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Recent developments of the ERL-based design of future high luminosity electron-hadron collider eRHIC focused on balancing technological risks present in the design versus the design cost. As a result a lower risk design has been adopted at moderate cost increase. The modifications include a change of the main linac RF frequency, reduced number of SRF cavity types and modified electron spin transport using a spin rotator. A luminosity-staged approach is being explored with a Nominal design (L ~ 10³³ cm⁻² s⁻¹) that employs reduced electron current and could possibly be based on classical electron cooling, and then with the Ultimate design (L > 10³⁴ cm⁻² s⁻¹) that uses higher electron current and an innovative cooling technique (CeC). The paper describes the recent design modifications, and presents the full status of the eRHIC ERL-based design., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

HOM damping is a challenge for high-current SRF linacs possibly generating HOM power at a level of 10 KW per cavity. A rectangular waveguide used as a natural high pass filter is a good option as high power, large spectrum HOM damper. However, its size is too big, causing a big challenge for the cooling and cryogenic system. A reliable, compact HOM damping scheme using a ridged waveguide is being developed to damp high power (> 10 kW), large spectrum HOMs ( up to 40 GHz) that may be generated in the 647 MHz 5-cell eRHIC ERL SRF linac. The size of a ridged waveguide is less than a quarter of the regular waveguide, which alleviates the thermal issue. This paper presents the design of a ridged waveguide and estimated HOM damping results using a ridged waveguide. The thermal or cooling design of the ridged waveguide will also be addressed., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

The ERL-ring eRHIC adopts the electron accelerator design of a multi-pass energy recovery linac (ERL), with fixed field alternating gradient (FFAG) recirculating passes. To ensure the beam quality in the accelerating and decelerating stage and the energy recovery efficiency, detailed start-to-end simulation is required to evaluate the various beam dynamics effects, such as synchrotron radiation, wake fields, coherent synchrotron radiation. In this paper, we present the eRHIC ERL start-to-end simulation strategy with various simulation codes and the current status., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

High current Superconducting Radiofrequency (SRF) 5-cell cavities are essential for the proposed ERL-based electron-ion collider eRHIC in BNL. The HOM power generated when a single bunch traverses the cavity is estimated by the corresponding loss factor. Multiple re-circulations through the ERL create a specific bunch pattern. In this case the loss factor can be different than the single bunch loss factor. The HOM power generation can be surveyed in the time and frequency domains. We estimate the average HOM power in the eRHIC 5-cell cavity with different ERL bunch patterns using both methods. We also discuss possible solutions to reduce this HOM power., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

The 45thAdvanced Beam Dynamics Workshop on Energy Recovery Linacs, hosted by the Brookhaven National Labo- ratory, was held at the Stony Brook University Campus, June 7-12, 2015. It was the sixth in the series of international workshops covering accelerator physics and technology of Energy Recovery Linacs. The workshop was organized into five working groups. Working group 2 (WG2) covered beam dynamics and optics issues of ERLs in operation as well as of proposed and future ones. Key aspects of some representatively selected talks together with the outcome of the working group discussion are summarized in this report., Proceedings of the 56th ICFA Advanced Beam Dynamics Workshop on Energy Recovery Linacs, ERL2015, Stony Brook, NY, USA

The chromaticities in the eRHIC linear non-scaling Fixed Field Alternating Gradient (FFAG) lattice are very large. Therefore, particles will decohere in phase space given the presence of lattice errors. The decoherence causes a deviation of the orbit response which is the basis for orbit corrections. In this report we will present a study of the linearity of the orbit response in a lattice with large chromaticity, a comparison of the results of orbit corrections for several cases together with a conclusion that correcting the average orbit with a measured orbit response works as good as an orbit correction for on-momentum particles., Proceedings of the 6th Int. Particle Accelerator Conf., IPAC2015, Richmond, VA, USA

An ampere class 20 MeV superconducting Energy Recovery Linac (ERL) is presently under commissioning at Brookhaven National Laboratory (BNL) for testing of concepts relevant for high-energy coherent electron cooling and electron-ion colliders. The injector subsystems tests and installation were finished in fall 2013. The injector includes: SRF photoelectron gun with 1 MW amplifier, 10W green drive-laser system, multi-alkaline cathode deposition system, cathode transport system, beam instrumentation and control. * The first photo current from ERL SRF gun has been observed in fall 2014 after second attempt. Completion of the ERL returning loop components installation is scheduled for April 2015 following full power ERL commissioning. After ERL commissioning in BLDG912 the ERL will be relocated to RHIC IP2 to be used as low energy RHIC electron cooler., Proceedings of the 56th ICFA Advanced Beam Dynamics Workshop on Energy Recovery Linacs, ERL2015, Stony Brook, NY, USA

The talk shall cover two SRF photoemission electron guns under commissioning at BNL: a 704 MHz elliptical ERL gun and a 112 MHz quarter-wave gun for coherent electron cooling experiment. In particular, the speaker shall report on generating first photoemission beam current from the 704 MHz SRF gun, multipacting issues in the SRF guns, photocathode behavior as well as other commissioning experiences and results., Proceedings of the 17th International Conference on RF Superconductivity, SRF2015, Whistler, BC, Canada

