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Blow-out regime of RF photoinjectors

A.K.A. Dynamically Self-Optimized Beam Distributions

For 40 years, uniformly filled ellipsoidal beam distributions have been studied theoretically, as they hold the promise of generating self-fields linear in the coordinate offset in all three directions. Recently, a scheme for producing such distributions, based on the strong longitudinal expansion of an initially very short beam under its own space-charge forces, has been proposed.

Luiten et al. [PRL, 2004], reconsidering an idea proposed by Serafini, have shown that to obtain a final ellipsoidal distribution there is essentially no requirement on the initial laser pulse shape other than it be much shorter than the final bunch length after expansion. On the other hand, formation of the ellipsoidal beam distribution is sensitive to the radial laser profile, with the ideal proposed profile having radial dependence of half-circle distribution. This mode of operation, where the optimized distribution is obtained by dynamical evolution from an initial ultrashort beam, has been termed the blowout regime of photoinjector operation, to emphasize the strong longitudinal space-charge expansion.

At the Pegasus laboratory we verified for the first time experimentally the existence of this regime. direct measurement of the beam temporal profile and spatiotemporal distribution, obtained with the aid of an X-band deflecting cavity Increasing the surface charge density at the cathode to a level where the longitudinal space-charge field becomes comparable with the rf accelerating field, we have also observed the ellipsoidal beam shape evolve into an asymmetric distribution having an elongated tail. The development of this beam tail is predicted well by particle dynamics simulations; it is due to the decelerating effect of the beam’s image charge at the cathode. The transverse quality of the beam has been measured and found—as expected—strongly correlated to the linearity of the space-charge fields and thus to the symmetry of the ellipsoidal beam distribution.