A Simple Technique for Increasing the Temperature Range of a Cryogenic Frozen-Gas Pellet Injector for Operation with Various Gases

On the Madison Symmetric Torus magnetic fusion plasma experiment, frozen pellet injection is an established method of depositing deuterium fuel into the core of the plasma. To freeze deuterium gas into pellets, the injector is cooled to 10 K with a cryogenic helium refrigerator. To exhaust residual frozen deuterium following injection of each pellet, the injector is warmed by resistive heating to >18.7 K, the triple point of deuterium. Motivated by the desire to inject carbon-containing pellets, the injector was modified to allow the freezing and injection of methane. The triple point of methane, 90.7 K, is well beyond the capability of the resistive heating hardware. To supplement the resistive heating, a small, steady flow of room-temperature helium was introduced as a heat source. The flow rate was optimized to provide minimum and maximum injector temperatures of 24 and 95 K, respectively, sufficient for methane pellet formation and exhaust. The flow rate can easily be optimized for other gases as well.