The 704 MHz superconducting RF gun successfully generated the first photoemission beam on Nov. 17 2014. This paper will report the latest results of SRF beam commissioning, including the SRF cavity performance, cathode QE measurements, and beam parameter measurements. The beam commissioning setup is described in the paper as well., Proceedings of the 6th Int. Particle Accelerator Conf., IPAC2015, Richmond, VA, USA

RHIC has provided polarized proton collisions from 31 GeV to 255 GeV in past decade. To preserve polarization through numerous depolarizing resonances through the whole accelerator chain, harmonic orbit correction, partial snakes, horizontal tune jump system and full snakes have been used. In addition, close attentions have been paid to betatron tune control, orbit control and beam line alignment. The polarization of 60% at 255 GeV has been delivered to experiments with 1.8×10¹¹ bunch intensity. For the eRHIC era, the beam brightness has to be maintained to reach the desired luminosity. Since we only have one hadron ring in the eRHIC era, existing spin rotator and snakes can be converted to six snake configuration for one hadron ring. With properly arranged six snakes, the polarization can be maintained at 70% at 250 GeV. This paper summarizes the effort and plan to reach high polarization with small emittance for eRHIC., Proceedings of the 6th Int. Particle Accelerator Conf., IPAC2015, Richmond, VA, USA

The energy-recovery electron beam recirculator, part of the eRHIC electron-ion collider project complex at BNL, is subject to feasibility studies in an FFAG arc based version. We develop here on tracking simulations and their analysis, regarding end-to-end polarized e-bunch transport in presence of synchrotron radiation, magnet alignment and field errors. Simulations include the evolution of energy, orbits, emittances, polarization profiles., Proceedings of the 6th Int. Particle Accelerator Conf., IPAC2015, Richmond, VA, USA

The proposed eRHIC electron-hadron collider uses a "non-scaling FFAG" lattice to recirculate 16 turns of different energy through just two beamlines located in the RHIC tunnel. This paper presents lattices for these two FFAGs that are optimised for low magnet field and to minimise total synchrotron radiation across the energy range. The higher number of recirculations in the FFAG allows a shorter linac (1.322GeV) to be used, drastically reducing cost, while still achieving a 21.2GeV maximum energy to collide with one of the existing RHIC hadron rings at up to 250GeV. eRHIC uses many cost-saving measures in addition to the FFAG: the linac operates in energy recovery mode, so the beams also decelerate via the same FFAG loops and energy is recovered from the interacted beam. All magnets will constructed from NdFeB permanent magnet material, meaning chillers and large magnet power supplies are not needed. This paper also describes a smaller prototype ERL-FFAG accelerator that will test all of these technologies in combination to reduce technical risk for eRHIC., Proceedings of the 6th Int. Particle Accelerator Conf., IPAC2015, Richmond, VA, USA

eRHIC is a future electron-hadron collider proposed at BNL. It will collide high-intensity hadron beams from one of the existing rings of RHIC with a 50-mA electron beam from a multi-pass 10-GeV superconducting RF (SRF) Energy Recovery Linac (ERL). A novel approach to the multi-pass ERL utilizing a non-scaling FFAG was recently proposed. It has many advantages over the previous designs including significant cost savings. The current design has 11 passes in two FFAG rings. To mitigate various beam dynamics effects, it was proposed to lower RF frequency of the SRF linac from 704 MHz used in the previous design. In this paper we consider different effects driving the frequency choice of the SRF ERL and present our arguments for choosing lower RF frequency., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

The accelerator design of future high-energy high-luminosity electron-hadron collider at RHIC, called eRHIC, is presented. We plan adding energy recovery linacs to accelerate the electron beam to 20 (potentially 30) GeV and to collide the electrons with hadrons in RHIC. The center-of-mass energy of eRHIC will range from 30 to 200 GeV. The luminosity exceeding $10^{34}$ cm$^{-2}$ s$^{-1}$ can be achieved in eRHIC using the low-beta interaction region with a 10 mrad crab crossing. The important eRHIC R&D items include the high-current polarized electron source, the coherent electron cooling and the compact magnets for re-circulating passes. A natural staging scenario is based on step-by-step increases of the electron beam energy by building-up of eRHIC's SRF linacs., Proc. of DIS 2012, 1051 - 1054; DESY-PROC-2012-02; ISSN 1435-8077

We present our studies on various aspects of the beam dynamics in ’racetrack’ design of the first stage electron-ion collider at RHIC (eRHIC), including transverse beam break up instabilities, electron beam emittance growth and energy loss due to synchrotron radiation, electron beam losses due to Touschek effects and residue gas scattering, beam-beam effects at the interaction region and emittance growth of ion beam due to electron bunch to bunch noises. For all effects considered above, no showstopper has been found., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

Beam-beam effects in eRHIC have a number of unique features, which distinguish them from both hadron and lepton colliders. Due to beam-beam interaction, both electron and hadron beams would suffer quality degradation or beam loss from without proper treatments. Those features need novel study and dedicate countermeasures. We study the beam dynamics and resulting luminosity of the characteristics, including mismatch, disruption and pinch effects on electron beam, in additional to their consequences on the opposing beam as a wake field and other incoherent effects of hadron beam. We also carry out countermeasures to prevent beam quality degrade and coherent instability., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

