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Physics Results from the National Spherical Torus Experiment

Office of Science. Supported by. Physics Results from the National Spherical Torus Experiment. 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 JAERI Ioffe Inst RRC Kurchatov Inst TRINITI KBSI

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Physics Results from the National Spherical Torus Experiment

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  1. Office of Science Supported by Physics Results from the National Spherical Torus Experiment 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 JAERI Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep U Quebec 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 PSI Princeton U SNL Think Tank, Inc. UC Davis UC Irvine UC Los Angeles UC San Diego U Colorado U Maryland U Rochester U Washington U Wisconsin M.G. Bell for the NSTX Group Princeton Plasma Physics Laboratory Princeton University, Princeton, NJ 1

  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, qa = 12 Conventional Tokamak A ≈ 3, qa = 4 Field lines 2

  3. NSTX Designed to Study High-Temperature Toroidal Plasmas at Low Aspect-Ratio Conducting platesfor MHD stability Slim center columnwith TF, OH coils Aspect ratio A 1.27 Elongation  2.5 (3.0) Triangularity  0.8 Major radius R0 0.85m Plasma Current Ip 1.5MA Toroidal Field BT0 0.6 (0.55) T Pulse Length 1.5s Auxiliary heating: NBI (100kV) 7 MW RF (30MHz) 6 MW Central temperature 1 – 3 keV 3

  4. NSTX Research Contributes to Fusion Energy Development, ITER Physics and Plasma Science • Determine the physics principles of ST confinement • Limits, scaling, control, heating schemes, integration • Complement and extend conventional aspect-ratio tokamaks • Utilize low aspect ratio and high-bT to address basic physics of toroidal confinement • Support preparation for burning plasma research in ITER • Participate in the ITPA and USBPO • Complement ITER by exploring possibilities for an attractive Component Test Facility and Demonstration Power Plant 4

  5. In Addition to High , New Physics Regimes Are Expected at Low Aspect Ratio • Intrinsic cross-section shaping ( > 2, BP/BT ~ 1) • Large fraction of trapped particles √(r/R)) • Large gyro-radius (a/ri ~ 30–50) • 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) 5

  6. Key ST Physics Issues to Explore in NSTX 6

  7. T = 2µ0<p>/BT02 (%) Normalized current Ip/a·BT (MA/m·T) NSTX Extends the Stability Database Significantly • 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) • Seeing benefits of • Low aspect ratio • Cross-section shaping • Stabilization of external modes by conducting plates 7

  8. Optimized Plasma Shaping Can Increase bP and Bootstrap Current Fraction at High bT • High elongation k reduces BP,av = µ0Ip/∫Cdl, increases bootstrap current • Sustained k 2.8 for many twall by fast feedback • Higher triangularity  and proximity to conducting wall allows higher N • Plasma rotation maintains stabilization beyond decay-time of wall current Divertor coil upgrade k = 3.0, dX= 0.8 li = 0.45 2004 2005 Vertically unstable fNI=100% target k Vertically stable operating space li D. Gates et al., NF 46 (2006) 17 8

  9. NSTX Approaches 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 = 1.2m, IP = 8 – 12MA bN ~ 5 %.m.T/MA, H98y,2 = 1.3 bT = 15 – 25% fBS = 45 – 50% 9

  10. High Performance Has Been Sustained For Several Current Redistribution Times at High Non-Inductive Current Fraction Non-inductive current fractions (TRANSP with classical thermalization) • p and NBI current drive provide up to 65% of plasma current • Bootstrap current accounts for ~50% • High bNH89P now sustained for up to ~5 current relaxation times D. Gates, PoP 13, 056122 (2006) 10

  11. Both Internal and External Modes Can Limit b Non-linear M3Dsimulations consistent with experiment Discharge collapses as rotation flattens and decreases Resistive Wall Modes can limit bT at low-q Maintaining high plasma rotation is key to stabilizing both modes bT (%) ff(0) (kHz) Mode Bq (G) q0 (EFIT w/o MSE) S. Sabbagh et al., NF 44 (2004) 560 11

