X-1 undulator movie

To better understand why it is important to align the beamline using an undulator peak, I have made a "movie" of the intensity distribution as a function of wavelength. This movie takes some time to load, and it requires that you have a web browser that supports GIF animations (most newer ones do).

The movie shows at left the the intensity distribution as a false color image. Note that the X and Y axes have different scales; the intensity distribution is broader than it is tall. At right is shown a plot of the on-axis intensity, and a line shows you where you are on the spectrum at any given time. Note that the undulator output of the X-1 undulator is affected quite a bit by electron beam emittance; this calculation was done for a horizontal emittance of 1.1x10^-7 meter radians, and a vertical emittance of 1.3x10^-9 meter radians.

The calculations were done using the program SMUT^1,2,3 which I wrote some years ago. Other, faster programs exist nowadays, but SMUT does the job and I know how to run it...

The important thing to notice is that the intensity distribution peaks above the undulator axis when you are not at undulator peaks. Care must be taken to be at the wavelength of an undulator peak before one aligns things like the vertical exit slit.

You can also view a movie of the zero-emittance distribution (though the wavelength steps are not fine enough to nail the third harmonic peak on-axis) by clicking here.

References

1. C. Jacobsen, "X-ray holographic microscopy of biological specimens using an undulator," PhD thesis, Department of Physics, SUNY Stony Brook, 1988.
2. C. Jacobsen and H. Rarback, "Predictions on the performance of the soft x-ray undulator," in R. Tatchyn and I. Lindau, eds., Insertion Devices for Synchrotron Sources, Proceedings of the SPIE 582, 201-212 (1985).
3. H. Rarback, C. Jacobsen, J. Kirz, and I. McNulty, "The performance of the NSLS mini-undulator," Nuclear Instruments and Methods in Physics Research A 266, 96--105 (1988).