seminar / 040514 / MGB

  6. Now Operating With a New Center Bundle for TF Coil • Original irrepairably damaged by joint failure in February ‘03 • Operated for > 4500 plasma pulses, including 140 with BT ≥ 0.5T • New bundle constructed after redesign, modeling and review • Joints are now continuously monitored and are operating stably at 0.45T • Maintaining contact resistances below industrial norm MIT seminar / 040514 / MGB

  7. Establish physics basis & tools for high performance for ∆t >> tskin Extend knowledge base of plasma science • establish solenoid-free physics & tools • high bT & tE, ∆t > tE • high bN & tE, ∆t >> tE The 2003 Five-Year Plan for NSTX Aims to Assess the ST for Fusion Energy Development ‘09 Optimization & integration • high bT and JBS near with-wall limit for ∆t >> tskin ‘08 Use highb & low A to deepen understanding through measurement, theory & comparison with conventional A Develop control strategies for toroidal systems ‘07 • Advanced control & • High-b physics ‘06 ‘05 ‘04 Physics exploration & passive limits - Identify needed control tools ‘03 MIT seminar / 040514 / MGB

  8. High-Performance Steady-State OperationPoses Many Scientific Challenges • MHD Stability at High bT and bN • High pressure at low toroidal field with high bootstrap current fraction • bT ~ 40%, bN ~ 8 for a ST power plant • Good confinement at small size, a/ri • HITER-98pb(y,2) ~ 1.4 – 1.7 with good electron confinement for ignition • Power and Particle Handling • Small major radius increases Ploss/R by factor 2 – 3 • Solenoid-Free Start-Up and Sustainment • Eliminate solenoid for compact reactor design • Integrating Scenarios • Achieve all this together MIT seminar / 040514 / MGB

  9. NSTX Rapidly Achieved High-b With NBI Heating • Data from 2001 – 3 bT 2m0p / BT02 bT = 30 - 35 % bN= 5-6 at IP/aBT0 = 5-6 Pulse-length = 0.3-0.4s Bootstrap fractionup to50% bN= 5-6.2 at IP/aBT0  3 Pulse-length up to 1s Menard, Sabbagh (Columbia) MIT seminar / 040514 / MGB

  10. Measured Dependence of Beta-Limit in 2001–3 Motivated Shaping Enhancements • Capability for higher Ip at high d contributes to strong dependence • Planning to modify inboard PF coils to increase d at higher  2003 data 2002 data 2001 data • Reducing error fields and routine H-modes (broader profiles) improved performance in 2002 Menard, Gates MIT seminar / 040514 / MGB

  11. High bT 1.2MA High bP 0.8MA 2001 2002-3 2004 k li Reduced Latency in Vertical Position Control & Earlier H-modes Opened Operating Window • Propagation latency through digital control system reduced to ~700µs • Pause in current ramp & lower gas puffing promoted early H-mode • Lower internal inductance allows higher elongation • Reduced flux consumption extends pulse Gates, Menard MIT seminar / 040514 / MGB

  12. Long H-modes Provided a Route toHigh b in Recent Experiments • Use He pre-conditioning to control recycling • Initiate plasma at high BT for most quiescent ramp-up • Early NBI and pause in Ip ramp trigger H-mode • High Te & low li allow high k, Ip • Stage PNBI increase to avoidbN limit & excessive density • Ramp down TF to increase bT • ELMs develop as TF falls • reduce edge density • bT maximum as It = Ip (q95 = 4) Menard, Wade (GA) MIT seminar / 040514 / MGB

  13. Pre-conditioning, Minimal Gas & ELMs Reduce Edge Density and Allow Good NBI Penetration 112600, 0.55s 112600, 0.55s • High-resolution CHERS confirms large gradients in Ti, vi • Control of ELMs will be critical to optimizing b • A = 1.5 • k = 2.3 • dav = 0.6 • q95 = 4.0 • li = 0.6 • bN = 5.9%·m·T/MA • bT = 40% (EFIT)34% (TRANSP) R.Bell, LeBlanc, Sabbagh, Kaye MIT seminar / 040514 / MGB