  12. 2 1 0 NSTX ITER -1 Control coils Blanketmodules Stabilizingplates -2 Z(m) ITER shapeboundaryvessel 0 1 2 R(m) Error Field Correction by External CoilsExtends Duration of High-Performance Plasmas Plasma rotation sustained by correction of intrinsic error fields 48 InternalBP, BR sensors 6 ExternalControl Coils Copper stabilizing plates J. Menard et al., IAEA 2006 12

  13. Resistive Wall Mode (RWM) Actively Stabilized at Low, ITER-Relevant Rotation • Reduce NB-driven rotation by magnetic braking • Apply non-resonant n=3 field perturbation • RWM becomes unstable when rotation drops below critical value • No-wall b-limit computed by DCON code using measured profiles • Data from internal sensors detects mode in real-time • Feedback system applies correcting n=1 field perturbation with appropriate amplitude and phase S. Sabbagh et al., PRL 97 (2006) 04500 13

  14. NSTX Provides a Novel Vantage Point from which to View Plasma Transport and Turbulence • Operates in a unique region of dimensionless parameter space: R/a, bT, (r*,n*) • Large range of bT spanning e-s to e-m turbulence regimes • Dominant electron heating with NBI • Relevant to a-heating in ITER • Strong rotational shear affects transport • Ion transport aproaches neoclassical • Electron transport anomalous • Localized electron-scale turbulence measurable (re ~ 0.1 mm) 14

  15. H-mode Confinement Scaling Experiments Have Isolated the BT and Ip Dependences Scans carried out at constant density, injected power (4 MW) 0.50 s 0.50 s S. Kaye et al., IAEA 2006 15

  16. ... Revealing Differences from Conventional Tokamaks Strong dependence of tE on BT Weaker dependence on Ip H98y,2 ~ 0.9 → 1.1 → 1.4 H98y,2 ~ 1.4 → 1.3 → 1.1 4 MW 4 MW tE,98y,2 ~ BT0.15 tE,98y,2 ~ Ip0.93 NSTX exhibits stronger tE scaling at fixed q:tE ~ Ip1.3-1.5 than ITER H-mode scaling:tE,98y,2 ~ Ip1.1 16

  17. Dimensionless Parameter Scans Address High-Priority ITPA Issues b-scan at fixed q, BT - b-dependence important to ITER advanced scenarios (Bt98y2~b-0.9) - Factor of 2-2.5 variation in bT - Degradation of tE with b weak on NSTX ne*-scan at fixed q - Factor of >3 variation in ne* - Strong increase of confinement with decreasing collisionality 20% variation in re, ne* k=2.1 d=0.6 S. Kaye et al., IAEA 2006 17

  18. Near-Neoclassical Ion Transport Primarily Governs Ip Scaling GTC-Neo calculation includes finite banana width effects (non-local) ci,GTC-NEO (r/a=0.5-0.8) 18

  19. Variation of Electron Transport Primarily Responsible for BT Scaling Broadening of Te & reduction in ce outside r/a=0.5 with increasing BT Ions near neoclassical Neoclassical 19

  20. Calculations Suggest ETG May Play an Important Role in Determining Electron Transport at Low BT • Non-linear simulations (up to 250re) show formation of radial streamers • FLR-modified fluid code [W. Horton et al., PoP 11 (2004)] • GS2 calculations show ETG linearly unstable only at lowest BT • 0.35T: R/LTe 20% aboveTe,crit • ≥0.45T: R/LTe 20-30% belowTe,crit 0.35 T GS2 • Good agreement between experimental and theoretical saturated transport level at 0.35 T • Experimental ce profile at 0.35 T consistent with prediction from e-m ETG theory [W. Horton et al., NF 45 (2005)] • Not consistent at higher BT J.H. Kim et al., APS-DPP, Philadelphia, Oct. 2006 20

  21. ELMs r/a Now Beginning to Make Measurements of Turbulence Extending to Electron Gyro-radius Scale 1mm microwave forward scattering system shows fluctuations (ñ/n) reduced in upper ITG/TEM and ETG k-ranges during H-mode Both ion and electron transport decrease at transition from L- to H- mode ~ Electron transport remains anomalous Ion transport close to neoclassical during H-phase • Localized measurement (r ≈ 6cm) • Adjustable from axis to outer edge H. Park et al., APS-DPP, Philadelphia, Oct. 2006 21

