Flows and Instabilities
Associated with an
Extremely Narrow Current Sheet
Presented byStephen Vincena
April 20, 2004University of Maryland
Second Workshop on Thin Current Sheets
Collaborators:Walter Gekelman and Patrick PribylUniversity of California, Los Angeles
Large Plasma Device LaboratoryDOE/NSF Basic Plasma Science Facility
Cathode discharge plasmaHighly Ionized plasmas n ≈ 3 x 1012 /cm3
Reproducible, 1Hz operation> 4-month cathode lifetimeUp to 3.5kG DC Magnetic Field on axisPlasma column up to 2000Rci across diameterOver 450 Access ports, with 50 ball jointsComputer Controlled Data AcquisitionMicrowave InterferometersLaser-Induced FluorescenceLarge variety of probes
Now a national user facilityhttp://plasma.physics.ucla.edu/
Overview of the experimental device
Electron current sheet generation with ‘Slot’
Cu plate:Length: 18cm=0.6i, 45e or i
Width: 1.9cm, 5e or i
Thickness: 0.15, 0.4e or i, 13 e
Glass-coveredCopper rod
3/8” stainlessSteel shaft
epoxy
Probes:
3-axis, differentially wound magnetic induction probesfor fluctuating magnetic fields.
Langmuir probes biased to collection ion saturation currentyields (assuming Te fixed) density fluctuationswhen calibrated with a microwave interferometer.
Multi-sided Langmuir probes (Mach probes) for ion flow.Example:
DownstreamSat
UpstremSat
s
flow
I
I
c
vM ln2
11.2mm
4-sided mach probe
Tungsten faces
Fixed magnetic probe
Current sheet antenna x =2mm
Movable flow probe
Fixed flow probe
Transistor switch
capacitors
Current = 70A
Voltage = 75 V
He, B=500G…1.5 kG
Photograph Tshutter = 1 s
View down axis of machine
Geometry andGeometry and
Philosophy of data collectionPhilosophy of data collection
Electron current drawn by slot
Am
pere
s/10
t/ (L/vA), L=20m
y(cm)
x (cm)
Machnumber
+0.05
-0.25
Movie of Parallel Ion Flow in a Perpendicular Plane
Rci = 4.1 mm
= 3.8 mm
y
x
z, B
This movie is available at http://plasma.physics.ucla.edu/bapsf/vincena/umd04/movie1.avi
Time of peak flow
y(cm)
x (cm)
Machnumber
+0.05
-0.25
T=690sec Parallel Ion Flow in a Perpendicular Plane
Rci = 4.1 mm
= 3.8 mm
Parallel Ion Flow in a Perpendicular Plane
y(cm)
x (cm)
Machnumber
+0.05
-0.25
T=1000sec
Time during spontaneous fluctuations
Peak parallel ion flow at times of maximum density gradient
Fluctuations (Drift-Alfven waves) and peak current associated with relatively filled-in density profiles->cross-field transport
x/i
Density (/cc)
Parallel Ion Flow in a Perpendicular Plane
y(cm)
x (cm)
Machnumber
+0.05
-0.25
T=1000sec
DownstreamSat
UpstremSat
I
IM ln2
1||
Time during spontaneous fluctuations
CorrelationMeasurementsare Madein this Region
Density Fluctuations Due to Drift Waves
+10
-10
(%)n
n
Frequency:0.2Fci
y(cm)
x (cm)
This movie is available at http://plasma.physics.ucla.edu/bapsf/vincena/umd04/movie2.avi
x/i
Density
Parallelion flow(Vz /Cs)
Perpendicularion flow(Vy /Cs)
M-p
arM
-per
p#/
cc
By snapshot, allfrequencies but dc
Gradient ofIsat (density)
spectrum along this line segment
By averaged powerspectrum along line above
averaged gradient scale lengthfor the six x-lines
10cm wavelengthk_y=0.63/cm
Time Series (Bx)Time Series (Bx) FFT FFT
frequency (Hz)
arb
units
x=0, y=0 (center)
He, B = 500 GHe, B = 500 G3 axis magnetic probe inside current 3 axis magnetic probe inside current sheetsheet
Spontaneous fluctuations
Coherency Spectrum *| |
| || |xm xf
xm xf
B B
B B
x (cm)
m : movable probe
f = fixed probe
z =2.24 m
First glimpse of electron solitary structures ?First glimpse of electron solitary structures ?
1 Ghz amplifier
Probe tip 12 X 27
To 4 GHz digitizer
Summary & Future work
• Observed strong parallel ion flows associated with a thin electron current sheet
• This flow is periodically disrupted by the formation of steep density gradients and the onset of drift wave turbulence.
• Building 3D mm-scale mach probe.• Independently measure perpendicular ion flow using laser-induced
fluorescence.• Developing micro-scale electric field probes to study fine-scale
(Debeye-length) electron structures (electron phase space holes) within the current sheet.
• Quantitative, scaled comparisons with drift wave theory and numerical predictions.
