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Physics Results From the National Spherical Torus Experiment (NSTX)

Office of Science. Supported by. Physics Results From the National Spherical Torus Experiment (NSTX). College W&M Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U SNL

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Physics Results From the National Spherical Torus Experiment (NSTX)

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  1. Office of Science Supported by Physics Results From the National Spherical Torus Experiment (NSTX) College W&M Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U SNL Think Tank, Inc. UC Davis UC Irvine UCLA UCSD U Colorado U Maryland U Rochester U Washington U Wisconsin Culham Sci Ctr U St. Andrews York U Chubu U Fukui U Hiroshima U Hyogo U Kyoto U Kyushu U Kyushu Tokai U NIFS Niigata U U Tokyo JAEA Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep U Quebec Stanley M. Kaye for the NSTX Group Deputy Program Director, NSTX PPPL, Princeton University, Princeton, NJ GA Seminar Jan. 14, 2008 General Atomics San Diego, Cal. 1

  2. R/a~1.3, k=2, qa=12 R/a~4, k=2, qa=4 A Spherical Torus is a Low Aspect Ratio Tokamak New physics regimes are expected at low aspect ratio • Strong toroidicity – field line length in good curvature region maximized - Benefits stability and confinement •Intrinsic cross-section shaping (k>2, BT/Bp ~ 1) - Enhanced stability (higher b) - High bootstrap current fraction (>50% of total) • Large fraction of trapped particles • Large gyroradius (a/ri ~ 30 – 50, re~0.1 mm) • Large plasma flow and flow shear (M ~ 0.5) • Supra-Alfvénic fast ions (vbeam/VAlfvén ~ 4) 2

  3. NSTX Addresses Key Issues for Plasma Science, Fusion Energy Development and ITER Physics • To provide the physics basis for future ST-based devices, such as NHTX, ST-CTF, and ST-Demo • To broaden the physics basis for Burning Plasma experiments, actively participating in ITPA and US BPO • To advance the understanding of toroidal magnetic confinement NSTX/ST Strength: • Exceptionally wide operating plasma parameter space • High degree of facility flexibility • Highly accessible plasmas, comprehensive set of diagnostics Major Radius R0 0.85 m Aspect Ratio A 1.3 Elongation k 2.8 Triangularity d 0.8 Plasma Current Ip 1.5 MA Toroidal Field BT 0.55 T Pulse Length 1.5 s NB Heating (100 keV) 7 MW bT,tot up to 40% 3

  4. Highly Accessible Plasmas and Extensive Diagnostics Enable NSTX’s Contributions 30 Ch, 60Hz MPTS for Te(r), ne(r) 51 Ch CHERS for Ti(r), V(r) 16 Ch MSE for q(r) (19 ch planned) Innovative design enables MSE in kG range for the first time Tangential High-k Scattering (3 MHz) Dr=3 cm 70 Ch Poloidal CHERS for Vp(r) Fast Ion Da Camera using P-CHERS views 4

  5. What are the Key ST Physics Issues to Address? 5

  6. NSTX Plasmas Approach the Normalized Performance Needed for a Spherical Torus - Component Test Facility (ST-CTF) WL= 1 MW/m2 4 MW/m2 Approximate Bootstrap Fraction fbs =0.3e0.5bpol • Design optimization for a moderate Q driven ST-CTF: • Minimize BT required for desired wall loading  Maximize <p>/BT2 = bT • Minimize inductive current  Maximize fbs 0.5P • Do this simultaneously  Maximize fbsbT 0.5PbT Goal of a driven ST-CTF: DT neutron flux = 1 – 4 MW/m2 Achievable with: A = 1.5, k= 3, R0 = 1.2 m, IP = 8 – 12 MA bN ~ 5 %-m-T/MA, H98y,2 = 1.3 bT = 15 – 25% fBS = 45 – 50% 6