  22. Strongly Reversed Magnetic Shear L-mode Plasmas Have Higher Te and Reduced Transport Linear GS2 calculations indicate reduced region of micro-tearing instability for RS plasma GS2 also indicates ETG stabilized by RS F. Levinton, invited talk ZI1.3, APS-DPP, Philadelphia, Nov. 2006 22

  23. Pellet Perturbations Used to Probe Relation of Critical Gradient Physics to q-Profile Soft X-ray array diagnoses fast DTe R/LTe t=440→444 ms H-mode with monotonic q-profile exhibits stiff profile → Te close to marginal stability R/LTe t=297→301 ms Reversed magnetic shear L-mode shows steepening of profile with increasing central Te D. Stutman, Phys. Plasmas 13, 092511 (2006) 23

  24. NSTX Accesses Fast-Ion Phase-Space Regime Overlapping With and Extending Beyond ITER • ITER will operate in new regime for fast ion transport • Fast ion transport expected from interaction of many modes • NSTX can access multi-mode regime via high bfast / btotal and vfast / vAlfven Multi-mode TAE bursts in NSTX induce larger fast-ion losses than single-mode bursts 1% neutron rate decrease 5% neutron rate decrease E. Fredrickson, Phys. Plasmas 13, 056109 (2006) 24

  25. Alfvén Cascades (RSAE) Observed at Low be on NSTX • Frequency chirping indicates evolution of qmin • Analysis with successive modes verifies profile reconstruction with MSE constraint • Modes also observed on MAST 25

  26. “Angelfish” MHD Phenomenon Identified as Form of Hole-Clump, Consistent with Theory • Mode satisfies Doppler-shifted resonance condition for TRANSP calculated fast ion distribution • Growth rate from theory in reasonable agreement with observation • Engineering of fast-ion phase space can suppress deleterious instabilities H. Berk, IAEA-FEC, Chengdu, (2006) 26

  27. lqSOL, m-p (cm) collisionless Counsell collisional Kallenbach PLoss (MW) Divertor Power Loading Critical Issue for the ST – High P/R Peak heat flux increases with poweras outer leg becomes connected Midplane heat flux SOL in NSTXbroader than models predict Conduction Dominated Divertor Radiation Dominated Divertor R. Maingi et al., PSI Conf. 2006 27

  28. Peak Heat Flux Can Be Reduced By Plasma Shaping • Compare configurations with different triangularity at X-point X • lower single-null, X ≈ 0.4 • double-null, X ≈ 0.4 • double-null, X≈ 0.75 high triangularity • Flux expansion decreases peak heat flux1 :0.5:0.2despite reduced radius • ELMs:Type IMixed Type V Measure heat flux to divertor with IR thermography of carbon tiles R. Maingi et al., PSI Conf. 2006 28

  29. Gas Puffing Near X-point Can Produce Radiative Divertor Without Affecting Core Confinement V. Soukhanovskii et al., IAEA 2006 29

  30. Lithium Evaporated on PFCs Produced Particle Pumping and Improved Energy Confinement in H-mode Plasmas Lithium Evaporator H98y,2 = 1.1→ 1.3 Effect transient and did not increase with amount of lithium R. Majeski et al., IAEA 2006; H. Kugel et al. APS-DPP, Philadelphia, Nov. 2006 30

  31. Imaging of Plasma Edge Contributing to Understanding Edge Turbulence Phenomena (Blobs, ELMs) ELM dynamics and rotation have been measured Measurements of “blob” propagation connect to evolving theory R. Maqueda, R. Maingi, S. Zweben, D. Stotler 31

  32. 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 Ways to Initiate, Ramp-up and Sustain Plasma Current without Reliance on Central Solenoid Critical for the ST CHI: Co-Axial Helicity Injection ECH/EBW: 28/15.3 GHz, 200 kW system planned (2009) HHFW: 30 MHz (~20th D harmonic), 6 MW NBI: effective with enough current to confine ions 32

  33. CHI Can Initiate Plasma Current Without Induction • Initially investigated and developed in HIT and HIT-II devices at U. Washington • Toroidal insulating breaks between inner, outer vacuum vessel in NSTX • Transient CHI: Axisymmetric reconnection during decay of injected current leads to formation of closed flux surfaces 33

