Collective Effects near the Ion Space-Charge Limit of the Orbitron

The Orbitron is a novel plasma reactor in development at Avalanche Energy, which uses static crossed fields to co-confine both ions and electrons. In our effort to optimize the ion loading process we aim to demonstrate reaching the ion space-charge limit of the electrostatic trap in the absence of electrons. We present here recent experiments and simulations in which we observe collective modes resembling so-called "negative mass" instabilities at a critical ion density. These modes place a limit on ion density about an order of magnitude lower than the theoretical space-charge limit, in the absence of co-confined electrons or other mitigation. We characterize these modes and describe methods to mitigate them in the Orbitron.