At small undulator gaps, necessary for generation of low energy photons, large magnetic fields give rise to very intense higher harmonics. For example, an undulator gap corresponding to a photon energy of 10 eV will also emit light at 20, 30,...eV. Since a grating will not eliminate higher order harmonics, the beamline utilizes a gas filter, which eliminates higher photon energies through absorption. The filter is a 4.5” long pipe filled to about 30 Torr with a rare gas. Light above the ionization energy of the rare gas is fully absorbed, creating a low-pass filter. The extent of suppression can be calculated using Beer's Law:
where I and I0 are, respectively, the transmitted and incident photon intensities, σ is the photoabsorption cross section , I is the path length, and N is the gas density. Thus, if the cross section is 10-18 cm2, the path length is 12 cm, and the gas density at 30 Torr is 1018 atoms/cm3, then I/I0 is 10-5. This result indicates that the higher harmonics are suppressed by five orders of magnitude, see Figure 1 for the experimental verification.
Figure 1: Experimental spectrum showcasing the suppression
of high order harmonics in the gas filter
Due to strict vacuum requirements (10-9 torr) for the undulator, three stages of differential pumping are utilized to create the gas filter, see Figure 2. The first stage reduces the pressure from 30 torr to 10-3 torr, followed by a reduction in pressure in the second stage to 10-7 torr, and finally to 10-9 torr.
Figure 2: Diagram of the differential pumping in the gas filter.
The photon image is focused to a spot size of about 100 microns in diameter in the center of the gas filter chamber, which permits the termination of the high pressure region with conductance limiting 1.0 and 0.5 mm tubing. The ends of the tubes are cut at angles to prevent the formation of a molecular beam directed along the axis of the light path. In addition, small holes are located on the side of the tube to enhance turbulence and thus minimize the molecular beam formation.
Only inert gases are used to fill the gas filter. The following table indicates the operating pressures for the standard gases used at the beamline. Please note that Helium and Argon are readily available. If another inert gas is required, please make arrangements with BL staff ahead of scheduled beamtime.
|Gas||Cut-off Energy (eV)||Gas Cell Pressure
P.A. Heimann, M. Koike, C. Hsu, M.D. Evans, K. Lu, C.Y. Ng, A.G. Suits, and Y.T. Lee "Performance of the chemical dynamics beamline at the Advanced Light Source," Rev. Sci. Instrum., 68, 1945 (1997).
A.G. Suits, P.A. Heimann, X. Yang, M.D. Evans, C. Hsu, K. Lu and Y.T. Lee, "A differentially pumped harmonic filter on the chemical dynamics beamline at the Advanced Light Source." Rev. Sci. Instrum., 66, 4841 (1997).
P. A. Heimann, M. Koike, C. W. Hsu, M. Evans, C. Y. Ng, D. Blank, X. M. Yang, C. Flaim, A. G. Suits and Y. T. Lee, "Performance of the VUV High Resolution and High Flux Beamline for Chemical Dynamics Studies at the Advanced Light Source," In Optics for High-Brightness Synchrotron Radiation Beamlines II, Proc. SPIE., 2856, 90 (1996).