Probes in non-uniform plasma

Summary form only given. Mach probes and Langmuir probes are investigated experimentally in a non-uniform, unmagnetized plasma, drifting in a wall presheath. A Mach probe consists of two single sided Langmuir probes back to back with an insulator between them. Two different probe diameter sets 0.125" and 0.375", (smaller than the ion collision length ∼3 cm at 1 mTorr) are used. The upstream side of the Mach probe (which faces the bulk plasma) indicated higher plasma potential than the local plasma potential in the absence of the probe. The plasma potential and density indicated by the upstream side of the Mach probe is almost the same plasma potential and density found an ion collision length (presheath length) from the probe. The downstream side of the Mach probe indicated lower plasma potential and density than the unperturbed local plasma potential and density. But the absolute value of the deviations from the local plasma potential and density were the same value for both sides of the Mach probe. The shadowing effect for the downstream side of the Mach probe is investigated when the probe is biased negatively and near the plasma potential. The potential and density profiles are also obtained by an emissive probe and a Langmuir probe near the upstream side and the downstream side of the Mach probe.