The recently observed in‐flight capture of keV antiprotons and protons in a Penning trap requires that the −3‐kV potentials on electrodes of a Penning trap near 4.2 K be switched on and off with switching times less than 20 ns. These rapidly switched potentials are applied via transmission lines which are not terminated at the trap, thereby avoiding unacceptable heat load on the helium Dewar. Simple high‐voltage switching circuits are constructed using krytrons and reed relays. A krytron provides the rapid switching and stays on just long enough for a reed relay to kick in and maintain the switched state indefinitely.