Experimental investigations of the nonlinear dynamics of a complex space-charge configuration inside and around a grid cathode with hole

Abstract

By negatively biasing a metallic grid with a small hole, down to a critical value of the applied potential a complex space-charge structure appears inside and around the grid cathode. The static current–voltage characteristic of the discharge shows one or two current jumps (the number of current jumps depending on the working gas pressure), one of them being of hysteretic type. Electrical probe measurements show a positive potential inside the grid cathode with respect to the potential applied on it. This is interpreted as being due to the hollow cathode effect. Thus, the inner fireball appears around the virtual anode inside the grid cathode. For more negative potentials, the electrons inside the cathode reach sufficient energy to penetrate the inner sheath near the cathode, passing through the hole and giving rise to a second fireball-like structure located outside the cathode. This second structure interacts with the negative glow of the discharge. The recorded time series of the discharge current oscillations reveal strongly nonlinear dynamics of the complex space-charge structure: by changing the negative potential applied on the grid cathode, the structure passes through different dynamic states involving chaos, quasi-periodicity, intermittency and period-doubling bifurcations, appearing like a competition of different routes to chaos.

Spectral and electrical diagnosis of complex space-charge structures excited by a spherical grid cathode with orifice

“…Similarly as in [1], comparative experiments have been performed in two plasma experiments, one at the University of Innsbruck, and the other one at the University of Iasi, both of them described in [1]. The main differences between the two devices are the material of the wall (stainless steel in Innsbruck and glass in Iasi) and the dimension of the spherical grid cathode.…”

“…This is probably due to the rather high density plasma and high background pressure which leads to an approximate local thermodynamic equilibrium and to more reliable characteristics. Moreover, attempts to measure the plasma potential inside and outside the spherical grid by an emissive probe turned out to be extremely difficult and unreliable due to strong perturbations of the space charge structure by the emissive probe [1].…”

“…In a recent paper [1] we reported on experimental results concerning the generation and dynamics of complex space charge structures forming inside and around a spherical grid cathode with an orifice. This structure consists of a plasma bubble [2][3][4][5] inside the grid cathode which interacts with a fireball [6][7][8][9][10] located outside the grid cathode near the orifice and which is connected with the negative glow of the discharge.…”

“…Here we report on experimental investigations of the spectral and electrical properties of a complex space charge structure such as described in [1]. Through the orifice axial profiles of optical emission spectra, plasma potential, electron temperature and electron, ion and excited neutral densities were obtained by emission spectroscopy and Langmuir probe measurements.…”

Concentric double hollow grid cathode discharges. Spectral investigations and phenomenological approach

“…With increasing applied negative bias on the grids, the electrons acquire sufficient energy to produce volume ionizations. In this geometry, complex SCS appear inside and around each grid once the elementary processes assure the necessary electric charge equilibrium [1,2]. The extent of the spatial configurations varies with the applied cathode biases and background pressure [2].…”