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kklmn avatar kklmn commented on August 17, 2024

The result of (1) strongly depends on how you calculate the linear and angular source sizes. A few approaches based on the Wigner function (a very rough approximation to phase-space distribution) are given e.g. in R.P.Walker, Phys. Rev. Accel. Beams 22, 050704 (2019). You can see there that the definitions vary a lot.

The result of (2) can also vary depending on what source characteristics you include in the calculations. In particular, it depends on the electron beam energy spread, far- or near-field way of calculations, the (de)tuning of the undulator and the inspected field of view (beamline acceptance). You should also check that you reach convergence, i.e. the number of one-electron fields is sufficient.

I wonder if I have make misunderstanding on these calculations.

And yes, the people's understanding of the coherence properties also has a non-zero spread :)

I have collected all this material into plots and tables and I hope within a few months I will write a paper on these issues. In that paper, I compare (a) direct 1:1 source imaging by means of wave propagation with (b) back phase propagation into the source plane. And I analyze the coherence properties by means of (1) Wigner function, (2) eigenmode decomposition and (c) double-slit diffraction.

The paper you mention is very limited in the implementation and self-consistency. We, therefore, canceled our participation in it. So I am also not quite sure how and what they did in the calculations.

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yangfg-bsrf avatar yangfg-bsrf commented on August 17, 2024

Thanks for your suggestion. Looking forward to reading your work,
Now, we are trying to evaluate the error of different methods.
As we know, the analytical result given by the Gauss approximation is good for determining the acceptance of the system for coherent application, especially for the coherence length. If the approximation fails as Walker's paper said, I wonder how to give evaluation.
For the mode decomposition calculation, I just run the example and did not change any paramter, and following Walker's euqation, but the difference of coherence fraction is still big.

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