Polarized proton beams are accelerated in RHIC to 250 GeV energy with the help of Siberian Snakes. The pair of Siberian Snakes in each RHIC ring holds the design spin tune at 1/2 to avoid polarization loss during acceleration. However, in the presence of closed orbit errors, the actual spin tune can be shifted from the exact 1/2 value. It leads to corresponding shift of locations of higher-order ("Snake") resonances and limits available betatron tune space. The largest closed orbit effect on the spin tune comes from the horizontal orbit angle between the two snakes. During RHIC Run in 2009 dedicated measurements with polarized proton beams were taken to verify the dependence of the spin tune on the local orbits at the Snakes. The experimental results are presented along with the comparison with analytical predictions., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

In this paper, we analyzed the linac optics design requirement for a multi-pass energy recovery linac (ERL) with one or more linacs. A set of general formula of constrains for the 2-D transverse matrix is derived to ensure design optics acceptance matching throughout the entire accelerating and decelerating process. Meanwhile, the rest free parameters can be adjusted for fulfilling other requirements or optimization purpose. As an example, we design the linac optics for the future MeRHIC (Medium Energy eRHIC) project and the optimization for enlarging the BBU threshold., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

A fast transition instability presents a limiting factor for ion beam intensity in RHIC. Several pieces of evidence show that electron clouds play an important role in establishing the threshold of this instability. In RHIC Runs 7 and 8 dedicated measurements of the instability, using different beam instrumentation tools (Button BPM, Wall Current Monitor, transition monitors) were done in order to observe the instability development over hundreds turns. The papers presents and discusses the results of those measurements in time and frequency domains., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

During the second half of Run-9, the Relativistic Heavy Ion Collider (RHIC) provided polarized proton collisions at two interaction points with both longitudinal and vertical spin direction. Despite an increase in the peak luminosity by up to 40%, the average store luminosity did not increase compared to previous runs. We discuss the luminosity limitations and polarization performance during Run-9., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

We present a medium-energy (4 GeV) electron ion collider (MeRHIC) lattice design for the Relativistic Heavy Ion Collider (RHIC). MeRHIC represents a staged approach towards the higher energy eRHIC, with MeRHIC hardware being reused for eRHIC. The lattice design includes two Energy Recovery Linacs (ERLs), multiple isochronous arcs connected to the ERLs, an interaction region design, a low energy ERL with a polarized electron source, and connecting beam lines., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

The Relativistic Heavy Ion Collider (RHIC) has two main operating modes with heavy ions and polarized protons respectively. In addition to a continuous increase in the bunch intensity in all modes, two major new systems were completed recently mitigating the main luminosity limit and leading to significant performance improvements. For heavy ion operation stochastic cooling mitigates the effects of intrabeam scattering, and for polarized proton operation head-on beam-beam compensation mitigated the beam-beam effect. We present the performance increases with these upgrades for heavy ions and polarized protons, as well as an overview of all operating modes past and planned., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

An ampere class 20 MeV superconducting Energy Recovery Linac (ERL) is presently under commissioning at Brookhaven National Laboratory (BNL). This facility enables testing of concepts relevant for high-energy coherent electron cooling, electron-ion colliders, and high repetition rate Free-Electron Lasers. The ERL will be capable of providing electron beams with sufficient quality to produce high repetition rate THz and X-ray radiation. When completed the SRF photoinjector will provide 2 MeV energy and 300 mA average beam current. The injector for the R&D ERL was installed in 2012, this includes a 704MHz SRF gun* with multi-alkali photocathode, cryo-system upgrade and a novel emittance preservation zigzag-like low energy merger system. We describe the design and major components of the R&D ERL injector then report the first experimental results and experiences learned in the first stage of beam commissioning of the BNL R&D ERL., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

For improved reproducibility of good operating conditions and ramp commissioning efficiency, new dual-plane slow orbit feedback during the energy ramp was implemented during run-10 in the Relativistic Heavy Ion Collider (RHIC). The orbit feedback is based on steering the measured orbit, after subtraction of the dispersive component, to either a design orbit or to a previously saved reference orbit. Using multiple correctors and beam position monitors, an SVD-based algorithm is used for determination of the applied corrections. The online model is used as a basis for matrix computations. In this report we describe the feedback design, review the changes made to realize its implementation, and assess system performance., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

After running uranium-uranium and copper-gold collisions in 2012, the high energy heavy ion run of the Relativistic Heavy Ion Collider (RHIC) for Fiscal Year 14 (Run14) is back to gold-gold (Au-Au) collisions at 100 GeV/nucleon. Following the level of performance achieved in Run12, RHIC is still looking to push both instantaneous and integrated luminosity goals. To that end, a new 56 MHz superconducting RF cavity was installed and commissioned, designed to keep ions in one RF bucket and improve luminosity by allowing a smaller beta function at the interaction point (IP) due to a reduced hourglass effect. The following presents an overview of these changes and reviews the performance of the collider., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany