Some recent PPCD results in MST

1
Some recent PPCDresults in MST

• B. Chapman for the MST group
• and collaborators

2
Introduction

• Large fraction of the PPCD gain in b and tE due
  to increased Te
• Ti usually changes very little
• Ion heating during PPCD believed small
• Recent MST improvements have allowed increased Ip
• PPCD at 575 kA exhibits large Ti
• Most data in this talk from 575 kA plasmas

3
(Re)introduction to PPCD

• b(core)/B 0.2
• b(n 9-15) PPCD-like from 10-12 ms

at plasma surface
m 1 n 7-15
m 0 n 1-4
central chord
central chord
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HXR emitted over plasma core

• Integrated over PPCD duration

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From previous measurements, Te grew large, Te gtgt
Ti

• Te(0) from x-ray data
• Ti(0) from CHERS, CVII
• Ion heating small?
• Electron-ion equil. time 10s of ms

6
Recent x-ray tomography confirms Te(core) 1 keV

• Double filter technique (Be filters of 478 and
  140 mm)
• Believed to be underestimate

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Recent PPCD plasmas exhibit Ti(0) gt 1 keV

• CHERS trustworthy up to 2 keV
• Likely that Ti gtgt Te early in PPCD
• Neutron count spikes at crash

sawtooth crash
m 1 m 0
from CHERS
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Neutron detector schematic
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Otherwise similar PPCD shots exhibit different
Ti(0)

• ltnegt 9x1018 m-3
• Te might be higher perhaps due to ion-electron
  coupling?

previous shot comparison shot
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Ti(deuterons) Ti(impurities)

• Ion-ion equilibration time 100 ms

Rutherford scattering CHERS
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Why is Ti higher in some PPCD plasmas?

• PPCD directly responsible for increased Te
• PPCD apparently not responsible for increased Ti
• Ti large, or not, before PPCD begins

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Ion heating occurs early and strongly in the core
Ip starts ramping up at t 0
Reversal parameter
Crashes

• Ion heating linked to
• m 0 instability

13
Heating occurs early and strongly in the edge as
well
Crashes

• Globally, estimate 10s
• of MW of ion heating,
• over a few 100 ms

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Ingredient(s) for hot-ion PPCD?

• Large heating during startup, during and maybe
  between crashes (Ti normally falls off rapidly
  between crashes)
• Large Ip sometimes leads to larger crash
  heating
• But not all large-Ip shots have large startup Ti
  -- why?
• Partial answer heating varies from crash to
  crash
• While PPCD does not cause large Ti, it may help
  to confine large Ti

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Could this possibly be useful for the future
(reactor...)?

• Pre-PPCD heating fine for now, but not as
  important for, e.g., substantially longer pulse
  PPCD (Ti will gradually decay)
• What about during PPCD?
• Short, controlled mode destabilization (impulsive
  heating)
• Avoid substantial confinement degradation? like
  ELMing H-mode

• During low-b period
• Mostly edge modes
• SXR change is small
• Maybe a slight gain?
• But whats Ti(r) doing?

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Some plans for/related to PPCD

• PPCD upgrade (longer, different PPCD waveform) --
  next week
• Test our ability to generate reliably and confine
  large Ti
• Large Ti could be important boost for planned
  higher-ne PPCD
• Programmable Bt power supply
• Arbitrary PPCD waveform
• May allow higher Ip
• Multipoint CHERS single-shot profile measurement
• Final commissioning of multipoint Thomson
  scattering
• X-ray tomography Te(r,t) measurement

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Summary/conclusions

• RFP plasma can contain Te and Ti gt 1 keV
• Large Ti implies improvement in b, and maybe tE
• Case of RFP reconnection contributing to fusion
  goal
• Many questions and challenges, e.g., how high can
  we push Ti?