[Paper Review] Status of polarization control experiment at Shanghai deep ultraviolet free electron laser

Haixiao Deng, Tong Zhang+16 more|arXiv (Cornell University)|Aug 13, 2012

Particle Accelerators and Free-Electron Lasers2 references3 citations

TL;DR

This paper presents the design and status of a polarization control experiment at the Shanghai Deep Ultraviolet Free Electron Laser (S-DUV FEL) test facility, utilizing a pair of crossed planar undulators with an electromagnetic phase-shifter to generate fully coherent, circularly polarized radiation. Simulations show the system can produce 100 nJ pulse energy, 5 ps pulse duration, and over 90% circular polarization degree, demonstrating a viable path to tunable, high-quality FEL radiation with controlled polarization.

ABSTRACT

A polarization control experiment by utilizing a pair of crossed undulators has been proposed for the Shanghai deep ultraviolet free electron laser test facility. Numerical simulations indicate that, with the electromagnetic phase-shifter located between the two crossed planar undulators, fully coherent radiation with 100 nJ order pulse energy, 5 picoseconds pulse length and circular polarization degree above 90% could be generated. The physical design study and the preparation status of the experiment are presented in the paper.

Motivation & Objective

• To develop a method for generating fully coherent, tunable circularly polarized radiation in the deep ultraviolet range using a free electron laser.
• To design and implement a polarization control system based on crossed planar undulators with an electromagnetic phase-shifter.
• To achieve high polarization degree (>90%) with stable, coherent output suitable for advanced scientific applications.
• To validate the physical design through numerical simulations and prepare for experimental realization at the S-DUV FEL test facility.
• To enable future applications requiring controllable polarization states in the deep UV spectral region.

Proposed method

• A pair of crossed planar undulators is used to generate transverse electromagnetic modes with orthogonal polarization states.
• An electromagnetic phase-shifter is placed between the two undulators to control the relative phase between the two orthogonal modes.
• The phase-shifter enables dynamic tuning of the polarization state from linear to circular by adjusting the phase difference.
• Numerical simulations are performed to model the FEL gain, radiation power, and polarization characteristics under various operating conditions.
• The system is designed to operate in a self-seeded mode to enhance temporal and spectral coherence.
• The physical layout and component tolerances are optimized to maintain high polarization purity and pulse quality.

Experimental results

Research questions

• RQ1 Can a crossed undulator configuration with a phase-shifter produce fully coherent radiation with high circular polarization degree in the deep UV range?
• RQ2 What are the optimal parameters for the phase-shifter and undulator geometry to achieve >90% circular polarization?
• RQ3 What pulse energy and duration can be achieved while maintaining high polarization purity?
• RQ4 How does the phase-shifter affect the temporal and spectral properties of the output radiation?
• RQ5 Can the system be experimentally realized with the current S-DUV FEL test facility infrastructure?

Key findings

• The system is capable of generating fully coherent radiation with a pulse energy on the order of 100 nJ.
• The simulated pulse duration is 5 picoseconds, suitable for ultrafast science applications.
• The circular polarization degree exceeds 90%, indicating high-quality polarization control.
• Numerical simulations confirm the feasibility of achieving stable, tunable circular polarization using the phase-shifter between crossed undulators.
• The physical design is well-advanced, with detailed preparation underway for experimental implementation at the S-DUV FEL test facility.
• The results demonstrate a promising pathway toward generating high-brightness, fully polarized deep UV FEL radiation.