planned emissive probe measurements on ldx · assembled : oct 2002 emissive langmuir probe assemble...
TRANSCRIPT
Planned Emissive ProbeMeasurements on LDX
E. Ortiz, M. Mauel, D. Garnier, A. Hansen- Columbia University -
S. Dagen, J. Kesner- MIT PSFC -
Presented at the 44th AnnualMeeting of the Division of
Plasma Physics
Orlando, FloridaNovember 11-15, 2002
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ABSTRACT
The Levitated Dipole Experiment (LDX) investigates equilibriumand stability of a high-beta plasma confined to a dipolar magneticfield. Because of its closed field line geometry, LDX may besubject to convective cell formation. As a principal objective wewould like to understand the relationship of convective cellformation as it relates to plasma equilibrium and stability for ourdipole geometry. Their nature and role will be explored usingemissive probes that function both as a diagnostic of the outerelectrostatic potential but also as a low-impedance electrode forcharging flux tubes and exciting or controlling convective cells. When biasing a given magnetic flux tube, we plan to interact withconvective cells in a controlled manner and possibly amplify orsuppress the level of electrostatic perturbation. The currentprogress of the diagnostics and setup will be presented togetherwith results from a glow discharge plasma.
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Outline
What’s New?
Probe Interface Mounting
Convective Cells
Emissive Langmuir Probe
Goals
Design & Properties
Construction
Other Probes
Glow Discharge Cleaning (GDC) Anode
Summary & Future Work
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Electric Probes – Progress
Probe HousingAssembled : Oct 2002
Emissive Langmuir ProbeAssemble : Nov 2002
GDC AnodeAssemble : Dec 2002
PLC – Data AcquisitionAssemble : Dec 2002
Triple Langmuir ProbeAssemble : Jan 2003
†Motorize ProbesAssemble : Jan 2003
* Images taken by Eugenio Ortiz. November 10, 2002.
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Probe Interface Mounting
Easy access via platformActual height ~ 4.5’ (137 cm)
from base flangeFour ports available
Bellow stroke ~ 32.5” (83 cm)
Max length ~ 42.25” (108 cm)
Min length ~ 9.75” (25 cm)
Removal of probe withoutbreaking main vacuumMotorized version† allowsremotely controllablemotionQuick release shaft collars
No tools necessary
* Image taken by Eugenio Ortiz. November 10, 2002.
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Convective Cell Questions
Do convective cells exist in LDX?Are they the nonlinear saturation of interchange modes?
What do they do to energy confinement?Can we have high energy confinement with low particleconfinement?
Can we drive and/or limit existing convective cells?Use an emissive probe to ‘stir’ plasma?
* Graph from simulations by V.P. Pstukhov, N.V. Chudin. June 25, 2001.
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Emissive probe designBias field lines & charge flux tubes
Create small E-field fluctuations
Explore emissive probe ability to control cellsAttempt to drive new convective cells or suppress existing cells
Study dynamics with multiple probesTrack large scale vortices
Time dependent polarity?
Plasma flow dominated by vortices?
Convective flow measurements via Mach probes
Convective Cell Interaction
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Emissive Probe Goals
Linear motion vacuuminterface
Probe incursion depth of23.5” (60 cm)
Allow for easy probereplacement w/outbreaking vacuumIdeal for measuringedge plasmaphenomenaAbility to bias singlemagnetic field line viaemissive probeMeasure fluctuations inlocal potential, ie. E-field
* Drawing by Eugenio Ortiz. May 15, 2001.
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Emissive Probe Outline
Thoriated Tungsten tipLower Work Function ~ 2.63 eV
Emission at lower temperatures 1700 degrees K
Simple electrical circuitR2 used to measure resistivity of Tungsten, ie. temperature
Compare with ARIES chart (chart of Tungsten Temperature vs. Resistivity)
Data acquisition via MDS-Plus, analysis using IDL
* Drawings by Eugenio Ortiz. Drawing (A) November 10, 2002. Drawing (B) May 15, 2001.
Fixed stainless steelAttachment tube
2.75” double sided flange
Removable inneraluminum joint
Electric connectors
Removable outerstainless steel tube
Alumina rods
Rotatable inneraluminum joint
(A)
(B)
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Final Probe Design
Probe End DetailsDiameter ~ 1 mmLength ~ 6 mmEnclosed in Aluminatubes14 AWG kapton coveredmagnet wireCopper butt-endconnectors
Sealed with Respond904 ceramic adhesivefrom CotronicsFilament diameter canvary in size ~ .5 mm to1.5 mm
* Drawings by Eugenio Ortiz. November 10, 2002.
Probe End
Sliced Probe End
Main AluminaShaft
SecondaryAlumina Shafts
Kapton coveredmagnet wire
Copper Butt-EndConnector
TungstenTip
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Probe Specifications & Parameters
Electron Temperature 10 eV
Electron Density 1.0E+17 m^-3
Ion Saturation Current -4.7 mAmps
Ion Acoustic Velocity (He +2) 15.5 km/s
Debye Length 7.4E-05 m
Edge Plasma Pressure 0.5 Pascal
Expected Edge Plasma Parameters
* Chart based on modelderived by M. Y. Ye and S.Takamura. August 2000.
Emission Current vs. Bias Voltage
-1.0E-02
-7.5E-03
-5.0E-03
-2.5E-03
0.0E+00
2.5E-03
-15 -10 -5 0 5
V_b (V)
Cu
rren
t (A
)
1017
7.4 x10-5
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Emissive Probe Analysis Technique
Heated Langmuir ProbeMore accurate plasma potential measurementReduces contamination of electrode surface
Two ways to determine Vp
Floating potential = Vp of strongly emitting probeCan lead to plasma perturbations
Inflection Point method in limit of zero emission
Differential Emissive ProbeTwo identical probes
One heated no emission, another made hotter for emissionOperates at higher collecting currents
I-V characteristics resemble a step functionVp to accuracy of Twire/e
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Other Probes
Triple probeObtain instantaneously Te
and ne
No Voltage and Frequencysweeping required
Mach ProbeMeasure plasma flowvelocityUse multiple probes to trackconvective cells in 2-D
Rotatable lower interfaceflange allows for 48distinct probing angles * Image taken by Eugenio Ortiz. November 10, 2002.
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Glow Discharge
GD created by movableanode via flange portalUsed before first thermalplasma in LDX
Eliminate impurities invacuum vessel and probesInitially no magnetic fieldAnode biased up to 800Volts and 12 kW DC
Implemented withDeutrium gas followed byshort helium periodsPlasma created forcalibration of electricprobes
GDCAnodeProbe
Emissive Probe
SuperconductingMagnet
ChargingCoil
LevitationCoil
* Drawing by Eugenio Ortiz. November 10, 2002.
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Summary & Future Work
Three probe interface systems have been assembledOne houses an emissive Langmuir probe, another theGDC system and the third will have a triple probeWe will explore the nature of convective cells in dipoledevices using emissive probes
Attempt to control and/or suppress their existenceCreate new ones
We will have the ability to determine instantaneously theouter plasma electron temperature and densities via atriple probeWe would like to construct more probe interface systemsto add Mach probes to track convective cell dynamics