Use of 3D fields for ELM Pace-Making in NSTX Lithium Enhanced H-Modes PFC Community Meeting Cambridge, MA July 8-10, 2009 NSTX Supported by 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 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

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, 2009 Motivation: Lithium wall conditioning improves pulse length, increases τ E, suppresses ELMs, but shows impurity accumulation

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, 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 Large ELMs 3D field applied->ELMs No lithium coatings in these shots

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, 2009 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

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, External midplane coils produce a weakly resonant n=3 magnetic field Motivation: Understand 3D field effects on pedestal stability and transport Provide mechanism for impurity control in Lithium-enhanced ELM-free H-modes Poloidal Mode Number m Island overlap parameter: –σ CH > 1 for ψ N > 0.6 (vacuum) –σ CH > 1 for ψ N > 0.9 (IPEC) n=3

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, Midplane coil current scan shows threshold for destabilization without Li conditioning Threshold coil current for ELM-triggering is ~950 A ->ΔB/B = 6  –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 t n=3 = 0.4 s t n=3 = 0.3 s I n=3 = 900 A I n=3 = 950 A I n=3 = 1000 A I n=3 = 1300 A t - t n=3 (s) Natural ELM time

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, T e ped increases when n=3 field is applied Blue profiles: no n=3 applied Red profiles: 20 ms after n=3 applied (before ELMs) Pedestal electron profiles Multiple profiles, all TS data mapped to ψ N No n=3, ELM-free With n=3, 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 Pedestal ion profiles No lithium coatings in these shots

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, 2009 Typical behavior with Li wall conditioning ELMs suppressed P rad ramps to ~2 MW; P NBI = 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 relatively unaffected Magnetic ELM triggering has been applied to Lithium enhanced ELM-free H-modes

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, 2009 Typical behavior with Li wall conditioning ELMs suppressed P rad ramps to ~2 MW; P NBI = 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 relatively unaffected Magnetic ELM triggering has been applied to Lithium enhanced ELM-free H-modes

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, 2009 Triggered ELMs tend to be large ELMs occur near end of 11 ms pulses I n=3 = 1.2 kA Largest ELMs occur after a pulse fails to trigger Average triggered ELM size for a typical discharge is = 10% Very large energy excursions occur on an ELM after the previous n=3 pulse fails to trigger In these cases can be 20% or more Need to maintain high triggering reliability and frequency

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, 2009 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 I n=3 = 1.2 kA I n=3 = 2.4 kA

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, 2009 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

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, 2009 ELM size can be decreased by raising triggering frequency ELMs are very large (ΔW/W tot ~ 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 I p = 1 MA I p = 0.8 MA

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, 2009 Lower triggering frequency may be optimal for impurity control without adversely affecting energy confinement Stored energy averaged over t= s P rad at t=0.4 P rad at t=0.75 P rad at t= Hz 30 Hz 50 Hz

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, 2009 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

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, 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

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, 2009 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

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, 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 n=3, ELM-free With n=3, before ELMs Electron Temperature and DensityIon Temperature and Rotation No n=3, ELM-free With n=3, before ELMs No lithium coatings in these shots

NSTX PFC 09– ELM Pacing during Li discharges in NSTX (Canik)July 8, 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 PEST n=3 mode structure ELITE Stability Boundary