Theta-pinch preionization and trapped flux in FRC formation

Abstract

Summary form only given. It is well known that it is desirable to increase trapped flux in Field Reversed Configurations (FRCs) to increase both temperature and lifetime. It is thus important to understand the mechanism that determines the amount of trapped flux during formation. FRCs are formed by first trapping an initial bias field in an ionized plasma and then applying a large field opposite to the bias field. This reversal wraps the field lines around the ionized plasma forming a toroidal configuration that is then compressed by further increase of the reversed field. The most common method for ionizing the plasma and trapping the flux in it is to embed a magnetic field in a cold gas, and then lower and raise that field abruptly by pulsing the azimuthal coil generating it. The induced electric field breaks down the gas and traps the flux in it. The observed breakdown occurs in the experiment only after the initial bias coil current falls to zero and begins to rise again, possibly because the magnetic field limits the energy gained by free electrons and inhibits breakdown in the bulk of the gas and along the insulating wall. We performed 2-d magnetohydrodynamic simulations of this process with MACH2 using a physical model that allows us to control the time of ionization. These simulations show that the flux trapped inside the first ionized plasma is very small compared to the bias flux. Upon a moment's reflection it is easy to see that this must be true in the experiment since the magnetic field is near zero then. The subsequent rise of the current back to and above the bias level drives a theta pinch which is also experimentally observed. In this case, only the flux that diffuses into the plasma during that pinch is available for trapping in the FRC. Our simulated FRCs formed in this fashion trap only a modest fraction of the bias flux and are similar to those observed in experiments. They are also much less robust than those in simulations where the time of first ionization is artificially delayed until later in the preionization pulse when the magnetic field is high. We will show these simulations and comparisons to experiments. We hope the understanding gleaned from these simulations will help us trap more flux in our FRCs. Our simulated FRCs formed in this fashion trap only a modest fraction of the bias flux and are similar to those observed in experiments. They are also much less robust than those in simulations where the time of first ionization is artificially delayed until later in the preionization pulse when the magnetic field is high. We will show these simulations and comparisons to experiments. We hope the understanding gleaned from these simulations will help us trap more flux in our FRCs.