The Reversed Field Pinch: on the path to fusion energy S.C. Prager September, 2006 FPA Symposium

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)

Why the RFP as a fusion configuration? Toroidal confinement in the limit B T 0 at the plasma surface

Why the RFP as a fusion configuration?

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

RFP Physics Challenges for Fusion Confinement Beta limits Resistive wall instabilities Current sustainment

RFP Physics Challenges for Fusion Confinement at q < 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)

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

Electron confinement improved improved standard T e (keV) MST Energy confinement increases tenfold

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

Ion heating in standard plasma Ion heating with improved confinement Improved confinement reconnection event T i (eV) T i (eV)

Ions hotter over entire plasma T i (eV) r/a MST

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

Confinement of fast ions Fast ion confinement time > 20 ms From neutral beam injection toroidal angle radius ion trajectory

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

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

Beta increased, for example, from upper limit not yet known

Resistive Wall Instabilities Recent: All instabilities suppressed by feedback (RFX, Italy; T2, Sweden)

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

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

Oscillating field current drive 10% current drive Ohmic efficiency Next:increased power, test efficiency and confinement compatibility MST

Current RFP Program Emphases Europe: Resistive wall instabilities Single helicity states US/Japan:Confinement Beta limits Sustainment

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