An electric probe is a simple conducting wire inserted into plasma to measure plasma characteristics by collecting the electric current. A single Langmuir probe is commonly used and can measure plasma potential, electron temperature, and electron and ion densities. It operates by forming a sheath between the probe and plasma. A double probe avoids disturbing the plasma by limiting current collection. An emissive probe can directly measure plasma potential by emitting electrons from a heated filament.

1. PLASMA DIAGNOSTIC BY ELECTRIC
PROBE(SINGLE AND DOUBLE PROBE)

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

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 (107 to 1014 cm-3 for lab plasma)
4. Incident particle flux
5. % of Gas Ionization
Advantage
 Local measurement
 Simple to use
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4. DESIGN OF SINGLE LANGMUIR PROBE
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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)
Fig :Structure of the sheath regions at a plane probe

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)
Vb= Probe voltage
Vp= Plasma potential
Vf= Floating potential
Fig2: A typical I-V characteristic of
Lagmuir probe

7. EFFECT OF MAGNETIC FIELD
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 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 Fig :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
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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
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
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Fig7. Emissive probe circuit Fig8. I-V curve for emissive probe

11. Made By:
Anita Malviya