Electron lenses for head-on beam-beam compensation in RHIC

Two electron lenses ($e$-lenses) have been in operation during the 2015 RHIC physics run as part of a head-on beam-beam compensation scheme. While the RHIC lattice was chosen to reduce the beam-beam-induced resonance-driving terms, the electron lenses reduced the beam-beam-induced tune spread. This has been demonstrated for the first time. The beam-beam compensation scheme allows for higher beam-beam parameters and therefore higher intensities and luminosity. In this paper, we detail the design considerations and verification of the electron beam parameters of the RHIC $e$-lenses. Longitudinal and transverse alignments with ion beams and the transverse beam transfer function measurement with head-on electron-proton beam are presented.

Figure 1

The layout for head-on beam-beam compensation in the RHIC. Head-on proton-proton beam-beam interactions occur in IP6 and IP8, while head-on electron-proton beam-beam compensation is achieved in IP10.

Figure 2

Schematic not-to-scale illustration of the two $e$-lens locations. The top figure shows the plan view, while the bottom figure shows the side view, which indicates the vertical offset between the blue (left) and yellow (right) $e$-lenses.

Figure 3

Schematic layout of the RHIC electron lens.

Figure 4

Electron gun design and optimization.

Figure 5

Transverse beam profile measurement.

Figure 6

Electron gun perveance for the 4.1 mm radius cathode (left) and 7.5 mm radius cathode (right).

Figure 7

Schematic layout of the two RHIC $e$-lenses and shared warm solenoid power supply connection.

Figure 8

Cross section of the RHIC electron lens superconducting magnet.

Figure 9

Field straightness measurements of the main superconducting magnet.

Figure 10

Diagram of the RHIC $e$-lens potentials.

Figure 11

Electron beam current caused by cathode bias.

Figure 12

Measurement of drift space perveance with different GS1 currents.

Figure 13

Beam rise and fall time measurement via the anode voltage (left) and signals from the four BPM electrodes (right).

Figure 14

Longitudinal alignment of the electron beam pulse within the ion beam abort gap.

Figure 15

Anode modulator signals and ion beam intensity during the parasitic mode.

Figure 16

Alignment and collision with the last several bunches.

Figure 17

Demonstration of the electron and ion beam overlap.

Figure 18

Separation bumps and electron beam current.

Figure 19

Separation bump around the rings.

Figure 20

Typical store with an electron lens.

Figure 21

Blue electron beam and ion beam alignment procedure for the horizontal plane.

Figure 22

Incoherent tune distributions for various electron beam sizes and an electron beam current of 900 mA. The beam distributions are measured with beam transfer functions and are aligned to the left with an offset for better visibility of the effect.