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Use of 3D fields for ELM Pace-Making in NSTX Lithium Enhanced H-Modes

NSTX. Supported by. Use of 3D fields for ELM Pace-Making in NSTX Lithium Enhanced H-Modes. J.M. Canik, ORNL

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Use of 3D fields for ELM Pace-Making in NSTX Lithium Enhanced H-Modes

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  1. NSTX Supported by Use of 3D fields for ELM Pace-Making in NSTX Lithium Enhanced H-Modes J.M. Canik, ORNL R. Maingi (ORNL), T. Evans (GA), R.E. Bell (PPPL), S.P. Gerhardt (PPPL), H.W. Kugel (PPPL), B.P. LeBlanc (PPPL), J. Manickam (PPPL), T. Osborne (GA), J.-K. Park (PPPL), S.A. Sabbagh (Columbia U), P.B. Snyder (GA), E.A. Unterberg (ORISE) and the NSTX Research Team College W&M Colorado Sch Mines Columbia U CompX General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U Purdue U SNL Think Tank, Inc. UC Davis UC Irvine UCLA UCSD U Colorado U Illinois 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 Hebrew U Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST POSTECH ASIPP ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep U Quebec PFC Community Meeting Cambridge, MA July 8-10, 2009

  2. Motivation: Lithium wall conditioning improves pulse length, increases τE, suppresses ELMs, but shows impurity accumulation

  3. Application of 3D field can destabilize ELMs in NSTX • 3D fields often used for ELM control via ELM suppression • In NSTX, 3D field trigger ELMs • Can be used as a tool for impurity control in ELM-free discharges Note: when applied to NSTX ELMy H-mode, 3D fields modify ELMs, but no suppression No lithium coatings in these shots No Large ELMs 3D field applied->ELMs

  4. Outline • Characteristics of 3D applied fields in NSTX • Coil set used to produce perturbation • Spectrum, island width analysis • ELM destabilization by 3D fields • Changes to plasma profiles • Threshold perturbation to cause ELMs • Magnetic ELM-pacing with Li conditioning • ELM suppression with Li-coated PFCs • 3D fields trigger ELMs, reduce impurity accumulation

  5. External midplane coils produce a weakly resonant n=3 magnetic field -20 0 20 Poloidal Mode Number m n=3 Motivation: Understand 3D field effects on pedestal stability and transport Provide mechanism for impurity control in Lithium-enhanced ELM-free H-modes • Island overlap parameter: • σCH > 1 for ψN > 0.6 (vacuum) • σCH > 1 for ψN > 0.9 (IPEC)

  6. Midplane coil current scan shows threshold for destabilization without Li conditioning Natural ELM time • Threshold coil current for ELM-triggering is ~950 A ->ΔB/B = 610-3 • No triggering at 900 A (natural ELMs start at ~0.5s in control discharge) • Intermittent ELMs at 950 and 1000 A • ELM frequency appears to increase with n=3 field magnitude • ELMs become more regular • Tendency clouded by tendency of plasma to lock high currents-too much braking In=3 = 900 A tn=3 = 0.4 s In=3 = 950 A tn=3 = 0.3 s In=3 = 1000 A tn=3 = 0.3 s In=3 = 1300 A tn=3 = 0.3 s t - tn=3 (s)

  7. Teped increases when n=3 field is applied • Blue profiles: no n=3 applied • Red profiles: 20 ms after n=3 applied (before ELMs) • No density pumpout is observed • Te, pressure gradient increases after n=3 field is applied • Tanh fitting gives ~30% increase in peak pressure gradient • PEST shows edge unstable after n=3 application No lithium coatings in these shots Pedestal electron profiles Pedestal ion profiles Multiple profiles, all TS data mapped to ψN No n=3, ELM-free With n=3, before ELMs

  8. Magnetic ELM triggering has been applied to Lithium enhanced ELM-free H-modes Typical behavior with Li wall conditioning ELMs suppressed Prad ramps to ~2 MW; PNBI = 3 MW Square wave of n=3 fields applied to LITER discharge 10 ms pulses, f=20 Hz, amp. = 1.2 kA ELMs triggered on 8 of 10 pulses Total radiated power reduced by a factor of ~2 Density ramp rate also decreased Stored energy relativelyunaffected

