High-brightness electron beams for linac-based bunched beam electron cooling

Abstract

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.

Modern and Future Colliders

“…RHIC physics searches for a critical point in the nuclear matter phase diagram [343] required operation below the nominal injection energy of 10 GeV/u. In order to reach the integrated luminosity goals, the first bunched beam electron cooler, with electrons from a high-current highbrightness RF accelerator [344], was successfully commissioned for the lowest RHIC energies [345].…”

“…Two cooling schemes are currently under consideration: a somewhat conventional cooling with a bunched electron beam, or variations of coherent electron cooling, where an electron beam is used as a pickup and kicker in a very high bandwidth stochastic cooling scheme [412]. The feasibility of the former, based on the electron RF photoinjector system has recently been demonstrated [344,345], but the required high beam intensities for a bunched beam electron cooler for eRHIC by far exceed the capabilities of present-day electron guns. An alternative scheme, in which the electron beam is stored in a small storage ring equipped with strong damping wigglers, is being evaluated.…”

Experimental Demonstration of Hadron Beam Cooling Using Radio-Frequency Accelerated Electron Bunches

“…A * fedotov@bnl.gov † Presently at Fermi National Laboratory natural approach of accelerating electron bunches to high energies is RF acceleration. The Low Energy RHIC electron Cooler (LEReC) is the first electron cooler based on RF acceleration of electron bunches [6,7]. With this approach now experimentally demonstrated, one can consider its application to much higher energies opening new possibilities by producing high-quality hadron beams.…”

“…A high-current high-brightness electron accelerator (layout shown in Fig. 1) was successfully commissioned in 2018 [7]. In 2019 cooling of Au ions was commissioned using electron beams with kinetic energy of 1.6 MeV to cool ions at 3.85 GeV/nucleon total energy and then using 2 MeV electron beams to cool ions at 4.6 GeV/nucleon.…”

“…In such a mode, only a single ion bunch is cooled effectively because of an energy droop from one electron macro-bunch to another due to beam loading in the RF cavities, thus providing good velocity matching for only a single macrobunch of electrons. With low laser beam power in such a mode and thus no thermal effects on the laser beam distribution, it was expected that the resulting electron beam parameters should be similar to those measured at 1 Hz, which was the frequency used for electron beam quality characterization [7].…”