Optimization of capillary source geometry for maximum pellet exit velocity in electrothermal plasma launchers

Abstract

Deep fueling for large-scale tokamak fusion reactors requires the use of high-velocity pellet injectors. The fuel pellets consist of frozen deuterium or deuterium-tritium ice. A fuel pellet must be launched into the fusion plasma at a high enough velocity to reach the core of the plasma before too much of the pellet melts, ablates and disintegrates. For tokamak fusion reactors, the fusion plasma is predicted to operate at temperatures of up to 15×107 °C at its core. At these extreme temperatures, the fuel pellets need to be launched at large velocities to prevent excessive melting and vaporization before reaching the core of the plasma. Obtaining this exit velocity is possible through the use of electrothermal (ET) plasma guns. Electrothermal plasma guns use a capillary tube where plasma is sparked inside the source and is allowed to continue travelling in an extension acceleration barrel. The plasma is sparked inside the source via the discharge of a capacitor bank that is charged up to 10 kV or higher. A liner material inside the source is ablated and forms plasma that draws discharge current (several kA) over 100 microseconds and can propel a pellet to velocities exceeding 3 km/s. Using a 1-D time-dependent computer code, a variety of computational predictions can be made on the plasma, as well as the pellet, as it moves through the barrel until it leaves towards the fusion core. Characteristics of different geometric configurations of the plasma gun can be simulated using the code to predict the optimal geometric configuration that will maximize the pellet's exit velocity. Previous research studies have been conducted by varying the length of the acceleration barrel and computing the effect on the pellet's exit velocity. In the present research, to complement the prior studies, the one dimensional computer code, ETFLOW, was used to simulate the effect of varying the source geometry, i.e. length, radius, and aspect ratio, on the pellet's exit velocity. For optimal geometries, pellet exit velocities of up to 3.9 km/s were computed. This is an increase of nearly 12% from previous computational studies. Computed pellet velocities are correlated to source geometry and plasma parameters.