We report the first production of high beta plasma confined in a fully levitated laboratory dipole using neutral gas fueling and electron cyclotron resonance heating. The pressure results primarily from a population of energetic trapped electrons that is sustained for many seconds of microwave heating provided sufficient neutral gas is supplied to the plasma. As compared to previous studies in which the internal coil was supported, levitation results in improved particle confinement that allows higher-density, high-beta discharges to be maintained at significantly reduced gas fueling. Elimination of parallel losses coupled with reduced gas leads to improved energy confinement and a dramatic change in the density profile. Improved particle confinement assures stability of the hot electron component at reduced pressure. By eliminating supports used in previous studies, cross-field transport becomes the main loss channel for both the hot and the background species. Interchange stationary density profiles, corresponding to an equal number of particles per flux tube, are commonly observed in levitated plasmas.

V =!

d!

B

MHD stability from plasma compressibilityIf p1V1

γ = p2V2γ , then interchange does

not change pressure profile.

Drift wave stability for interchange invariant profileFor η = d lnT

d ln n=

23

, density and

temperature profiles are also stationary.

Interchange mixing leads to constant entropy peaked profilesn ! V !1 and p ! V !!

Testing a New Approach to Fusion and

Laboratory Plasma ConfinementLevitated Dipole Reactor

60 m500 MW

D-D(He3) Fusion

Kesner, et al. Nucl. Fus. 2002 Internal ring

Steady state

Non-interlocking coils

Good field utilization

Possibility for τE > τp

Advanced fuel cycle

Abstract

The Levitated Dipole Concept

Uncommon Plasma Topology

No magnetic shear‣ ExB convective cells are

possible Non-linear evolution of

interchange may lead to convective cells‣ Explore non-linear

evolution of interchange like instabilities

Near marginal stability, convective cells do not necessarily transport energy

Possible Fusion Power Application

1.1 MA Floating dipole coil‣Nb3Sn superconductor‣ Inductively charged by 10 MJ

charging coil‣Up to 2 hour levitation using

active feedback on upper levitation coil

Two component plasma created by multi-frequency ECRH‣2.5 kW, 2.45 GHz‣2.5 kW, 6.4 GHz‣10 kW, 10.5 GHz

Magnetic equilibrium‣ flux loops, Bp coils, Hall effect sensors, levitation system trackers

Fast electrons‣ 4 Channel x-ray PHA, x-ray detector, 137 GHz radiometer

Core parameters‣ interferometer, visible cameras, visible diode and array, survey spectrometer

Fluctuations‣ Edge Isat and Vf probes, Mirnov coils, visible diode arrays, interferometer, fast

visible camera, floating probe array

Edge parameters‣ swept and Mach probes

Unstable Regime: ‣ Fast electron radial transport‣ Low density‣ Low diamagnetism (low β)

High Beta Regime: ‣ Large diamagnetic current ‣ Measurable density. ‣ β loss events accompanied by

xray bursts‣ Low frequency edge electric

and magnetic fluctuations Afterglow: (no input power)

‣ Low density‣ Slow diamagnetism decay‣ Quiescent with instability

bursts

0.0

2.5

5.0

kW

Heating Power (total)6.4 GHz 2.45 GHz

0

4

8

a.u.

Visible Intensity

0

2

4

a.u.

X-Ray Intensity

-20

2

4

a.u.

Edge Ion Saturation Current

0 1 2 3 4 5Time(sec)

0.0

0.4

0.8

mW

Diamagetic Flux

4500 A / 50 V Power supply‣ Resistive coil allows for rapid shutdown

Realtime digital control computer‣ Allows different control methods to

be implemented ‣ Matlab/Simulink Opal-RT

development environment‣ 4 kHz feedback loop‣ Failsafe backup for upper fault

Programmable Logic Controller‣ Slow fault conditions‣ Interlocks‣ Coil and Bus temperature and resistance

monitoringLDX Control Room

Laser Position Detectors

QNXReal Time Computer

(RTC)High Speed I/OFeedback Loop

150 kW Power Supply

Operator Interface

LDX Data

System

PLC

Low Speed I/OCoil and Bus Temperature

and Joint Voltage MontoringInterlocks

STOP

LauncherCatcher

Magnetic Loops

L-Coil

F-Coil

LDX Plasma DAQ System

Plasma Diagnostic Set

Typical Supported Mode Shot

LDX experiment. The vacuum vessel is 5 m in diameter and the 560 kg superconducting dipole coil is 1.2 m in diameter.

With supports, high fueling needed to stabilize HEI, increase density, and increase beta‣ Unstable regime evolves gas from

vessel walls by surface heating

Once stable, less fueling is needed to maintain stability‣ Without continued puffing, plasma

pumps required gas from chamber

Consistent with theory showing background density stabilizing

Krall (1966), Berk (1976)...

!

d ln n̄ h

d ln V< 1+

m 2

"

24

!d h

!c i

n̄ i

n̄ h

0.0

2.5

5.0

kW

Total Heating Power

01234

10-6 T

orr Neutral Pressure

0

20

40

a.u.

Visible Intensity

0.0

0.5

1.0

a.u.

X-Ray Intensity

0 2 4 6Time(sec)

0.0

0.2

0.4

mW

Diamagetic Flux

0.0

2.5

5.0

kW

Total Heating Power

0

1

2

3

4

10-6 T

orr

Neutral Pressure

0

1020304050

a.u.

