Optimum Operation of Split Rf Photoinjectors

We. describe how to achieve minimum transverse emittance in RF photoinjectors by applying theoretical predictions from a fully analytical model of beam dynamics in a split injector. This device consists of a short two cell (full+partial) RF gun followed by a drift and a booster RF linac. Matching the beam in the booster to the invariant envelope, an equilibrium mode of laminar beam flow, is shown to be the basis of emittance correction. Analytical predictions are compared to numerical simulations, finding excellent agreement. We also show how a further improvement of the beam quality can be obtained by matching the beam out of the booster into a Brillouin flow with proper control of the envelope oscillations. If these are actually coherent plasma oscillations in the laminar regime, then the normalized rms transverse emittance can be even further reduced. I. THE INVARIANT ENVELOPE EQUILIBRIUM OF RELATIVISTIC LAMINAR BEAMS The beam dynamics in RF photoinjectors is mainly characterized by an rms beam envelope behavior which is almost insensitive to the initial temperature emittance εnth this is set up by the photoemission processes at the photo-cathode surface but is mostly affected by the equilibrium set up by the space charge outward pressure on the beam and the counteracting combined focusing effects of the ponderomotive RF focusing, the external solenoid and the adiabatic damping caused by strong acceleration in the device. The resulting equilibrium represents a laminar beam flow described by an exact analytical solution of the rms envelope equation ′′ + ′ ′ + − − = σ σ γ γ σ κ ζ σγ ε σ γ Kr s nth ( )