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

​Langmuir Probe:

• Langmuir probes are used to measure the electron plasma density n_e, the space potential V_sp, and the electron temperature K_BT_e 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:

• The probe is immersed into the plasma and polarized to the potential V_p by an external circuit.
• This bias to V = V_p − V_sp the probe with respect to the local space plasma potential V_sp.
• Then, the drained current I_p = I(V_p) 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 I_p by the probe for each bias potential V_p.
• 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 ≫ r_p, λ_D), and also the electron temperature KBTe ≫ KBTi ≃ KBTa is higher than those of ions and neutrals.