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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