First results from commissioning of low energy rhic electron cooler (lerec)*

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