Electrical Breakdown of Ice at Cryogenic Temperatures
An ice or ice-silica compound system is attractive for electrical insulation at cryogenic temperatures. A sample of ice without silica was found to have a breakdown strength of 0.3 MV/cm for ac voltages, and 0.6 MV/cm for impulse voltages. The breakdown strength of an ceicesilica compound system was 0.7 MV/cm for ac voltages. There were no visible macroscopic cracks in the sample, but microscopic cracks and voids may exist, since partial discharges were observed at about half the value of the ac breakdown voltage. The relationship between partial discharges and microscopic voids, and the voltage-time characteristics of this insulation system were studied.
Some Characteristics of Dielectric Materials at Cryogenic Temperatures for HVDC Systems
This dielectric study was performed as part of the dc Superconducting Power Transmission Line Program. During a screening test program, the dc breakdown strength of 12 dielectric materials, in sheet form, was determined at 12 K under 1.38 MPa of helium pressure, the operating conditions of the cable design. Tests were also made with four of these materials impregnated with mineral oil. Further dc breakdown tests were performed on five of these dielectric materials at various temperatures (12 to 298 K) and pressures (0.69 and 1.38 MPa) of helium. Two sets of experiments were performed on cellulose paper impregnated with distilled water at 83 K and 1.38 MPa of helium pressure. Five dielectric materials were then selected for tests as cable samples with dc and impulse voltages at 12 K and 1.38 MPa of helium. Data are presented on these tests, as well as on those of the electrical conductivity of four dielectric materials at cryogenic temperatures, and of the surface flashover strength of an epoxy bushing at 12 K and 298 K under 0.69 MPa and 1.38 MPa of helium pressures. The characteristics of the dielectric behavior at cryogenic temperatures are discussed. Impregnation of cellulose and copaco papers and PP/C with mineral oil improved their dc withstand strengths significantly, even at cryogenic temperatures. The dc breakdown field (uniform field con figuration) of cellulose and copaco papers (with and without oil impregnation) and PP/C varied as the exponential of the inverse of temperature.
PULSE: a high-repetition-rate linac driver for x-ray FELs
We describe a staged concept for a linac-based freeelectron laser providing coherent tuneable VUV and soft X-ray output with pulse lengths down to 10 fs or less. Use of 1.3 GHz cryomodules operated with CW RF will allow flexible pulse patterns at potentially very high repetition rates up to 1 MHz. We consider the options for FELs that may be included in a facility of this type.