  7. NSTX Plasmas Many High-Performance Plasma Features Simultaneously for Extended Pulse NSTX “Hybrid”-Like Scenario as Proposed for ITER High Confinement Multiplier High Non-inductive Fraction High NormalizedBeta Stable Boundary tCR 7

  8. T = 2µ0<p>/BT02 (%) (30) Normalized current Ip/a·BT (MA/m·T) (24) (6) Macroscopic Stability • NSTX is developing predictive understanding and control of performance-limiting MHD • Improved low-f mode detection improves RWM and EF control • n > 1 error fields discovered & corrected, improving plasma sustainment • Low-A, low BT locked mode threshold scaling favorable for ITER, ST’s • NTM onset b increases with increased rotation – important for next-steps 8

  9. RWM Feedback Stabilization Experiments at Low Toroidal Rotation are Validating VALEN Code • Optimal 270 phase difference between mode BP and applied BR consistent with VALEN • Also investigating role of plasma rotation in RWM feedback control • “Mode deformation” reduced (not shown here) using combined U/L sensors in mode-ID for feedback  More robust feedback with additional sensors 9

  10. Discovered High-n Error Fields (n=3) Important at High bN • Pulse-length depends on polarity of applied n=3 • Anti-corrective polarity disrupts Ip and b • Plasmas operate above n=1 no-wall limit  RFA • slows rotation  • destabilizes n=1 RWM • Correction current magnitude for n=3 similar to that for n=1 correction • Applied n=3 |BR| is  6 G at outboard midplane • Fortuitous phase match between intrinsic n=3 EF and field coils can apply 0A -300A +300A • n > 1 error fields not commonly addressed in present devices, or in ITER 10

  11. Simultaneous Multiple-n Correction Improves Performance(Active Feedback Control of n=1 RFA + Pre-Programmed n=3 Correction) • Long period free of core low-f MHD activity • Plasma rotation sustained over same period • Core rotation decreases with increasing density (fGW 0.75), but… • R > 1.2m rotation slowly increases until large ELM at t=1.1s •  Record pulse-length at IP=900 kA (bn~5 maintained) 0.65s 11

  12. Transport and Turbulence • NSTX offers a novel view into plasma transport & turbulence properties • Unique parameter space (R/a, b) covering e-s to e-m turbulence • Dominant electron heating (expected for a-heating in BP devices) • Strong rotational shear can influence transport • Excellent lab in which to study e- transport (ions neoclassical,localized measurements of e- scale turbulence w/ re~0.1 mm) • NSTX can contribute to developing a first-principles understanding of the transport of electron energy, particles, and angular momentum • Confinement scalings differ from those of conventional aspect ratio • b dependence of H-mode confinement is shape/ELM dependent • High-k fluctuations consistent with expectation from ETG theory • Microtearing modes can contribute to electron thermal transport • Impurity particle transport is neoclassical • Drift effects on ITG/TEM may induce angular momentum pinch 12

  13. Ion Transport Often Found to Be Near Neoclassical In H-modes Linear GS2 calculations indicate possible suppression of low-k turbulence by ExB shear during H-phase - Supported by non-linear GTC results Controls tE scaling with Ip Neoclassical (r/a=0.5-0.8) Neoclassical levels determined from GTC-Neo: includes finite banana width effects (non-local) ci anomalous in high density L-modes 13

  14. NSTX Is In an Excellent Position to Study and Understand the Key Areas of Electron Turbulence and Critical Gradient Physics High-k scattering diagnostic (Dr=3 cm) k-range of fluctuations in ETG/high-k TEM range Expt’l R/LTe HHFW heating Jenko et al., critical R/LTe for ETG (empirical approximation) Linear GS2 analysis High fluctuation level when R/LTe is greater than critical value for ETG 14

  15. Electron Internal Transport Barrier Associated With Strongly Reversed Magnetic Shear Reduction in electron transport with reversed magnetic shear (i.e., ce drops as Te peaks) Fluctuations in ETG k-range (kqre=0.23) increase with decreased shear reversal 15

