progress and plans on the maryland centrifugal experiment · calculated timescales for comparison...
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Progress and plans on the
Maryland Centrifugal Experiment
R.F. Ellis, A.Case, R. Clary, A.B. Hassam, R. Lunsford,
C. Teodorescu, R. Elton, J. Ghosh,
University of Maryland, College Park MD
Highlights
• Supersonic rotation : MS = 1-4
• Steady discharges : τLIFE and τMOM >> τinterchange
• Measurements of velocity shear sufficient for
MHD stability
• Velocity exceeding Alfven ionization limit in HR
mode
• Hα emission much lower in HR mode (tokamak
like)
• Hints of centrifugal confinement in HR mode
Upgrades this year
• Capacitor bank voltage increased from 10 to 20
kV, energy increased from 176 to 708 kJ
• New azimuthal magnetic probe arrays
• New 5 chord Doppler spectroscopy system
• New Hα emission detectors
• 2 color interferometer almost completed
Basic idea : centrifugal confinement scheme requires
supersonic rotation and velocity shear
Major Goals• Supersonic rotation(yes-sonic Mach # 1 <MS <4)
• Velocity shear stabilization (maybe- new results this meeting)
• Centrifugal confinement(hints)
V applied
Schematics of MCX and discharge circuit
V
insulatorinsulator
Initial vacuum E·B => breakdown, V~ 10 kV
Metallic core
MCX Cutaway : Stainless steel core is high voltage
electrode, disc insulators terminate plasma region
Disc Insulators
Disc InsulatorStainless Core
4 m
B ~ 1T , V ~ 10 kV
Rm ~ 9 (Bmid = 0.2 - 0.3 T)
ap= 0.20 m ~ 102 ρρρρi , Lp = 1.40 m
Hydrogen, Static Fill, P0 = 5-10 mtorr
ni ~ few ×××× 1020 m-3 (fully ionized)
T ~ 20 – 60 eV
V applied 5-20 kV, pulse length 1-10 ms
vrot ≡≡≡≡ uExB ≥≥≥≥ 100 km/s
Plasma Parameters
MCX has two operating modes
O (ordinary mode)
Sonic Mach # Ms = vROT /vti ~ 1
HR (high rotation mode)
Ms ~ 3
3 x larger stored energy
3 x longer momentum confinement time
Alfven Mach # MA = vROT /vA ~ 1
Plasma voltage remains steady for 1000’s of
MHD instability times
• MHD instability growth time τMHD ~ 2 - 20µs.
• Measured momentum confinement time τmom ~ 200µs.
• MHD stable due to velocity shear?
• HR mode:
MS ~3
• O mode:
1< MS <2
MS= vrot/ cS
VP
IP
0 1 2 3 4 5 6-10
-8
-6
-4
-2
0
2
Load
Voltag
e (
kV
)
Shot #: MCX031111-12 ; Dated: 11/11/2003 03:05:53 PM
0 1 2 3 4 5 6-4
-2
0
2
4
6
8
Load
Cu
rren
t (k
A)
Time (ms)
HRO
tstart tcrowbar
τmomτMHD
Calculated timescales for comparison to MCX
discharge duration (> 5 ms) and momentum
confinement time( 200 µµµµs)
Axial Alfven time ~ LP/v
A 5µs
Period of rotation ~ (2πR/uφ) 10µs
Interchange growth time ~ [(aPL
P)/(T/M
p)]1/2 10µs
Axial electron heat conduction time ~ (LP/λ)2 τ
e30µs
Axial sonic time ~ LP/(T/M
p)1/2 30µs
Electron-ion heat exchange time ~ (Mp/m
e)τ
e40µs
Classical viscous damping time ~ (aP/ρ)2 τ
ii8000µs
( n = 2x1020 m-3, T = 30 eV, B = 0.2 T)
2
3
4
5
6
7
6 8 10 12 14 16
Bank Voltage (kV)
Maxim
um
P
lasm
a V
oltage (k
V) - H
R
Avera
ge P
lasm
a V
oltage (k
V) - O
- HR Mode
- O Mode
Plasma Voltage -vs- Bank Voltage
MCX Phase Map
0.5 1 1.5 2 3 4 5 6 7 8
5 kV
6 kV
7 kV
8 kV
9 kV
10 kV
11 kV
12 kV
13 kV
14 kV
15 kV
16 kV
17 kV
18 kV
Series Resistance (W)
Capacitor Bank
Voltage
Alfven Ionization Condition
A proposed limit on velocity with which an ion cloud can move thru a
neutral cloud without ionizing the neutrals
(½ miv
i2)
CRIT= eΦ
Ioniz
In MCX it applies at the boundary between the rotating plasma and the end insulators and maps to the midplane plasma
disc
insulator
z
rω constant on a field line
vins = vmid(rins/rmid)
vROT/vCRIT at the disc insulator
0
1
2
3
0 50 100 150 200
vi/v
c
O Mode
HR Mode
Distribution of Alfven Mach Numbers
0
20
40
60
0.1-0.3 0.3-0.5 0.5-0.7 0.7-0.9 >0.9
MA Range
Nu
mb
er O MODE
HR MODE
• Supersonic rotation
• Alfven ionization
limit exceeded in HR
