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.

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-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.

In this paper we describe a new microbunching instability occurring in charged particle beams propagating along a straight trajectory. The nature of these exponentially growing plasma oscillations gave the reason for its name: plasma-cascade instability. Such instability can strongly amplify longitudinal microbunching originating from the beam's shot noise, even to the point of saturation. Resulting random density and energy microstructures can drastically reduce beam quality. Conversely, such instability can drive novel high-power sources of broadband radiation or can be used as a broadband amplifier. We discovered this phenomenon in a search for such amplifier in the coherent electron cooling scheme [Phys. Rev. Lett. 102, 114801 (2009)] without separation of electron and hadron beams. In this paper we present a brief analytical theory of this new phenomenon, detailed numerical studies, the results of experimental demonstration as well as control of the longitudinal plasma-cascade instability.

The brand new non-magnetized bunched beam electron cooler (LEReC) has been built to provide luminosity improvement for the Beam Energy Scan II (BES-II) physics program at the Relativistic Heavy Ion Collider (RHIC). The LEReC accelerator includes a photocathode DC gun, a laser system, a photocathode delivery system, magnets, beam diagnostics, an SRF booster cavity, and a set of Normal Conducting RF cavities to provide sufficient flexibility to tune the beam in the longitudinal phase space. This highcurrent high-power accelerator was successfully commissioned in the period of March -September 2018. Beam quality suitable for cooling has been achieved which led to the first demonstration of bunched beam electron cooling of hadron beams in April 2019. In this paper we discuss achieved results and experience learned during commissioning. INTRODUCTION A new, state of the art, electron accelerator for cooling low energy RHIC hadron beams (LEReC) was built and is being commissioned at BNL. The purpose of LEReC is to provide luminosity improvement for the RHIC operation at low energies to search for the QCD critical point (Beam Energy Scan Phase-II physics program) [1-2]. Unlike all electron coolers to date, LEReC uses bunched electron beams accelerated to the required energies using RF cavities [3]. To achieve efficient cooling, the electron beam must not only be optimized for low transverse emittance but, more importantly, for low energy spread. The LEReC accelerator includes a photocathode DC gun with a high power laser system, magnets, beam diagnostics, an SRF booster cavity, and a set of normal conducting RF cavities to provide sufficient flexibility to tune the beam in the longitudinal phase space. LEReC uses a DC photocathode gun similar to the one used at the Cornell University [4]. The gun itself was built by the Cornell University. The gun tests with beam started in 2017 when it operated up to 10 mA average current [5]. Electron beams are generated by illuminating a multi-alkali (CsK2Sb or NaK2Sb) photocathode [6] with green light (532 nm) from a high-power fiber laser [7] by utilizing sophisticated laser transport and stabilization [8]. To optimize operational time and minimize the cathode exchange time three multi-cathode carriers were built. Each cathode carrier, which can hold up to 12 pucks of photocathodes, is attached to the gun in a 10-11 Torr-scale vacuum (for details of design see [9]). Figure 1: Layout of the LEReC accelerator. The red contour box indicates DC gun test area. The layout of LEReC is shown in Fig. 1. The 350-400 keV electron beam from the gun is transported via a 704 MHz SRF booster cavity and a 2.1 GHz 3rd harmonic linearizer normal conductive cavity. Electron beams can be accelerated to maximum kinetic energy of 2.6 MeV. The electron bunch is ballistically stretched to the required bunch length in the transport line. The accumulated energy ___________________________________________ * Work supported by Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy † dkayran@bnl.gov 10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW Publishing ISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-MOPRB085 MC1: Circular and Linear Colliders A19 Electron-Hadron Colliders MOPRB085 769 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 19 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I

Coherent electron cooling experiment is aimed for demonstration of the proof-of-principle demonstration of reduction energy spread of a single hadron bunch circulating in RHIC. The electron beam should have the required parameters and its orbit and energy should be matched to the hadron beam. In this paper we present the achieved electron beam parameters including emittance, energy spread, and other critical indicators. The operational issues as well as future plans are also discussed.

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.

The low energy RHIC Electron Cooling (LEReC) accelerator will be running with electron beams of up to 110 kW power with CW operation at 704 MHz. Although electron energies are relatively low (< 2.6 MeV), at several locations along the LEReC beamline, where the electron beam has small (about 250 um) RMS radius design size, it can potentially hit the vacuum chamber with a large incident angle. The accelerator must be protected against such a catastrophic scenario by a dedicated machine protection system (MPS). Such an MPS shall be capable of interrupting the beam within a few tens of microseconds. In this paper we describe the current conceptual design of the LEReC MPS. LEREC LAYOUT AND PARAMETERS The LEReC accelerator [1] consists of the 400 keV DC photo-gun followed by the 1.6-2.4 MeV SRF Booster, the transport line, the merger that brings the beam to the two cooling sections (CS1 and CS2) and the cooling sections followed by the 140 kW dump. The LEReC also includes two dedicated diagnostic beamlines: the low-power beamline capable of accepting 10 kW beam and the RF diagnostic beamline. The LEReC layout is schematically shown in Fig. 1. We are planning to start the gun commissioning in the winter of 2017 with the short beamline that does not include the SRF Booster and ends at 10 kW beam dump. The LEReC beam train consists of 9 MHz macrobunches. Each macro-bunch consists of Nb=30 bunches repeated with 704 MHz frequency. The length of each bunch at the cathode is 80 ps. The charge per bunch (Qb) can be as high as 200 pC. We will have the ability to work with macro-bunch trains of various length t), various number of macrobunches per train (Nmb), and various time delay (T) between the trains. Also, as an alternative to our nominal operational mode with continuous train of 9 MHz macro-bunches, we will have the capability to run a continuous wave (CW) of 704 MHz bunches. Table 1: LEReC Beam Modes ! " # " #

An FEL-based Coherent electron Cooling (CeC) has a potential to significantly boosting luminosity of high-energy, high-intensity hadron-hadron and electron-hadron colliders. In a CeC system, a hadron beam interacts with a cooling electron beam. A perturbation of the electron density caused by ions is amplified and fed back to the ions to reduce the energy spread and the emittance of the ion beam. To demonstrate the feasibility of CEC we pursue a proof-of-principle experiment at Relativistic Heavy Ion Collider (RHIC) using an SRF accelerator and SRF photoinjector. In this paper, we present status of the CeC systems and our plans for next year.