NSLS2 U20 Tapered Spectrum Quick Simulation

Calculations by Toshi Tanabe, May 14, 2008

Page author: Bruce Ravel

Spectrum of the tapered U20

The following tapered (~20% field change over 3m, small gap B=0.8T) U20 spectrum is calculated with SPECTRA.

About this image, Toshi says:

I have to point out that spectra shown ... is spatial
flux density (photons/s/mm^2/0.1%BW) at 20m away.  

I have noticed that when people use "Flux", it is sometimes 
photons/s/0.1%BW, sometimes (for BM users?) it is 
photons/s/0.1%BW/mrad.

If what you need is total flux at a certain spectral bandwidth,
DW source might be much superior to any undulator by 
collecting the light in big aperture.

So very true!!

Close up views of the third and fifth harmonics

Magnified in the third harmonics. It looks like the value is smaller than that by DW.

Fifth harmonics

At the NSLS-II source properties page we see that the flux for U20 is down compared to DW90 by anywhere from a factor of 2 to a more than an order of magnitude between 6 and 25 keV. Tapering the U20 will reduce its flux at a given energy by an additional 1 or 2 orders.

Screenshots of the SPECTRA program

Toshi is using SPECTRA to simulate the profile of the tapered undulator.

Paramters used are show in the following figs. (gap value “20mm” is irrelevant)