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...
Experiments are reported on oscillations that arise in a double plasma device when plasma production is restricted to the source chamber and the separating grid between the two chambers is biased negatively. The free oscillating system shows periodic pulling which is a typical behavior of driven van der Pol type oscillators. The second interacting frequency is identified to be half the ion plasma frequency at the sheath edge on the source side. With the help of particle in cell simulations the concept of virtual anode oscillations (VAO’s) as the underlying oscillation mechanism is investigated and the van der Pol character of these is revealed. When applied to the experimental conditions, the VAO-model predicts correct oscillation frequencies. It gives a new interpretation of the scaling of these with plasma density and grid bias, and is compatible with earlier findings.
The oscillation of ion bunches around a strongly negative grid in a double plasma device is studied with one–dimensional particle–in–cell simulation. The system exhibits feedback amplified virtual anode oscillations in the target chamber. The undriven system is shown to perform relaxation processes which can be described by the van der Pol equation for high nonlinearity. The periodically driven system exhibits nonlinear phenomena like mode locking and periodic pulling, which are in quantitative agreement with theoretical predictions for the driven van der Pol oscillator. For large driver amplitudes period doubling and weak chaos are found.
The discharge modes and the nonlinear potential relaxation instability of a thermionic argon discharge operated at pressures of 0.02-0.2 Pa are studied experimentally and compared to particle-in-cell (PIC) simulations. The plasma produced in a steel vessel with a filamentary cathode is confined by a magnetic box arrangement of permanent magnets. Depending on the discharge voltage, the neutral gas pressure, and the filament heating current, the discharge shows two stationary states with a pronounced hysteresis in the current-voltage characteristic. For a certain range of discharge parameters, sawtooth oscillations can be observed in the discharge current. One-dimensional (ID) particle-in-cell simulations reveal details about the mechanism of the oscillations which are confirmed by experiment. The piasma potentiai whIch reiiects the microscopic mechanisms is measured by means of emissive probes with high spatial and temporal resolution. The coherence of the self-oscillating system is improved by mode locking it with an external driver. The results of the simulation-specifically formation of an electron hole and its transformation into a double layer-are confirmed experimentally.
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