Technique for measuring D ₂ pellet mass loss through a curved guide tube using two microwave cavity detectors

Two microwave cavity mass detectors have been used to measure the mass loss of deuterium (D ₂) pellets transported through a curved guide tube. The test tube was a mock-up of the pellet injection guide tube for the proposed ITER experiment, which will be used to transport pellets, including deuterium-tritium (D-T), from the pellet acceleration device to the inner wall (or magnetic high-field side) of the large tokamak for pellet injection and core fueling of plasmas. An accurate estimate of the mass loss is particularly important for D-T injection, because the inventory of the radioactive isotope (T) for ITER is limited and accountability and recycling will be crucial issues. In the laboratory, frozen cylindrical D ₂ pellets of nominal 5.3-mm diameter were shot through the stainless steel test tube (≈10 m in length and 10-mm inside diameter), with each end equipped with a microwave cavity. As the pellet passes through each tuned microwave cavity, the peak output signal from the electronics is directly proportional to the pellet mass. An absolute calibration of the cavities, which can be problematic, is not needed for the nondestructive technique described here. Instead, a cross calibration of the two cavities with pellets of varying masses provides the relationship to determine mass loss more precisely than any other technique previously reported. In addition, the individual output signals from the cavities can be used to identify intact pellets (a single signal peak) or broken pellets (multiple signal peaks). For the pellet speed range tested in this study (100-500 m/s), the mass loss for intact pellets was directly dependent on the pellet speed, with ≈10% mass loss at 300 m/s. The microwave cavities and the associated electronics, as well as some basic theory, are described; calibration and experimental data are presented and discussed.