We present measurements of enhanced quantum efficiency (QE) in thin film photocathodes due to optical interference in the cathode-substrate multilayer. Modulations in the quantum efficiency of Cs$_{3}$Sb films grown on multilayer 3C-SiC substrates are observed over a range of visible wavelengths, and are shown to increase the QE by more than a factor of two at certain wavelengths. We derive a model to describe the modulations in QE based on a three step photoemission process incorporating cases of both constant density of states and density functional theory (DFT) derived density of states, which is shown to be in good agreement with the measurements. Predictions from the model show that the QE can be enhanced by more than a factor of four by optimizing the cathode and substrate layer thicknesses. We also find that by optimizing layer thicknesses of the cathode-substrate system in the calculation, optical interference can enhance the QE beyond optically dense films. Advantages of this interference effect for electron accelerator sources are discussed.

We present measurements of quantum efficiency (QE) modulations in CsSb and Cs3Sb photocathodes that arise from optical interference of reflections from the underlying substrate that has multiple semi-transparent layers. The photocathode films are grown on a cubic silicon carbide layer (3C-SiC) which itself is grown epitaxially on Si(100) during fabrication. We find that the QE modulates by up to a factor of two over a laser wavelength range of 30 nm, and that a modulation peak can be tuned to coincide with a desired laser wavelength by changing the thicknesses of both the photocathode and the silicon carbide layer in the substrate. A model for the QE modulations is derived and fitted to QE measurements of CsSb and Cs3Sb films, which have different indices of refraction, in addition to QE measurements of Cs3Sb films grown on 3C-SiC substrates with two different silicon carbide layer thicknesses. Good agreement is found between the model and measurements, confirming the optical interference effect can be exploited to enhance quantum efficiency at desired visible wavelengths.

The Strong Hadron Cooling (SHC) for electron ion collider project requires a state-of-the-art high brightness DC electron gun. The gun is required to deliver high average current (98.5 mA), 1 mm∙mrad normalized transverse emittance and 1-2.5 nC bunch charge. In this paper, we describe the high voltage design of a DC gun with an operating voltage of 550 KV, to be conditioned up to 600 KV. Unique features of this gun includes the use of inverted ceramic in this level of voltage, active cooling for the cathode and use of large/single crystal multialkali cathode grown on Silicon-Carbide substrate. A test beam line for high current test is also described.

Alkali-based semiconductor photocathodes are widely used as electron sources and photon detectors. The prop-erties of alkali-based semiconductor materials such as crystallinity and surface roughness fundamentally de-termine the performance merits like quantum efficiency and thermal emittance. In BNL, pulsed laser deposition (PLD) was utilized to assist the growth of alkali-based photocathode materials, providing precise control of material growth and improving film quality. In the pre-sented work, films prepared with thermal and PLD sources are compared. The film quality of K2CsSb, Cs3Sb and Cs2Te grown with PLD assisted technique are reported.

Properties such as high quantum efficiency (QE), low thermal emittance, and longevity are crucial features for the rapidly developing electron accelorators. Compared to the traditional sequential deposition, the co-evaporation method is reported to yield better surface roughness, film crystallinity, and high quantum efficiency for photocath-ode materials. Here we present the effort in upgrading the coherent electron cooling (CeC) photocathode deposition system to adapt the co-evaporation growth method, the development of the co-evaporation recipe, and the prepa-ration of K-Cs-Sb photocathode using the developed system. QE of about 6.3% at wavelength 532 nm was obtained for co-deposited K2CsSb photocathode, where stoichiometry was determined by the deposition rate of each element. The system upgrade also enables the prepa-ration of GaAs photocathodes activating with Cs-Te. In our study, both CsTe and CsTe/CsO activated GaAs are prepared using the “yo-yo” method. QE of about 3.6% and 5% at wavelength 532 nm are obtained respectively. Lifetime measurements are performed and results are reported.

Photocathodes play an integral role in the development of electron accelerators and photon detectors. The emitted beam brightness can be limited by the surface and bulk disorder of the polycrystalline photocathode material. Epitaxial growth of photocathodes has the potential to overcome this problem and achieve high brightness electron beam. This work demonstrates the epitaxial growth of K2CsSb photocathode on varied single crystal substrates. In our study, streaky pattern aligned with latticed matched substrates from reflection high energy electron diffraction (RHEED) were observed from the K2CsSb thin film. Further, azimuthal angular dependence of the crystalline structure of the K2CsSb thin film was also observed for RHEED, which confirms the growth of the epitaxial layer with flat surface and high crystallinity. We obtained quantum efficiency (QE) of about 4.5 % at wavelength 530 nm light from the ~ 20 nm film with a roughness of ~ 0.8 nm. The stoichiometry and crystallinity of the K2CsSb thin films are confirmed by X-ray diffraction (XRD) and X-ray fluorescence (XRF). High QE over 9 % at 530 nm has been achieved for epitaxial K2CsSb photocathode thin films.