Three-dimensional rendered drawing (rear view) of ORION 1.6 cell RF gun assembly, with downstream solenoid and associated supports.
Three-dimensional rendered drawing (rear view) of ORION 1.6 cell RF gun assembly, with downstream solenoid and associated supports.
UCLA PBPL and collaborating laboratories must employ solenoids when cylindrically-symmetric focusing is needed. The most prominent application is for the so-termed emittance-compensation process, where the solenoid is used for control transverse beam size and associated beam-plasma oscillations after the RF gun component of the photoinjector. The ORION gun and solenoid, both manufactured by PBPL as a component of the ORION collaboration, are shown in a rendering to the right. The ORION solenoid, as built, is also shown in a photograph below.
A new version of this somewhat bulky and inflexible device has been designed by UCLA for the SPARC rf photoinjector. This design as shown in the POISSON output of this assembly can reproduce the field flatness of the now-standard BNL/ORION design which is important for benchmarking experimental results from SPARC against similar facilities — the beam dynamics for SPARC are nominally nearly identical to the LCLS photoinjector, which is designed to be the highest brightness electron source to date.
The most compelling feature of this new design is in fact the adjustability of the longitudinal position of the lens center; however, in which case we introduce unequal currents in each coil, linearly ramping them up or down in z. This scheme was originally introduced in the Neptune photoinjector using two iron-separated coils. In the new design, one may produce a shift of over 5 cm in the center of the lens using only asymmetry in the coil currents. This adjustability will allow the fine tuning of the emittance compensation process. The PBPL has also deployed solenoids for matching of beams into plasma wakefield acceleration experiments at ANL and FNAL, where one often desires the beam density to exceed that of the plasma and one must match the beam focusing to beta functions in the plasma channel of less than 1 cm. The spot sizes produced with solenoids in these experiments are typically smaller than those achieved with quads by a factor of two.