Abstract
With the increasing development of laser electron accelerators, electron energies beyond a GeV have been reached and higher values are expected in the near future. A conventional beam dump based on ionization or radiation loss mechanisms is cumbersome and costly, not to mention the radiological hazards. We revisit the stopping power theory of high-energy charged particles in matter and discuss the associated problem of beam dumping from the point of view of collective deceleration. The collective stopping length in an ionized gas can be several orders of magnitude shorter than that described by the Bethe-Bloch formulas and associated with multiple electromagnetic cascades in solids. At the same time, the tenuous density of the gas makes the radioactivation negligible. Such a compact beam dump without radioactivation works well for short and dense bunches, as they are typically generated from a laser wakefield accelerator. In addition, the nonuniform transverse wakefield can induce microbunching of the electron bunch by betatron oscillation. The microstructure could serve as a prebunched source for coherent radiation or feeding a free electron laser.
Highlights
As accelerators of particles acquire higher energies and fluence, the issue of the radiological safety for the operation of such accelerators is increasingly important
With the increasing development of laser electron accelerators, electron energies beyond a GeV have been reached and higher values are expected in the near future
The beam density needs to be comparable to the plasma density, just as for the collective deceleration discussed in the present paper, but the temporal and/or spatial scales on which these instabilities develop are different
Summary
As accelerators of particles (electrons and ions) acquire higher energies and fluence, the issue of the radiological safety for the operation of such accelerators is increasingly important. We shall find that short and dense bunches of electrons and other particles such as positrons are amenable under appropriate conditions to be stopped over distances many orders of magnitude shorter than in a conventional beam dump with solid matter. The required plasma density is low so that hazardous radioactivation due to individual nuclear collisions can be reduced significantly This compact and safe beam dump becomes more important for particle energies beyond a GeV, a regime in which secondary particles, like muons, can be generated which are heavy and need a longer distance for stopping in condensed matter. In order to make the accelerator and its associated beam dump system compact and safe, we can marshal collective interaction that can far surpass in magnitude over the conventional individual forces, provided that proper conditions are met.
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