PIC-MCC SIMULATION OF THE ELECTRICAL BREAKDOWN IN A PLASMA FOCUS DEVICE

The breakdown phase in a Plasma Focus (PF) device is not yet a well-understood phenomenon, even if widely known to be of great importance in the subsequent evolution of the discharge. A Particle-In-Cell (PIC) method coupled with a Monte-Carlo-Collision (MCC) module has been developed to study the time evolution of electron density and Electron Energy Distribution Function (EEDF) up to few tens of nanoseconds. An innovative dynamic control on the simulation particles number has been introduced. A merging technique based on a Sorted Hierarchical Agglomerative Sub-Clustering is performed to collapse clusters of particles in the phase space. Moreover, in order to avoid particles depletion in the cathode-fall region, an additional splitting procedure will be discussed: it is intended to preserve a sufficiently high sample representativeness for the stochastic secondary emission events and to limit local fluctuations in the electric field solution. The algorithms not only accelerate the numerical solution and guarantee better statistics, but are also intended to preserve the position, charge-mass, direction of flight, momentum and energy of the starting set of particles. As it arises from simulations, the EEDF development shows a deviation from the Maxwellian distribution function: an anomalous number of high-energy electrons enhances the ionization rate of the working gas towards the development of a localized plasma sheath on the insulator sleeve at the electrodes closed end. After few nanoseconds from the beginning of the discharge, a fundamental role is played by secondary electrons produced at the cathode wall by photoelectric effect due to molecules excited states de-excitation. The numerical model describes the breakdown ignition more precisely than existing hydrodynamic ones.