  16. Electron Transport May Be Controlled by Multiple Mechanisms Low-k microtearing important in low shear/“Hybrid” discharges Suppressed with strongly reversed magnetic shear Heat flux due to high-k electron modes (ETG) consistent with levels inferred from TRANSP in L- & H-modes for r/a>0.5 16

  17. Momentum Transport Studies Focus on High-Rotation Plasmas, Relation to Energy Transport cf << ci, is common in NSTX, unlike at higher R/a Perturbative momentum transport studies using magnetic braking indicate significant inward pinch cf from momentum balance – no pinch tf ~ 150 ms ~ 3-4 tE cf > cf,neo, even though ci ~ ci,neo Experimental pinch scales with theoretical estimates based on low-k turbulence driven pinch Is momentum transport the best probe of low-k turbulence? Gi,turb < Gi,neo ; Gf,turb > Gf,neo 17

  18. Fast-Ion Driven MHD • Unique combination of fast-ion instability drive flexibility and complete diagnostic coverage continues to lead to new discoveries in NSTX • Fast ion density profile is significantly broadened by strong MHD • The TAE stability space has been fully mapped for first time • 3D non-linear simulations are elucidating multi-mode TAE properties • Beta dependence of Cascade/GAM coupling demonstrated for first time • New Beta-induced Alfvén Acoustic Eigenmodes identified, characterized 18

  19. NSTX Accesses ITER-Relevant Fast-Ion Phase-Space Regime • ITER will operate in new, small r* regime for fast ion transport • k^r ≈ 1 means "short" wavelength Alfvén modes • Fast ion transport expected from interaction of many modes • NSTX can access multi-mode regime via high bfast / btotal and vfast / vAlfven NSTX observes that multi-mode TAE bursts induce larger fast-ion losses than single-mode bursts: 1% neutron rate decrease: 5% neutron rate decrease: E. Fredrickson, Phys. Plasmas 13, 056109 (2006) 19

  20. Beam neutral density footprint Scan elevation radius at RTAN=80cm NPA sightline Vertical Scanning Capability of Neutral Particle Analyzer (NPA) Used to Diagnose Fast-Ion Transport in an “MHD-active” Discharge • Discharge with low-f kink-type + bursting fishbone-like MHD activity + “sea of Alfvénic modes (morass of MHD)”  very strong effect on fast ions expected 20

  21. TRANSP Simulation of NPA Vertical Scan Requires Anomalous Fast Ion Diffusion and Shows Outward Redistribution of Core Fast Ions • Fast ion density profile near EINJ evolves from monotonic to hollow Consistent with NBI current redistribution measured (Menard et al., PRL 2006) Anomalous diffusion much weaker/absent for majority of NSTX discharges 21

  22. NSTX Observations Support Recent Theoretical Models of (RS) Alfvén Cascade Modes Coupling to Geodesic Acoustic Modes e(0.25s)  5% e(0.25s)  1.2% TAE frequency  vAlfven • Onset near GAM frequency (lower red curve) • Frequency sweeps upwards • Saturates near TAE frequency (blue curve) GAM frequency  cs (cs/vA)2 ~ b f=(m-nqmin)/qminR MSE data agrees with qmin deduced from mode frequency sweeps 22

  23. Boundary Physics • NSTX is unique in the world as the only diverted tokamak studying Li for particle pumping and power handling, while retaining a broad boundary program studying divertor heat flux, edge turbulence, pedestal physics, ELMs, and fueling • Li can improve energy confinement and increase particle pumping • Partial divertor detachment reduces peak divertor heat flux while remaining compatible with high performance plasmas • SOL width scalings being developed to extrapolate to next-steps • Progress in developing predictive capability for SOL turbulence 23

  24. Line-average density (cm-3) NOTE slower rate of rise Lithium Evaporator (LITER) Can Produce Particle Pumping and Improved Energy Confinement in H-mode Plasmas Li effective in suppressing ELMs No Li Li 24

