Pulse-mode measurement of electron beam halo using diamond-based detector

Using a diamond-based detector, the electron beam halo in a high-energy accelerator can be measured with a lower detection limit than that using other instruments, such as a core monitor, a dose meter, or an optical fiber. We have successfully measured an electron beam halo using diamond-based detectors operating in the ionization mode, which were installed in the beam duct to measure the intensity of the beam halo directly. Pulse-by-pulse measurements were adopted to suppress the background noise efficiently. Feasibility tests on the diamond-based detector and beam halo monitor were performed in the beam dump area of the 8 GeV SPring-8 synchrotron booster and at the 250 MeV SPring-8 Compact SASE Source test accelerator for the SPring-8 Angstrom Compact free electron LAser (SACLA), respectively. We achieved a lower detection limit of $2\times {10}^3\mathrm{electrons}/\mathrm{pulse}$ for single-shot measurement, which corresponds to a ratio of about ${10}^{-6}$ relative to the typical charge of the beam core of 0.3 pC. We also confirmed the feasibility of the electron beam halo monitor for use as an interlock sensor to protect undulator permanent magnets used in SACLA from radiation damage.

Figure 1

Principle of the diamond-based detector.

Figure 2

Structure of the diamond-based detector.

Figure 3

Setup of the electron beam tests performed in the SPring-8 synchrotron beam dump area.

Figure 4

Pulse shape of current signal of the diamond-based detector.

Figure 5

Linearity of output signal.

Figure 6

Photographs: (a) diamond-based detector on the holder, (b) configuration of the detectors in the vacuum chamber, and (c) beam halo monitor installed at the SCSS test accelerator.

Figure 7

Layout of SCSS test accelerator.

Figure 8

Pulse shape of the beam halo monitor (blue line: one-shot measurement; red line: 10-shot average).

Figure 9

Profiles of a weak beam (blue line: upper detector; red line: lower detector).

Figure 10

Waveform caused by induction current. The gaps between the upper and lower detectors are indicated.

Figure 11

Spectrum of the signal caused by the induction current.

Figure 12

Effect of induction current before and after suppression. The lower (red) waveform is with LPFs and the upper (blue) waveform is without LPFs.

Figure 13

OTR images. The spots indicated by yellow arrows are due to satellite bunches.

Figure 14

Signals from the beam halo monitor.

Figure 15

Effect of distance from beam center to diamond-based detector on FEL oscillation. The intensity of FEL oscillation (vertical axis) was calculated from an output signal of a photodiode.

Figure 16

Scanning measurements for various spatial slit widths. The numbers are the widths of the spatial slit. (The aperture size of the energy filter was fixed to 10 mm.)

Figure 17

Scanning measurements with various aperture size of the energy filter. The numbers are the aperture sizes of the energy filter. (The width of the spatial slit was fixed to 10 mm.)

Figure 18

Schematic diagram of the beam halo monitoring system.

Figure 19

Typical waveform stored in database. The upper (green) waveform is from the upper detector and the lower (red) waveform is from the lower detector.

Figure 20

Result of half-day measurement during the machine study period. The left axis shows the charge signals from the upper (green) and lower (red) detectors of the beam halo monitor, and the right axis shows the intensity of the FEL oscillation (blue). The gap between the detectors is 4 mm.

Figure 21

Result of stability test. The left axis shows the integral output signal of the beam halo monitor (upper: green; lower: red), and the right axis shows the intensity of the electron beam (blue). The gap between the detectors is 4 mm.