Production of uniform micropellets of hydrogen and hydrogen pellet injection on ORMAK

An experimental study was undertaken to determine the interaction of a hydrogen pellet with a high temperature plasma. An apparatus was developed which produced 70 micrometer diameter droplets of hydrogen by the acoustic disintegration of a liquid jet. The liquid drops were frozen and injected into vacuum at a speed of 95 M/sec. The pellet injection apparatus was mounted on ORMAK, a tokamak magnetic confinement device located at Oak Ridge National Laboratory. Hydrogen pellets were injected into the ORMAK plasma. The pellet-plasma interaction was monitored by the enhanced neutral line emission of the ablating molecular hydrogen excited by the plasma electrons. The data indicated a pellet lifetime of 350 to 450 μ sec which implied an injection distance of 3.3 to 4.1 cm into the edge of the ORMAK plasma along the injection path which was at an angle of 450 from a radial line. FASTAX movies were taken of the pellets entering the plasma. A luminous cloud much larger than the pellets was observed sequentially on four frames. A model was developed in which the ablation rate of the pellets was regulated by the dynamic formation of a dense shielding cloud surrounding the pellet. The cloud thickness is such that the electrons from the plasma lose most of their energy before striking the pellet surface. The model was in good agreement with both the experimental pellet lifetime and the observed ablation cloud. This model predicts that a 400 micrometer diameter pellet injected at 2.5 x 103 M/sec would penetrate to the center of the ORMAK plasma