# Multiple-Collision Free-Electron Laser Compton Backscattering for a High-Yield Gamma-Ray Source
## 3 Results  
### Experimental Setup Using the Infrared FEL System in the Niji-Iv  
The infrared FEL system in the storage ring NIJI-IV was used to observe the spatial distribution and the spectrum of FEL-Compton backscattering gamma rays at multiple collision points. Figure 2 shows the scheme of the infrared FEL system and the measurement configuration of the FEL-Comptonbackscattering experiments. The NJIJ-IV can store up to 16 electron bunches in its circumference of 29.6 m [29]. The energy of the electron is usually operated at 310 MeV in infrared FEL experiments. The fundamental FEL oscillations in the wavelength region of 0.84-2.68 \(\upmu\)m have been demonstrated with this system [30,31]. An ETLOK-III planar optical klystron was incorporated into the infrared FEL system as an insertion device [32]. It has two identical undulator sections of length 1.4 m and one dispersive section of length 0.72 m. The undulator period in the undulator section is 0.2 m, and the number of the periods is 7. Although the value of the deflection parameter \(K\) is tunable from 1.28 to 10.4, it was fixed at 3.03 in the demonstration experiments of the multiple-collision FEL-Compton backscattering. As shown in Figure 3, the wavelength of the FEL oscillation used in these experiments was 1.53 \(\upmu\)m. The finesse of the optical cavity was \(2.6\times 10^{3}\) at the wavelength of 1.53 \(\upmu\)m. The energy spread of the electron beam increased up to approximately \(10^{-3}\) due to bunch heating caused by the FEL oscillations. However, the electron beam sizes did not increase at the collision points of the FEL-Compton backscattering on oscillations because the dispersion function was almost zero in the long straight section where the ETLOK-III was installed. Because the optical cavity length of the infrared FEL system is 14.8 m, it can store up to 16 FEL pulses as well as the electron bunches. However, the maximum number of electron bunches that could oscillate in the FEL was four due to the coupled-bunch instability of the electron beam in the  
Figure 3: Spectra of the infrared FELs at the FEL-Compton backscattering experiments in (**a**) the two-bunch operation and (**b**) the three-bunch operation. Figure 2: Schematic layout of the FEL-Compton backscattering experiments in the storage ring NJIJ-IV. The cross symbols are the collision points in the optical cavity and the numerals under the names of collision points are the numbers of FEL-Compton backscattering processes per revolution period at each collision point. The collision points without magnetic field are shown in red. NJJI-IV [31; 33]. The monochromaticity of the FEL-Compton backscattering gamma rays is degraded by the magnetic field at the collision points [34]. By controlling the arrangement of electron bunches, multiple collision points can be set outside the optical klystron ETLOK-III or quadrupole magnets. As shown in Figure 2, there are three collision points (C\({}_{3}\), C\({}_{-2}\), C\({}_{-3}\)) without magnetic field in the optical cavity. At least two electron bunches are required for generating gamma rays via backward Compton scattering with the storage ring FELs. Because an electron bunch collides with only one FEL pulse in the optical cavity in the two-bunch operation of the NJJI-IV, the number of collisions per revolution period is two, which is the same as the number of electron bunches. The FEL-Compton backscattering gamma rays were observed behind the downstream mirror chamber. A lead collimator with a bore diameter of 10 mm and thickness of 100 mm was positioned at a distance of 0.4 m from the downstream cavity mirror in the air. A Ta\({}_{2}\)O\({}_{5}\)/SiO\({}_{2}\) multilayer mirror, which had a quartz substrate, was used as the cavity mirror [35; 36]. The gamma-ray beam passing through the collimator was measured using a LaBr\({}_{3}\)(Ce) scintillation detector with diameter and length of 25 mm and 51 mm [34], respectively. The output signals from this detector were divided and fed into a constant fraction discriminator and a shaping amplifier.