  34. Transient CHI Has Now Produced 160 kA of Closed-Flux Current in NSTX Toroidal plasma current remaining after ICHI→0 flows on closed surfaces Once ICHI0, EFIT reconstruction using external magnetic sensors tracks dynamics of detachment from injector & resistive current decay Current decay rate consistent with resistivity of plasma 10 – 20 eV R. Raman et al., PRL 97 (2006) 175002 34

  35. 0.3 0.2 Apparent coupling efficiency 0.1 0.0 0.0 0.2 0.4 Apparent coupling efficiency Time (s) EBW Coupling to External Antenna Investigated Using Mode Conversion of Thermal EBW in Plasma • Conventional ECCD not possible in “overdense” (pe > ce) ST plasmas • Electron Bernstein waves (EBW) can propagate and be absorbed in plasma to produce current drive for NSTX goal, but • EBW-CD relies on mode conversion from externally launched e.m. waves Coupling via B-X-O conversionwell modeled in L-mode at fce Coupling in H-modelower than predicted G. Taylor; S. Diem APS-DPP, Philadelphia, Nov. 2006 35

  36. Constant PRF≈2MW k||=3 m-1 We (kJ) 2 7 m-1 BRF (a.u.) 14 m-1 1 0.1 0.2 0 Time (s) Heating Efficiency of HHFW Improved at High BT and k|| • Need directed waves with k|| = 3.5 – 7m-1 for HHFW-CD current drive At fixed BT = 0.45T, electron heating efficiency degrades at lower k|| (higher vph) Magnetic probe at edge detects RF signal from Parametric Decay Instability At fixed k|| = 7m-1, electron heating efficiency improves at higher BT J. Hosea, APS-DPP, Philadelphia, Nov. 2006 36

  37. NSTX Achieves Many High-Performance Plasma Features Simultaneously for Extended Pulses NSTX “Hybrid”-like scenario as proposed for ITER High Confinement High Non-inductive Fraction High N Stable Boundary tCR 37

  38. MHD-Induced Redistribution of NBI Current Drive Contributes to NSTX “Hybrid”-Like Scenario Proposed for ITER qmin>1 for entire discharge, increases during late n=1 activity • Fast ion transport converts peaked JNBI to flat or hollow profile • Redistribution of NBICD makes predictions consistent with MSE n=1 mode onset n=1 mode onset • High anomalous fast ion transport needed to explain neutron rate discrepancy during n=1 J. Menard, PRL 97, 095002 (2006) 38

  39. Integrated Modeling Points to Importance of Shaping, Reduced ne, and Increased Te/tE for Higher fNI and High bN • Achieved (116313) • n20(0)=0.85 • =2.2 • H98=1.1 • N = 5.6 • q(0) = 1.15 Total Total NICD Bootstrap NBCD p • Lower density • n20(0)=0.36 • =2.2 • H98=1.1 • N = 5.6 • q(0) = 1 @ 0.8 s • Higher , E • n20(0)=0.75 • =2.55 • H98=1.35 • bN = 6.6 • q(0) = 1.4 n(0)=0.75e20 C. Kessel, Invited talk, APS-DPP, Denver, Oct. 2005 39

  40. NSTX Normalized Performance Approaches Required Level for ST-CTF • Advanced mode stabilization methods and diagnostics are being applied to improve performance • Dynamic Error Field Correction and RWM feedback suppression • Unique tools available to study transport and turbulence • Excellent laboratory in which to study core electron transport • Investigating fast-ion instabilities with full diagnostics, including MSE for q-profile • Capability to mimic ITER situation • Developing non-inductive startup and sustainment schemes • CHI, EBW, HHFW • Developing methods for heat flux and particle control • Lithium, radiative divertors • NSTX also contributes to several high-priority issues for ITER 40

  41. STs Can Lead to Attractive Fusion Systems • Component Test Facility (CTF) will be needed after ITER to carry out integrated DEMO power testing and development • ST enables highly compact CTF with full remote maintenance and high duty factor, and it provides potentially attractive reactor configuration M. Peng et al, PPCF 47,B263(2005) 41

