Low velocity boron micro-pellet injector for edge and core impurity transport measurements

Abstract

A simple low velocity boron micro-pellet injector has been under development for Current Drive Experiment Upgrade (CDX-U) spherical torus edge and core impurity transport measurements, and wall conditioning. The injector consists of 16 barrels on a rotatable turret. Each barrel can be loaded with boron powder particles of diameters ranging from 1 to 40 μm diameter in amounts ranging from less than 0.25 mg to more than 2 mg. A selected barrel is manually rotated into firing position using a vacuum precision rotary/linear motion feedthrough. A piezoelectric valve gas feed system triggered by CDX-U discharge timing is used to control H2 or D2 propellant gas at a cylinder pressure of 5.8×10−3  Pa (40 psi) or less. The injector barrel-to-CDX-U plasma edge distance is 0.47 m. Initial low mass injections of neutral boron beams were performed into CDX-U plasmas at a velocity of 23 m/s. Measurements were obtained with a filtered gated charge coupled device TV camera, bolometry, visible spectroscopy, and ultrasoft x-ray diagnostics. This work is in support of the present CDX-U research program and possible applications on National Spherical Torus Experiment.