Radio-Frequency Noise of an Immersed Langmuir Probe

Abstract

The rf noise power spectrum at a Langmuir probe in a cold-cathode neon discharge is observed over a 50-kc to 10-Mc range. The statistical noise amplitude distribution function is found to be Gaussian and the probe noise can be analyzed in terms of shot noise of the probe electron and ion currents using a substantially constant probe sheath impedance at different probe potentials. The noise current spectral density increases at low frequencies in agreement with hot-cathode discharge probe observations, has a minimum at about the shot noise current level, and has a high-frequency increase corresponding to that observed by Shimada [K. Shimada, Proc. I.R.E. 49, 632 (1961)]. It is believed that freedom from wall effects can be attained, since relative insensitivity of the noise to discharge pressure, current, and purity is observed. The introduction of stationary striations does not influence the power spectrum other than to superimpose very sharp oscillatory peaks which are easily altered by changing discharge current or pressure. Low-frequency fluctuations in the plasma can be largely eliminated by using large diameter discharge tubes.