Narrow-band photon beam via laser Compton scattering in an energy recovery linac

Narrow-bandwidth photon beams in the x-ray and $\gamma$-ray energy ranges are expected to be applied in various fields. An energy recovery linac (ERL)-based laser Compton scattering (LCS) source employing a laser enhancement cavity can produce a high-flux and narrow-bandwidth photon beam. We conducted the first experiment of an ERL-based LCS source in combination with a laser enhancement cavity. We obtained LCS photons with an energy of $6.95\pm 0.01\mathrm{keV}$ by colliding an electron beam of 20 MeV with a laser of 1064 nm wavelength. The photon flux at the interaction point was evaluated to be $(2.6\pm 0.1)\times 10^7\mathrm{photons}/\mathrm{s}$ with an average beam current of $58\mu \mathrm{A}$ and an average laser power of 10 kW. The energy bandwidth was evaluated to be 0.4% (rms) with an opening angle of 0.14 mrad. The technologies demonstrated in this experiment are applicable for future ERL-based LCS sources.

Figure 1

Schematic of laser Compton scattering.

Figure 2

Calculated LCS x-ray spectra (a) simulated with ${ϵ}_n=1\mathrm{mm}\mathrm{mrad}$ and (b) ${ϵ}_n=5\mathrm{mm}\mathrm{mrad}$.

Figure 3

Layout of the Compact ERL. COL1-COL5 are movable collimators to scrape off the beam halo.

Figure 4

Beam evolution in the injector optimized for 0.77 pC bunch charge: normalized rms emittances (top), transverse beam sizes and bunch length (bottom) versus position in the injector.

Figure 5

Beam envelopes designed to obtain a tight focused electron beam at the IP, ${\beta }_x^{*}=0.11\mathrm{m}$ and ${\beta }_y^{*}=0.037\mathrm{m}$.

Figure 6

Schematic drawing of the LCS beam line.

Figure 7

Schematic of the bow-tie 4-mirror cavity.

Figure 8

The optical cavity structure.

Figure 9

Dependence of the spot size at the IP on distance between the concave mirrors.

Figure 10

Spot size of laser beam along the propagation in the cavity. Vertical dotted lines show the position of the mirrors.

Figure 11

Schematic drawing of the cavity system. The whole optical system was mounted on a mover, an optical bench movable in the two transverse directions of the electron beam.

Figure 12

Schematic figure of the inner layout of the vacuum chamber. A removable screen monitor was located at the IP.

Figure 13

Setup of the optical system. $\lambda /2$: half-wave plate, $\lambda /4$: quarter-wave plate, PBS: polarizing beam splitter, PD: photodiode, isolator consists of Faraday rotator and PBS.

Figure 14

Block diagram of the feedback system.

Figure 15

The intensity of the transmitted light and the error signal.

Figure 16

Laser beam profile measured at behind M2. A regular fringe structure is due to neutral density filters for the laser beam profiler.

Figure 17

Observation of resonance peaks.

Figure 18

Stability of the stored laser power in the optical cavity.

Figure 19

Stability of the laser phase in phase locking feedback.

Figure 20

Tuning of the electron beam sizes by the $Q$-scan using screen monitor at the IP.

Figure 21

Measurement of emittance with LCS operation mode. The red line is the fitted curve with a parabolic function.

Figure 22

Snapshots of the screen monitor located at the IP. (a) shows the laser profile without an electron beam. (b) shows the electron beam profile.

Figure 23

Detected energy versus phase difference between the cERL rf and laser.

Figure 24

LCS signal rate as a function of the vertical position of the laser beam (top) and timing of laser phase (bottom).

Figure 25

Energy spectrum of the observed LCS photons with a linear scale (top) and a logarithmic scale (bottom).

Figure 26

Setup of the LCS x-ray imaging at the cERL.

Figure 27

LCS x-ray imaging of a hornet. The distance from the sample to the detector is 2.5 m for refraction contrast imaging.

Figure 28

LCS x-ray flux during image acquisition.

Figure 29

X-ray spectra (a) measured with HP Ge detectors and (b) simulated with the EGS5 Monte Carlo computer code.

Figure 30

Calculated energy spectra of the LCS photons at the detector. The solid line shows a spectrum with a perfectly aligned detector and the dotted line is for a detector having position error of 1.5 mm in the horizontal and vertical directions.