Design of the Mid-infrared Oscillator Free Electron Laser at Chiang Mai University

Abstract

At the PBP-CMU Electron Linac Laboratory (PCELL) of the Plasma and Beam Physics Research Facility (PBP), Chiang Mai University (CMU), the upgrade of the elec- tron accelerator system to produce mid-infrared free-electron laser (MIR-FEL) and super- radiant terahertz free electron laser (THz-FEL) is in progress. In this thesis, the MIR-FEL oscillator system is designed and the generation of the FEL is studied using computer sim- ulations. In the design of an optical cavity, the laser waist position is set at the middle of the permanent magnet undulator, which has a total length of 1.6 m. There are two concave mirrors placed upstream and downstream the waist at the positions of 2.956 and 2.450 m, respectively. This configuration yields the cavity length of 5.406 m, which con- tains 102 FEL pulses with an RF repetition rate of 2,856 MHz. Then, the setup is used in FEL simulation with the GENESIS 1.3 code, which is modifed for this research. The electron beam with kinetic energy of 25 MeV and the measured 1.6-m undulator magnetic field distribution with the average maximum K-value of 1.07 are used in FEL simulation. Thus, the MIR-FEL with the central wavelength of 13.01 pim is investigated. The opti- mum upstream and downstream mirror curvatures of 3.091 and 2.613 m are respectively obtained, which provide the Rayleigh length of 0.631 m for the 13.01-pim FEL wavelength. These designed values are represented by the g-parameter of 0.80 that has the 10-% mirror misalignment tolerance of the transverse offset with the tilt of 士0.25 mm and +0.15 mrad, respectively. The FEL simulation result shows that the optical cavity length should be extended with the slippage length that is 20 times of the FEL wavelength to reduce the slippage effect. Then, the optical cavity length detuning by reducing the cavity length of an FEL wavelength is mandatory for the appropriate FEL lasing. This suggests that the longitudinal moving step length of both two mirrors would be within a few microns. To generate the MIR-FEL, the electron beam requires rms transverse size of smaller than 1.0 mm at the undulator entrance with focusing point at the 50 - 75 % of the undulator length from the entrance. The FWHM bunch duration of not larger than 1 ps and the peak cur- rent of higer than 50 A are required for generation of the MIR-FEL with the intra-cavity peak power from sub-Watt to sub-GW scale at the FEL saturation point. 'Two sets of electron beam properties are considered to estimate the MIR-FEL properties. The first considered beam has the kinetic energy of 25 MeV and the bunch charge of 60 pC with the peak current of 123 A and the transverse beam sizes of 0.2 - 0.5 mm at the undulator entrance. The second one has the kinetic energy of 22 MeV and the bunch charge of 60 PC with the peak current of 53 A and the transverse beam sizes of 0.8 - 1.0 mm at the undulator entrance. As a result, the MIR-FEL with the wavelength range of 9.5 - 12.9 pm is obtained from the 25-MeV electron beam using the undulator K-value of 0.53 - 1.07. The 22-MeV electron beam provides the FEL wavelength range of 13.5 - 16.6 ps with the undulator K-value of 0.75 - 1.07. The results obtained from both two electron beams are used for optimizing the upstream mirror hole radius for the radiation coupling out from the oscillator system. The study gives the optimum hole radius of 1.3 mm, which yields the FEL power coupling ratio of 0.13 - 0.15 %. As a result, the optimized optical cavity design with optimum parameters of these two electron beams provides the FEL power of up to sub-GW scale with the FEL saturation within 5 ps, which is within the expected maximum electron macro-pulse duration of 6 ps. At the saturation time, the extracted FEL pulse energies at the mirror hole are 0.3 - 0.4 p.J for 25-MeV beam and 0.15 - 0.2 p.J for 22-MeV beam.