Plasma lens focusing and plasma channel transport for heavy ion fusion

Abstract

The capabilities of adiabatic, current-carrying plasma lenses for the final focus problem in heavy-ion-beam-driven inertial confinement fusion are explored and compared with the performance of non-adiabatic plasma lenses, and with that of conventional quadrupole lenses. A final focus system for a fusion reactor is proposed, consisting of a conventional quadrupole lens to prefocus the driver beams to the entrance aperture of the adiabatic lens, the plasma lens itself, and a high current discharge channel inside the chamber to transport the focused beam to the fusion pellet. Two experiments are described that address the issues of adiabatic focusing, and of transport channel generation and stability for ion beam transport. The test of the adiabatic focusing principle shows a 26-fold current density increase of a 1.5 MeV potassium ion beam during operation of the lens. The lens consist of a discharge of length 300 mm, filled with helium gas at a pressure of 1 Torr and is pulsed with a current between 5 and 15 kA. The investigations of discharge channels for ion beam transport show that preionization of the discharge channels with a UV laser can be an efficient way to direct and stabilize the discharge.