Transport Window and Longitudinal Compression Scheme for Beam Bunching
in Final Stage of Heavy Ion Inertial Fusion
Takashi Kikuchi, Mitsuo Nakajima, Kazuhiko Horioka
Department of Energy Sciences, Tokyo Institute of Technology
tkikuchi
es.titech.ac.jp
Heavy ion inertial fusion (HIF) concepts require beam compression from long pulse (a few hundred ns) to short pulse (10 ns) at the final stage of the driver system.[1]
An induction voltage modulator is recommended for the final beam bunching in HIF. The bunching beam has a considerable velocity tilt Db/b between the head and the tail of the beam bunch.
[2] From this reason, the transverse mismatch of the beam especially through the bend region may cause the loss of beam particles.
At the first stage of design works for the final beam transport, the initial phase advance
s is decided to avoid resonance lines on the tune diagram. However, as the phase advance changes by
Db/b during the beam bunching, the beam reenters in the resonance condition. The transverse mismatch induced by those effects may cause a chaotic behavior, a halo formation and an emittance growth, during the compression process. We estimate the dynamics and transport window of the bunching beams by a Particle-Core Model (PCM). Figure 1 shows a Poincaré plot of the test particles in the typical unstable case. Influences of the resonance reentrance and the envelope mismatch due to Db/b are clearly shown in Fig.1. The stable transport region is discussed on a s-Db/b diagram from a consideration for avoiding the resonance reentrance. As Fig.2 shows, the transport window can be illustrated in the diagram, as a function of the phase advance and
Db/b.
We also suggested a concept of the induction beam buncher with recirculating configuration and quasi-equilibrium compression scheme.[3] The recirculating beam buncher is composed of a minimum number of linacs and bend regions.
The beam is applied the bunching voltage in the induction modulator and is slightly compressed during the linear drift spaces. The applied Db/b is controlled, so that the tilt becomes very small by the space charge force, in the linear drift space. The space charge force and the longitudinal emittance are generally balanced with the compression force, during the whole compression process. The beam is gradually compressed with quasi-equilibrium condition. Consequently, the effect of transverse mismatch due to
Db/b is expected to suppress at minimum level in this beam buncher.
Details of the proposed beam buncher and transport window of bunching beams will be discussed.
[1] J.J. Barnard, et al., Nucl. Instrum. Methods in Phys. Res. A415, 218 (1998).
[2] E.P. Lee and J.J. Barnard, Proc. of the 2001 Particle Accelerator Conf., Chicago, (IEEE, Piscataway, NJ, 2001), Vol.4, p.2929.
[3] T. Kikuchi, M. Nakajima and K. Horioka, J. Plasma Fusion Res. 78, 1 (2002).
