Diversified Technologies, Inc. (DTI) designed and built a 600 kVDC, 150 mA Multiplier Power Supply (MPS) for Brookhaven National Laboratory (BNL). DTI's design met the defined MPS specifications and proposed performance values for the High Voltage Power Supply (HVPS). The system is comprised of a Multiplier Assembly, Pressure Vessel, Inverter Assembly, Cooling Manifold / Flow Sensors and Control / Monitoring System. The high voltage is generated by a 50 kV AC inverter feeding a 12-stage Cockcroft-Walton multiplier, regulated by high-speed analog control loops for stability and precision. The bleeder resistors and diode boards are located adjacent to the associated capacitors. Each logical diode is 8 series connected, 12 kV modules. An independent voltage divider stack, consisting of combined resistor and resistor-capacitor network designed for good high frequency response, is located near the voltage multiplier stack. This custom voltage feedback is crucial to accomplishing the high precision specifications of 1% accuracy 0.16% droop at full voltage and full load. All the high voltage parts are enclosed in the pressure vessel, capable of withstanding up to 90 PSI N2, nominally operating at 75 PSI. The pressure vessel encloses the multiplier stack and the high voltage divider network.

Polarized electrons play an important role in high-energy and nuclear physics, and their properties have also been exploited in ultrafast electron microscopy. Currently, gallium arsenide crystals illuminated by circular polarized infrared laser light are commonly used for generating polarized electrons. However, the achievable accelerating voltage and the gradient of these electrostatic sources limit the beam quality and quantity. A solution could be to combine gallium arsenide photocathodes with radio-frequency electron guns, which are capable of accelerating beams with significantly higher gradients and voltage. Here we report the successful operation of a gallium arsenide photocathode in a superconducting radio-frequency gun. Our findings are relevant for future sources of polarized electrons.

SRF CW accelerator constructed for Coherent electron Cooling (CeC) Proof-of-principle (POP) experiment at Brookhaven National Laboratory has frequently demonstrated record parameters using 1.5 nC 350 ps long electron bunches, typically compressed to FWHM of 30 ps using ballistic compression. We report experimental demonstration of CW electron beam with parameters fully satisfying requirements for hard X-ray FEL and significantly exceeding those demonstrated by APEX LCLS II electron gun. This was achieved using a 10-year-old SRF gun with a modest accelerating gradient of $\sim$15 MV/m, a bunching cavity followed by ballistic compression to generate 100 pC, $\sim$15 ps FWHM electron bunches with a normalized slice emittance of $\sim$0.2 mm-mrad and a normalized projected emittance of $\sim$0.25 mm-mrad. Hence, in this paper, we present an alternative method for generating CW electron beams for hard-X-ray FELs using existing and proven accelerator technology. We present a description of the accelerator system settings, details of projected and slice emittance measurements as well as relevant beam dynamics simulations.

Superlattice GaAs photocathodes are vital for producing polarized electron beams for the Electron-Ion Collider (EIC) at Brookhaven National Laboratory. The electron pre-injector at the EIC requires a 7 nC bunch with at least 85% spin polarization from a GaAs-based superlattice cathode. The doping density of the very surface layer of the cathode needs to be optimized to extract a high bunch charge beam from the high-voltage DC gun. The polarization axis of the emitted beam is longitudinal, and it will be rotated to transverse direction using two Wien filters, each rotating the spin by 45 degrees. In this paper, we report our progress in recent R&D efforts for polarized photocathodes, and spin considerations for the EIC.

The polarized electron source is a critical component in accelerator facilities such as the electron–ion collider, which requires a polarized electron gun with higher voltage and higher bunch charge than existing sources. One challenge we faced was the surface charge limit of the distributed Bragg reflector GaAs/GaAsP superlattice (DBR-SL-GaAs) photocathode. We suppressed this effect by optimizing the surface doping and heat cleaning procedures. We achieved up to 11.6 nC bunch charge of polarized electron beam. In this report, we discuss the performance of tests of a DBR-SL-GaAs photocathode in the high voltage direct current gun. Possible reasons for the observed peak quantum efficiency wavelength shift are analyzed, and we addressed it by using a wavelength tunable laser. In addition, the impact of the DBR layer and laser on the lifetime is investigated in this paper. The optimal DBR-SL-GaAs operating zone has been proposed, which gave us a long lifetime and high polarization at 30 μA operation. The success of this polarized gun will be key to the future of the nuclear sciences.

