Richard D'Arcy

Abstract:

This work represents the first long-term (µs+) simulations of capillary discharge devices, made feasible by the computationally inexpensive QUEST algorithm [4]. We have demonstrated that QUEST gives comparable results to full plasma fluid simulations both during and long after the discharge has terminated. Using QUEST, the heat flow through the plasma-wall interface was simulated for discharge current conditions relevant to the FLASHForward experiment [2], operated at kHz-MHz repetition rates. The model showed that 1 kHz and 10 kHz repetition rates could be sustained indefinitely, but that 100 kHz and MHz rates quickly exceeded the sapphire capillary melting point. By reducing the pulse length and amplitude, MHz repetition rates can feasibly be sustained while providing plasma conditions suitable for accelerator applications.

Abstract:

Electron beams to be accelerated in beam-driven plasma wakes are commonly formed by a photocathode and externally injected into the wakefield of a preceding bunch. Alternatively, using the plasma itself as a cathode offers the possibility of generating ultrashort, low-emittance beams by trapping and accelerating electrons from the ambient plasma background. Here, we present a beam-driven plasma cathode realized via laser-triggered density-downramp injection, showing stable beam formation over more than a thousand consecutive events with an injection probability of 95%. The plasma cathode is highly tunable, resulting in the injection of electron bunches of tens of pC of charge, energies of up to 79 MeV, and relative energy spreads as low as a few percent. The stability of the injected beams was sufficiently high to experimentally determine their normalized emittance of 9.3 μm rms with a multishot method.

Abstract:

A model describing the evolution of the average plasma temperature inside a discharge capillary device including Ohmic heating, heat loss to the capillary wall, and ionization and recombination effects is developed. Key to this approach is an analytic quasistatic description of the radial temperature variation which, under local thermal equilibrium conditions, allows the radial behavior of both the plasma temperature and the electron density to be specified directly from the average temperature evolution. In this way, the standard set of coupled partial differential equations for magnetohydrodynamic (MHD) simulations is replaced by a single ordinary differential equation, with a corresponding gain in simplicity and computational efficiency. The on-axis plasma temperature and electron density calculations are benchmarked against existing one-dimensional MHD simulations for hydrogen plasmas under a range of discharge conditions and initial gas pressures, and good agreement is demonstrated. The success of this simple model indicates that it can serve as a quick and easy tool for evaluating the plasma conditions in discharge capillary devices, particularly for computationally expensive applications such as simulating long-term plasma evolution, performing detailed input parameter scans, or for optimization using machine-learning techniques.