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

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

PFC Community MeetingCambridge, MAJuly 8-10, 2009

NSTXNSTX Supported by

College W&MColorado Sch MinesColumbia UCompXGeneral AtomicsINELJohns Hopkins ULANLLLNLLodestarMITNova PhotonicsNew York UOld Dominion UORNLPPPLPSIPrinceton UPurdue USNLThink Tank, Inc.UC DavisUC IrvineUCLAUCSDU ColoradoU IllinoisU MarylandU RochesterU WashingtonU Wisconsin

Culham Sci CtrU St. Andrews

York UChubu UFukui U

Hiroshima UHyogo UKyoto U

Kyushu UKyushu Tokai U

NIFSNiigata UU Tokyo

JAEAHebrew UIoffe Inst

RRC Kurchatov InstTRINITI

KBSIKAIST

POSTECHASIPP

ENEA, FrascatiCEA, Cadarache

IPP, JülichIPP, Garching

ASCR, Czech RepU Quebec

J.M. Canik, ORNLR. 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

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

Motivation: Lithium wall conditioning improves pulse length, increases

τE, suppresses ELMs, but shows impurity accumulation

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

3D field applied->ELMs

No lithium coatings in these shots


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


External midplane coils produce a weakly resonant n=3 magnetic field

Motivation:Understand 3D field effects on

pedestal stability and transportProvide mechanism for impurity

control in Lithium-enhanced ELM-free H-modes

-20 0 20Poloidal Mode Number m

Island overlap parameter:– σCH > 1 for ψN > 0.6 (vacuum)– σCH > 1 for ψN > 0.9 (IPEC)

n=3


Midplane coil current scan shows threshold for destabilization without Li conditioning

• Threshold coil current for ELM-triggering is ~950 A ->ΔB/B = 610-3

– 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

tn=3 = 0.4 s

tn=3 = 0.3 s

tn=3 = 0.3 s

tn=3 = 0.3 s

In=3 = 900 A

In=3 = 950 A

In=3 = 1000 A

In=3 = 1300 A

t - tn=3 (s)

Natural ELM time


Teped 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 ψN

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 profilesNo lithium coatings in these shots


Typical behavior with Li wall conditioningELMs suppressed

Prad ramps to ~2 MW; PNBI = 3 MW

Square wave of n=3 fields applied to LITER discharge10 ms pulses, f=20 Hz, amp. = 1.2 kAELMs triggered on 8 of 10 pulsesTotal radiated power reduced by a

factor of ~2Density ramp rate also decreasedStored energy relatively unaffected

Magnetic ELM triggering has been applied to Lithium enhanced ELM-free H-modes


Triggered ELMs tend to be large

ELMs occur near end of 11 ms pulses

In=3 = 1.2 kA

Largest ELMs occur after a pulse fails to trigger

• 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


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


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


ELM size can be decreased by raising triggering frequency

• 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= 1 MA

Ip= 0.8 MA


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=0.4Prad at t=0.75Prad at t=1.25

10 Hz

30 Hz

50 Hz


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


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


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


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-freeWith n=3, before ELMs

Electron Temperature and Density Ion Temperature and Rotation

No n=3, ELM-freeWith n=3 , before ELMs

No lithium coatings in these shots


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

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

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