Experimental Generation and Characterization of Uniformly Filled Ellipsoidal Electron-Beam Distributions

Abstract

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 (approximately 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.

Longitudinal phase space characterization of the blow-out regime of rf photoinjector operation

“…7, we observe the onset of beam quality degradation due to asymmetry discussed in Ref. [10]. When the surface charge density approaches 10% of the rf accelerating field, the space charge induced expansion becomes asymmetric due to image charge effects.…”

“…In order to generate a uniformly filled ellipsoidal distribution at the UCLA Pegasus laboratory we illuminate the cathode of a 1.6 cell SLAC/UCLA/BNL rf gun with a very short ( < 50 fs rms) UV (266 nm) laser pulse. We refer to our previous publication on the blow-out regime operation of a photoinjector [10] for the details of the beam distribution characterization. The gun is operated for this measurement at a slightly lower accelerating gradient (75 MV=m) due to the lower rf power output from an aging klystron.…”

“…By phasing the rf in the cavity so that the time centroid of the beam experiences no kick (zero phase), the longitudinal coordinate is thereby linearly mapped to the vertical transverse dimension on a fluorescent screen located downstream. In our previous paper [10], the deflector was used to retrieve the t À x ellipsoidal beam distribution. Exploiting the two dimensional nature of the images obtained with the rf deflector, the horizontal coordinate can be used to encode any information whose variation along the bunch temporal coordinate is under study.…”

“…Recently at the UCLA Pegasus photoinjector laboratory, we have experimentally demonstrated the blow-out regime of rf photoinjector operation. In this scheme, proposed a few years ago [8,9], a very short ( < 100 fs) laser pulse illuminates the cathode and the beam expands under its own self-forces from its initial pancake-like shape to create a nearly ideal uniformly filled ellipsoidal distribution [10]. The main advantage of such a final state is that the beam self-fields are approximately linear in each of the coordinate offsets, resulting in extremely linear beam dynamics and phase spaces [11].…”

Conceptual and Technical Design Aspects of Accelerators for External Injection in LWFA

“…• Illuminate the cathode with short laser pulse (duration < 100 fs), called pancake-like distribution, and use the longitudinal space charge driven expansion to create a nearly ideal uniformly filled ellipsoidal distribution [39,50,51]; • Illuminate the cathode with long (≈few ps) and skinny (<100 µm spot size) laser pulse, also called cigar-like, and use the transverse space charge to drive the beam expansion [52].…”