Linear accelerators have been employed in space.  While "voltage" is perhaps not the right term, precisely, these do generate very high energy beams.  

There are current proposals to [develop a 1MeV linac](https://www.frontiersin.org/articles/10.3389/fspas.2019.00035/full) for experiments in space.  Early experiments in the 1970s used standard DC high voltage supplies to generate beams of up to 40keV, injecting pulses into the ionosphere using sounding rockets.

[Spacelab-1](https://link.springer.com/chapter/10.1007/978-94-009-8440-0_31) also had a 7.5keV accelerator on board for a series of particle accelerator experiments. 

There was also the [BEAM experiment](https://ui.adsabs.harvard.edu/abs/1990liac.conf...10O/abstract) in 1989 that produced a 1MeV neutral hydrogen beam.  This had a 30keV injector to the RF accelerator - I'd expect this probably used a standard DC accelerator grid at a similar voltage but I'm not certain.