Recently, manipulation with the UV driving laser plays a significant role in high brightness electron beam production by the photocathode RF gun. The method based on pulse stacking with birefringent crystal serials was tried to longitudinally shape ultraviolet laser pulse. Using four or five pieces of α-BBO crystals to stack an input UV pulse with appropriate initial duration into 16 or 32 sub-pulses is able to form quasi flattop UV laser pulse, which can be applied for emittance optimization of the electron beam based on the photocathode RF gun. Moreover, the negative slop of the energy transmittance of aBBO serials is also revealed to be a passive stabilization mechanism for energy jitter reduction in the driving laser. With appropriate design of a-BBO serials, this method can fulfil the requirements for driving laser in a broad scope of applications such as x-ray FELs and high-power Terahertz(THz) radiation production.
It has been assumed that the initial temporal distribution of the electron beam is the same as that of the laser pulse, so electron beams with arbitrary temporal profile can be achieved by laser pulse shaping duo to the negligible emission time of the photocathode illuminated by the ultrashort driving laser pulses. The quasi flattop UV laser pulse formed by shaping method based on the birefringent α-BBO crystal serials can be applied to reduce the initial emittance of the electron beam from the photocathode RF gun. The beam length can also be controlled by changing the initial pulse duration. In practice, for S-band photocathode RF gun the optimal drive laser has duration of about 10ps and the rising/falling edge should be less than 1ps. In recent years, several shaping methods for ultrashort laser pulse have been developed such as frequency domain shaping technique represented by the AOPDF for temporally flattop pulse production. Other methods such as Neumann’s method by Fabre–Perot interferometer, pulse stacking by beam splitter plates, or by polarizing beam splitter cubes have also been proposed and tested. Due to lack of material that can transmit UV light, the feasibilities of using a-BBO crystals, which can transmit UV light has been investigated and employed.
For electron beams another key requirement besides the low emittance is that the rms beam charge jitter should be less than a certain value, typically 2%, so it demands that the UV laser pulse energy jitter should be less than 2% rms. Efforts have been tried to reduce the energy jitter of UV laser. Active stabilizing of the pump lasers is applied to meet the requirement for the UV energy jitter（2% rms ）to guarantee the peak current jitter (12% rms) at the undulator in the design of Linac Coherent Light Source(LCLS) ⅠandⅡ. In the SwissFEL, multiplexing of six ultra-stable diode-based pump lasers and complex pre-compensation loop are applied to achieve 0.54% energy jitter in IR for pulse energy of about 20mJ. In the FERMI system, the UV laser energy jitter is designed to be 3% rms to ease the parameter realization. In this paper, a passive mechanism for energy jitter reduction in UV is investigated by taking advantage of the negative slope of the UV energy transmittance in the cascaded pulse stacking system mentioned above employing α-BBO crystals.
This article comes from accelconf edit released