Development of photo injector employing Yb fiber laser for stimulus super-radiant THz FEL

THz wave is wonderful prove for materials. Additionally new nonlinear phenomena are expected in spintronic devices if the peak electromagnetic field is greatly higher than 10 MV/cm (3.3 T/cm). Although coherent transition radiation from short bunches is intense, it is very difficult to exceed 10 MV/cm. However we found a possibility of which oscillator FEL reaches ~100 MV/cm employing a pre-bunched configuration.* On a test accelerator (t-ACTS), we are going to introduce a photo-cathode to increase the bunch charge and synchronize with round-trip frequency of an FEL optical resonator. Currently t-ACTS supplies a bunch-charge of 5 pC, whereas the goal is to increase it more than 50 pC for the pre-bunched FEL. Since the intensity of coherent radiation is basically proportional to the square of the charge per bunch, increase of the bunch charge is much effective. The laser system includes a Yb fiber laser oscillator (wavelength: 1047 nm), a multi-pass amplifier, and a fourth harmonic generation (262 nm). A target synchronization accuracy is within 1 deg at 2856 MHz. The assembly of the oscillator has almost completed, and a work for synchronization with RF is on the way.