Photocathodes based on GaAs and other III–V semiconductors are capable of producing highly spin-polarized electron beams. GaAs/GaAsP superlattice photocathodes exhibit high spin polarization; however, the quantum efficiency (QE) is limited to 1% or less. To increase the QE, we fabricated a GaAs/GaAsP superlattice photocathode with a Distributed Bragg Reflector (DBR) underneath. This configuration creates a Fabry–Pérot cavity between the DBR and GaAs surface, which enhances the absorption of incident light and, consequently, the QE. These photocathode structures were grown using molecular beam epitaxy and achieved record quantum efficiencies exceeding 15% and electron spin polarization of about 75% when illuminated with near-bandgap photon energies.

The high intensity polarized electron source is a critical component for future nuclear physics facilities. The Electron Ion Collider (EIC) requires a polarized electron gun with higher voltage and higher bunch charge compared to any existing polarized electron source. At Brookhaven National Laboratory, we have built an inverted high voltage direct current (HVDC) photoemission gun with a large cathode size. We report on the performances of GaAs photocathodes in a high gradient with up to a 16 nC bunch charge. The measurements were performed at a stable operating gap voltage of 300 kV - demonstrating outstanding lifetime, and robustness. We observed obvious lifetime enhancement by biasing the anode. The gun also integrated a cathode cooling system for potential application on high current electron sources. The various novel features implemented and demonstrated in this polarized HVDC gun open the door towards future high intensity-high average current electron accelerator facilities.

High brightness, high charge electron beams are critical for a number of advanced accelerator applications. The initial emittance of the electron beam, which is determined by the mean transverse energy (MTE) and laser spot size, is one of the most important parameters determining the beam quality. The bialkali photocathodes illuminated by a visible laser have the advantages of high quantum efficiency (QE) and low MTE. Furthermore, Superconducting Radio Frequency (SRF) guns can operate in the continuous wave (CW) mode at high accelerating gradients, e.g. with significant reduction of the laser spot size at the photocathode. Combining the bialkali photocathode with the SRF gun enables generation of high charge, high brightness, and possibly high average current electron beams. However, integrating the high QE semiconductor photocathode into the SRF guns has been challenging. In this article, we report on the development of bialkali photocathodes for successful operation in the SRF gun with months-long lifetime while delivering CW beams with nano-coulomb charge per bunch. This achievement opens a new era for high charge, high brightness CW electron beams.

High-current low-emittance continuous wave (CW) electron beams are indispensable for nuclear and high-energy physics fixed target and collider experiments, cooling high energy hadron beams, generating CW beams of monoenergetic X-rays (in FELs) and gamma-rays (in Compton sources). Polarization of electrons in these beams provides extra value by opening a new set of observables and frequently improving the data quality. We report on the up-grade of the unique and fully functional CW SRF 1.25 MeV gun, built as part of the Coherent electron Cooling (CeC) project, which has demonstrated sustained CW operation with CsK 2 Sb photocathodes generating electron bunches with record-low transverse emittances and record-high bunch charge exceeding 10 nC. We will extend the capabilities of this system to high average current of 100 milliampere in two steps: increasing the current 30-fold at each step with the goal to demonstrate reliable long-term operation of the high-current low-emittance CW SRF guns. We also will test polarized GaAs photocathodes in the ultra-high vacuum (UHV) environment of the SRF gun, which has never been successfully demonstrated in RF accelerators.

