The Reversed Field Pinch: on the path to fusion energy

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The Reversed Field Pinchon the path to fusion
energy

• S.C. Prager
• September, 2006
• FPA Symposium

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Why RFP research?

• RFP as a fusion configuration
• For fusion energy science
• (magnetic transport, resistive wall
  instabilities, electrostatic transport with
  strong shear, high beta instabilities, fast
  particle instabilities..)
• For connections to plasma physics and
  astrophysics
• (benefits from hot, well-diagnosed fusion plasma)

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Why the RFP as a fusion configuration?

• Toroidal confinement in the limit
• BT 0 at the plasma surface

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Why the RFP as a fusion configuration?
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• low magnetic field
• ?
• High beta
• Very high engineering beta (low field at coils)
• Normal coils, reduced shielding
• High mass power density (compact)
• Efficient maintenance/disassembly
• Possibly free choice of aspect ratio

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RFP Physics Challenges for Fusion

• Confinement
• Beta limits
• Resistive wall instabilities
• Current sustainment

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RFP Physics Challenges for Fusion

• Confinement
• at q lt 1, large transport from magnetic
  fluctuations
• the historic RFP obstacle

• Beta limits
• intrinsically high limit not yet known

• Resistive wall instabilities
• multiple modes unstable without a conducting
  shell

• Current sustainment
• bootstrap current small
• (except possibly at low aspect ratio)

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Confinement

• Status
• Confinement improved by j(r) control
• Electron energy increases
• Energy confinement improves ten-fold
• Tokamak-like electron confinement
• But, so far demonstrated only transiently

Recent advances Ion energy and confinement
increases large orbit ions very
well-confined quasi-single helicity states
obtained
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Electron confinement improved
1.5
MST
improved
Te (keV)
1
0.5
standard



Energy confinement increases tenfold
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Ion energy increase

• Ions heated by reconnection event
• (self-heating)
• Ion energy retained by initiating improved
  confinement immediately following event

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Ion heating in standard plasma
Ti (eV)
reconnectionevent
Ti (eV)
Ion heating with improved confinement
Improved confinement
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Ions hotter over entire plasma
Ti (eV)
MST
r/a
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Confinement

• Status
• Confinement improvement by j(r) control
• Electron energy increases
• Energy confinement improves ten-fold
• Tokamak-like electron confinement, with low BT
• But, so far demonstrated only transiently
• Recent advances
• Ion energy and confinement increases
• large orbit ions very well-confined
• quasi-single-helicity states obtained

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Confinement of fast ions
From neutral beam injection
Fast ion confinement time
gt 20 ms
ion trajectory

radius
toroidal angle
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Confinement

• Status
• Confinement improvement by j(r) control
• Electron energy increases
• Energy confinement improves ten-fold
• Tokamak-like electron confinement, with low BT
• But, so far demonstrated only transiently
• Recent advances
• Ion energy and confinement increases
• large orbit ions very well-confined
• quasi-single-helicity states obtained

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Beta Limits

• Status
• Beta historically high, even with poor
  confinement,
• High beta also obtained with improved
  confinement

Recent With improved confinement, beta
increases with pellet injection
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• Beta increased, for example, from

upper limit not yet known
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Resistive Wall Instabilities
Recent All instabilities suppressed by
feedback (RFX, Italy T2, Sweden)
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Resistive Wall Instabilities
Recet All instabilities suppressed by
feedback (RFX, Italy T2, Sweden)

• Standard
• With feedback

toroidal mode number
Next optimize for engineering feasibility,
additional programming for enhanced control
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Current Sustainment

• Oscillating field current drive
• (or ac helicity injection, an oscillating
  inductive technique,
• possibly efficient, but possibly degrades
  confinement)
• Bootstrap-current driven at small aspect ratio
• Pulsed reactor scenarios

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Oscillating field current drive
10 current drive Ohmic efficiency
MST
Next increased power, test efficiency and
confinement compatibility
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Current RFP Program Emphases

• Europe Resistive wall instabilities
• Single helicity states
• US/Japan Confinement
• Beta limits
• Sustainment

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RFP Development

• In the past decade, the RFP has developed
  substantially, with modest (but significant)
  resources

• In the next decade, with reasonably enhanced
  capabilities, the RFP can be advanced to more
  fusion-relevant and new physics regimes

• If results are positive, the RFP could
  influence the fusion vision within the time to
  DEMO