Ultrafast Laser Laboratory

High Average Current Photoinjector

In recent years, there has been considerable interest in the generation of high brightness, high average current electron beams for high luminosity colliders and next generation light sources. High brightness electron beams with ~100 mA and at ~10 MHz PRF are needed to cool the RHIC ion beams. Such a high average current, but at a higher ~1 GHz frequency, are much sought for the Free Electron Laser (FEL) applications. Several issues such as the photocathode-laser system, high average power required to drive the RF cavity, and thermal management of the cavity in the presence of the high RF power need to be resolved for the realization of this injector. The research at the Instrumentation division is directed towards the first issue, namely photocathode-laser systems.

High average current, all niobium superconducting RF injector (SCRF):

One of the approaches to minimize RF power dissipated in the cavity and hence reduce the power driving the injector is to use a superconducting cavity. In this approach, the choice of the photocathode is limited to either Nb with low electron yield that forms the cavity or a high yield, short life-time material incorporated into the Nb cavity. Both of these schemes suffer significant drawbacks. The former results in low average current while the latter suffers from short life-time and compromised superconductivity. Recent advances in the surface preparation of Nb have led to significant increase in its electron yield. Laser treatment of the Nb surface has resulted in almost 100 fold increase in the electron yield. High RRR niobium samples subjected to buffered chemical polishing (BCP), electropolishing (EP), mechanical polishing (MP) all exhibited this enhancement. Using 1 Watt of laser power at the wavelength of 266 nm, it is now possible to generate >10 microampere average current from an electropolished Nb. A SCRF injector has been designed and built as a part of a SBIR/CRADA collaboration with Advanced Energy Systems. The electron beam will be generated at 81.25 MHz and characterized shortly.

High average current normal conducting RF injector:

The large work function of Nb necessitates the use of UV photons for photoemission, limiting the maximum current deliverable with commercial laser systems. A robust photocathode with high electron yield for <2 eV photons is required to futher increase the average electron current. Photocathodes that are currently used have either high yield or long life-time, but not both. Research program to improve these parameters has recently been initiated. A multialkali cathode deposition and testing system is shown here.

For more information contact Triveni Rao