  14. (with centrifugal effects) Measured (MPTS) (assuming density is a flux function) MA = 0.3 0 1 2 Time High vf/vA Affects MHD Equilibrium; Relevance to Stability Being Explored • Density shows in-out asymmetry • Effect of high Mach number of driven flow • Experiment: kinks saturate • Theory (M3D): for fixed momentum input, growth rate reduced by factor 2 - 3 MA = 0 Menard, Park, LeBlanc, Stutman MIT seminar / 040514 / MGB

  15. 8 6 dBp (G) 4 2 0 0.40 0.45 0.50 0.55 0.60 Time (s) BR BZ Installed Internal Sensors for Detecting Resistive Wall Modes • 24 each large-area internal BR, BZ coils installed before ‘03 run • Mounted on passive stabilizers • Symmetric about midplane • External n=1 detector signal lags the internal sensor by ~ twall • Internal sensor signal greater than external by factor of 5 • Internal sensors reveal clear up/down mode asymmetry Menard, Sabbagh MIT seminar / 040514 / MGB

  16. 0.535s 0.545s 0.555s 0.565s 0.575s CHERS rotation frequency (kHz) Radius (cm) 8 6 4 2 0 RWM Sensors Detect Mode in High bT Plasma • Global rotation collapse consistent with neoclassical viscosity due to ideal RWM perturbation n=1 frequency increases with edge rotation during collapse Frequency of Mirnov acitivity (kHz) RWM sensor cutoff dBp[n=1] (G), lower sensors Time (s) Sabbagh, Bell, Menard MIT seminar / 040514 / MGB

  17. Variety of Kinetic Instabilities Occurs with NBI • Some modes correlated with fast ion losses • TAE • "fishbones" • “Fishbones” are different at low aspect-ratio • Possibly driven by bounce-resonance • All modes interact Fredrickson MIT seminar / 040514 / MGB

  18. Detailed Stability Analysis Will Benefit from New MSE Measurements of q-Profile • Low field presents severe challenges for MSE technique • First two channels now operating, more being installed this run • Eventual aim is to be able to control profiles to optimize stability Levinton (NOVA) MIT seminar / 040514 / MGB

  19. PF5 coils (main vertical field) Developing Capability for Active Control of Resistive Wall Modes • 6 external correction coils being installed during this run • Operate as opposing pairs driven by three switching amplifiers • Planning to process sensor data in real-time through plasma control system for feedback control MIT seminar / 040514 / MGB

  20. Conventional Tokamak Confinement Trends Evident in Controlled Single Parameter Scans • Lower-Single-Null divertor plasmas (small difference in upper, lower X-point flux) • k = 2 – 2.2, d = 0.5 – 0.7 Kaye MIT seminar / 040514 / MGB

  21. Global Confinement Shows Similar NB Power and Current Dependence to ITER Scalings • Data for D-NBI heated H-modes at time of peak stored energy • BT = 0.45T, R0 = 0.84 – 0.92 m, A = 1.3 – 1.5, k = 1.7 – 2.5 • EFIT analysis using external magnetic data • Includes up to ~30% energy in unthermalized NB ions • No correction for unconfined orbit losses MIT seminar / 040514 / MGB

  22. 120 80 E<thermal> (ms) 40 0 0 40 80 120 E<ITER-H98p(y,2)> (ms) Global and Thermal Confinement Exceed Standard ITER Scalings • Compare with ITER scaling for total confinement, including fast ions • TRANSP analysis for thermal confinement • L-modes have higher non-thermal component and are more transient Kaye MIT seminar / 040514 / MGB

  23. Power Balance During NBI Heating Shows Ions Have Low Transport • Some shots show anomalously high Ti in region r/a ~ 0.6 – 0.8, yielding i < i<NC> • Analyze power balance with TRANSP code • Use measured profiles of Te, Ti, ne, nimp, Prad • Monte-Carlo calculation of NBI deposition and thermalization • i<NC> < i < e MIT seminar / 040514 / MGB

