Experimental generation and characterization of uniformly filled ellipsoidal electron-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. In this Letter we present the experimental demonstration of this scheme, obtained by illuminating the cathode in a rf photogun with an ultrashort laser pulse (� 35 fs rms) with an appropriate transverse profile. The resulting 4 MeV beam spatiotemporal (x; t) distribution is imaged using a rf deflecting cavity with 50 fs resolution. A temporal asymmetry in the ellipsoidal profile, due to image charge effects at the photocathode, is observed at higher charge operation. This distortion is also found to degrade the transverse beam quality. DOI: 10.1103/PhysRevLett.100.244801 PACS numbers: 29.25.Bx, 29.27.Bd, 41.75.Ht, 41.85.Ct State-of-the-art high brightness electron source designs are most commonly based on use of a high gradient rf photoinjector, with a properly longitudinally and radially shaped laser pulse to illuminate the cathode [1]. In these schemes, which rely on the emittance-compensation process [2,3], approximately uniformly filled, temporally flat, cylindrical distributions are usually assumed in order to minimize the space-charge induced emittance growth. Obtaining such laser pulse shapes introduces a high degree of complexity in the photocathode drive laser, which is a key, technologically challenging component of the system. Moreover, the ‘‘beer-can’’ beam distribution, as it is often referred to, presents large nonlinearities in the spacecharge field profiles at the beam longitudinal head and tail. These nonlinearities eventually give rise to irreversible emittance growth—as opposed to the reversible emittance growth dealt with by compensation —leading to the formation of bifurcated phase space distributions, as well as beam spatial halos [4]. The ideal beam distribution, where such irreversible beam dynamics would be entirely and self-consistently avoided, requires that the space-charge fields in each Cartesian dimension have linear dependence on the bunch coordinates. Such a condition is achieved for uniformly filled ellipsoidal distributions [5]. Despite their advantages, these distributions have not been considered a practical possibility until recently. One possibility for obtaining the uniform ellipsoidal density distribution is to similarly shape the laser pulse illuminating the cathode [6]; however, this solution further increases the complexity of the laser system. Another attractive option takes advantage of the ultrafast rearrangement of the beam as it expands longitudinally under the influence of strong space-charge forces. Luiten et al. [7], reconsidering an idea proposed by Serafini [8], 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