Nonlinear dynamical behavior of thermionic low pressure discharges. II. Experimental

Abstract

Strongly nonlinear relaxation oscillations of discharge current and plasma potential are investigated in a magnetized thermionic plasma discharge. The quasi-one-dimensional electron motion allows a direct comparison with one-dimensional models and computer simulations. Two different stable discharge modes can be established, the low-current space charge limited and the high-current temperature limited mode. Time resolved probe measurements of the plasma potential distribution demonstrate that the current oscillations result from a strongly nonlinear instability of the potential structure in the weak current discharge mode. This confirms the model based on particle-in-cell simulations [F. Greiner et al., Phys. Plasmas 2, 1810 (1995)]. The oscillation process consists of three distinct phases. The sequence of events and the observed parameter dependencies of the oscillation frequency is in accordance with the model. The periodically driven system shows the characteristic behavior of nonlinear oscillators: quasiperiodicity, mode-locking, and period doubling sequences towards chaos. It is possible to link the complex dynamical behavior to the details of the trigger mechanism that are revealed by the simulation.

Toroidal stellarator diode: discharge features and electron transport

“…The phenomena and processes associated with the nature of hysteresis and current jumps of CVCs in thermionic discharges with and without the longitudinal magnetic field have been described in [22][23][24][25][26][27][28][29][30][31][32][33][34][35]. In modulated thermionic plasma discharges, different modes of CVC hysteresis behaviour and transition to so-called chaos were the subjects for studies [26,28,[30][31][32].…”

“…Measurements of the plasma potential before and after the CVC hysteresis jump gave two different stable states of the discharge: firstly, "space-charge current mode" ("anode-glow mode" in classical terminology for thermionic discharges); secondly, "temperature-limited current mode" (Langmuir mode). These two modes were characterized by different plasma potential distributions [24,28,[30][31][32]. Note that the discharge is influenced by the negative space charge forming the so-called "virtual cathode" in front of the hot thermionic cathode and by a decrease in the space charge caused by positive ions produced in the anode sheath.…”

“…In modulated thermionic plasma discharges, different modes of CVC hysteresis behaviour and transition to so-called chaos were the subjects for studies [26,28,[30][31][32]. For the most part, the experiments were carried out in the devices with a cylindrical vacuum chamber and a magnetic layer composed of a row of cusps formed by the field of permanent magnets.…”

Self-similar space-time evolution of an initial density discontinuity

“…20 We formulate the problem and its boundary conditions so that it applies for the operation of a single ended Q-machine, 14,21 where electrons are emitted thermally, while ions are produced by surface ionization at a hot cathode. Under steady state conditions, the ion velocity distribution in the main plasma (outside the end-sheaths) is a truncated Maxwellian.…”

Numerical studies of a plasma diode with external forcing

“…The basic model corresponds to a single ended Q-machine, 1,2 where electrons are emitted thermally while ions are produced by surface ionization at a hot cathode. In the basic version of the Q-machine the ions are produced by contact ionization 3 of hot alkali metals with suitable work functions.…”