# Multiple-Collision Free-Electron Laser Compton Backscattering for a High-Yield Gamma-Ray Source
## 3 Results
### Demonstration of the Multiple-Collision FEL-Compton Backscattering  
In order to generate the multiple-collision FEL-Compton backscattering gamma rays, the infrared FELs in the storage ring NJJI-IV were oscillated in the three-bunch operation. Coupled-bunch instability appeared in the electron beam of the storage ring NJJI-IV when there were three or more electron bunches in the storage ring. However, the FEL oscillation stabilized in the three-bunch operation when the electron-beam current exceeded 13 mA. This is because the FEL oscillation caused bunch lengthening due to the bunch heating effect, and the electron density in the bunch decreased considerably [39]. In the above-mentioned current region, each electron bunch in the three-bunch operation emitted the same saturated FEL power as that in the single-bunch operation. Using an radio-frequency (RF) knock-out method, we filled RF-buckets #0, #5, and #10 with bunch intervals of 31 ns, 31 ns, and 37 ns, respectively [26]. The electron bunches #0, #5, and #10 collided with the FEL pulses at the points (C\({}_{3}\), C\({}_{-2}\)), (C\({}_{3}\), C\({}_{-3}\)), and (C\({}_{2}\), C\({}_{-3}\)), respectively. There was no external magnetic field except at the collision point C\({}_{2}\), and the yield of the gamma-ray beam passing through the lead collimator was high. Figure 5 shows the spectra of the FEL-Compton backscattering gamma rays generated by each electron bunch at the electron-beam current of 4.2 mA for bunch #0, 4.9 mA for bunch #5, and 5.4 mA for bunch #10. The FEL power extracted from the cavity mirror was 0.89 mW in the air. The yields of the gamma rays passing through the lead collimator were \(1.7\times 10^{5}\) photons/s in consideration of the efficiency of the measurement system. Figure 5 also shows the calculated spectra of the FEL-Compton backscattering gamma rays for electron bunches #0, #5, and #10.  
Figure 4: Measured energy spectra of the FEL-Compton backscattering gamma rays for the bunch-fill patterns of (**a**) (#0, #5) and (**b**) (#0, #6). The calculated spectra are indicated by the red broken lines. The opening half angles of the collimator were 0.47, 0.52, 0.84, and 1.0 mrad for the collision points of C\({}_{3}\), C\({}_{2}\), C\({}_{-2}\), and C\({}_{-3}\), respectively.  
The spatial distribution of the FEL-Compton backscattering gamma rays was also measured in the three-bunch operation. The bunch-fill pattern of the electron beam was (#0, #5, #10) as in the measurement of the gamma-ray spectra. Figure 6 shows the two-dimensional distribution of the gamma-ray beam measured by the IP with exposure time of one minute. The electron-beam current was 19.2 mA and the currents of each bunch were almost equal. The FEL power extracted from the cavity mirror was 1.34 mW in the air, and the total yield of the FEL-Compton backscattering gamma rays was evaluated to be approximately \(2\times 10^{6}\) photons/s. The diameter of the cavity mirror was 2 inches, and the effective area where the gamma-ray beam was not disturbed by the metallic mirror holder was in a circle with a diameter of approximately 50 mm.  
Figure 5: Measured energy spectra of the FEL-Compton backscattering gamma rays generated by electron bunches (**a**) #0, (**b**) #5, and (**c**) #10. The spectra evaluated using the calculated data are indicated by the red broken lines.