  25. Partially Detached Divertor (PDD) Achieves Significant Heat Flux Reduction at High k, d with No Decrease in H-mode Confinement (MW/m2) • Peak divertor heat flux reduced by factor 2-3 during PDD phase by injecting D2 in divertor • No increase in main plasma density • No degradation of confinement • Same WTOT for same PNBI = 6MW Extended results from low k, d (2006) to high k, d, high flux expansion shape for high performance scenarios 1 MA, 6 MW Heat flux mitigation techniques compatible with high performance crucial for next-steps 25

  26. SOLT Turbulence Code Shows Blob Formation and Ejectionand Reproduces Many Features of Boundary Turbulence Observed Experimentally Validation of code physics on several hierarchical levels  Working to resolve discrepancy between theory and exp’t on blob acceleration when disconnected Analytic work on effect of parallel electron physics, e-s vs electromagnetic Effects of non-adiabatic fluctuations (dF, dn out of phase) for driving blob formation 26

  27. Plasma Startup and Rampup • NSTX is contributing to wave heating and current drive techniques applicable to high-b, over-dense plasma conditions • Significant amounts of non-inductive startup current generated on closed flux surfaces by Co-Axial Helicity Injection • HHFW coupling improved by reduced surface wave excitation • EBW coupling improved by boundary changes and Lithium conditioning 27

  28. HHFW for ramp-up of low Ip plasma (bootstrap + FWCD) IP CHI or PF-only for plasma initiation and early ramp-up 200 kW ECH/EBWH time CHI HHFW HHFW+NBI Developing Means to Initiate Solenoid-Free Plasmas and Couple to Non-Inductive CD Methods Is Essential for ST HHFW + EBW + NBI CHI: Co-Axial Helicity Injection ECH/EBW: 200 kW 28/15.3 GHz system planned (2009) HHFW: 30 MHz, 6 MW, 20th harmonic NBI: Fast ions confined for Ip>600 kA 28

  29. 160 kA of Closed Flux Current Produced in NSTX by Transient Co-Axial Helicity Injection Evidence for high-IP flux closure: • IP=160kA remains after CHI injector current ICHI 0 at t=9ms • After t=9ms, plasma current decays away inductively • Once IINJ 0, reconstructions track dynamics of detachment & decay Current decay rate consistent with resistivity of plasma 10 – 20 eV 29 R. Raman et al., PRL 97 (2006) 175002

  30. Good Coupling Key to Success of Both EBW and HHFW • Heating more efficient at short l, high BT • Reduction in heating efficiency due to • propagation near edge when ne k||B/w L-mode observations show 60% conversion efficiency, consistent with theory Want to maximize k||, B Increase in EBW transmission efficiency in H-mode observed with Lithium conditioning 19 m g/min 11 mg/min 0 mg/min 30

  31. Integrated Scenarios • NSTX is ST world leader in integrating and sustaining high confinement, high-non-inductive current fraction, and high b • Breakdown, ramp-up, and plasma shape successfully modified to access high-qmin q profile aimed at increasing non-inductive fraction 31

  32. Stable & Fully Non-Inductive Target Scenario Utilizing Only NBI and BS Current Drive Has Been Identified Target scenario: Present high-fNI long-pulse H-modes: IP = 750kA, fNI < 65%, fBS < 55% IP = 700kA, fNI =100%, fBS= 80% bN < 5.6, bP < 1.5, bT < 17% bN = 6.7, bP = 2.7, bT =15% li = 0.6, qmin=1.3, BT=4.5kG li = 0.5, qmin = 2.4, BT=5.2kG k = 2.3, dX-L = 0.75, q*=3.9 k = 2.6, dX-L = 0.85, q*=5.6 Inductive current drive is replaced by: Higher JNBI from higher Te & lower ne Higher JBS from higher bP-thermal Self-consistent JTOT from TRANSP 32

