Spectral investigations of discharges on complex structured cathodes*

Plasmas can form complex 3D structures hard to investigate in terms of space and time. Frequently the physical access is limited, and the dynamic behavior requires high temporal resolution of the diagnostic. Emission spectroscopy of low temperature plasmas, usually situated inside a steel vacuum chamber, has to be carried out through a glass or plexiglass window or by fiber optics inserted into the chamber by feed-throughs. Fiber optics have the advantage that the light can be collected near the plasma. Drawbacks are possible deposition on the fiber, perturbation of the discharge, and often intricate variations of the position. Readings from outside of the chamber are more straightforward. The so-called eyepiece projection creates an image of the plasma either on a 2D mobile screen with fixed fiber position or on a screen with grid holes where the optical fiber can be shifted. Here we present a setup for measuring electron density and temperature along an axis. The cathode system consisted of two concentric gridded cylinders. The temperature was determined from the average of the Boltzmann plot slope. Electron density profiles have been calculated using relative intensities of neutral atomic lines and singly charged ionic lines according to Saha-Eggert.