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NSTX Physics Results and Opportunities S. M. Kaye Head, NSTX Physics Analysis

NSTX Physics Results and Opportunities S. M. Kaye Head, NSTX Physics Analysis PPPL, Princeton University Princeton N.J. 08550 Presented at: U. of Maryland 17 October 2001. Lodestar NYU Princeton Sci. Instruments UC Irvine UKAEA Tokyo U Kyushu Tokai U

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NSTX Physics Results and Opportunities S. M. Kaye Head, NSTX Physics Analysis

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  1. NSTX Physics Results and Opportunities S. M. Kaye Head, NSTX Physics Analysis PPPL, Princeton University Princeton N.J. 08550 Presented at: U. of Maryland 17 October 2001 Lodestar NYU Princeton Sci. Instruments UC Irvine UKAEA Tokyo U Kyushu Tokai U Himeji Inst. Science Tech Hiroshima Niigata U Tsukuba U Ioffe Inst Triniti (RF) KBSI (Korea)

  2. Outline • Potential benefits of low aspect ratio • Role of NSTX • Physics program, results and opportunities • Confinement and transport • MHD • Non-inductive current startup and sustainment • Conclusions and future plans

  3. R/a~4, k=2, qa=4 R/a~1.3, k=2, qa=12 The Spherical Torus (ST) is a low aspect ratio tokamak • ST maximizes the field line length in the good curvature (stable) region - strong toriodicity - Potential benefit to stability and confinement • Strong natural shaping -Enhance stability, relax magnet requirements

  4. NSTX Mission • Extend the understanding of toroidal physics to high-b, low-collisionality regimes at low aspect ratio (R/a ≤ 1.4) • Develop technologies necessary for a Spherical Torus (ST) power plant • Essential elements of mission • High-bt (≤ 40%), large bootstrap current fraction (fbs ≤ 0.7) • Non-inductive current generation and sustainment • Adequate power and particle handling • Integration of individual elements in long-pulse discharges

  5. National Spherical Torus Experiment (NSTX) Baseline Parameters (Achieved) Major Radius 0.85 m Minor Radius 0.68 m Elongation≤ 2.2(2.5) Triangularity ≤ 0.6(0.7) Plasma Current 1 MA(1.4 MA) Toroidal Field 0.3 to 0.6 T(≤0.45 T) Heating and CD 5 MW NBI(5 MW) 6 MW HHFW(6 MW) CHI Pulse Length ≤ 5 sec(0.5 sec)

