  Highly energetic ultrashort electron bunches have the potential to reveal the
ultrafast structural dynamics in relatively thicker in-liquid samples. However,
direct current (DC) voltages higher than 100 kV are exponentially difficult to
attain as surface and vacuum breakdown become an important problem as the
electric field increases. One of the most demanding components in the design of
a high-energy electrostatic ultrafast electron source is the high voltage
feedthrough (HVFT), which must keep the electron gun from discharging against
ground. Electrical discharges can cause irreversible component damage while
voltage instabilities render the instrument inoperative. We report the design,
manufacturing, and conditioning process for a new HVFT that utilizes ultra-high
molecular weight polyethylene (UHMWPE) as the insulating material. Our HVFT is
highly customizable, inexpensive, and has proven to be effective in high
voltage applications. After a couple of weeks of gas and voltage conditioning,
we achieved a maximum voltage of 180 kV with a progressively improved vacuum
level of 1.8 x $10^{-8}$ Torr.
