Equilibrium and stability studies of non-neutral and pair plasmas in curved magnetic field geometries*

Non-neutral plasmas (NNPs) are fascinating both for their application to research with antimatter (e.g., anti-hydrogen production, Ps chemistry, e-e+ “pair plasmas”, etc.) and for enabling precision experiments that can be compared to theory.  However, most of these have been done in uniform magnetic fields (Penning–Malmberg traps); we are now bringing these into the realm of non-uniform magnetic fields.  We have studied thermal equilibrium states of single-species plasmas confined in a dipole trap, numerically finding solutions for not only local thermal equilibria along magnetic field lines but also global thermal equilibria. These results agree with the analytic predictions. There is, in principle, no limit to the confinement time of a single-species plasma in a global thermal equilibrium state. In contrast to local thermal equilibrium states, global thermal equilibrium states are guaranteed to be stable since there is no free energy that could drive an instability. We have investigated the global stability properties of an e-e+ pair plasma at different positron to electron ratios in the linear regime, confined by the magnetic field of an infinitely long wire (i.e., the large-aspect-ratio limit of a levitated dipole coil). We expect that the most important instabilities are of low frequency and long wavelength compared to the cyclotron motion and are electrostatic in nature, since the plasma pressure is typically low compared to the magnetic pressure. In the non-neutral case, the plasma supports a diocotron mode; as the plasma approaches neutrality, the diocotron mode vanishes and is replaced by an interchange mode.  By extending NNP theories to non-uniform magnetic fields, we not only explore new territory in fundamental plasma physics but also build a predictive/interpretive framework for experiments, e.g.,  from the APEX (A Positron Electron eXperiment) Collaboration, that seek to combine e+ pulses with e- plasmas to create and study confined e+e- plasmas.