Beamline 9.0.2 Optics

Harmonic Filter (Gas Filter)

At small undulator gaps necessary for generation of low energy photons, the large magnetic fields give rise to very intense higher harmonics. Thus if the undulator gap is set to 10 eV, light at 20, 30,...eV is passed as well. The higher harmonics cannot be stopped by a grating because gratings also pass these energies in higher orders. The gas filter has been installed in order to eliminate these higher harmonics by gas absorption. The filter consists of 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 by the gas and thus acts as a high energy cut-off filter. The extent of suppression can be calculated using Beer's Law:

where I and I_o are the transmitted and incident photon intensities, s is the photoabsorption cross section , lis the path length, and d is the gas density. 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, the I/I_o is 10^-5. Thus, the higher harmonics will be suppressed by five orders of magnitude. As shown in the spectrum, this is experimentally verified.

The vacuum requirements for the undulator are severe. The base pressure in the beamline before the undulator must be kept to a pressure below 10^-9 Torr. As shown in the gas filter diagram, three stages of differential pumping reduce the pressure in steps from 30 Torr to 10^-3 Torr, to 10^-7 Torr to 10^-9 Torr. An essential feature is the good focusing property of the undulator light, which allows the photon image to be focused to a spot size of about 100 microns in diameter in the center of the gas filter chamber. This 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 an angle to prevent the formation of a molecular beam that is 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.

In addition to the gas filter, MgF2 windows are available for the non-monochromatized light going to Terminal 1. This window may be inserted into the beamline providing a lower energy cut-off.

The picture below shows the gas filter in action.  The rare gas being ionized by the higher harmonics results in the emission of light, this is a similar process to what goes on inside a neon lamp.

References

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).