A new emissive probe circuit for the EPaX experiment at the University of San Diego

A new low temperature plasma experiment is coming online at the University of San Diego (USD) that attempts to explore (measure) internal sheaths in electronegative plasma, called the Electronegative {PlAsma Sheath EXperiment, or EPaX. Plasma potentials are to be measured with emissive probes built for the new experiments. New emissive probe circuits must also be built, and instead of reproducing the existing circuit in use at USD, we attempt to change the design from a current controlled bipolar junction transistor (BJT) based design [Yan, et al. Rev. Sci. Instrum. 67 4130 (1996)] to a voltage controlled MOSFET based design, with a view to achieving significant simplification and improved noise reduction. The nominal operating parameters for our current circuit is approximately 1.2 ma of emission for 300 ma of heating current, for a 6mm long tungsten filament that is 25.4 $\mu m$ in diam. We compare the two circuits for for experiments performed in weakly collisional (λ_{mean-free-path}>>λ_D), low pressure (P_neutral ≤ 1 mTorr), low temperature (kT_e of order 1 eV), single ion species Ar plasma.