Study of self‐organised criticality in a coaxial plasma source with gridded cathode

Abstract

Multiple luminous sheath structures develop in the localised plasma formation inside a cylindrical gridded cathode plasma source. These structures gradually expand into the whole vessel and undergo a critical state through self‐organised criticality (SOC) along with pronounced sheath oscillations during the variation of anode potential. The present investigation emphasises the stability properties associated with the oscillations of the sheath potential structures in the cylindrical‐gridded cathode in glow discharge regime through SOC phenomena through non‐linear time series analysis. The non‐linear temporal and spatial analysis of potential structure and dynamics of charged particles are correlated with SOC in cylindrical‐gridded cathode in glow discharge regime through the dynamics of multiple sheath structure formations. The existence of flicker noise and long‐range correlation in the time series suggests the existence of SOC behaviour in the system, the multiple sheath structures evolve through reduction and addition of layers. Cylindrical‐gridded cathode discharge initiation during formations of multiple sheath structures, switching of discharge regimes with anode potential, layer reduction and addition inside the sheath structures through SOC behaviour, renders uniqueness to the present regime of investigation. Self‐organised pattern formations and self‐organised criticality behaviour investigated in the present glow discharge regime using the cylindrical‐gridded cathode source are of prime importance to understand the complex behaviour of plasmas in the field of hollow cathode discharges.

Study of nonlinear characteristics in hollow cathode discharge in unmagnetized plasmas

“…The plasma potential is obtained from the first derivative of the Langmuir probe I-V trace, as shown in Figure 3. [8,24] The probe voltage varies from −35 to +35 V with respect to the grounded variation near the centre (V f1 ), 1.2 cm away from the centre (V f2 ) and near the anode surface (V f3 ). The variation of floating potential at different locations indicates the spatial potential variation.…”

Formation of multiple anodic fire spots inside an electrostatic cavity

“…The H‐exponent is a self‐similar parameter that determines the long‐term correlation in the recorded time series. [ 8,10,12,33 ] Generally, 0.5 < H < 1 indicates the persistency of the signal, H = 0.5 indicates an uncorrelated process, while 0 < H < 0.5 indicates the anti‐persistence of the data series. To examine how far the data set reflects long‐term correlation, the whole signal is divided into non‐overlapping bins and the H‐coefficients are obtained for each bin, shown in Figure 9a for V A ˜ 340 V. The R/S analysis provides H = 0.61 for the time lags from 12 to 300 while H = 0.83 for time lags from 300 to 1100, which indicates the existence of long‐term memory in the time series, and the trend persists in the next blocks of the data set.…”

“…The plasma potential is calculated from the probe I‐V characteristics, measured using a modified shielded Langmuir probe. [ 10,23 ] The modified Langmuir probe is placed inside a cylindrical floating electrode; the probe tip connects to the oscilloscope through an RC‐coupled circuit. [ 24 ] The typical sheath radius varied within (0.1–0.3) mm during the present study.…”

“…To minimize the fluctuating potential, an RC filter circuit is connected parallel to the probe tip. [ 10,23,24 ] The RC circuit consists of 10 kΩ resistance and 1000 pf capacitor, placed parallel and connected to the probe tip. After positioning the Langmuir probe(s) well inside the plasma formation during the continuous glow discharge, a progressively increasing dc bias voltage is applied to the Langmuir probe(s) and the resulting probe current is noted for each bias voltage step.…”

“…[ 1–5 ] However, in almost all of these cases, an ambient cathode plasma was produced by filament within a certain gas pressure range, in the absence of a magnetic field as well as in the presence of a magnetic field. [ 6 – 9 ] Self‐organized criticality (SOC) behaviour was investigated earlier during Double Layer (DL) formations in various confinement devices, mostly near the electrodes, [ 8,10,12 ] owing to its vital importance in the investigation and understanding of complexities in space plasma and fusion plasmas. [ 13 – 15 ] The SOC indicates the existence of self‐similarity in the discharge states and helps in understanding the fractal dimensions during such a discharge regime.…”