Thermal distortion limits on the performance of XUV free-electron lasers configured with a multifacet-mirror ring resonator

摘要

XUV (10 nm $LSEQ $lambda $LSEQ 100 nm) free-electron lasers (FELs) are potential light sources for materials research applications (to complement future synchrotron light sources) and soft x-ray projection lithography applications for the manufacture of electronic microcircuits. Requirements for compact physical dimensions for these devices, together with relatively low reflectivity mirrors and small optical beam sizes at these wavelengths, make thermal loading of the mirrors of XUV FEL oscillators a potentially serious problem. In this paper, we consider the effects of thermal distortion of the mirrors on the performance of an XUV FEL designed specifically as a 60 nm light source for lithographic applications. We formulate a mathematical model of the relevant physical processes and solve that model with the aid of a 3-D FEL simulation code. The numerical solutions demonstrate the existence of an unusual optical mode of the FEL oscillator in which the steady-state resonator power decreases linearly as the thermal distortion coefficient of the mirrors increases. The stabilizing effect of a scraper mirror as a mode control aperture is discussed. Limits on the magnitude of the mirror thermal distortion coefficient, for which near ideal FEL performance is retained, are discussed.