  33. Individual Elements for Achieving Fully Non-Inductive Target Are in Place rtEFIT isoflux control produces desired shape Can elevate qmin using via high-k breakdown, LITER 125006, 550ms (LRDFIT06) High qmin TARGET • = 2.6, dX-L = 0.85 dRSEP = -2mm • = 2.75, dX-L = 0.85 dRSEP = -8mm MHD fast ion redistribution can help maintain elevated qmin Enhanced confinement using LITER 33

  34. Summary • NSTX normalized performance approaching ST-CTF level • Advanced mode stabilization tools and diagnostics for DEFC/RWM suppression • ELM suppression using external midplane coils • Unique tools for understanding core and edge transport and turbulence • Excellent laboratory in which to study core electron transport • Momentum transport subject of Joule milestone • Uniquely able to mimic BP fast-ion instability drive with full diagnostics • Developing understanding and unique tools for heat flux and particle control • Two LITERs for 2008 campaign, LLD for 2009 • Non-inductive startup schemes utilizing CHI, EBW, HHFW being developed • Understanding of EBW/HHFW coupling essential • Elements for achieving fully non-inductive, high-b plasma are in place 34

  35. FY08 Research Plans • Macroscopic stability • Assess active and passive RWM stabilization physics for improved mode control (milestone) • Evaluate sources of plasma viscosity and assess impact of rotation on stability, including NTMs (Joule milestone) • Transport and Turbulence • Assess the role of flow shear using poloidal CHERS (milestone) • Evaluate the plasma rotation and momentum transport, and assess impact on confinement (Joule milestone) • Study source of electron transport combining high-k measurements and code V&V • Non-solenoidal startup and rampup • Demonstrate ohmic flux savings using CHI • Couple inductive ramp-up to CHI plasmas (milestone) • Boundary physics • Characterize divertor heat flux and access to detachment (milestone) • Compare divertor SOL to midplane SOL and edge turbulence characteristics • Determine relationship of ELM properties to discharge boundary shapes and Lithium conditioning, and compare stability of pedestal/ELMs with model calculations • Waves and Energetic particles • HHFW/EBW - Understand and improve coupling to and heating of H-mode deuterium NBI-heated plasmas • Fast Ion MHD - Assess fast ion transport from TAE avalanches and compare to non-linear TAE simulations • Advanced scenarios and control • Characterize non-inductive current drive fraction versus density, shaping and q • Attempt to achieve long-pulse density control using improved fueling and Lithium conditioning • Vertical stability limits in highly elongated plasmas 35

  36. Backup Vugraphs 36

  37. 1.0 Ip (MA) 0 bN 6 4 2 0 10 wf/2p (kHz) 0 0 IA (kA) -1.0 DBpun=1(G) 40 20 0 DBrun=1(G) 8 4 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 t (s) Record Pulse Length at IP=900 kA Achieved Through Combined n=1,3 Active Control • Feedback used combined upper/lower Bp sensors with spatial phase offset • Moderate plasma rotation keeps DBr in check • n=3 DC field phased to best maintain wf 37

  38. Type III ELM-free Type I Global Studies Have Established Dependences for Confinement and L-H Threshold Power • L-H threshold power • Apparent Ip dependence, L-H easier with high-field-side fueling • Triangularity, X-point height, configuration important • Global confinement dependences differ from those at higher aspect ratio • tE ~ BT0.9 ↔tE98y,2 ~ BT0.15; tE ~ Ip0.4 ↔tE98y,2 ~ Ip0.9 • Improvement in global confinement with Lithium evaporation b-scaling high priority ITPA topic:Pedestal/ELMs matter! Type III/No → Type I ELMs Type I: DW/W~7% Small Type V ELMs k=1.85 d=0.4 k=2.1 d=0.6 38

  39. ELM-Induced Cold Pulse Propagation Used to Probe Critical Gradient Physics of ETG Modes Increase in high-k fluctuations as cold pulse propagates inward from edge, increasing Te 39

