Technical Papers and Presentations

Pulsed capillary discharges are fundamental to generating high-density plasma waveguides for laser wakefield acceleration (LWFA) [1] and very high energy electron (VHEE) applications [2]. However, their transient nature—spanning rapid electrical breakdown, quasi-steady current flow, and recombination decay—involves non-equilibrium kinetics poorly captured by simplified analytical models. This work develops and validates a predictive simulation framework for the specific argon-filled capillary configuration at the I-LUCE (INFN-Laser indUCEd Radiation Production) facility [3-4] at INFN-LNS, incorporating the driving resistor-inductor-capacitor (RLC) circuit (R=20 Ω, L=600 nH, C=5 nF), capillary geometry, and variable gas pressures.

We implement a two-dimensional axisymmetric model in COMSOL Multiphysics [5], coupling the Plasma Module’s [6] drift-diffusion formulation for electron, ion, and neutral transport with finite-rate argon chemistry, while Poisson’s equation self-consistently computes the electric potential. The external discharge driver is integrated via the Electrical Circuit interface using measured component values. The plasma chemistry adopts the well-validated argon reaction set from the COMSOL Application Libraries. The time-dependent solver, with adaptive time stepping, resolves dynamics from the nanosecond breakdown phase to the microsecond current pulse, while parametric sweeps systematically scan applied voltages and gas pressures.

Our results yield detailed spatio-temporal profiles of electron density, electron temperature, ion density, reduced electric field, and electric potenital. Electron densities range from E17 to E18 cm⁻³, increasing with both voltage and pressure [7], and the temperature varies between 0-5-6 eV. Figure 1 shows an example of typical profiles of electron density and electron temperature. Crucially, the RLC circuit model accurately reproduces the measured current waveform (rise time, peak amplitude, decay tail), validating our approach against experimental data from literature [8].

This work establishes the first validated COMSOL-based predictive framework for our capillary discharge configuration, offering quantitative insights into the coupling between circuit dynamics, gas conditions, and plasma profiles. The framework directly supports the development of stable, tunable electron beams for preclinical radiotherapy research [9] at user facilities like I-LUCE and lays the groundwork for future extensions incorporating non-uniform density profiles and gas dynamics for advanced beam manipulation.