The Reversed Field Pinch:on the path to fusion energy

S.C. PragerSeptember, 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 limitBT 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/disassemblyPossibly free choice of aspect ratio

RFP Physics Challenges for Fusion

• Confinement

• Beta limits

• Resistive wall instabilities

• Current sustainment

RFP Physics Challenges for Fusion

• Confinementat 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 increasesEnergy confinement improves ten-foldTokamak-like electron confinementBut, 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

1

1.5

0.5

improved

standard

Te (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

reconnectionevent

Ti

(eV)

Ti

(eV)

Ions hotter over entire plasma

0

200

400

600

800

1000

1200

1400

1600

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Following reconnectionWith no reconnection

Impurity ion temperature (eV)

r/a

Ti

(eV)

r/a

MST

Confinement

Status: Confinement improvement by j(r) controlElectron energy increasesEnergy confinement improves ten-foldTokamak-like electron confinement, with low BTBut, so far demonstrated only transiently

Recent advances:Ion energy and confinement increaseslarge orbit ions very well-confinedquasi-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) controlElectron energy increasesEnergy confinement improves ten-foldTokamak-like electron confinement, with low BTBut, so far demonstrated only transiently

Recent advances:Ion energy and confinement increaseslarge orbit ions very well-confinedquasi-single-helicity states obtained

Beta LimitsStatus

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

€

βtotal =p

V

Bs2 /2mo

» 15%

€

βtotal » 27%

€

βtoroidal =p

V

BT ,vac2 /2mo

» 80%

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

€

÷ B r

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 instabilitiesSingle helicity states

US/Japan: Confinement Beta limitsSustainment

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