Ion Beam Properties after Transport through an Extended Plasma Channel

Beam transport in plasma channels is a transport mechanism especially suited for the transport of high current beams or beams of poor quality (e.g. large momentum spread). It is currently studied as an alternative to ballistic focusing for final transport in ion beam driven inertial fusion reactors. Plasma channels are created by a discharge in a background gas. The channels can be initiated either by laser heating of the gas along the axis or by sending an ion beam pulse through the gas-filled vessel before the discharge, which ionizes the gas on the axis. The initial GSI experiments used a discharge chamber with a length of 50 cm. In 2003, the experimental setup has been refitted to accommodate a longer chamber to be able to produce longer plasma channels. A 50 cm extension tube was inserted. Within this extended chamber, 1 m long channels could be created. However, it became evident that the former design of having the chamber walls on one half of the cathode potential had to be improved, because the electric field strength on the axis in the longer chamber is very low with just one wall potential. Calculations showed that different potentials on the three wall parts increase the field strength [1]. Insulator flanges of 3 cm thickness and an array of resistors were included in order to set the three parts of the new chamber separately to different percentages of the full cathode potential. The resistors are used as potential dividers, so that the total voltage is divided evenly between the wall parts, see fig. 1.