The design of the Electron Ion Collider (EIC) electron pre-injector to generate a 4 x 7 nC bunch pattern to meet the requirements for injection into the Rapid Cycling Synchrotron (RCS) has been designed. The major challenges are the generation and transport of the high charge polarized electron beam, while achieving small energy spread. The pre-injector design includes the polarized electron source, bunching section, traveling wave plate (TWP) LINAC, zigzag phase space manipulation and a spin rotator. In this proceeding, we discuss the RF frequency selection, and achieving energy spread as low as 0.25% by longitudinal phase space manipulation. We also report the results of beam dynamics simulation. EIC PREINJECTOR INTRODUCTION The EIC pre-injector should produce 85% polarized electron beam with 8 bunches in a repetition frequency of 1 Hz with up to 7 nC of charge. The polarized electron beam will be generated from a high voltage (HV) DC gun with a strained superlattice GaAs(SL-GaAs) photocathode. A prebuncher with ballistic compression will be used to compress the bunch length to 10 ps. The TWP LINAC will use standard 2.856 GHz S-band normal conducting TWP to boost the beam energy up to 400 MeV. Then, a longitudinal matching section will be placed between the LINAC and the spin rotator to assure beam stability in the RCS. The longitudinal matching section includes a zig-zag section for rotating the beam in longitudinal phase space and a de-chirp cavity to minimize the energy spread. The electron beam spin orientation is longitudinal from the cathode. A dipole solenoid spin rotator will be placed before injecting into the RCS [1]. Two Mott polarimeters will be used, one at the photocathode cathode preparation system and another at the gun beam diagnostic beam line. Table 1 shows the beam requirements at the exit of the pre-injector. The photocathode operational lifetime is expected to be on the order of weeks. Figure 1 shows a schematic of the 400 MeV pre-injector. POLARIZED ELECTRON SOURCE The Stanford Linear Collider (SLC) polarized gun achieved up to 16 nC bunch charge in the 1990’s, however, it operated at lower voltage and using a more complex cathode exchange system as desired for the EIC. Instead, we have developed an inverted HV load lock DC gun, based upon the polarized gun operating at TJNAF [2]. We redesigned the ∗ wange@bnl.gov Table 1: EIC Pre-injector Beam Requirements

Continuous-wave photoinjectors operating at high accelerating gradients promise to revolutionize many areas of science and applications. They can establish the basis for a new generation of monochromatic x-ray free electron lasers, high-brightness hadron beams, or a new generation of microchip production. In this Letter we report on the record-performing superconducting rf electron gun with CsK_{2}Sb photocathode. The gun is generating high charge electron bunches (up to 10  nC/bunch) and low transverse emittances, while operating for months with a single photocathode. This achievement opens a new era in generating high-power beams with a very high average brightness.

This article reports the design, three-dimensional magnetostatic simulation results, coil fabrication, assembly, and low-temperature test results of a C-frame, iron-dominated high-temperature superconductor dipole magnet for ultra high vacuum magnetic-sensitive spectroscopy techniques at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The magnet is required to provide fields up to similar to 1 T along two spatial directions. With an added capability to rotate about its vertical axis of symmetry, the magnet system is capable of delivering field along all three principal axes. The combination of this magnet and fast-switched, elliptically polarized soft X-rays produced by the NSLS elliptically polarized wiggler forms the basis for an excellent tool for polarization-sensitive studies of magnetic materials. The prototype magnet consists of a set of pancake coils fabricated from BSCCO tape reinforced with stainless steel. Those pancake coils are clamped in a low-carbon AISI 1006 grade steel pole and yoke assembly. For testing purposes, a cold He gas environment provided cooling the superconducting magnet to subtransition temperatures; for operation in an ultra high vacuum endstation environment, cooling can be provided by an external cryocooler. The low-temperature tests establish the safe-operating parameters needed to achieve the target magnetic field values. The measured magnetic field values are in close agreement with the magnetic simulation results, thereby confirming that the magnet design fulfills the performance requirements. The final section of this article lists some suggestions to improve the magnet performance.

CW photoinjectors operating at high accelerating gradients promise to revolutionize many areas of science and applications. They can establish the basis for a new generation of monochromatic X-ray free electron lasers, high brightness hadron beams, or a new generation of microchip production. In this letter we report on the record-performing superconducting RF electron gun with $\textrm{CsK}_{2}\textrm{Sb}$ photocathode. The gun is generating high charge electron bunches (up to 10 nC/bunch) and low transverse emittances, while operating for months with a single photocathode. This achievement opens a new era in generating high-power beams with a very high average brightness.

