The electron ion collider (EIC) Hadron Storage Ring (HSR) will reuse most of the existing superconducting magnets (SC) from the RHIC storage ring. However, for some sectors of the machine, a modification of the accelerators optics will be required. To do this, the existing RHIC magnet electrical circuits will have to be modified and some superconducting current leads will need to be used at higher current. A work has been conducted to understand the current leads design parameters and their operational flexibility around these parameters, in particular for use at higher current. This paper details the study of the existing RHIC current leads, their potential for use at higher current and where required the modifications to extend their operational range.

The Beam Energy Scan phase II (BES-II), performed in the Relativistic Heavy Ion Collider (RHIC) from 2019 to 2021, explored the phase transition between quark-gluon plasma and hadronic gas. BES-II exceeded the goal of a fourfold increase in the average luminosity over that achieved during Beam Energy Scan phase I (BES-I), at five gold beam energies: 9.8, 7.3, 5.75, 4.59, and 3.85 GeV/nucleon. This was accomplished by addressing several beam dynamics effects, including intrabeam scattering, beam-beam, space charge, beam instability, and field errors induced by superconducting magnet persistent currents. Some of these effects are especially detrimental at low energies. BES-II achievements are presented, and the measures taken to improve RHIC performance are described. These measures span the whole RHIC complex, including ion beam sources, injectors, beam lifetime improvements in RHIC, and operation with the world's first bunched beam Low Energy RHIC electron Cooler (LEReC).

For RHIC to operate at its top energy (100 GeV/n) while protecting the future sPHENIX detector [1], spontaneous and asynchronous firing of abort kicker modules (pre-fires) have to be avoided [2]. A new triggering circuit for the abort kickers was implemented with relatively slow mechanical relays in series with the standard fast thyratron tubes [3]. The relays prevent unwanted pre-fires during operation, but come at the expense of a long latency about 7 milliseconds between the removal of the beam permit and the actual firing of the abort kickers. Protection considerations of RHIC’s superconducting magnets forbid delaying energy extraction from the main dipoles and quadrupoles for too long after a quench. The beam has thus to circulate in both RHIC rings for a few milliseconds as the current in dipole and quadrupole circuit is being extracted. We present the results of delayed abort experiments conducted in July 2018 with the analysis of fast orbit and tune measurements and discuss the safety implications of this implementation for future RHIC operation.

which increases the actual bending radius of the machine. However, this measure also affects the damping distribution in the machine, and may even result in anti-damping, making the lattice not suitable as a storage ring. In the following we demonstrate how to convert a separated-function lattice to a combined-function lattice while simultaneously preserving the damping distribution.

RHIC provided Au+Au collisions at beam energies of 5.75 and 4.59 GeV/nucleon for the physics program in 2020 as part of the Beam Energy Scan II experiment. The machine configuration and operational experience at these energies are presented in this report with emphasis on their unique features which include but are not limited to the addition of a second RF system to enable large longitudinal acceptance and to reduce the intrabeam scattering rate at 5.75 GeV/nucleon, the exploration of the tune space for better performance, the use of lower frequency cavities for alleviating space charge effects, and the world-first operation of cooling on colliding beams with an RF-accelerated bunched electron beam.

As part of the Beam Energy Scan phase-II (BES-II) program, RHIC operated in the fixed target mode with Gold beam at energies 5.75, 7.3, 9.8, 13.5, 19.5 and 31.2 GeV/nucleon in 2020. The gold beams at these energies were moved vertically to scrape the halo on a gold fixed target. In addition to beam orbit control, tune and chromaticity adjustments and external excitation were used to produce and maintain the event rate. This paper will review the operational experience of RHIC in the fixed target mode at various energies in 2020.

The relativistic heavy ion collider (RHIC) at BNL uses low-energy RHIC electron cooling (LEReC) to conduct experiments to search for the quantum chromodynamic critical point. The first ever electron cooling based on the RF acceleration of electron beams was experimentally demonstrated on April 5, 2019 using LEReC at BNL. The first critical step in obtaining successful 3D non-magnetized cooling of the Au ion bunches in the RHIC cooling section was matching the electron beam energy with a relative error less than 5 x 10(-4) to the ion beam energy. Part of the LEReC beamline is a dipole magnet that bends the electron beam 180 degrees. One of the most outstanding measurement challenges is that the dipole field is so low (approximate to 200 G). Most of the existing NMR probes can only measure fields >400 G. A lower signal-to-noise ratio at low fields requires the use of larger sample volumes. Working with CAYLAR, the NMR probe has been redesigned and optimized for these low field measurements with high resolution. We report the methods, challenges and results for extensive magnetic field mappings of the 180 degrees dipole magnet. A combination of NMR and Hall sensors was successfully implemented to measure uniform field regimes inside the magnet center area and non-uniform field regimes at the magnet ends. Detailed measurement and mapping were performed at five radii and five heights along the beam trajectory. Meanwhile, a finite element magnetic modeling simulation of the magnet using Opera software was performed. The calculated and measured data were compared, and the calculated data are a good reference for the measured data over long length mapping from the magnet edge to the center. The measured magnetic measurement data are directly useful for beam instrumentation, diagnostics and operation.

The Low Energy RHIC electron Cooling (LEReC) project at Brookhaven National Laboratory recently demonstrated for the first time cooling of hadron bunches with radio-frequency (rf) accelerated electron bunches. LEReC uses a high-voltage photoemission electron gun with stringent requirements for beam current, beam quality, and stability. The electron gun has a photocathode with a high-power fiber laser, and a novel cathode production, transport, and exchange system. It has been demonstrated that the high-voltage photoemission gun can continually produce a high-current electron beam with a beam quality suitable for electron cooling. We describe the operational experience with the LEReC dc photoemission gun in RHIC and discuss the important aspects needed to achieve the required beam current, beam quality, and stability.

