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GaAs-based DC high voltage photo-guns used at accelerators with extensive user programs must exhibit long photocathode operating lifetime. Achieving this goal represents a significant challenge for proposed high average current facilities that must operate at tens of milliamperes or more. Specifically, the operating lifetime is dominated by ion back-bombardment of the photocathode from ionized residual gas. While numerous experiments have been performed to characterize the operating lifetime under various conditions, detailed simulations of the ion back-bombardment mechanism that explains these experiments are lacking. Recently, a new user routine was implemented using the code General Particle Tracer (GPT) to simulate electron impact ionization of residual beam line gas and simultaneously track the incident electron, the ejected electron, and the newly formed ion. This new routine was benchmarked against analytical calculations and then applied to experiments performed at the CEBAF injector at the Thomas Jefferson National Accelerator Facility to study the effectiveness of limiting ions from entering the cathode-anode gap using a positively biased anode. These simulations were performed using detailed 3D field maps produced with CST Microwave Studio describing the photo-gun electrostatics. Discussion of the experiment and the application of this new GPT routine to model the experiments will be presented at the workshop.

Extending the charge lifetime of today’s spin polarized GaAs photoelectron guns from tens to thousands of Coulombs is a requirement for long-duration operation at milliampere beam current. There are two main approaches frequently considered to achieve this goal. The first is improving photo-gun vacuum, which provides a direct means to minimize the ill effects of ion bombardment and associated QE decay. In this contribution, we demonstrate the second: enhancing the charge lifetime at milliampere beam current by using large laser spots which serves to distribute ion damage over a larger region of the photocathode. The efficacy of this approach was presented at past workshops, but using bulk GaAs and green laser light without RF structure. In this contribution, highly polarized beam was produced from strained-superlattice GaAs/GaAsP photocathodes inside the 130 kV photo-gun at the CEBAF photoinjector, at beam currents from 0.5 to 1.5 mA and with a clear indication of lifetime enhancement using laser beams as large as ~8 mm$^2$. However, when the laser beam size was increased beyond this diameter, further lifetime enhancement was not observed which highlights the importance of proper cathode/anode design to maintain loss-free beam delivery and the preservation of static vacuum conditions. In addition, this contribution presents electron beam polarization measurements at currents to 1 mA.

The ion beams of the proposed Jefferson Lab Electron Ion Collider (JLEIC) must be cooled to achieve the required collision luminosity. In general, cooling is accomplished when an electron beam co-propagates with an ion beam moving at the same average velocity, but with different temperature, where the energy of chaotic motion of the ion beam is transferred to the cold electron beam. The cooling rate can be improved by about two orders of magnitude if the process occurs inside a solenoidal magnetic field – so-called magnetized cooling - that forces the electrons to follow small helical trajectories thereby increasing the interaction time with ions and improving the cooling efficiency. However, one of the challenges associated with implementing this cooling technique relates to the fringe field of the cooling solenoid which imparts a large unwanted azimuthal kick onto the electron beam that prevents the electron beam from traveling in the desired tight, well-defined volume within the solenoid. As proposed by Derbenev, the ill-effect of this fringe field can be cancelled if the electron beam is born in a similar field and encountering a fringe field upon exiting the electron gun that produces an azimuthal kick in the opposite direction, such that the two kicks cancel. Besides requiring magnetized beam, the JLEIC re-circulator cooler design requires an electron beam with very high average current and high bunch charge: 140 mA and with nanoCoulomb bunch charge. This contribution describes the latest milestones of a multiyear program to build a magnetized electron beam source based on a 350 kV DC high voltage photogun with inverted insulator geometry.