A 112 MHz SRF electron photoinjector (gun) was developed at Brookhaven National Laboratory to produce high-brightness and high-bunch-charge bunches for the coherent electron cooling proof-of-principle experiment. The gun is designed to deliver electrons with a kinetic energy of up to 2 MeV. Electrons are generated by illuminating a high quantum efficiency (QE) K2CsSb photoemission layer with a green laser operating at a wavelength of 532 nm. The gun was able to generating 3 nC bunches at 1.7 MeV. The design goals, fabrication, performance and operational experience are reported here.

Charge lifetime of strained superlattice GaAs photocathodes in DC guns is limited by ion back bombardment. It needs to be improved at least an order of magnitude to meet the requirements for future colliders such as Electron-Ion Collider (EIC). In this work, we propose and present simulation results for an offset anode scheme to increase charge lifetime in DC guns. This scheme eliminates the bombardment of high energy ions on the cathode and enables maximum usage of the available cathode area. Depending on the size of the available cathode area, this method can increase the charge lifetime by an order of magnitude compared to the current best alternative method. An anode assembly capable of in-vacuum movement is required for this method, which has been designed and fabricated at Brookhaven National Laboratory.

To facilitate the SAC 2015 Long Range Plan for Nuclear Science: a high-energy high-luminosity polarized Electron-Ion Collider (EIC) as the highest priority for new facility construction following the completion of FRIB. Brookhaven National Laboratory (BNL) is proposing to build a high luminosity electron-hadron collider called e-RHIC which incorporates a new electron synchrotron with the existing Relativistic Heavy Ion Collider (RHIC). A low risk conventional technology based design is being adopted for the majority of the accelerator components. The e-RHIC electron source will produce a highly polarized beam current of up to 50 mA with > 80% polarization at an energy of up to 18 GeV with a luminosity > 1034 cm-2s-1. The prototype e-RHIC beam source is currently under development at BNL and Stony Brook University. This paper presents a conceptual design of the e-RHIC machine, how polarized beam will enhance the physics program and plans to address the remaining challenges associated with the construction of e-RHIC. In order to construct a future electron ion collider with high luminosity, a high average current and high bunch charge polarized electron source is under development at Brookhaven National Laboratory. We present the R&D plan for achieving the required charge and current in the polarized eRHIC gun.. The plan involves developing a large single cathode gun to generate 5.3 nC and 6 mA polarized electrons beam. We report the progress of large cathode prototype gun development, the beam line design and plan for measuring gun charge lifetime for high bunch charge, high current operation.

Crab crossing is essential for high-luminosity colliders. The high-luminosity Large Hadron Collider (HL-LHC) will equip one of its interaction points (IP1) with double-quarter wave (DQW) crab cavities. A DQW cavity is a new generation of deflecting rf cavities that stands out for its compactness and broad frequency separation between fundamental and first high-order modes. The deflecting kick is provided by its fundamental mode. Each HL-LHC DQW cavity shall provide a nominal deflecting voltage of 3.4 MV, although up to 5.0 MV may be required. A proof-of-principle (POP) DQW cavity was limited by quench at 4.6 MV. This paper describes a new, highly optimized cavity, designated the DQW SPS series, which satisfies dimensional, cryogenic, manufacturing, and impedance requirements for beam tests at the Super Proton Synchrotron (SPS) and operation in the LHC. Two prototypes of this DQW SPS series were fabricated by U.S. industry and cold tested after following a conventional superconducting radio-frequency surface treatment. Both units outperformed the POP cavity, reaching a deflecting voltage of 5.3-5.9 MV. This voltage-the highest reached by a DQW cavity-is well beyond the nominal voltage of 3.4 MV and may even operate at the ultimate voltage of 5.0 MV with a sufficient margin. This paper covers fabrication, surface preparation, and cryogenic rf test results and implications.

The Coherent Electron Cooling Proof-of-Principle (CeC PoP) experiment [1, 2] employs a high-gradient CW photo-injector based on the superconducting RF cavity. Such guns operating at high accelerating gradients promise to revolutionize many sciences and applications. They can establish the basis for super-bright monochromatic Xray and gamma ray sources, high luminosity hadron colliders, nuclear waste transmutation or a new generation of microchip production. In this paper we report on our operation of a superconducting RF electron gun with a record-high accelerating gradient at the CsK2Sb photocathode (i.e. ~ 20 MV/m) generating a record-high bunch charge (above 3 nC). We give short description of the system and then detail our experimental results.