Manipulation of Laser Distribution to Mitigate the Space-Charge Effect for Improving the Performance of a THz Coherent Undulator Radiation Source

Abstract

A THz coherent undulator radiation (THz-CUR) source has been developed at the Institute of Advanced Energy, Kyoto University. A photocathode Radio-Frequency (RF) gun and a bunch compressor chicane are used for generating short-bunch electron beams. When the electron beam energy is low, the space-charge effect strongly degrades the beam quality, such as the bunch length and the energy spread at the high bunch charge condition at around 160 pC, and results in the reduction of the highest frequency and the maximum radiated power of the THz-CUR. To mitigate the space charge effect, we have investigated the dependence of the electron beam quality on the laser distribution in transverse and longitudinal directions by using a numerical simulation code, General Particle Tracer GPT. The manipulation of the laser distribution has potential for improving the performance of the THz-CUR source. The electron bunch was effectively compressed with the chicane magnet when the laser transverse distribution was the truncated Gaussian profile, illuminating a cathode. Moreover, the compressed electron bunch was shortened by enlarging the laser pulse width. Consequently, an enhancement of the radiated power of the THz-CUR has been indicated.

Theory and design consideration of a THz superradiant waveguide FEL

“…Bunched beam superradiance is an attractive concept for attaining intense THz radiation in a compact FEL scheme [1,3,[14][15][16]]. Here we presented theory and design consideration of a superradiant waveguide FEL based on a compact (60 cm long) hybrid photocathode RF gun.…”

“…In the presented design the bunch duration was limited by space charge effects in the undulator section. It may be possible to mitigate these space-charge effects by proper shaping of the beam transverse and longitudinal profiles at the gun and at the entrance to the undulator [14] and thus restore the emission energies predicted for an ideal beam.…”

“…( 7)) would diminish the superradiant radiation at frequencies beyond 3 THz. Attainment of higher frequency superradiance in future superradiant FEL designs requires technological advance in reduction of the intrinsic energy spread of the gun or enhancement of the modulation (chirp) factor S, (that would require also a shorter drift length to the undulator) and mitigation of space charge effects in the undulator region, possibly by optimizing the dimensions and distribution shape of the beam [14].…”

Considered design and radiation calculation for a compact THz coherent radiation source

“…The space charge effect may be the main cause to limit the power of radiation. For mitigating the space charge effect, the manipulation of the laser distribution reported in [16] will be one of possibility used in the process of beam dynamics optimization to achieve high qualities of electron beam. For on-axis radiation (seen in figure 5(b)), odd higher harmonics of the desired radiation will always appear under the condition of 0.1 ps electron bunch length.…”

Design and Beam Dynamic Studies of an Injector for a Compact THz Coherent Radiation Source

“…A bunch charge of 1.0 nC/bunch was assumed to be generated from the RF gun. Techniques, such as optimizing the laser pulse shape, can be employed to address concerns related to space charge effects, as described in reference [12][13][14]. Although solenoid focusing can compensate for the linear space-charge force, it may not be sufficient to address the nonlinear space-charge force on the cathode surface.…”