  6. National Spherical Torus Experiment (NSTX)

  7. U.S. National NSTX Research Team Collaboration and International Research Cooperation Princeton Plasma Physics Laboratory: M. Ono, E. Synakowski, S. Kaye, M. Bell, R. E. Bell, S. Bernabei, M. Bitter,* C. Bourdelle, R. Budny, D. Darrow, P. Efthimion, D. Ernst, J. Foley, G. Fu, D. Gates, L. Grisham, N. Gorelenkov, R. Kaita, H. Kugel, K. Hill, J. Hosea, H. Ji, S. Jardin, D. Johnson, B. LeBlanc, Z. Lin, R. Majeski, J. Manickam, E. Mazzucato, S. Medley, J. Menard, D. Mueller, M. Okabayashi, H. Park, S. Paul, C.K. Phillips, N. Pomphrey, M. Redi, G. Rewoldt, A. Rosenberg, C. Skinner, V. Soukhanovskii, D. Stotler, B. Stratton, H. Takahashi, G. Taylor, R. White, J. Wilson, M. Yamada, S. Zweben Oak Ridge National Laboratory:M. Peng, R. Maingi, C. Bush, T. Bigelow, S. Hirshman,* W. Houlberg, M. Menon,* D. Rasmussen,* P. Mioduszewski, P. Ryan, P. Strand, D. Swain, J. Wilgen University of Washington: R. Raman,T. Jarboe, B. A. Nelson, A. Redd, D. Orvis, E. Ewig Columbia University:S. Sabbagh, F. Paoletti, J. Bialek, G. Navratil, W. Zhu General Atomics:J. Ferron, R. Pinsker, M. Schaffer, L. Lao, B. Penaflor, D. Piglowski Johns Hopkins University:D. Stutman, M. Finkenthal, B. Blagojevic, R. Vero Los Alamos National Laboratory:G. Wurden, R. Maqueda, A. Glasser* Lawrence Livermore National Laboratory:G. Porter, M. Rensink, X. Xu, P. Beiersdorfer,* G. Brown* UC San Diego:T. Mau, J. Boedo, S. Luckhardt, A. Pigarov,* S. Krasheninnikov* UC Davis:N. Luhmann, K. Lee, B. Deng, B. Nathan, H. Lu UC Los Angeles:S. Kubota, T. Peebles, M. Gilmore Nova Photonics:F. Levinton Massachusetts Institute of Technology:A. Bers, P. Bonoli, A. Ram, J. Egedal* UC Irvine:W. Heidbrink Sandia National Laboratory:M. Ulrickson,* R. Nygren,* W. Wampler* Princeton Scientific Instruments: J. Lowrance,* S. von Goeler* Lodestar: J. Myra, D. D’Ippolito NYU:C. Cheng* University of Maryland: W. Dorland* Dartmouth University:B. Rogers* U.K.,EURATOM UKAEA Culham: A. Sykes, R. Akers, S. Fielding, B. Lloyd, M. Nightingale, G. Voss, H. Wilson JAPAN,Univ. Tokyo:Y. Takase, H. Hayashiya, Y. Ono, S. Shiraiwa; Kyushu Tokai Univ.:O. Mitarai; Himeji Inst of Science & Technology:M. Nagata; Hiroshima Univ.: N. Nishino; NIFS.:T. Hayashi; Niigata Univ.:A. Ishida; Tsukuba Univ.:T. Tamano Russian Federation, Ioffe Inst.:V. Gusev, A. Detch, E. Mukhin, M. Petrov, Y. Petrov, N. Sakharov, S. Tolstyakov, Dyachenko, A. Alexeev; TRINITI: S. Mirnov, I. Semenov, Korea,KBSI: N. Na ______________________________________________ *In cooperation with DOE OFES Theory, OFES Technology, Astrophysics, or SBIR programs

  8. Confinement and Transport • Moved quickly from global confinement to local transport studies • Local studies require well-calibrated profile diagnostics (Te, ne, vf, Ti, …) • Global confinement well-enhanced over L-, and H-modes • Local transport often shows good ion confinement • Power balance puzzle – anomalous ion heating? • How good is classical collisional framework? • H-mode studies a big part of last run period • Operational scenarios for accessing H-mode routinely • L-H threshold studies • Power threshold 10x that predicted by ITER scaling • Edge parameters show little ordering with respect to various theories

  9. Global Confinement Scaling • Generally similar parametric scalings as higher R/a - Stronger Ip scaling (thermal + fast ion effect?) - Enhanced confinement observed • 15-20% discrepancy between external magnetics and kinetic stored energy - Kinetic EFITs being prepared 2x 2x 1x 1x 0.5x 0.5x

  10. Background view NB Active view Te(0) confirmed by 3 independent measurements Well-Calibrated Profile Diagnostics Key to Local Analysis Observed R. Bell Fitted line Background • Ti, Vf: development of direct background measurement • Required for time-dependent measurements • Required cross-calibrations performed; analysis techniques being developed LeBlanc, R. Bell; Bitter, Hill

  11. Diagnostic Validation (cont’d) • TS high throughput, small error bars. ne consistent with reflectometry • Ti: CHERS, NPA consistent with each other Kubota, Gilmore, Peebles (UCLA); LeBlanc, R. Bell S. Medley, R. Bell

  12. Generally good agreement between kinetic and magnetic stored energies • Kinetic stored energy determined from measured profiles, guiding center calculation with FLR (TRANSP) • Magnetic stored energy determined from EFIT with external magnetics (flux loops, Bpol probes)

  13. Rotation speeds near Mach 1! What is role of rotation on equilibrium reconstruction?

  14. Power Balance Indicates Electrons as PrimaryLoss Channel

  15. Ion Thermal Diffusivity Can Be Close to Neoclassical

  16. Power Balance Is Sometimes Puzzling!

  17. 0.5 6 0. 4 4 0.3 2 0.2 0 0.0 0.4 0.2 2.5 • • Theory of stochastic wave heating of corona developed (White) • • Application of theory to ST has begun • • Vbeam >> VAlfven key 2.0 1.5 1.0 0.5 0.0 0.1 0.2 0.3 Gates, Gorelenkov, White Astrophysics and observed MHD may hold one clue to the power balance puzzle 103932/103936 TF ne • Being investigated: Compressional Alfven Eigenmodes • Modes excited by fast ions; waves transfer energy to thermal ions neL (1015 cm2) Tesla TF Frequency (MHz) Time (sec) Fredrickson (PRL 2001)

  18. Beam-driven Compressional Alfven Waves may heat ions on NSTX (keV/s) • Simulations of compressional Alfven modes give stochastic ion heating. • e.g.-- with 20 modes centered at half Alfven frequency • Possible relevance to interpretation of ion-heating on NSTX D. Gates, N. Gorelenkov, R. White (PRL, 2001)

  19. Measured Ti is matched with neoclassical transport and majority of beam power channeled to ions • ci assumed to be ci,Chang-Hinton ~ ci, NCLASS • 85% of heating power channeled to the ions - Typical electron:ion heating is 2:1

  20. (Innermost) (Outermost) Measured and Calculated Neutron Rates Are in Rough Agreement •Fast ion confinement appears to be near classical (L. Roquemore) • Fast ion loss probes show much lower loss than expected (D. Darrow)

  21. Impurity transport rates are small in the core • After puff, almost no neon penetrates the core until MHD event near 260 ms • Modelling suggests core diffusivity < 1 m2/s, rivaling neoclassical theory • Low ion particle transport consistent with low ion thermal transport Signals from difference of similar plasmas w/ and w/o neon D. Stutman, JHU

  22. Long wavelength modes may be suppressed • Long wavelengths: growth rate lower than ExB shear rate • Large l associated with ion thermal transport • Short wavelengths: growth rates large • Responsible for electron thermal transport? • Non-linear simulations begun C. Bourdelle (PPPL), W. Dorland (U. MD)

  23. Electron ITB formation in HHFW Heated Plasmas Te Pressure ne

  24. Reduced electron transport correlated with high Te • Core ce drops as high Te develops • Steep gradients due to transport changes, not source • Conductivity still very high! • Heating source from HPRT ray tracing (Rosenberg) 105830a03 1000 100 m2/s Preliminary 10 1 0.3 0.5 0.1 0.7 0.9 r/a B. LeBlanc

  25. Turbulence theory suggests testable trend for transport experiments Conduct a theory experiment: vary Ti/Te, keeping other profiles constant Growth rates measured ETG • Theory indicates: • High Ti/Te stabilizes ion modes • High Te/Tistabilizeselectron modes • Do we see signatures of this in measured confinement trends? Shearing rate ITG C. Bourdelle

  26. Both ELM-free and ELMy H-modes have been observed in NSTX ELMing ELM-free • HHFW and NBI • Exploration for scenario development, transport, and boundary physics

  27. EBW mode conversion efficiency increases at L-H transition when edge density increases • Mode conversion of EBW from core occurs in scrapeoff in H mode • Measured EBW conversion efficiency agrees relatively well with calculated values • On CDX-U, limiter inserted to control Ln demonstrates conversion efficiency can be controlled • Possible tool for plasma startup, island healing • Joint research with U. Wisconsin

  28. H-Modes with RF • LSN • Lower current (350 - 500 kA) • He and D • ELMy, ELM-free • bp = 1 observed • Large bootstrap? • Large dip in surface voltage

  29. LeBlanc • Fluctuations reduced at H mode transition • Change in edge transport evident in density profile :  Transitions to H mode state exhibit typical signatures but high power thresholds Visible light, false color Movie not available • NBI: Power required > 10x that predicted from empircal scaling laws: • Strong magnetic shear? • Poloidal damping? Wall neutrals? Maingi, Bush (ORNL); LeBlanc; PRL (to be published) Fast camera: Maqueda (LANL)

  30. Imaging of edge reveals qualitative differences in H- and L-mode turbulence pol. rad. Movie not available • Helium puffed; emission viewed along a field line • He0 emission observed with a fast-framing, digital, visible camera • 1000 frames/sec, 10 ms exposure each frame Los Alamos NATIONAL LABORATORY Maqueda, LANL; Zweben

  31. Theory indicates complex turbulent structures may exist in NSTX edge • BOUT code: turbulence modeling • 2-fluid,3D Braginskii equation code Xu (LLNL)

  32. MHD: beta-limiting • Range of operations • Towards assessing physics of RWM’s • Locked mode coils/error field detection • NTM’s • Current driven kinks

  33. Profile variations have been performed to map out operational space for stability studies bN = 4li • Maximum bN increases, then saturates with increased current profile peaking • Fast beta collapses observed at all values of li • High pressure peaking • Beta saturation coincident with tearing activity at higher bN, bp ~ 0.45

  34. Candidates for RWM have a fast collapse when rotation frequency is below a small value • “Critical” rotation frequency < 2 kHz at the plasma core at time of collapse, consistent with DIII-D observations of RWM and theory • Rotation decrease occurs across the entire plasma (preliminary CHERS analysis)

  35. up midplane 200.500 201.500 202.500 204.500 203.500 down 235.000 236.000 237.000 238.000 239.000 240.000 USXR data shows a mode structure resembling a global kink in RWM candidate shots 106168 • RWM Candidate • Fast collapse • No core islands and no 1/1 mode before reconnection event (RE) • More likely a kink mode onset t (s) 106169 up RE • Plasma with island activity • Reconnection event leads to 1/1 mode at 237 ms • Less likely a kink mode onset midplane 1/1 down D. Stutman t (s)

  36. Wall-plasma gap scan, dB r and rotation measurements reveal clues regarding RWM • Smaller gap  longer time between start of rapid growth phase of dBr and beta collapse  stronger wall mode interaction • With clear tearing activity, no observed dBr precursor to collapse At least two possible complications • Theory suggests dBr largest on inboard side for these bN’s,  weaker wall interactions compared to DIII-D • Locked mode coils  large field error from PF 5

  37. PF5 is apparently responsible for a large n=1 field error 10cm gap 0 cm gap k=1.8 plasma boundary • Error field is localized to lower PF5 coil • 30-50 Gauss n=1 error field for typical PF5 current and 0 cm gap • 20-30 Gauss peak error field for 10cm gap Gauss

  38. Slow-growing, b-limiting internal modes are likely NTM’s • Coincident with saturation of beta • Limits bp ~ 0.4 • Modes appear near bcrit • Further analysis required D. Gates, E. Fredrickson

  39. Co-Axial Helicity Injection (CHI) Generates Toroidal Current Non-Inductively C L •Inject poloidal current on open field lines in lower divertor • Plasma moves up into main chamber • Toroidal current develops to maintain force-free configuration • Flux surfaces may close through magnetic reconnection Plasma Startup w/CHI R. Raman

  40. High currents, strong MHD observed in CHI start-up studies • Up to 390 kA of toroidal current was produced, with 14 times current multiplication • Discharges sustained for 330 ms • Strong n = 1 oscillations observed

  41. Spectroscopy used to measure Ti and edge plasma rotation in CHI plasmas • Direction of rotation is clockwise (same as HIT-II) • Ti ~ 30 - 50 eV, midplane view • Vf ~ -10 to -20 km/s

  42. Flux closure is being assessed via further analysis of magnetics and spectroscopy • USXR emission increases at the higher CHI currents • MFIT consistently shows modest closed flux regions when CHI-driven current is sufficiently high • Not seen in absence of n=1 MHD • Encouraging, but is not proof of closed mean-field surfaces Stutman, JHU Schaffer, GA

  43. Transport: confinement picture intriguing and potentially important • New heating physics specific to the ST? • Electron thermal channel may dominate • Edge turbulence measurements made; comparison to modelling begun • HHFW: effective heating in all regimes • Fast ion interactions observed • CD studies begun; no obvious signatures of HHFW-CD yet • MHD: Beta-limiting modes being investigated • RWM studies underway: well along towards ‘02 Milestone • Importance of locked mode uncovered • NTM’s, current-driven kinks both being studied

  44. Physics central to the ST mission is being investigated (con’t) • Boundary physics extended to heat and particle flux studies • Comparisons to modelling begun • Enabling techniques extended: plasma boronization • H mode power threshold established for one condition: ~ 10x usual scaling • CHI research produced record toroidal current, observed critical MHD activity • Ti of 30 - 50 eV measured • MFIT analysis consistent with closed flux, not conclusive • Absorber arcs reduced, but continue to be an issue • First CHI-to-ohmic, ohmic-to-CHI experiments performed; technical exploration stage

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