Which Plasma Analytical technique involves placement of conducting probe smaller than particle mean free path directly in the plasma?

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UGC NET Paper 2: Electronic Science Nov 2020 Official Paper
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  1. Actinometry
  2. Lesser Induced Fluorescence
  3. Retarding Grid
  4. Langmuir technique

Answer (Detailed Solution Below)

Option 4 : Langmuir technique
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UGC NET Paper 1: Held on 21st August 2024 Shift 1
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Detailed Solution

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Langmuir Probe:

  • Langmuir probes are used to measure the electron plasma density ne, the space potential Vsp, and the electron temperature KBTe in cold low-density plasmas.
  • The electric probes measure the local plasma parameters by using stationary or slow time-varying electric (and/or magnetic) fields to emit or collect charged particles from the plasma.
  • These measuring techniques are particularly well suited for low-density cold plasmas, as low-pressure electric discharges, ionospheric and space plasmas.

 

The collecting Langmuir probe is a metallic electrode with a well-defined geometry (planar, cylindrical, or spherical) as shown:

F1 Shubham Madhuri 30.04.2021 D8

  • The probe is immersed into the plasma and polarized to the potential Vp by an external circuit.
  • This bias to V = V− Vsp the probe with respect to the local space plasma potential Vsp.
  • Then, the drained current Ip = I(Vp) for different probe potentials Vp is monitored, and the plasma parameters are calculated from these voltage-current (IV) characteristic curves.
  • The current-voltage curves (IV) are obtained by measuring the drained current Ip by the probe for each bias potential Vp.
  • In order to give a qualitative interpretation, we will consider an idealized nonequilibrium collisionless, Maxwellian, and unmagnetized plasma.
  • Thus, the collisional mean free paths of all particles are larger than all characteristic lengths (λ ≫ rp, λD), and also the electron temperature KBTe ≫ KBTi ≃ KBTa is higher than those of ions and neutrals.
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