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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 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)
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
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
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
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)
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
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
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
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
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
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)
Rotation speeds near Mach 1! What is role of rotation on equilibrium reconstruction?
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)
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)
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
(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)
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
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)
Electron ITB formation in HHFW Heated Plasmas Te Pressure ne
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
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
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
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
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
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)
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
Theory indicates complex turbulent structures may exist in NSTX edge • BOUT code: turbulence modeling • 2-fluid,3D Braginskii equation code Xu (LLNL)
MHD: beta-limiting • Range of operations • Towards assessing physics of RWM’s • Locked mode coils/error field detection • NTM’s • Current driven kinks
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
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)
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)
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
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
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
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
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
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
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
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
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