A high-current high-brightness electron accelerator for low-energy RHIC electron cooling (LEReC) was successfully commissioned at Brookhaven National Laboratory. The LEReC accelerator includes a dc photoemission gun, a laser system, a photocathode delivery system, magnets, beam diagnostics, a superconducting rf booster cavity, and a set of normal conducting rf cavities to provide enough flexibility to tune the beam in the longitudinal phase space. Cooling with nonmagnetized rf accelerated electron beams requires longitudinal corrections to obtain a small momentum spread while preserving the transverse emittances. Electron beams with kinetic energies of 1.6 and 2.0 MeV with a beam quality suitable for cooling were successfully propagated through 100 m of beam lines, including dispersion sections, maintained through both cooling sections in RHIC and used for cooling ions in both RHIC rings simultaneously. The beam quality suitable for cooling RHIC beams was achieved in 2018, which led to the first experimental demonstration of bunched beam electron cooling of hadron beams in 2019.

Cooling of beams of gold ions using electron bunches accelerated with radio-frequency systems was recently experimentally demonstrated in the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. Such an approach is new and opens the possibility of using this technique at higher energies than possible with electrostatic acceleration of electron beams. The challenges of this approach include generation of electron beams suitable for cooling, delivery of electron bunches of the required quality to the cooling sections without degradation of beam angular divergence and energy spread, achieving the required small angles between electron and ion trajectories in the cooling sections, precise velocity matching between the two beams, high-current operation of the electron accelerator, as well as several physics effects related to bunched-beam cooling. Here we report on the first demonstration of cooling hadron beams using this new approach.

Persistent currents in superconducting magnet introduce magnetic errors especially at low operating fields. In addition, their decay causes magnetic field variations and therefore drifts of the beam orbits, tunes, and chromaticities. To reduce field errors and suppress magnetic field variations, a new magnetic cycle was proposed for the low-energy beam operation at the Relativistic Heavy Ion Collider (RHIC). In the new magnetic cycle, the magnet current oscillates around the nominal operating current with diminishing amplitude a few times before it settles. The new magnetic cycle has been demonstrated experimentally to reduce field errors and the amplitude of magnetic field variations significantly and is essential for the ongoing RHIC Beam Energy Scan II (BES-II) program. This article will present beam-based experimental studies of the persistent current effects with the new magnetic cycle, and discuss its application in RHIC.

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

The design effort for the electron-ion collider eRHIC has concentrated on electron-proton collisions at the highest luminosities over the widest possible energy range. The present design also provides for electron-nucleon peak luminosities of up to 4.7·10³³ cm⁻²s^{−1} with strong hadron cooling, and up to 1.7·10³³ cm⁻²s^{−1} with stochastic cooling. Here we discuss the performance limitations and design choices for electron-ion collisions that are different from the electron-proton collisions. These include the ion bunch preparation in the injector chain, acceleration and intrabeam scattering in the hadron ring, path length adjustment and synchronization with the electron ring, stochastic cooling upgrades, machine protection upgrades, and operation with polarized electron beams colliding with either unpolarized ion beams or polarized He-3.

RHIC will provide Au-Au collisions at beam energies of 5.75, 4.59 GeV/nucleon for physics program in 2020, and at beam energy of 3.85 GeV/nucleon for physics program in 2021 as part of the BeamEnergy Scan II (BES-II). The oper- ational experience gained in the first year (2019) of BES-II operation will be applied toward operations in the coming years. This article will present some technical details and the outlook of the BES-II operations in the coming years.

RHIC operated in fixed target mode at beam energies 4.59, 7.3, and 31.2 GeV in 2019 as a part of the Beam Energy Scan II program. To scrape beam halo effectively at the fixed target which is 2.05 m away from the center of the STAR detectors, lattice design with relative large beta function at STAR was implemented at the two lower energies. The kickers of the baseband tune (BBQ) measurement system were engaged to dilute the beam transversely to maintain the event rate except for 31.2 GeV/nucleon. In addition, beam orbit control and/or tune chromticity adjustment were used to level the event rate. This paper will review the operational experience of RHIC in fixed target mode at various energies.

The Low Energy RHIC electron Cooler (LEReC) was recently commissioned at BNL. The LEReC is the first electron cooler based on RF acceleration of electron bunches (previous electron coolers all used DC beams). Bunched electron beams are necessary for cooling hadron beams at high energies. The challenges of such an approach include generation of electron beams suitable for cooling, delivery of electron beams of the required quality to the cooling sections without degradation of beam emittances and energy spread, achieving the required small angles between electrons and ions in the cooling sections, precise velocity matching between the two beams, high-current operation of the electron accelerator, as well as several physics effects related to bunched beam cooling. Following successful commissioning of the electron accelerator in 2018, the focus of the LEReC project in 2019 was on establishing electron-ion interactions and demonstration of the cooling process. Here we report on the first demonstration of Au ion cooling in RHIC using this new approach.

Non-magnetized bunched electron cooling of low energy RHIC requires electron beam energy in range of 1.6-2.6 MeV, with average current up to 45 mA, very small energy spread, and low emittance [1]. A 400 kV DC gun equipped with photocathode and laser delivery system will serve as a source of high quality electron beam. Acceleration will be achieved by an SRF 704 MHz booster cavity and other RF components that are scheduled to be operational in early 2018. The DC gun testing in its installed location in RHIC will start in early 2017. During this stage we plan to test the critical equipment in close to operation conditions: laser beam delivery system, cathode QE lifetime, DC gun, beam instrumentation, high power beam dump system, and controls. In this paper we describe the gun test set up, major components, and parameters to be achieved and measured during the gun beam test.