Visible Intensity

0 2 4 6 8 10Time (sec)

0.00.20.40.60.81.01.21.4

mW

b

Diamagetic Flux

0.0

2.5

5.0

kW

Total Heating Power

0

1

2

3

4

10-6 T

orr

Neutral Pressure

-202468

a.u.

Visible Intensity

0 2 4 6 8 10Time (sec)

0.0

0.2

0.4

0.6

mW

b

Diamagetic Flux

Well controlled flight‣ After launch, < .2 mm z excursions‣ small effect of ~ 10 plasma shots‣ oscillation in x and y caused by slow

rotation gravitational well in toroidal rotation caused by

incomplete balance 6 minute period with very small damping

Control algorithm‣ P - I - D - A control of voltage PS voltage feedback fast Small operating space of reliable gains

‣ Full state control under development (Extended) Kalman filter Optimal control Coupling between coils affected by plasma beta!

300032003400360038004000

L-coil Current (Amps)

-200

0200400600800

Launcher Force/g (kg)

-20-15-10

-505

Z position (mm)

-10

-8-6-4-202

X position (mm)

0 1000 2000 3000 4000Time (sec)

-4-3-2-101

Y position (mm)

Hot Electron Interchange (HEI)

Elimination of parallel loss channel

Improvement of bulk electron confinement‣Higher bulk density‣Single peaked profiles with

near constant number of particles per flux tube‣Improved energy confinement ‣Improved stability for hot electron interchange mode

Similar supported (left) and levitated (right) shots. Numerous small hot electron

interchanges are followed by a disruptive HEI in the afterglow. In comparison, the levitated shot undergoes no HEI’s even though operated well below the unstable limit for supported operation. In general, reduced HEI activity are observed when levitated and no major

disruptions have been observed in the heated phase when levitated, even while operated at much lower neutral pressures.

With levitation, observed density profile is dominated by radial transport

Interchange mixing likely cause of profiles with near constant particles per flux tube

We observe low frequency oscillations, both broadband and quasi-coherent that may be representative of interchange mixing

Ongoing examination of relationship of observed turbulence with changes in plasma profiles as well as the effect of plasma turbulence on the plasma profiles.

Steady state central chordal density measurement versus neutral pressure for different conditions. The effect of levitation for deuterium plasmas with 5 kW of input ECRH is shown as increased density by a factor of 2-4. (Also depicted are significant power and species dependencies at high neutral pressure).

Diamagnetic flux for supported and levitated operation during density scan of run 80321. Both operations show an inverse scaling between fast particle confinement and bulk density. Levitat ion al lows high beta operation at higher densities.

0

1

2

3

4

0 2 4 6 8

Dia

magnetic F

lux (

mW

eber)

Line Average Density ( * 1016 m-2)

Levitated

Supported

Similar shots‣With (solid) and without

(dashed) levitation‣Similar gas fueling3 x density ‣much more peaked

profile with levitation2 x stored energy

Plot of edge floating probe array, showing quasi-coherent structure of edge turbulence. Dominant is a 2 kHz, m=1 mode, rotating in the electron diamagnetic drift direction. (Which is also the ExB direction.)

Time (samples at 80khz)

Prob

e Nu

mbe

r

Floating Potential (VAC), Shot 81001018

500 1000 1500 2000 2500 3000

5

10

15

20−0.06

−0.04

−0.02

0

0.02

0.04

Correlation weighted histogram of phase relationship between photodiode array channels during heating phase of levitated plasma. Channels are separated by 90 degrees azimuthally. Same quasi-coherent m=1 mode is observed.

-3 -2 -1 0 1 2 3Phase (Radians)

0

1

2

3

4

5

6

kHz

phd_array_1-phd_array_2-visible-chan_08

81003018

0.0

3.3e-06

6.7e-06

1.0e-05

Computation of the density profile for supported (diamond) and levitated (square) discharges under similar conditions with 15 kW of ECRH.

The levitated case has a singly peaked profile with near constant number of particles per flux tube.

n δV (Particles/Wb)

Density (Particles/cc)

Closed Field Lines

0

2•1018

4•1018

6•1018

S805150086.75000 sec

0.6 0.8 1.0 1.2 1.4 1.6 1.8Radius (m)

0

1•1011

2•1011

3•1011

4•1011

S80515025

Levitated

Supported

Deviation parameter (Δ) from constant number of particles per flux tube profile.

Levitated plasmas with multi-source ECRH and sufficient neutral fueling exhibit density profiles close to the interchange stationary profile. 0.0

0.2

0.4

0.6

0.8

1.0

0 5 10

!"

Neutral Pressure (µTorr)

Levitated (15 kW)

Supported (15 kW)

Investigating Radial Transport and Turbulence

5

2m

Hoist

InductiveCharging

Levitation Coil

2.45 GHz

6.4 GHz

1 m

Pastukhov, Chudin, Pl Physics 27 (2001) 907.

coil

wall

r

φ

Ricci, Rogers, Dorland, PRL 97, 245001 (2006).

coil

wall

r

φ

0

2

4

6

kW

(a)

0

1

2

3

4

10

-6 T

orr

(b)

0

2

4

6

8

< 1

016 m

-3 >

(c)

0 2 4 6 8 10 12 14Time (sec)

0

1

2

3

10

-3 W

eber (d)

ECRH Power

Neutral Pressure

Line Averaged Density

Diamagnetic Flux

Levitated Dipole Experiment (LDX) Confinement Improves with Levitation

Levitation System

Fast camera showing radial structure of interchange turbulence.

coil r

φ