Nuclear generated plasmas in noble gas thermionic converters

Abstract

The generation of a plasma by fission fragment ionization in noble gas thermionic converters was investigated in a series of inpile experiments at the University of Michigan reactor. The plasma generated in Ne : A (1000 : 1) at a pressure of 20 mm Hg was investigated in a plane parallel diode with electrically heated emitter and collector of barium impregnated tungsten. The fission fragment source was uranium foil mounted on a third electrode. As calculated from diode V- I characteristics, ion densities of order 10 10 cm −3 were generated by fission fragments in the space surrounding the converter, and ion densities of order 10 11 cm −3 were present in the interelectrode gap due to the formation of ion trapping sheaths at the electrode surfaces during normal converter operation. The effect on converter operation of cesium generated in the fission process was investigated in a tube with a diode structure and filling as above, but with a fission product source shielded from the interelectrode space. No cesium effects were observed during the 11 1 2 hr of tube operation. The ion density due to the reactor gamma flux was calculated from the V- I characteristic to be of the order 10 8–10 9 cm −3. Fission product poisoning of the collector was observed at temperatures less than 1000°K; the collector could be reactivated. To investigate the plasma generated by fission fragments in Xe at a pressure of 8 mm Hg, a plane parallel diode was constructed with a nuclear heated emitter of uranium carbide-rhenium and a barium impregnated tungsten collector. The emitter operating temperature was 1950°K in a neutron of 5 × 10 12 cm −2 sec −1. Discontinuities were observed in the forward current characteristic between 6–7 V. The effects between 6 and 7 V were attributed to the formation of xenon metastable atoms in the collector sheath which could then produce additional ionization. The discontinuity at 10 V was attributed to Xe breakdown. The ion density calculated from V- I characteristic before the first discontinuity was of order 10 9 cm −3.