the voltage to grid 22 sensing of voltage of terminal a relative to ground or of the beam current i o may be employed to vary a voltage source in grid voltage bias lead 24 in addition remote control of pulse voltages to grids 19 and 22 may be provided if desired the method of voltage control according to the present invention can be applied to both negative and positive terminal accelerators in the case of a positive terminal accelerator the pulse current i o is produced by a beam of positive ions and the compensating current i cc is produced either by a negative particle source located in the accelerator terminal and causing negatively charged particles to flow from a to b or by another positive ion source located at the high voltage terminal b and causing positive ions to flow from b to a on the other hand if the terminal voltage is negative i o can be produced by a beam of negative ions or electrons and i cc can be produced either by an electron source located at the high voltage terminal b or by a positive ion source located in the accelerator terminal a the relative positioning of the compensating current source is thus important due to its influence on the accelerator terminal a from the above it will be appreciated.