Emittance Compensation Studies of Photoinjector Beams with Angular Momentum*

Beam dynamics studies on the FNPL photoinjector that seek to optimize the transport of intense electron beams with large values of canonical angular momentum have been performed. These studies investigate the effect of solenoid emittance compensation on beams that evolve under the combined influence of intense space charge forces and large angular momentum. We present details of experimental measurements and supporting simulations of beam envelope evolution. INTRODUCTION Flat beams generated in rf photoinjectors have been proposed for the generation of short-pulse hard x-rays [1] based on earlier studies for linear collider applications [2]. The flat beams are generated by a technique that uses a magnetized cathode to create strong coupling between the horizontal and vertical phase spaces. The coupling is removed downstream by a skew quadrupole lattice which produces a beam with large ratio between the vertical and horizontal emittances. Emittance compensation techniques [3][4] are now widely used in the design and operation of photoinjectors to minimize the normalized emittance of the beam as it is transported from the injector. Here, we study the modifications of the technique for beams that carry large amounts of angular momenta . THEORETICAL DEVELOPMENT Emittance In an azimuthally symmetric system, the normalized 4D transverse emittance can be expressed as [5] ε4D = 1 4 γβ ( ) r ′ r 2 + r ′ θ ( ) − r ⋅ ′ r 2 − r ′ θ 2 [ ] (1) where ′ r = dr /dz and ′ θ = dθ /dz . This emittance measures the volume of the full 4-D transverse phase space. Photoinjector beams can be classified as rigid rotators such that θ is constant over all radii. In this case, the normalized radial emittance εnr = 1 2 γβ ( ) r ′ r 2 − r ⋅ ′ r 2 [ ] (2) provides the full measure of the beam phase space. Beam Envelope Evolution The envelope equation of the RMS radial beam spot size (σr = r 2 ) can be expressed as [5] *This work was supported by the Department of Energy under Contract No. DE–AC03–76SF00098. ′ ′ σ r + ′ σ r ′ γ γβ 2 