Abstract
Since the recent publication of a practical recipe to create ``pancake'' electron bunches which evolve into uniformly filled ellipsoids, a number of papers have addressed both an alternative method to create such ellipsoids as well as their behavior in realistic fields. So far, the focus has been on the possibilities to preserve the initial ``thermal'' transverse emittance. This paper addresses the linear longitudinal phase space of ellipsoidal bunches. It is shown that ellipsoidal bunches allow ballistic compression at subrelativistic energies, without the detrimental effects of nonlinear space-charge forces. This in turn eliminates the need for the large correlated energy spread normally required for longitudinal compression of relativistic particle beams, while simultaneously avoiding all problems related to magnetic compression. Furthermore, the linear space-charge forces of ellipsoidal bunches can be used to reduce the remaining energy spread even further, by carefully choosing the beam transverse size, in a process that is essentially the time-reversed process of the creation of an ellipsoid at the cathode. The feasibility of compression of ellipsoidal bunches is illustrated with a relatively simple setup, consisting of a half-cell S-band photogun and a two-cell booster compressor. Detailed GPT simulations in realistic fields predict that 100 pC ellipsoidal bunches can be ballistically compressed to 100 fs, at a transverse emittance of $0.7\text{ }\ensuremath{\mu}\mathrm{m}$, with a final energy of 3.7 MeV and an energy spread of only 50 keV.
Highlights
The most important process limiting the quality of pulsed, high-brightness electron beams is the unrecoverable distortion of the six-dimensional (6D) phase-space distribution due to nonlinear space-charge forces
In 2004 Luiten et al showed that waterbag bunches can be created in practice because any temporal laser profile is acceptable as long as the laser pulse is sufficiently short, and that the ideal initial radial profile is a half circle [4]
In this paper we focus on the longitudinal phase-space behavior of waterbag bunches in realistic fields, in particular, the realization of high currents combined with lowenergy spread
Summary
The most important process limiting the quality of pulsed, high-brightness electron beams is the unrecoverable distortion of the six-dimensional (6D) phase-space distribution due to nonlinear space-charge forces. Contrary to all other charge density distributions, waterbag bunches do not require fast acceleration to relativistic speeds in order to prevent degradation of bunch quality due to space-charge forces This allows longitudinal ballistic compression at sub or mildly relativistic energies, which is much more efficient because far less energy spread is required compared to the high-energy alternatives used in present day systems [9]. Another research area that could greatly benefit from the availability of mildly relativistic ultrashort high-brightness bunches is laser-wakefield acceleration with external injection [12,13]: As long as the energy of the electron bunch is sufficiently high to be trapped by the plasma-wakefield, additional injection energy will only marginally increase the dephasing length Apart from these existing applications, new applications appear at the horizon which require mildly relativistic, or even nonrelativistic energies.
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