  40. NSTX Is In a Strong Position to Study and Understand Electron Transport High-k fluctuations seen to increase with increasing R/LTe Electron transport anomalous: controls BT scaling k-range of fluctuations in ETG/high-k TEM range High-k scattering diagnostic (Dr=3 cm) 40

  41. Internal Transport Barriers • ETG critical gradients greatly exceeded for reversed shear, but electron transport remains low 41

  42. Theory/Gyrokinetic Calculations Suggest ETG May Play an Important Role in Determining Electron Transport at Low BT ETG linearly unstable only at lowest BT - 0.35 T: R/LTe 20% above critical gradient - 0.45, 0.55 T: R/LTe 20-30% below critical gradient Non-linear simulations indicate formation of radial streamers (up to 200re):FLR-modified fluid code [Horton et al., PoP 2005] GS2 0.35 T Kim, IFS Saturated ETG level TRANSP Good agreement between experimental and theoretical saturated transport level at 0.35 T Qe (kW/m2) 42

  43. Near Neoclassical Ion Transport Primarily Governs Ip Scaling GTC-Neo neoclassical: includes finite banana width effects (non-local) ci,GTC-NEO (r/a=0.5-0.8) 43

  44. Local Transport Studies Reveal Sources of Energy Confinement Trends Electrons primarily responsible for strong BT scaling in NSTX (tE~BT0.9) Variation in near-neoclassical ion transport primarily responsible for Ip scaling (tE~Ip0.4) Electrons anomalous Ions near neoclassical Neoclassical Neoclassical levels determined from GTC-Neo: includes finite banana width effects (non-local) 44

  45. Core Momentum Diffusivities Up to An Order of Magnitude Lower Than Ion Thermal Diffusivities Is momentum diffusivity tied more to electron diffusivity when ions are neoclassical? What is cf,neo? 45

  46. Momentum Diffusivity NOT Neoclassical Even Though Ion Thermal Diffusivity ~Is Inward neoclassical momentum flux driven by Ti cf,neo << cf cf, neo can be negative! Relation to source of intrinsic rotation? 46

  47. R~1.32 m R~1.15 m Dedicated Perturbative Momentum Confinement Experiments Recently Carried Out Perturbative tf, cfcan be obtained from transient application of nRMP • Use dL/dt = T – L/tf relation to determine instantaneous tf • model spin-up to determine perturbative tf using • L(t) = tf * [T – (T-L0/tf) * exp(-t/tf)], where • L = Angular momentum • T = Torque (NB torque only) • L0 = Angular momentum at time of nRMP turn-off Maximum plasma response at location of maximum torque (R~1.32 m) Steady-state tE ~ 50 ms tf >>tE in NSTX (consistent with cf<<ci) (w/Solomon) 47

  48. Perturbative Momentum Transport Studies Using Magnetic Braking Indicate Significant Inward Pinch • Can determine vpinch only if w, w decoupled • Assume cfpert, vpinchpert constant in time • Expt’l inward pinch generally scales with theoretical estimates based on low-k turbulence-driven pinch • Peeters: vpinch/cf= [-4-R/Ln]/R (Coriolis drift) • Hahm: vpinch/cf= -3/R (B, curvature drifts) 48

  49. Dedicated Perturbative Momentum Confinement Experiments Recently Carried Out • Use non-resonant n=3 magnetic perturbations to damp plasma rotation • Previously been used to slow plasma rotation for ITER-relevant RWM stabilization experiments (Sabbagh et al.) Observed rotation damping consistent with neoclassical toroidal viscosity (NTV) theory Steady-state & transient application 49

  50. Identification of b-Induced Alfvén-Acoustic Eigenmodes (BAAE) • EP driven modes often seen at frequencies lower than those expected for TAE • Couples two fundamental MHD branches (Alfvén & acoustic) – new • Joint studies with JET Reflectometers needed for measuring localization and mode structure How does this scale to ITER? 50

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