Design and test bench operation results of a solid hydrogen pellet injector developed for SST-1 tokamak

Abstract

A single barrel pellet injection system is developed for pellet injection related studies targeting SST-1 tokamak plasma. Conventional pipe gun concept is adopted for designing the injector. The hydrogen ice is formed in-situ in the gun barrel, and a high pressure light propellant gas is used to dislocate the pellet from the freezing zone and accelerate it to higher speed. This system uses a cryogen free, closed loop compact cryocooler, which provides ease of handling and operational reliability to the pellet freezing process. Pellet parameters such as size and speed are considered by using the density limit and neutral gas shielding (NGS) model based calculations, respectively. The designed size and speed (Vp) of the pellet is fixed at (1.6 mm length [ℓ] × 1.8 mm diameter [φ]) and Vp < 800 m/s, respectively. A programmable logic controller (PLC) based control system and a graphical user interface (GUI) is developed and installed to operate the injector remotely. This paper presents the design and test results of the injector such as pellet freezing capability, effect of temperature gradient along the barrel on pellet formation process and its speed control with propellant pressure.