Generation of Ellipsoidal Beam through 3d Pulse Shaping for a Photoinjector Drive Laser*

In this paper we present a 3D laser pulse shaping scheme that can be applied for generating ellipsoidal electron bunches from a photoinjector. The 3D shaping is realized through laser phase tailoring in combination with chromatic aberration in a focusing optics. Performance of an electron beam generated from such shaped laser pulses is compared with that of a uniforma ellipsoidal, a uniform cylindrical, and a Gaussian electron beam. PARMELA simulation shows advantage of this shaped beam in both transverse and longitudinal performances.. INTRODUCTION The emittance of an electron beam is governed by the emittance at its birth and the growth during its propagation. If the beam is only subjected to linear force, the latter can be fully recovered with proper beam compensation. It is well known that an ellipsoidal beam with uniform charge distribution has a linear space-charge force [1-3] and hence the most expected distribution for modern high-brightness beams. Recently, several researchers looked at practical ways of generating such ellipsoidal beams, including self evolving [4], cold electron harvesting [5], and laser pulse manipulations including spectral masking, pulse stacking, and dynamic spatial filtering [6]. In-depth analysis shows that in practical situations, the ellipsoidal beams do generate beam with lower emittance than Gaussian and cylindrical beams [1, 6-8]. Applications for such high-brightness beam include next-generation light sources such as the Linac Coherent Light Source (LCLS), high-energy colliders such as the International Linear Collider (ILC), as well as energy recovery linacs (ERLs). LASER PULSE SHAPING To generate an ellipsoidal beam directly from the photocathode, the laser pulse has to be shaped in 3D. It is well known that the longitudinal laser pulse shape can be manipulated by controlling the phase space using techniques such as DAZZLER [9] or SLIM [10]. One essence of this phase modulation is to control the phase and amplitude at certain frequencies at the same time so that the pulse can have a particular phase and amplitude. In the meantime, we notice that the instant frequency of a laser pulse is related to the phase by ω(t)=dφ(t)/dt. This gives a way of actively controlling the focal size of the laser as a function of time using the chromatic aberration of a common lens, of which the focal length can be expressed as [11] [ ] ⎟ ⎟