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.