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

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.

Figure 1

Layout of the LEReC accelerator. The red contour box indicates the dc gun test area.

Figure 2

LEReC cathode pucks with deposited cathode material. Left: Active cathode area of 12 mm diameter in the center used during the gun commissioning in 2017. Right: Active cathode area of 6 mm diameter shifted by 4 mm from the center used during LEReC high-current operation in 2018 and 2019.

Figure 3

Simulated rms bunch length and rms energy spread along the beam line from the gun to the beam dump for a bunch charge of 75 pC and a kinetic energy of 1.6 MeV.

Figure 4

LEReC and RHIC beam time structure in the cooling sections. The left shows two electron bunches spaced by 1.42 ns (corresponding to the 704 MHz LEReC laser maximum repetition frequency). The center shows one LEReC macrobunch consisting of 30 electron bunches (blue) placed over a single ion bunch (red). The right shows a train of three electron macrobunches spaced by 110 ns (corresponding to the 9.1 MHz RHIC repetition frequency) overlapped with the three ion bunches.

Figure 5

The rf diagnostic line layout.

Figure 6

Longitudinal phase-space measurements at the rf diagnostic line with a transverse deflecting cavity and a YAG profile monitor for bunch charges of 75 pC. The beam images (left) show at the top a single macrobunch and at the bottom a train of six macrobunches. The intensity profiles (right) are taken over the indicated windows and are used to determine the energy distribution, including differences along the six-macrobunch train.

Figure 7

Result of the longitudinal phase-space optimization. The sliced energy spread is measured at the rf diagnostic line YAG profile monitor for a bunch charge of 75 pC with a FWHM of 400 ps.

Figure 8

Phase-space measurements in the injection line using a multislit mask for a bunch charge of 75 pC. Top: Vertical intensity profile at the YAG profile monitor when a multislit mask is inserted upstream. Bottom: Result of the transverse phase-space reconstruction at the multislit mask location with the listed geometric emittance $ϵ$ and the Twiss parameters $\alpha$ and $\beta$.

Figure 9

Transverse phase-space measurements in the two cooling sections using movable slits: (a),(b) horizontal and vertical phase space in the yellow cooling section and (c),(d) horizontal and vertical phase space in the blue cooling section. Each plot lists the respective geometric emittance ($ϵ$), the rms beam size ($\sigma$), and the rms beam angular spread (${\sigma }^{\prime }$).

Figure 10

High-current 9 MHz CW operation (up to 30 mA) with laser beam intensity feedback. Top: Cathode QE (black). Bottom: Average laser power delivered to the cathode (green) and beam current measured by the DCCT (red).

Figure 11

Marks on the high-power beam dump surface as a result of a high-power beam operation with 20 mA beam current and 1.6 MeV kinetic energy during the 2018 run. Left: Beam dump entrance flange with marks at both sides highlighted by red circles. Right: Enlargement of the end of the cone.

Figure 12

High-current 9 MHz CW operation to the high-power beam dump at the end of run 2019 with an average beam current of up to 18 mA and 2 MeV kinetic energy. Top: Vacuum in the extraction line and near the beam dump. Middle: Temperatures of the different sides of the beam dump entrance pipe. Bottom: Beam current measured by the DCCT. Intensity feedback was turned on at 01:00 AM, and the fast intensity fluctuations were reduced from $5\times {10}^{-2}$ to $5\times {10}^{-4}$ peak to peak.

Figure 13

Emittance reduction of a single RHIC ion bunch in the RHIC yellow ring as a result of LEReC providing cooling in the 76 kHz (orange) and in the 9 MHz (blue) modes. Cooling started around the 200 s mark.

Figure 14

Bunch length reduction in both RHIC rings as a result of longitudinal electron cooling running in the 9 MHz CW mode. Top: Bunch length of the first three and the last three bunches in the blue ring. Bottom: Bunch length of the first three and the last three bunches in the yellow ring.