Electron-orbit Control Using a Postdiode Magnetic-Field Structure

For many applications, control and manipulation of the electron orbits in a high-current electron beam is desirable. This is especially true when a weakly-self-pinched, multi-MV electron-beam is used to make bremsstrahlung radiation. In this case, the radiation pattern is highly peaked along the direction that the electron beam makes when it strikes the x-ray target. Therefore, to maximize the number of photons in the forward direction, it is desirable that the electrons strike the x-ray target as close to normal with as little spread in the beam angles as possible. In this paper, a method for controlling the macroscopic angle of a high-power electron beam using a post-diode magnetic-field structure is presented. The idea is to extract the electron beam into a vacuum cavity through a thin, low-mass foil where a portion of the return-current flows through a central post. The amount of current that flows through the central post and therefore the amount of beam straightening is controlled by inductively splitting the return current so that a portion of it returns through the central post and a portion returns outside the beam. By adjusting the balance between these two currents one can alter the electron orbits and achieve a wide range of angles that the electron beam makes with the target without the need for plasma or an external pulser. 1 Particle-in-cell simulations have been performed to determine the parameters required to straighten an 8-MV, 200-kA, 23-cm-diameter hollow electron beam with an inward 20° macroscopic (average) angle so that it approaches the x-ray target at normal incidence. The simulations show an increase in the forward photon spectrum by up to a factor of 3. Experiments with similar beam parameters using the Mercury Inductive-Voltage Adder at the Naval Research Laboratory have shown an increase of a factor of two in the forward dose using this technique and are in good qualitative agreement with the simulations. Additional simulations and experiments are planned to optimize the forward dose and will be reported on during this talk.