Heavy Ion Fusion Sources*

In Heavy-Fusion and in other applications, there is a need for high brightness sources with both high current and low emittance. The traditional design with a single monolithic source, while very successful, has significant constraints on it when going to higher currents. With the Child-Langmuir current-density limit, geometric aberration limits, and voltage breakdown limits, the area of the source becomes a high power of the current, A~I^8/3. We are examining a multi-beamlet source, avoiding the constraints by having many beamlets each with low current and small area. The beamlets are created and initially accelerated separately and then merged to form a single beam. This design offers a number of potential advantages over a monolithic source, such as a smaller transverse footprint, more control over the shaping and aiming of the beam, and more flexibility in the choice of ion sources. A potential drawback, however, is the emittance that results from the merging of the beamlets. We have designed injectors using simulation that have acceptably low emittance and are beginning to examine them experimentally. MULTIBEAMLET INJECTOR A requirement of heavy ion fusion (HIF) is a source the produces a beam with high brightness --having both high current and low emittance. Traditionally in the HIF program, the sources that have been used are monolithic, solid, hot plate sources. While these have performed quite successfully over the years, they do have limitations. They have poor scaling when going to higher currents and have limited lifetimes before the ions are depleted. Going to multiple beamlets circumvents the scaling problem at high current and allows use of a plasma source which does not have the problem of ion depletion. The multibeamlet injector concept has been extensively studied, with a focus on understanding and minimizing the emittance growth[1]. As part of that work, a procedure for designing a multibeamlet injector was laid out and several examples given. In this paper, that work is extended, using an improved layout of the beamlets and further examining some details. HIGH CURRENT SCALING When going to higher currents, the single beam injectors do not scale well. Taking into consideration the space-charge limited current density given by the ChildLangmuir relation, voltage breakdown limits, and limits on the geometry to minimize aberrations, the source radius varies as a high power of the current and the current density varies inversely with the current[1]. The poor scaling can be circumvented by using multiple beamlets --each beamlet has a low current and avoids the poor scaling. Fixing the total area of the source (the sum of the area of the beamlets), the current density becomes proportional to the total current. The inherent emittance of an injected beam from the temperature of the emitter varies as the square root of the product of the temperature and the beam area. Since the total area of the beamlet source can be much smaller than the area of a single source, the temperature of the emitter can be higher for a multibeamlet injector. This allows use of plasma type sources, which have higher operating temperatures than solid sources.