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

We attempt to improve the design of our existing emissive probe circuit that is based on a current controlled current source for heating the filament (1 mil Tungsten, 6mm long) of our emissive probes. A new low temperature plasma experiment called the Electronegative Plasma Sheath EXperiment, or EPaX, is coming on-line at the University of San Diego (USD), that attempts to measure internal sheaths in electronegative plasma. Plasma potentials are to be measured with emissive probes built for the new experiments. A new circuit design based on a voltage controlled current source is explored, using a MOSFET rather than a BJT, that could significantly simply the existing circuit and improve DAQ interface issues (eliminate buffer amplifiers), and possibly reduce noise. By measuring current-voltage characteristics of a variety of MOSFETS, we present our work aiming to find a suitable match for the power and heating and emission current requirements for a first test of the new circuit for an emissive probe used 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.