  9. Magnetic ELM triggering has been applied to Lithium enhanced ELM-free H-modes Typical behavior with Li wall conditioning ELMs suppressed Prad ramps to ~2 MW; PNBI = 3 MW Square wave of n=3 fields applied to LITER discharge 10 ms pulses, f=20 Hz, amp. = 1.2 kA ELMs triggered on 8 of 10 pulses Total radiated power reduced by a factor of ~2 Density ramp rate also decreased Stored energy relativelyunaffected

  10. Triggered ELMs tend to be large • Average triggered ELM size for a typical discharge is <ΔW/Wtot> = 10% • Very large energy excursions occur on an ELM after the previous n=3 pulse fails to trigger • In these cases<ΔWtot/W> can be 20% or more • Need to maintain high triggering reliability and frequency Largest ELMs occur after a pulse fails to trigger ELMs occur near end of 11 ms pulses In=3 = 1.2 kA

  11. Increasing the n=3 perturbation strength triggers ELMs faster • With 1.2 kA pulses of in perturbation coils, ELMs are triggered in ~8 ms • At 2.4 kA, ELM onset is reduced to ~3 ms • Limited by field penetration time through vessel (estimated to be ~4 ms) • Internal coils may trigger much faster • Provides a means for improving triggering efficiency for fixed pulse duration In=3 = 1.2 kA In=3 = 2.4 kA

  12. Maximizing the n=3 pulse amplitude allows high frequency triggering with very high reliability • ELM frequencies up to 62.5 Hz have been achieved while maintaining 100% triggering efficiency • Allows average ELM size to be reduced • Internal coils should allow faster triggering, higher frequency • Time-average magnetic braking of rotation is strong at high frequencies • Can also be greatly improved with internal coils

  13. ELM size can be decreased by raising triggering frequency Ip= 1 MA • ELMs are very large (ΔW/Wtot ~ 15%) when triggered at 10 Hz • Average ELM size can be reduced to ~5% by increasing triggering frequency to 60 Hz • Some outliers remain • Triggering reliability drops at high frequency, might be improved with internal coils • Some evidence that triggered ELMs are smaller at reduced plasma current • Evident at highest frequencies Ip= 0.8 MA

  14. Lower triggering frequency may be optimal for impurity control without adversely affecting energy confinement Stored energy averaged over t=0.5-0.75s Prad at t=0.4 Prad at t=0.75 Prad at t=1.25 10 Hz 30 Hz 50 Hz

  15. Combining ELM pacing with optimized fueling successful in producing quasi-stationary conditions • Fueling from a slow valve on the center stack was reduced, replaced with a puff with faster response • Allows fuelling to be turned off quickly following startup • Applying n=3 pulses arrested the line-averaged density and total radiated power for 0.3 s • Discharge performance was limited by n=1 rotating MHD n=1 mode

  16. Summary • Application of n=3 fields can destabilize Type-I ELMs • n=3 reduces rotation, increases pedestal electron pressure in discharge without lithium coatings • Preliminary stability calculations show pedestal is near limits, more research needed to explore transition from stable to unstable • Triggering not well understood, profile data lacking in lithium-conditioned discharges • ELM triggering has been used for magnetic ELM pace-making in Li-enhanced ELM-free H-modes • Li coatings suppress ELMs, improve confinement, but problems with impurity accumulation • ELMs are controllably introduced with n=3 fields, reducing density and radiated power • ELMs are large, can be reduced by raising triggering frequency • High frequency may not be necessary for impurity control, or optimal for keeping high confinement

  17. Maximizing the n=3 pulse amplitude improves the triggering efficiency, allowing high frequency ELMs • Triggering efficiency less than 50% with 2kA n=3 pulses • Efficiency nearly 100% at 3kA (max allowable coil current) • Allows short pulses to be used (4 ms) • Currently appear to be limited by vessel penetration time (also ~4ms) • Much shorter pulses might be possible with internal coils

  18. Core n,T profiles unchanged, toroidal rotation drops when n=3 field is applied • Blue profiles: no n=3 applied • Red profiles: 20 ms after n=3 applied (before ELMs) No lithium coatings in these shots Electron Temperature and Density Ion Temperature and Rotation No n=3, ELM-free With n=3 , before ELMs No n=3, ELM-free With n=3, before ELMs

  19. Calculations with PEST and ELITE show that pedestal is near stability boundary • PEST calculations of n=3 stability shows change from stable to unstable after 3D field is applied • ELITE shows 3D field-applied case is near peeling stability boundary • Modes should be strongly diamagnetically stabilized • Trend towards instability with 3D field application is less clear-more data needed ELITE Stability Boundary PEST n=3 mode structure

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