  24. Experimental Uncertainties Propagated Through TRANSP Power Balance • Increased confidence in analysis for confinement zone LeBlanc MIT seminar / 040514 / MGB

  25. Low Aspect-Ratio Appears to Provide a Unique Test-Bed for Studying Electron Transport • GS2 linear analyses of microstability show • Long-l ITG modes stabilized by E  B shearing • Short-l ETG modes are not stabilized & may dominate transport • Usually insignificant modes with tearing parity may play a role • Non-linear studies of turbulence with GS2 are underway • Additional studies planned with FULL, GYRO, GTC • Plan to install high-k microwave scattering diagnostic in 2005 to probe k = 2 – 30 cm-1 fluctuations Bourdelle, Redi, Rewoldt , Dorland MIT seminar / 040514 / MGB

  26. Studying Fueling With Several Gas Injectors; Other Techniques Are Being Developed • Low-field side and lower divertor injectors provide programmable flow rates • High-field-side injectors fully discharge external plenums through a pipe (1 – 2 m) Later in this run period two additional methods will be commissioned: • Room temperature solid pellet injector • 1 – 8 pellets per discharge, 20 – 400 m/s • Li, B, C, LiD (~10mg for lithium, ~1021 Li) • Supersonic gas injector • Graphite Laval nozzle close to plasma edge • Gas at ~1.8km/s • Up to 6 x 1021 D in 300ms MIT seminar / 040514 / MGB

  27. Deuterium Gas Fueling Efficiency is Low but Neutral Beams Fuel Efficiently • LFS, HFS gas fueling have similar efficiency in matched plasmas • Accumulated significant NBI fueling in recent long-duration H-mode plasmas • Regression analysis of data indicates: • Incremental gas fueling efficiency of 4 – 6 % • NB fueling efficiency 90 – 105 % MIT seminar / 040514 / MGB

  28. With Gas Fueling, Densities are Approaching the Greenwald Limit • Observe little degradation in confinement at high density MIT seminar / 040514 / MGB

  29. Planning Additional Methods for Controling Recycling • Density control needed for long pulses in “advanced” modes combining • Transport barriers (H-mode and possible ITBs) • High fraction of bootstrap current (dominated by density gradient) • RF current drive (dependent on Te) • Currently prepare and condition carbon plasma-facing surfaces with • 350°C bakeout (1 - 2 per run) • Boronization with glow discharge in He/D-TMB [(CD3)3B] (~2 weeks) • Between-shots He-GDC (8 – 12 min) • Helium discharge cleaning - investigation just started • Effects of “mini” boronization between shots - this week • Lithium (boron) pellet conditioning - this run • Lithium evaporator (CDX-U development) - next year • Divertor cryo-pump - 2006 • Lithium surface pumping module - 2007 400-BarrelRoom-Temperature- Solid Pellet Injector MIT seminar / 040514 / MGB

  30. Gas Puff Imaging is Producing a Wealth of Data on Edge Turbulence • Looks at Da(656 nm) from gas puff: I  none f(ne,Te) • View ≈ along B field line to see 2-D structure  B • Use PSI-5 camera at 250,000 frames/sec (4 µs/frame) viewing area ≈ 20x25 cm Zweben MIT seminar / 040514 / MGB

  31. Clear Differences in Edge Turbulence Between L- and H- Modes in NSTX • H-modes look quieter in NSTX than in C-Mod, perhaps because GPI sees further in past separatrix in NSTX MIT seminar / 040514 / MGB

  32. Exploring Methods for Generatingand Sustaining Toroidal Plasma Current • STs need non-inductive current • space for transformer solenoid in center is very limited • Exploit the neoclassical “bootstrap” current at high  • Requires a collisionless plasma • Use RF waves which interact with the electrons • Fast waves at high harmonics of ion cyclotron frequency (HHFW) • Electron Bernstein Waves (EBW) at low harmonics of electron cyclotron frequency • Coaxial Helicity Injection (CHI) can initiate toroidal current • Create linked toroidal and poloidal magnetic flux (helicity) by injecting poloidal current which relaxes to form closed magnetic surfaces MIT seminar / 040514 / MGB

