The Z-accelerator at Sandia National Laboratories (SNL), is a high-current pulsed
power machine used to drive a range of high energy density physics (HEDP) experiments [1]. To achieve peak currents of >20MA, in a rise time of ~100ns, the current is split over four levels of transmission line, before being added in parallel in a
double-post-hole convolute (DPHC) and delivered to the load through a single inner
magnetically insulated transmission line (MITL). The electric field on the cathode electrode, >107Vm-1, drives the desorption and ionisation of neutral contaminants to form
a plasma from which electrons are emitted into the anode-cathode (a-k) gap. The current addition path in the DPHC forms magnetic 'null' regions, across which electrons
are lost to the anode, shunting current from the inner MITL and load. In experiment,
current losses of >10% have been measured within the convolute; this reduces the power
delivered to the load, negatively impacting the load performance, as well as complicating the prediction of the Poynting flux used to drive detailed magneto-hydrodynamic
(MHD) simulations [2, 3]. In this thesis we develop 3-dimensional (3D) Particle-in-Cell
(PIC) and hybrid fluid-PIC computer models to simulate the plasma evolution in the
DPHC and inner MITL. The expected experimental current loss at peak current was
matched in simulations where Hydrogen plasma was injected from the cathode elec-
trode at a rate of 0.0075mlns-1 (1ml=1015cm-2), with an initial temperature of 3eV.
The simulated current loss was driven by plasma penetrating the downstream side of
the anode posts, reducing the effective a-k gap spacing and enhancing electron losses
to the anode. The current loss at early time (<10MA), was matched in simulations
where space-charge-limited (SCL) electron emission was allowed directly from the cathode; to match the loss over the entire current pulse, a delay model is motivated. Here,
plasma injection was delayed after the start of SCL emission, based on realistic plasma
expansion velocities of ~3cmμs-1. The PIC model, which was necessary to accurately
simulate the kinetic behaviour of the lower density plasma and charged particle sheaths,
was computationally intensive such that the spatial resolutions achieved in the 3D simulations were relatively poor. With the aim of reducing the computational overhead,
allowing finer spatial resolutions to be accessed, we investigate the applicability of hybrid techniques to simulating the cathode plasma in the convolute. Our PIC model was
both implemented in the resistive MHD code, Gorgon, where part of the plasma was
modelled in the single fluid approximation, and extended to include an inertial two-fluid description of the plasma. The hybrid models were applied to the DPHC simulations,
the results from which are used to motivate a three component model; here, the densest
part of the convolute plasma is modelled using the single fluid MHD approximation,
transitioning to a fully kinetic PIC description of the lower density plasma and charged
particle sheaths, linked by a two-fluid description.