Thermal-buble initiated breakdown characteristics of liquid helium and nitrogen at atmospheric pressure
Experiments show that electric field promotes to form a vapor locking in a gap with a hot spot and in turn the formation of locked vapor produces the serious reduction of breakdown voltage. Especially it is argued that the dielectric strength of liquid helium under serious hot spot conditions decreases below that for saturated gas.
Dielectric Breakdown of Liquid Helium
The dielectric strength of liquid ${\mathrm{He}}^{4}$ under its saturated vapor pressure has been determined from 1.2 to 4.2\ifmmode^\circ\else\textdegree\fi{}K. With $\frac{3}{8}$-inch diameter spherical steel electrodes 0.15 mm apart the average breakdown field, ${\overline{E}}_{b}$, is approximately 1 Mv/cm, nearly independent of temperature. At spacings of 0.5 and 1 mm, ${\overline{E}}_{b}$ is lower, and appears to decrease with decreasing temperature below 2.5\ifmmode^\circ\else\textdegree\fi{}K. The mechanism of breakdown is not clear, but field emission of electrons from the cathode is probably involved, because the average breakdown voltage ${\overline{V}}_{b}$, obtained with a point and plane electrode system when the point is negative, is about half that obtained when the point is positive, both above and below the $\ensuremath{\lambda}$-point. Pre-breakdown currents were never detected, and were probably less than ${10}^{\ensuremath{-}13}$ amp, implying an electrical resistivity for liquid helium of ${10}^{18}$ ohm-cm.
A slowly rotating coil system for AC field measurements of Fermilab Booster correctors
A method for measurement of rapidly changing magnetic fields has been developed and applied to the testing of new room temperature corrector packages designed for the Fermilab Booster Synchrotron. The method is based on fast digitization of a slowly rotating tangential coil probe, with analysis combining the measured coil voltages across a set of successive magnet current cycles. This paper presents results on the field quality measured for the normal and skew dipole, quadrupole, and sextupole elements in several of these corrector packages.