PLASMA DIAGNOSTIC BY ELECTRIC PROBE

NITIN MINOCHAM.Tech. N.S.T., Delhi University

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OVERVIEW OF PRESENTATION

1. INTRODUCTION2. DESIGN OF SINGLE PROBE3. PROBE-PLASMA INTERACTION4. I-V CHARACTERISTICS 5. EFFECT OF MAGNETIC FIELD6. OTHER ELECTRIC PROBES:• DOUBLE PROBE• EMISSIVE PROBE

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INTRODUCTION

• Electric probe is just a simple conducting wire inserted in plasma to collect electric current from plasma and hence measure plasma characteristics.

• It is mainly used to study lab plasma.• It is also used in Tokamaks to study edge plasma.Physical parameters measured by Electric probe:1. Plasma potential2. Electron temperature (0.1 eV to 30 eV)3. Electron & Ion density (107 to 1014 cm-3 for lab plasma)4. Incident particle flux5. % of Gas Ionization Advantage• Local measurement• Simple to use

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DESIGN OF SINGLE LANGMUIR PROBE

REQUIREMENTS & SOLUTIONS• Sustain high heat loads from plasma Probe tip is made of a high temperature material (Pt,W, Mo)• Should not disturb the plasma globally Thin insulating layer surround the probe (Alumina, Fused silica, SiC)• Should not erode by sputtering Low Z materials as probe tip (Graphite) • Less reflection of incoming particles Probe is made of absorbing material

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Fig1. A typical single Langmuir probe

PROBE –PLASMA INTERACTION(SHEATH FORMATION)

Fig3:Structure of the sheath regions at a plane probe

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SINGLE PROBE I-V CHARACTERISTICS

IDEAL CHARACTERISTICS1. Ion saturation (Iis)2. Sharp transition region 3. Electron saturation (Ies)

REAL CHARACTERISTICS• Rounded knees due to oscillations

in Vp• Effective area of charge

collection by probe increases with Vb

• Increase in saturation current (Ies & Iis) with applied voltage(Vb)

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Vb= Probe voltageVp= Plasma potentialVf= Floating potential

Fig2: A typical I-V characteristic of Lagmuir probe

EFFECT OF MAGNETIC FIELD

• Electron current impedes and hence Ies decrease

• Ies/Iis ≠ √(mi/me)• Effective charge collection area

of probe is decreased• Collisionless theory is invalid

and hence Quasicollisionless comes into picture

• Bohm current is modified to take into account the effect of magnetic field

Fig4:Schematic representation of sheath and pre-sheath in strong magnetic field

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OTHER ELECTRIC PROBES

Problem with single probe• Withdraw large current(Ies) from plasma and hence disturb the plasma• High particle flux on probe may result in probe damage

Need of other probes• Avoid disturbance to plasma• Avoid large particle flux on probe

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DOUBLE PROBEDesign • Two probes biased wrt each other• Both probes are at negative potential wrt

plasma to repel electrons• Current is limited to Iis I-V Characteristics• At V(V1-V2)=0, both probes are at Vf, hence I=0• As V , more electrons go to probe1 and

ions goes to probe 2• As V , more electrons go to probe2 and

ions goes to probe 1Advantages• Less heat flux on probe• Less disturbance to plasmaDisadvantages• Only fast e- in tail are collected • Bulk e- are not sampled

9Fig6. I-V curve for double probe

Fig5. Double probe circuit

EMISSIVE PROBE

• Consists of a tungsten filament heated by passage of current• At +ve bias wrt plasma potential, emitted electrons come back• At -ve bias wrt plasma potential, emitted electrons escape and contribute IiLimitation• Emission can’t be increase indefinitely because of space charge formation Advantage• Direct measure of plasma potential

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Fig7. Emissive probe circuit Fig8. I-V curve for emissive probe

THANK YOU

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