The plasma howitzer concept

"(Casaba Howitzer)...allegedly operates at 10
percent efficiency up to about 1 megaton, although with only about .01 radian
beam directivity. Speeds of 1,000 kilometers per second are inevitably
accompanied by ionization, and because charged particles curve in the earth's magnetic field, they wouldnot be useful for long-range applications. Velocities up to 200 kilometers per second,however, are believed possible without vaporization."