The inverse Smith-Purcell effect in the subillimeter region

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The inverse Smith-Purcell (ISP) effect is one of the candidates for laser-driven linacs with a metallic grating as an interaction circuit to produce acceleration gradient of a few GeV/m. The purposes of the project are to demonstrate experimentally the ISP effect by using a submillimeter wave (SMM) laser and confirm validity of theoretical predictions on the effect.We performed the theoretical analyses of the ISP effect in the SMM region in order to design the experimental equipents necessary to demonstrate it. The optimum groove dimensions of the grating with rectangular profile were determined for various SMM laser lines by analyzing the electric field distribution just in front of the grating surface. Also with computer simulations, it has been found that a laser power of lW can produce [[30eV increase in the electron energy spectrum.We have constructed the experimental system based upon the theoretically analyzed results. The system consists of an optically pumped SMM laser and the electron beam system which includes the metallic grating fabricated on a milling machine according to the theoretical design. The SMM CH_3F molecular laser with the wavelength of 496<mu>m was pumped by an EQ- switched CO_2 laser newly developed for this project. The CH_3F laser had the maximum peak power of 46W, the repetition rate of up to 9.2kHz, and regular output pulse shape under the fundamental transverse mode of operation.In early 1987, we have successfully observed the ISP effect for the first time. The experimental results showed that lW laser power produces an increase of approximately 18eV in the full width for the energy spectrum of electrons. Furthermore. we measured electron energy change as a function of the laser power and of the initial electron energy. These experimental results gave good agreement with the theoretical perdictions.