We have conducted a series of experiments designed to measure the flashover strength of various azimuthally symmetric $45\ifmmode^\circ\else\textdegree\fi{}$ vacuum-insulator configurations. The principal objective of the experiments was to identify a configuration with a flashover strength greater than that of the standard design, which consists of a $45\ifmmode^\circ\else\textdegree\fi{}$ polymethyl-methacrylate (PMMA) insulator between flat electrodes. The thickness $d$ and circumference $C$ of the insulators tested were held constant at 4.318 and 95.74 cm, respectively. The peak voltage applied to the insulators ranged from 0.8 to 2.2 MV. The rise time of the voltage pulse was 40--60 ns; the effective pulse width [as defined in Phys. Rev. ST Accel. Beams 7, 070401 (2004)] was on the order of 10 ns. Experiments conducted with flat aluminum electrodes demonstrate that the flashover strength of a crosslinked polystyrene (Rexolite) insulator is $(18\ifmmode\pm\else\textpm\fi{}7)%$ higher than that of PMMA. Experiments conducted with a Rexolite insulator and an anode plug, i.e., an extension of the anode into the insulator, demonstrate that a plug can increase the flashover strength by an additional $(44\ifmmode\pm\else\textpm\fi{}11)%$. The results are consistent with the Anderson model of anode-initiated flashover, and confirm previous measurements. It appears that a Rexolite insulator with an anode plug can, in principle, increase the peak electromagnetic power that can be transmitted across a vacuum interface by a factor of $[(1.18)(1.44){]}^{2}=2.9$ over that which can be achieved with the standard design.
