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