1. Supported by Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics NYU ORNL PPPL PSI SNL UC Davis UC Irvine UCLA UCSD U Maryland U New Mexico U Rochester U Washington U Wisconsin Culham Sci Ctr Hiroshima U HIST Kyushu Tokai U Niigata U Tsukuba U U Tokyo Ioffe Inst TRINITI KBSI KAIST ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching U Quebec 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

2. MIT seminar / 040514 / MGB 2 “Spherical Torus” Extends Tokamak to Extreme Toroidicity Motivated by potential for increased  (Peng & Strickler, 1980s)  max (= 2  0  p  /B T 2 ) = C·I p /aB T  C·  Aq B T : toroidal magnetic field on axis;  p  :average plasma pressure; I p : 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

3. MIT seminar / 040514 / MGB 3 In Addition to High , New Physics Regimes Are Expected at Low Aspect Ratio Intrinsic cross-section shaping (B P /B T ~1) Large gyro-radius (a/  i ~ 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 (v NBI /v Alfvén ~ 4) High dielectric constant (  ~ 30–100)

4. MIT seminar / 040514 / MGB 4 NSTX Designed to Study High-Temperature Toroidal Plasmas at Low Aspect-Ratio Aspect ratio A1.27 Elongation  2.5 Triangularity  0.8 Major radius R 0 0.85m Plasma Current I p 1.5MA Toroidal Field B T0 0.6T Pulse Length1s Auxiliary heating: NBI (100kV)7 MW RF (30MHz)6 MW Central temperature1 – 3 keV Experiments started in Sep. 99

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

6. 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 B T ≥ 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

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

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

9. MIT seminar / 040514 / MGB 9 NSTX Rapidly Achieved High-  With NBI Heating Data from 2001 – 3 Bootstrap fraction up to 50%  N = 5-6.2 at I P /aB T0  3 Pulse-length up to 1s  T = 30 - 35 %  N = 5-6 at I P /aB T0 = 5-6 Pulse-length = 0.3-0.4s  T  2  0  p  / B T0 2 Menard, Sabbagh (Columbia)

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

11. MIT seminar / 040514 / MGB 11 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 2001 2002-3 2004 High  T 1.2MA High  P 0.8MA lili 

12. MIT seminar / 040514 / MGB 12 Long H-modes Provided a Route to High  in Recent Experiments Use He pre-conditioning to control recycling Initiate plasma at high B T for most quiescent ramp-up Early NBI and pause in I p ramp trigger H-mode High T e & low l i allow high , I p Stage P NBI increase to avoid  N limit & excessive density Ramp down TF to increase  T ELMs develop as TF falls –reduce edge density  T maximum as I t = I p (q 95 = 4) Menard, Wade (GA)

13. MIT seminar / 040514 / MGB 13 Pre-conditioning, Minimal Gas & ELMs Reduce Edge Density and Allow Good NBI Penetration High-resolution CHERS confirms large gradients in T i, v i Control of ELMs will be critical to optimizing  A = 1.5  = 2.3  av = 0.6 q 95 = 4.0 l i = 0.6  N = 5.9%·m·T/MA  T =40% (EFIT) 34% (TRANSP) 112600, 0.55s R.Bell, LeBlanc, Sabbagh, Kaye

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

15. MIT seminar / 040514 / MGB 15 Installed Internal Sensors for Detecting Resistive Wall Modes BRBR BZBZ 24 each large-area internal B R, B Z coils installed before ‘03 run –Mounted on passive stabilizers –Symmetric about midplane External n=1 detector signal lags the internal sensor by ~  wall Internal sensor signal greater than external by factor of 5 Internal sensors reveal clear up/down mode asymmetry Menard, Sabbagh 0.400.450.500.550.60  B p (G) 0 2 4 6 8 Time (s)

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

17. 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

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

19. MIT seminar / 040514 / MGB 19 Developing Capability for Active Control of Resistive Wall Modes PF5 coils (main vertical field) 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

20. 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) –  = 2 – 2.2,  = 0.5 – 0.7 Kaye

21. 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 B T = 0.45T, R 0 = 0.84 – 0.92 m, A = 1.3 – 1.5,  = 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

22. MIT seminar / 040514 / MGB 22 Global and Thermal Confinement Exceed Standard ITER Scalings TRANSP analysis for thermal confinement  E (ms) 12080400 120 80 40 0 Compare with ITER scaling for total confinement, including fast ions L-modes have higher non-thermal component and are more transient Kaye

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

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

25. 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- ITG modes stabilized by E  B shearing –Short- 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

26. 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, ~10 21 Li) Supersonic gas injector –Graphite Laval nozzle close to plasma edge –Gas at ~1.8km/s –Up to 6 x 10 21 D in 300ms

27. 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 %

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

29. 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 T e ) Currently prepare and condition carbon plasma-facing surfaces with –350°C bakeout (1 - 2 per run) –Boronization with glow discharge in He/D-TMB [(CD 3 ) 3 B] (~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-Barrel Room-Temperature- Solid Pellet Injector

30. MIT seminar / 040514 / MGB 30 Gas Puff Imaging is Producing a Wealth of Data on Edge Turbulence viewing area ≈ 20x25 cm Looks at D  (656 nm) from gas puff: I  n o n e f(n e,T e ) View ≈ along B field line to see 2-D structure  B Use PSI-5 camera at 250,000 frames/sec (4 µs/frame) Zweben

31. 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

32. MIT seminar / 040514 / MGB 32 Exploring Methods for Generating and 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

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

34. MIT seminar / 040514 / MGB 34 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 –k T = ± (3-14) m -1 Field line in edge High-Harmonic Fast-Wave System Designed to Provide Both Heating & Current Drive  D = 9 – 13 Expect little direct wave absorption on thermal ions Wave velocity matched to thermal velocity of electrons

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

36. MIT seminar / 040514 / MGB 36 Evidence for Current Drive by HHFW with k T ≈ ±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)

37. MIT seminar / 040514 / MGB 37 Neutral Particle Analyzer Shows Interaction of HHFW with Energetic Ions Produced by NBI 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 ln (flux / Energy 1/2 ) 8 10 12 6 4 2 14012010080604020 Energy (keV) beam injection energy B 0 =4.5 kG,  t = 5.2% B 0 =4.0 kG,  t = 6.6% B 0 =3.5 kG,  t = 8.6% B 0 =4.5 kG B 0 =4.0 kG B 0 =3.5 kG NBI HHFW NPA Medley, Rosenberg

38. MIT seminar / 040514 / MGB 38 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 -k T ≈ -7m -1 waves do not interact directly with ions Hot component Biewer

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

40. MIT seminar / 040514 / MGB 40 NSTX Explores Plasma Confinement in a Unique Toroidal Configuration Potential for high  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)

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