mode
• Significant Alfven
Mach #
Shot Index
H alpha emission measured at various axial locations
employing fast photodiodes
HR-Mode shows much lower Hα emission than O mode :
similar to tokamaks
• Mid-Plane B-: 3 kG
• Bank Voltage: 8 kV
• Series Resistor: 2.0 Ω
Midplane Hα detector signal
HR mode
Discharge current and voltage
Axial variation of Hα emission in HR & O-Mode
• Mid-plane Hα much less in HR-mode for 3kG parameter
• Bank Voltage: 7 & 8 kV
• Series Resistor: 1.0 Ω, 1.5 Ω, & 2.0 Ω
• Error bars show standard deviatioin for 14 shots
3 kG 2 kG
Doppler shifts show rotation consistent with ExB
broadening gives approximate ion temperature
Midplane Midplane
Spectroscopy Spectroscopy
SetupSetup
C IV Spectra
Unshifted line
Single Chord Measurements
Multi-chord spectroscopic measurements of
Doppler shifts at midplane
spectrometer
Fiber optic cables
Spectrometerwith
ICCD camera
• High-resolution spectrometer:
0.182 nm/mm or 0.692 nm/mm.
• 5-chord measurements across
plasma column at midplane.
• Signal is a superposition of signals
from each 5 user-defined annular
zones:
∑<
+=ji
ijijijiiiiiiiii vISvISvIS ),,(),(),( i, j = 1,...,5
• Deconvolution yields intensity I(R) and velocity v(R) radial profiles.
12
34
5
Plasma angular velocity peaks in the
middle of the plasma column
• E×B drift is the dominant motion.
•MCX plasma shows no solid-body rotation
0
1
2
3
4
5
6
4 10 16 22 28
radial position R (cm)
ΩΩ ΩΩ (
x 1
05 r
ad
/s)
C3+
C2+
C+
H
R outR int
Neutrals and impurities are largely depleted
in the middle of the plasma
0
0.2
0.4
0.6
0.8
1
1.2
4 10 16 22 28
radial position R (cm)
I (a
rb.
un
its)
C3+
C2+
C+
H
• Neutral drag is minimal inside the plasma core.
Shear is large enough for mode stabilization
during HR mode
during O mode
• Velocity shear from C+ Doppler shifts shows stability threshold is exceeded!
[ ] stable)( '2/13/1
int ⇔>Ω µγ RlnR
New azimuthal arrays of magnetic probes
Two arrays located axially
at z = +/- 63cm
16 probes per array,
located radially at last
outer closed flux surface
Complements movable
single probes
Magnetic probes
• 20 Channel δδδδBz array installed and tested,
operating at 1MHz sample rate.
• 4 x,y,z, δδδδB triple probes installed and tested.
• δδδδBz/Bz ranges from 0.3% to 3% depending on
operating mode
• Distinct differences in δδδδBz activity between
operating modes.
• HR mode δδδδBz activity is much higher frequency
than in other modes
• All modes show significant activity below
100 kHz (~rotation frequency).
• All modes show negligible cross correlations
between δδδδBz probes below 35 kHz850 900 950 1000 1050
−6
−4
−2
0
2
4
6
Raw data, Window #1, probes 1,5,9,13
Vo
lts (
plu
s o
ffse
ts)
time, microseconds
0 2 4 6 8 10−8
−7
−6
−5
−4
−3
−2
−1
0
1
Window #1Window #1 Window#2Window #1 Window#2 Window#3
Voltage Trace, shot MCX050826−2
time, ms
volta
ge, k
V
Wobble-free rotation ?
• Axial view of MCX plasma
• Phantom 7.1 camera
• t = 5 ms after breakdown
• dtexposure= 2 µs
• 91,000 frames/s
Picture courtesy of
Ricardo Maqueda, PPPL
End view - 4 consecutive frames 2 µs exposure
Next Steps and status
• Complete diagnostics to study stability and relation to velocity shear : multichord spectrometer(done), magnetic probe arrays(done), two color interferometer system( near completion).
• New diagnostics to study centrifugal confinement. (interferometer chord at mirror throat, diamagnetic loop at the mirror throat, endloss analyzers at disk insulators).
• Expand parameter space, assess scaling. (Surface conditioning, higher voltage 20 kV capacitor bank(done), increased magnetic field Bmid ~ 1.0 T, Bmirr ~ 2.6 T ).
• Direct injection of momentum.(funded experiment pulsed plasma gun – HyperV Corp - see poster this meeting).
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