Principles of non-Liouvillean pulse compression by photoionization for heavy-ion fusion drivers

Abstract

Photoionization of single-charged heavy ions has been proposed recently by Rubbia (1989) as a non-Liouvillean injection scheme from the linac into the storage rings of a driver accelerator for inertial confinement fusion. The main idea of this scheme is the accumulation of high currents of heavy ions without the usually inevitable increase of phase space. Here we suggest the use of the photoionization idea in an alternative scheme: if it is applied at the final stage of pulse compression (replacing the conventional bunch compression by an rf voltage, which always increases the momentum spread), there is a significant advantage in the performance of the accelerator. We show, in particular, that this new compression scheme can potentially relax the tough stability limitations, which were identified in the heavy-ion fusion reactor study HIBALL (Badger et al. 1984). Moreover, it is promising for achieving higher beam power, which is suitable for indirectly driven fusion targets (1016 W/g, in contrast with 1014 W/g for the directly driven targets in HIBALL).The idea of non-Liouvillean bunch compression is to stack a large number of bunches (typically 50–100) in the same phase-space volume during a change of charge state of the ion. A particular feature of this scheme with regard to beam dynamics is its transient nature, since the time required is one revolution per bunch. After the stacking the intense bunch is ejected and directly guided to the target. The present study is a first step in exploring the possibly limiting effect of space charge under the parameter conditions of a full-size driver accelerator. Preliminary results indicate that there is a limit to the effective stacking number (non-Liouvillean “compression factor”), which is, however, not prohibitive. Requirements on the power of the photon beam from a free-electron laser are also discussed. It is seen that resonant cross sections of the order of 10−15 cm2 lead to photon beam powers of a few megawatts.