Beam Dynamics in Heavy Ion Fusion

for current multiplication, and it is there that the main challenges of beam dynamics are found. It is being studied in Europe, Russia, and Japan, and is discussed in ref. [1]. A standard design for heavy ion fusion drivers under study in the US. is an induction linac with electrostatic focusing at low energy and magnetic focusing at higher energy. The need to focus the intense beam to a fewmillimeter size spot at the deuterium-tritium target establishes the emittance budget for the accelerator. Economic and technological considerations favor a larger number of beams in the low-energy, electrostatic-focusing section than in the high-energy, magnetic-focusing section. Combining four beams into a single focusing channel is a viable option, depending on the growth in emittance due to the combining process. Several significant beam dynamics issues that are, or have been, under active study are discussed: large space charge and image forces, beam wall clearances, halos, alignment, longitudinal instability, and bunch length control. . II. TARGET CONSTRAINTS ON THE DRIVER BEAM We consider here indirect-drive targets[2], which have a frozen deuterium-tritium fuel shell inside a radiation enclosure, or hohlraum. The beam energy is deposited in converter material, and secondary, soft x-radiation propagates through the hohlraum, uniformly irradiating and imploding the fuel. Direct-drive targets are heated by the driver beams. In the latter situation, the illumination uniformity on the capsule is tightly coupled to the geometry of the incoming ion beams, and is generally considered to be a less conservative target design. Target design studies show that the driver must deposit ~400 TW for ~10 ns with a ~20 ns prepulse of <100 TW in order to achieve an energy gain of 10-100, or sufficient to make the economics work out favorably for commercial energy production. I. HEAVY ION FUSION SYSTEM BASED ON INDUCTION LINEAR ACCELERATORS Working backwards from ballistic transport with little or no neutralization leads to ~10 GeV kinetic energy with an ion atomic mass of 200. The target constraints establish an emittance budget in the transverse and longitudinal planes, approximately 6 π-mm-mrad and 1 eV-s, respectively. A standard design for heavy ion fusion drivers under study in the U.S. is sketched in Fig. 1. An ion source and injector supplies 2-3 MeV beams to an electrostatically focused induction linac section (~64 beams). This is followed by a ~16 beam, magnetically focused induction linac section. The 64, later 16, beams are accelerated inside common induction cores. Finally, a compression section shortens the beams to a pulse length appropriate to the constraints of target ignition physics, and the last focusing elements bring the beams to a r=2-3 mm spot size. Common to most variants of this 'standard' design are the assumptions of conservatively designed, conventional focusing systems throughout the driver. Another conservative assumption is ballistic transport of unneutralized beams in the reactor chamber. Because the cost of the induction cores necessary for acceleration to 10 GeV is substantial, there is a premium on compact transverse packing of the parallel beams. Economic and technological considerations favor a larger number of beams in the low-energy, electrostatic-focusing section than in the high-energy, magnetic-focusing section. Combining four beams into a single focusing channel is a viable option, depending on the growth in emittance due to the combining process. (Other driver designs omit beam combining, and some of those use only electric or magnetic focusing, rather than both. ) More exotic final focusing and chamber transport systems that rely on significant charge and current neutralization could make beam quality control easier. They are being investigated mainly for application near the end of the driver as a means for transporting the beams into the reactor chamber and to the target. Because these techniques are not applicable to most of the driver, much of the accelerator design would remain unchanged with a neutralized final focus at the end. However, the constraints on the number of beams, ion kinetic energy, and emittance could be relaxed. Fewer beams would simplify the interface between the driver and the reactor. The techniques, which are beyond the scope of this paper, include plasma lens focusing [3], and electron co-injection using a grid cathode [4]. RF based accelerator technology is the alternative principal heavy-ion driver approach. Storage rings are used * This work was supported by the Director, Office of Energy Research, Office of Fusion Energy, U.S. Dept. of Energy, under Contract No. DE-AC03-76SF00098. 3159 © 1996 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.