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

2. OVERVIEW OF PRESENTATION 1.INTRODUCTION 2.DESIGN OF SINGLE PROBE 3.PROBE-PLASMA INTERACTION 4.I-V CHARACTERISTICS 5.EFFECT OF MAGNETIC FIELD 6.OTHER ELECTRIC PROBES: DOUBLE PROBE EMISSIVE PROBE 2

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: 1.Plasma potential 2.Electron temperature ( 0.1 eV to 30 eV) 3.Electron & Ion density (10 7 to 10 14 cm -3 for lab plasma) 4.Incident particle flux 5.% of Gas Ionization Advantage Local measurement Simple to use 3

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 4 Fig1. A typical single Langmuir probe

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

6. SINGLE PROBE I-V CHARACTERISTICS IDEAL CHARACTERISTICS 1.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) 6 V b = Probe voltage V p = Plasma potential V f = Floating potential Fig2: A typical I-V characteristic of Lagmuir probe

7. EFFECT OF MAGNETIC FIELD Electron current impedes and hence Ies decrease Ies/Iis ≠ √(m i /m e ) 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 7

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 8

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 I is I-V Characteristics At V(V 1 -V 2)= 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 9 Fig6. I-V curve for double probe Fig5. Double probe circuit

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 10 Fig7. Emissive probe circuitFig8. I-V curve for emissive probe

11. THANK YOU 11