DEVLOPMENT OF A LOW EMITTANCE DC GUN FOR SMITH-PURCELL BWO FEL

An electron DC gun capable for producing very low emittance beam is now under evaluation of beam qualities at Laboratory of Nuclear Science, Tohoku University. The DC gun employs a high voltage of 50 kV to extract electrons, which is suitable to drive Smith-Purcell backward wave oscillator free electron laser (BWO FEL) [1]. From a result of numerical simulation using a 3-D finite deference time domain (FDTD) method [2], the BWO FEL oscillation at the terahertz wavelength region maybe achieved by using the electron beam with an emittance lower than 1 π mm mrad. Average power is expected to be more than 100 W/mm2. In addition to which a very small cathode of LaB6 single crystal is employed for the gun [3], the geometrical structure is optimized to produce the lower emittance beam. A numerical calculation of the electro-static model for the DC gun to solve equilibrated beam envelope predicts a normalized beam emittance of 0.3 π mm mrad will be possible for the beam current of a couple of hundreds mA. Particularly by applying additional bias voltage between the cathode and the wehnelt, the transverse distribution of electrons is possibly becoming to be an ideal Kapchinskij-Vladimirskij (K-V) beam [4], so that the space charge effect will be minimized