To generate optical vortex with multiple topological charges, a simple scheme based on the phase mask shaping technique is proposed and applied in a seeded free electron laser. With a tailored phase mask, an extreme-ultraviolet (EUV) vortex with multiple topological charges can be produced. To prove the feasibility of this method, an eight-step phase mask is designed to shape the seed laser. The simulation results demonstrate that 100-MW, fully coherent EUV vortex pulses with topological charge 2 can be generated based on the proposed technique. We have also demonstrated the possibility of generating higher topological charges by using a phase mask with more steps.

In this study, we implement attosecond afterburners in an ultrashort soft X-ray free-electron laser (FEL) to improve the performance of generating attosecond pulses. In this scheme, the FEL pulse produced in the normal radiator section is dumped while the well bunched electron beam is reserved and reused in downstream afterburners. Subsequently, radiation in the afterburners gains rapidly as the bunching factor in the current spike is large, making the radiation pulse much shorter and cleaner than that from a normal radiator. Multi-shot simulations are carried out to demonstrate the performance and stability of the proposed technique.

Terahertz (THz) radiation has become a significant tool in cutting-edge research due to its superior properties. THz vortices with tunable orbital angular momentum (OAM) are particularly attractive to the scientific community due to their well-defined discrete azimuthal phase around the propagation axis. However, the generation of high-power THz radiation with OAM remains a challenge for most existing technologies. In this paper, a new method for generating coherent high-power THz vortices with tunable OAM and frequency is proposed by combining frequency beating and transverse phase mask shaping techniques. Theoretical analyses and numerical simulations indicate that this method can generate coherent THz vortices with peak powers in the tens of megawatts and tunable topological charge numbers.