  42. During Magnetic Braking, Rotation Profile Follows Neoclassical Toroidal Viscosity (NTV) Theory • First quantitative agreement with NTV theory • Due to plasma flow through non-axisymmetric field • Trapped particle, 3-D field spectrum important • Computed using experimental equilibria • Necessary physics for simulations of rotation dynamics in future devices (ITER, CTF) Magnetic braking due to applied n=3 field W. Zhu et al., PRL 96 (2006) 225002 42

  43. 121071 121083 Increased Ion Collisionality Decreases Critical Rotation Frequency Wcrit • Plasmas with similar vA • Consistent with neoclassical viscous dissipation model • at low g, increased ni leads to lower Wcrit • ITER plasmas with lower ni may require higher degree of RWM active stabilization (K. C. Shaing, Phys. Plasmas 11 (2004) 5525.) Further analysis aims to uncover RWM stabilization physics Sontag, et al., IAEA 2006 43

  44. Low A, High b Favorable for Study of NTM Seeding / Stabilization Sawtooth excitation of n = 2 • Several types of event can initiate low frequency MHD modes in NSTX • e.g. sawteeth*, RWMs** • Can led to soft beta limit, or rotation reduction resulting in disruption • Large q = 1 radius, high b, mode coupling at low-A facilitate seeding • NTM stabilization amplified at low-A (GGJ  e3/2) – NTM less deleterious • NTM study planned 2007 - 2009 • Characterize modes: NTMs, TMs, or internal kinks? • Exploit 12 channel MSE, reflectometer, fast USXR capabilities • Mitigate deleterious effects of modes n=1 • Sawtooth excites n = 2, but n = 2 can decrease post-crash *Fredrickson, et al., APS-DPP 2004; **Sabbagh, et al., NF 44 (2004) 560 44

  45. Reflectometry Data Reveals 3-wave Coupling of Distinct Fast-Ion Instabilities EPM  Energetic Particle Mode (bounce-resonant fishbones) TAE  Toroidal Alfven Eigenmode • Large EPMTAEphase locks to EPM • forming toroidally localized wave-packet • Low-f EPMs co-exist with mid-f TAE modes • Bi-coherence analysis reveals 3-wave coupling between 1 EPM and 2 TAE modes Influence of toroidal localization of TAE mode energy on fast ion transport and EPM/TAE stability presently being investigated N. Crocker, Phys. Rev. Lett. 97, 045002 (2006) 45

  46. Identification of b-Induced Alfvén-Acoustic Eigenmodes (BAAE) • Energetic particle driven modes often seen at frequencies lower than those expected for TAE • Couples two fundamental MHD branches (Alfvén & acoustic) • Joint studies planned with JET 46

  47. FY 07 Facility Enhancements • Higher temperature bakeout of divertor tile to improve plasma performance and to prepare for lithium • Lower divertor tiles to 350°C and upper tiles to 240°C • Faster lithium evaporation • x10 deposition rate to lower divertor plates • Evaporation between/during shots in normal cycles • Higher Mach number for supersonic gas injector to improve fueling for H-mode (LLNL) • Higher voltage for higher current CHI (U Washington) • Improved monitoring of voltage transients • Faster processors for real-time plasma control system (GA) • Aiming to operate in parallel by end of FY07 run 47

  48. FY 07 Diagnostic Enhancements • Poloidal CHERS (27 ch) for transport physics • MSE 12  16 channels for improved j(r) resolution (Nova) • Transmission grating x-ray spectrometer viewing across NBI for impurity transport(JHU) • FIDA (Fast Ion Dmeasurement) - fast, band-pass-filtered channels for the local fast-ion density (prototype channels late in ‘07 run) (UC Irvine) • FIReTIP 4  6 channels (500 kHz) for improved spatial resolution (UC Davis) • New collection mirror for high-k scattering system • Correlation reflectometer, fixed freq. reflectometer (3  6 ch), profile reflectometer (25  10s) and high-k backscattering (late in the run) (UCLA) • Improved high-frequency Mirnov coil system for energetic particle modes and segmented Rogowski coil for disruption study • Wider-angle view and local gas-puffing for EBW radiometers for H-mode coupling • 3 RF probes to measure surface waves during HHFW heating • Additional divertor filterscope fast channels (24-32 total) (ORNL) 48

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