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PLASMA DIAGNOSTIC BY ELECTRIC PROBE

PLASMA DIAGNOSTIC BY ELECTRIC PROBE. NITIN MINOCHA M.Tech. N.S.T., Delhi University. OVERVIEW OF PRESENTATION. INTRODUCTION DESIGN OF SINGLE PROBE PROBE-PLASMA INTERACTION I-V CHARACTERISTICS EFFECT OF MAGNETIC FIELD OTHER ELECTRIC PROBES: DOUBLE PROBE EMISSIVE PROBE. INTRODUCTION.

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PLASMA DIAGNOSTIC BY ELECTRIC PROBE

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  1. PLASMA DIAGNOSTIC BY ELECTRIC PROBE NITIN MINOCHA M.Tech. N.S.T., Delhi University

  2. OVERVIEW OF PRESENTATION • INTRODUCTION • DESIGN OF SINGLE PROBE • PROBE-PLASMA INTERACTION • I-V CHARACTERISTICS • EFFECT OF MAGNETIC FIELD • OTHER ELECTRIC PROBES: • DOUBLE PROBE • EMISSIVE PROBE

  3. 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: • Plasma potential • Electron temperature (0.1 eV to 30 eV) • Electron & Ion density (107 to 1014 cm-3 for lab plasma) • Incident particle flux • % of Gas Ionization Advantage • Local measurement • Simple to use

  4. 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 Fig1. A typical single Langmuir probe

  5. PROBE –PLASMA INTERACTION(SHEATH FORMATION) Fig3:Structure of the sheath regions at a plane probe

  6. SINGLE PROBE I-V CHARACTERISTICS IDEAL CHARACTERISTICS • Ion saturation (Iis) • Sharp transition region • 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) Fig2: A typical I-V characteristic of Lagmuir probe Vb= Probe voltage Vp= Plasma potential Vf= Floating potential

  7. 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

  8. 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

  9. DOUBLE PROBE Design • 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 1 Advantages • Less heat flux on probe • Less disturbance to plasma Disadvantages • Only fast e- in tail are collected • Bulk e- are not sampled Fig5. Double probe circuit Fig6. I-V curve for double probe

  10. 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 Ii Limitation • Emission can’t be increase indefinitely because of space charge formation Advantage • Direct measure of plasma potential Fig7. Emissive probe circuit Fig8. I-V curve for emissive probe

  11. THANK YOU

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