Langmuir Probe

I-V characteristic of a plasma probe — T_e, plasma density n, sheath physics

T_e = 2.0 eV | n = 1.0×10¹⁶ m⁻³ | V_f = -4.6 V | I_sat⁺ = −0.24 mA

T_e (eV): 2.0
n (×10¹⁶ m⁻³): 1.0
T_i (eV): 0.5
Mode:

A Langmuir probe measures I-V characteristics in plasma. For V ≪ V_plasma: ion saturation current I_i⁺ = −0.5enA√(kT_e/m_i) (Bohm criterion). In the transition region: I = I_e·exp(e(V−V_p)/kT_e) − |I_i⁺|. Electron temperature T_e = e/k · (ΔV/Δ(ln I)) from the semi-log slope. Floating potential V_f where I=0 gives V_f ≈ V_p − (T_e/2)·ln(m_i/2πm_e).