Experiments and particle-in-cell simulation on self-oscillations and period doubling in thermionic discharges at low pressure

Abstract

The mechanism of different discharge modes, self-oscillations, and the period-doubling route to chaos is studied by comparing experimental results from a filament cathode discharge with particle-in-cell simulations. The self-oscillation process invokes ion trapping by charge exchange, double layer formation, and ion depletion. The exhausting of resources which underlies the period-doubling route is identified with incomplete ion refilling. PACS numbers: 52.35.Fp, 47.52.+J, 52.65.+Z Discharges with filament cathodes at pressures of about 10 _I Pa are in widespread use in magnetic box devices [1], double plasma arrangements [2], or ion sources [3]. Their inherent hysteresis of the iW) characteristic [4,5] has attracted much interest from the viewpoints of catastrophe theory [6] and nonlinear dynamics [7]. Under certain discharge conditions thermionic discharges perform low-frequency oscillations [8], which can be driven to chaos either by varying the discharge parameters [9] or by applying external modulation [10]. Routes to chaos via intermittency [11] and period doubling [7,12] were observed. Cheung and Wong [7]have discussed the role of ion dynamics and electron neutral collisions in perioddoubling scenarios. In the present paper we intend to identify the detailed interplay of plasma processes, which lead to period-doubling transition to chaos.Our investigations are performed in a magnetic box device with filament cathode [12] (Fig. 1). This device has been selected as a typical representative of this class of discharge. The discharge is operated in argon at p = 0.03-0.3 Pa. The anode voltage can be modulated by additionally applying periodic pulses of a few kHz frequency. The spatiotemporal evolution of plasma parameters and plasma potential is measured with movable Langmuir and emissive probes.The general behavior of this discharge is compiled in Fig. 2. The /(£/) characteristic shows the well-known hysteresis curve. The upper branch represents the "temperature-limited mode" (TLM); the lower branch is the "anode-glow mode" (AGM) in the classical terminology for thermionic discharges [13,14]. In our case the states are similar to the above-mentioned collisional discharges despite the fact that the mean free path for electron and ion collisions with neutrals is comparable to the plasma dimensions. For these low pressure thermionic discharges the AGM is established by producing ions in the anode sheath and trapping them by charge exchange in the potential well of the virtual cathode. The resulting plasma potential distribution [ Fig. 2(b)] shows a cathodic plasma, which is close to the cathode potential, and an anode layer, where the plasma potential increases to anode potential. From the discharge current we estimate the electron density to be of the order n e 10 14 m In the TLM, the plasma potential distribution is homogeneous and close to anode potential [ Fig. 2(a)] except for a cathode sheath which is not accessible by emissive probes. The plasma parameters n e = 10 16 m ~3 and T e =2 eV are obtained from La...

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.…”

PIC‐Simulation Resolved Period‐Doubling Route to Chaos in the Classical Pierce Diode

“…Such weakly non-linear period-N oscillations in the Pierce diode have already been intensely studied theoretically [2][3][4][5], experimentally [6,7], and/or using Particle-in-Cell (PIC) simulations [6,8,9]. They have been considered, e.g., as mechanisms for generating high-power microwaves [2] or producing light-ion beams for inertial-confinement fusion [3].…”

Alfvén multistability: Transient and intermittent dynamics induced by noise

“…In laboratory plasmas, Cheung and Wong 1 reported the experimental observation of chaotic behavior in a pulsed plasma discharge, with a calculation of the Feigenbaum constant for a perioddoubling route to chaos. The period-doubling route to chaos was also studied by Greiner et al 2 by comparing results from a filament cathode discharge with particle-in-cell simulations. Sheridam 3 conducted the experimental characterization of chaotic dynamics in a complex ͑dusty͒ plasma of three particles, with the detection of an attractor with fractal dimension and a positive Lyapunov exponent.…”