Development of Small-sized Cherenkov FEL in the Far-Infrared by Using a Small Electron Beam Cathode

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The concern of many researchers on free electron laser (FEL) is toward large-sized FELs in the shorter wavelength such as X ray. However some of them are also interested in the THz frequency region because of the recent boom of THz technology. In order to generalize and apply FEL in the THz or far-infrared region, it is very important to develop a tabletop type FEL. Even an FEL with a micro-wiggler is not unfortunately so small that it can be installed in a usual laboratory of university. Therefore other type FEL must be considered and then we have been developing a small-sized Cherenkov FEL (C-FEL) by using a field emission electron beam source.In order to construct the C-FEL, we have developed/studied (1) a micro e-beam source, (2) e-beam focusing and propagation system, and (3) waveguide resonator and the estimation method of its optical constant. We also researched on the theory for C-FEL and its simulation computational code, and formation of single walled carbon nanotubes (SW-CNT … More s) as a candidate for new e-beam source.The simulation for small signal gain of C-FEL showed 〜10 % at the frequency of 46.4 GHz under the condition of the e-beam (50 keV and 1 mA) propagating between two Si plates with thickness of 0.65 mm. It also saturated after 〜300 round trips of the radiation inside the Si resonator. The e-beam in the magnetic flux density of 5 T would propagate with the diameter of 〜12μm. Referring to these results etc., the C-FEL has been designed/constructed in a superconducting magnet of 5 T. It is so small that the height and area are 2 m and 0.7x0.7 m^2, respectively. We would succeed in the C-FEL oscillation using a Spindt cathode in the near future. We also have been constructing another C-FEL which dose not use a superconducting magnet but electric field lens for e-beam.SW-CNTs were produced by an ablation method using an excimer laser. The purity of them was higher than those by other methods. It was observed that the diameter of SW-CNT became smaller with the wavelength of the excimer laser. Although the produced SW-CNTs were used as a field emission cathode, they did not give, at present time, a stable e-beam current. Less