Evaluation of a new VUV/soft x-ray toroidal grating monochromator with a movable exit slit

The performance of a toroidal grating monochromator can be significantly improved by compensating for the wavelength dependent defocusing aberration by means of a movable exit slit. Model calculations of a movable exit slit design show a resolving power that varies nearly linearly with wavelength over the scan range. The energy resolution follows the slope between two optimized resolution minima of a fixed slit TGM. The overall improvement in resolution due to the movable slit depends critically on the figure error of the toroid surface. The Al(LII,III) core edges provide a means to calibrate gratings and determine the resolution at 72.8 eV. A method is described for using the shift in core edge with exit slit position to enable final alignment of the exit slit motion providing stability of energy calibration of 2 meV over the scan range. Comparison of calculations and measured resolution suggests that slope errors remain the limiting factor in performance