Juhao Wu

Simulation Study of the NSRRC High Brightness Linac System and Free Electron Laser

A 263 MeV linac system has been designed to provide a high brightness electron beam for the NSRRC VUV FEL test facility. This system is equipped with a dogleg with linearization optics to compensate the effects of nonlinear energy chirps introduced into the system by the chirper linac voltage during bunch compression. In this study, we performed start-to-end simulation to illustrate the capability of this linac system to generate a beam that can be used to drive a SASE FEL to saturation within reasonable undulator length. It has been demonstrated that, for a 200 pC beam, such FEL has a saturated output power of ~200 MW at 6-m undulator length. Further optimization of bunch current profile and momentum spectrum is required.

Accelerator-based compact extreme ultraviolet (EUV) sources for lithography

Extreme ultraviolet lithography (EUVL) is a next-generation technology using light at 13.5 nm. We start with general discussion about accelerator based approaches for compact EUV source generation, including Laser Induced Microbunching (LIM) schemes, High-Gain Inverse Compton (HGIC) source, etc. Such accelerator based EUV sources are compared to laser-produced plasma (LPP) source. Besides the high average power required by EUVL; for industrial applications, EUV source’s stability is an important measure. A storage-ring based steadystate microbunching (SSMB) configuration (Chao, Int. J. Mod. Phys. A, 2015) is a very promising approach providing EUV source at kilowatts level average power meeting the high volume manufacturing requirements for EUVL. SSMB overcomes the large pulse-to-pulse power fluctuation commonly existing in single-pass high-gain systems, e.g., a self-amplified spontaneous emission (SASE) free-electron laser. In a reversible SSMB configuration, the generation of high peak power EUV source and keeping high repetition rate are nicely decoupled.

EFFICIENCY ENHANCEMENT IN A TAPERED FREE ELECTRON LASER BY VARYING THE ELECTRON BEAM RADIUS

Energy extraction efficiency of a free electron laser (FEL) can be increased when the undulator is tapered after the FEL saturation. By use of ray equation approximation to combine the one-dimensional FEL theory and optical guiding approach, an explicit physical model is built to provide insight to the mechanism of the electron-radiation coherent interaction with variable undulator parameters as well as electron beam radius. The contribution of variation in electron beam radius and related transverse effects are studied based on the presented model and numerical simulation. Taking a recent studied terawatt, 120 m long tapered FEL as an example, we demonstrate that a reasonably varied, instead of a constant, electron beam radius along the undulator helps to improve the optical guiding and thus the radiation output.

Transverse effects due to random displacement of resistive wall segments and focusing elements

In this paper, we study the single bunch transverse beam dynamics in the presence of random displacements of resistive wall segments and focusing elements. Analytical formulas are obtained for long-range resistive wall wake, together with numerical results for short-range resistive wall wake. The results are applied to the LCLS project and some other proposed accelerators.

Transverse effect due to short-range resistive wall wakefield

For accelerator designs with ultra short electron beams, beam dynamics study has to invoke the short-range wake- fields. In this paper, we first obtain the short-range dipole mode resistive wall wakefield. Analytical approach is then developed to study the single bunch transverse beam dynamics due to this short-range resistive wall wake. The results are applied to the LCLS undulator.