  33. Diamagnetic Neoclassical Bootstrap Effect Drives Substantial Fraction of Plasma Current • Achieved substantial fraction of NBI-driven and bootstrap current for ~ skin time in diamagnetic plasma • Vloop ≈ 0.1V for ~0.3s • Control of profiles of pressure & current needed to maximize stability & bootstrap current together MIT seminar / 040514 / MGB

  34. Field line in edge High-Harmonic Fast-Wave System Designedto Provide Both Heating & Current Drive • 6 MW at f = 30 MHz • Pulse length up to 5 s • 12 Element antenna • Fed by 6 RF sources • Active phase control between elements • kT = ± (3-14) m-1 • w/WD = 9 – 13 • Expect little direct wave absorption on thermal ions • Wave velocity matched to thermal velocity of electrons MIT seminar / 040514 / MGB

  35. HHFW Power Can Heat Electrons and Trigger H-modes • Antenna operated in phasing (0π)6 for slowest waves: kT ≈ 14m-1 LeBlanc MIT seminar / 040514 / MGB

  36. Evidence for Current Drive by HHFW with kT ≈ ±7m-1 in Co and Counter CD Phasing • Phase velocity matches 2keV electrons • 150 kA driven current from simple circuit analysis • Modeling codes calculate 90 – 230 kA driven by waves Ryan (ORNL) MIT seminar / 040514 / MGB

  37. 12 B0=4.5 kG B0=4.0 kG B0=3.5 kG B0=4.5 kG, bt = 5.2% B0=4.0 kG, bt = 6.6% B0=3.5 kG, bt = 8.6% 10 8 6 NPA beam injection energy 4 HHFW NBI 2 20 40 60 80 100 120 140 Energy (keV) Neutral Particle Analyzer Shows Interaction of HHFW with Energetic Ions Produced by NBI ln (flux / Energy1/2) • Ion “tail” above NB injection energy enhances DD neutron rate • Tail reduced at lower B: • Higher  promotes greater off-axis electron absorption reducing power available to central fast-ion population Medley, Rosenberg MIT seminar / 040514 / MGB

  38. Hot component He II (Majority) Ions in Edge Region Develop Hot Component During HHFW Heating • Hot component grows over ~300ms but decays in ~30ms • Possible indication of parametric decay of RF waves • kT ≈ -7m-1 waves do not interact directly with ions Biewer MIT seminar / 040514 / MGB

  39. Planning 3MW EBW System for Localized Current Drive • ~28GHz for 3fce launch and Doppler-shifted 4fce absorption • Use O-X-B mode conversion of waves launched below midplane at oblique angle to toroidal direction • Current driven by Ohkawa mechanism • Selective trapping of co-going electrons • Current can by driven in the critical mid-radius region Taylor, Efthimion, Harvey (CompX) MIT seminar / 040514 / MGB

  40. NSTX Explores Plasma Confinement in a Unique Toroidal Configuration • Potential for high b already demonstrated • Confinement with NBI heating exceeds expectations • Ions are well confined • Combined NBI-driven and bootstrap current up to 60% • Challenge is to achieve favorable characteristics simultaneously with non-inductive current drive • Self-consistent bootstrap current • Current sustainment by RF waves • Current initiation by coaxial helicity injection • Installing a capacitor bank to exploit “transient CHI” (HIT-II) MIT seminar / 040514 / MGB

  41. + DC Power Supply - CHI Has Generated Significant ToroidalCurrent Without Transformer Induction Toroidal Insulators J X B pol tor B T I inj • Goal to produce reconnection of current onto closed flux surfaces • Demonstrated on HIT-II experiment at U. of Washington, Seattle MIT seminar / 040514 / MGB

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