Breakdown law and post-breakdown current-voltage characteristics of emission-driven microplasmas

Abstract

Summary form only given. Microplasmas are traditionally defined as plasmas that are confined to gaps that are less than 1 mm and have found applications in nanoparticle synthesis, plasma medicine and flow control. More recently, there has been significant emphasis on microplasmas operating at even smaller gaps that are less than 100 μm. It is now well-established that the breakdown voltages at microgaps deviate from Paschen law due to field emission from the cathode [1]. Numerical simulations [2,3], experiments [2] and theoretical studies [1,3] have focused on both pre-breakdown and certain basic post-breakdown characteristics [4] of these discharges. This talk will present an extension to these studies by presenting a breakdown law for microplasmas that are driven by a combination of both thermionic and field emission. The emission of electrons from the cathode is governed by the thermo-field theory [5]. The cathode source current is enhanced due to ionization in the gap, the secondary electron emission and the enhancement in emission current due to the ions that reach the cathode. Equations are formulated to describe the steady-state current density of the pre-breakdown Townsend dark discharges. The condition for absence of a steady-state pre-breakdown state is used to determine the breakdown voltage as a function of cathode temperature and applied voltage and present a scaling law for thermo-field emission driven breakdown. The theoretical results for the pre-breakdown state and breakdown voltage are validated using one-dimensional particle-in-cell with Monte Carlo collisions (PIC/MCC) simulations using a modified version of XPDP1 code. Finally, a theoretical model for the post-breakdown state comprising a cathode-fall and a quasi-neutral region is formulated to obtain current density as a function of applied voltage and then validated using constant current density PIC/MCC simulations. The variation of electron energy distribution function across the gap is also presented.