multiscale turbulence excited by a pulsed current sheet walter gekelman,stephen vincena, patrick...
DESCRIPTION
Cathode discharge plasma Highly Ionized plasmas n ≈ 3 x /cm 3 Reproducible, 1Hz operation > 4-month cathode lifetime Up to 2.5kG DC Magnetic Field on axis Plasma column up to 2000R ci across diameter Over 450 Access ports, with 50 ball joints Computer Controlled Data Acquisition Microwave Interferometers Laser-Induced Fluorescence Large variety of probes Operates as a national user facility Overview of the experimental deviceTRANSCRIPT
Multiscale Turbulence excited by Multiscale Turbulence excited by a pulsed current sheeta pulsed current sheet
Walter Gekelman ,Stephen Vincena,
Patrick Pribyl , Brett Jacobs, Eric Lawrence(UCLA Department of Physics and Astronomy),
Paul Kintner (Cornell University,Dept. of Engineering)
Franklin Chiang (UCLA Dept of Engineering)
Noam Katz (Physics Dept, MIT)
Narrow Current sheets occur in many situations
Current sheetCurrent sheet
Ion flowIon flow
Reconnection experimentReconnection experiment
Current sheet
width :c
ω pe
Cathode discharge plasmaHighly Ionized plasmas n ≈ 3 x 1012 /cm3
Reproducible, 1Hz operation> 4-month cathode lifetimeUp to 2.5kG 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
Operates as a national user facilityhttp://plasma.physics.ucla.edu/
Overview of the experimental device
Orientation
Orientation
Making a Narrow Current Channel
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
data collectiondata collection
Density Density Perturbation as seen with Laser Induced Fluorescence
Electric Field ProbesElectric Field ProbesChallenge; They must be Debye
scale in sizeλD =
KTe4πne2
In space (auroral ionosphere) λD is 1-10 m
In LAPD λD is 30 microns
E Field Probe: E Field Probe: −∇Φ
Constructed at UCLA MEM’s lab
Gold wire traces
60-_m
Gold tips
Bond Pads
Probe TipsProbe Tips
E Field Probe
E probe and air-cooled circuit boxE probe and air-cooled circuit box
Circuit diagram (1 probe)Circuit diagram (1 probe)
Drives 50 Ohm cable
Electric field in current channel
Field Field ComponentsComponents
Spectra ESpectra Eperpperp
Spectra ESpectra Eparallelparallel
Wavelet Analysis E Perpendicular to Background Magnetic Field
fce/2
fpi-in
Wavelet AnalysisE Parallel to Background Magnetic Field
Enhanced HF
Probe in electron beam (low density <109 cm-3 plasma)
B0
LaB6 emitter3 mm dia
4 tip microprobe
e-beam
Background PlasmaBackground Plasma
Higher Frequencies- Langmuir Higher Frequencies- Langmuir WavesWaves
• The probe response was less than 2 GHz- in a separate experiment at lower density we searched for electron solitary structures.
• They are Debye scale size entities which move as fast as vte
Spiky turbulenceSpiky turbulence
Voltage
(Volts)
Time (ns)
V=4X107 cm/s
Vthe = 1X108 cm/s
Conditional AveragingConditional Averaging
Key issues raised in this Key issues raised in this experimentexperiment
1) Are low frequency phenomena (e.g. Drift Alfvén waves, flows) coupled to high frequency phenomena (lower hybrid, whistlers and Langmuir waves)?
2) What is the electron distribution function f(r,v,t) ?
3) What is the electron (high frequency) physics in a magnetoplasma?
Velocity Analyzer Grid Area (Cross Section)
Collector electrode (~.2 μm)
Polyimide(~15 μm)
Repel (~.2μm)
Shield (~.2μm)
Silicon Substrate
Design ParametersDesign Parameters• Angle of Acceptance (desire less than 10 deg):
– 2 µm hole, total thickness 30 µm half angle = 3.82 deg– 2.5 µm hole, total thickness 30 µm half angle = 4.76 deg
• Exposed area– 160x160 array of 4 μm2 holes, 4 µm between holes. – 1 mm x 1 mm head 10.24% open area
• Current being carried– Expected current density: 1000 mA/cm2 1x10-5 mA/μm2
– 160 * 160 * 4 * 1E-5 = 1.024 mA (maximum collected)• Gap closing force
– For g = 15 µm @ 25 V: F ≈ 10 µN
⎟⎟⎠⎞
⎜⎜⎝⎛
−= 2
2
2gAVF orεε