Development and implementation of a rail current optimization program

Efforts are underway to automate the operation of a railgun hydrogen pellet injector for fusion reactor refueling. A plasma armature is employed to avoid the friction produced by a sliding metal armature and, in particular, to prevent high-Z impurities from entering the tokamak. High currents are used to achieve high accelerations, resulting in high plasma temperatures. Consequently, the plasma armature ablates and accumulates material from the pellet and gun barrel. This increases inertial and viscous drag, lowering acceleration. A railgun model has been developed to compute the acceleration in the presence of these losses. The model suggests that, depending on the rail and insulator materials used, there is a point of diminishing returns. Namely, for a given current, there is an acceleration time beyond which little or no increase in pellet speed is produced. The optimal pulse width was determined by identifying the time at which the acceleration decreased to zero. In order to quantify these losses, the ablation coefficient, α, and drag coefficient, C_d, must be determined. These coefficients are estimated based on the pellet acceleration. The sensitivity of acceleration to α and C_d has been calculated using the model. Once α and C_d have been determined, their values are applied to the model to compute the appropriate current pulse width. An optimization program was written in LabVIEW software to carry out this procedure. This program was then integrated into the existing code used to operate the railgun system. Preliminary results obtained after test firing the gun indicate that the program computes reasonable values for α and C_d and calculates realistic pulse widths.