Elegant users forum

Hello,

I am looking into how slice Twiss parameters evolve through a bunch compressor. I do this by slicing the beam and assigning each particle to a slice, then I use sddsanalyzebeam on each slice to get the slice beta and alpha along the bunch compressor. Typically I do this with a 2,000,000 macro-particles and have used 20-600 longitudinal slices. I've noticed the slice Twiss parameters were larger than the whole beam Twiss parameters from twiss_output. I assigned the particles to a single slice to check whether the sddsanalyzebeam Twiss match that of twiss_output, in case there problem was the number of slices, but I still found the Twiss parameters from sddsanalyzebeam were much larger than from twiss_output. I find that more slices I use the larger the slice parameters get. The corrected geometric emittance and sigma beam size from sddsanalyzebeam match the sigma output file.

I have attached plots of beta, ecx and Sx along the bunch compressor using *twiss and *sigma output files, and using the equivalent single 'slice' beta, ecx and Sx from sddsanalyzebeam at each watch file. Also attached the *ele and *lte and initial bunch.

Do you understand why the beta from *twiss output and sddsanalyzebeam are different here?

Here are the figures comparing twiss and sigma output compared to those from sddsanalyzebeam. It also won't let me attached the initial bunch distribution.

Thanks,
Adam

Adam,

Are you using the -correctedOnly option on sddsanalyzebeam, to get the true beta functions (with suppression of dispersive effects)? Either that, or look at the betacx, betacy, etc, values.

--Michael

Hi Michael,

I'm not using the -correctedOnly option, but in the above beta function plot betax and betacx are the uncorrected and corrected values from sddsanalyzebeam. Where both are larger than the twiss_output (betax.twi) values.

Adam

Adam,

Can you provide the file input/BC2.W-INIT and any other input files I might need to run this?

--Michael

I have attached BC2.W-INIT. The script I used to slice the beam and track the slices, is similar to what you provided here - viewtopic.php?t=1201.

Adam

Adam,

The discrepancy appears to be due to strong chromatic effects. If I convert the entire lattice to use matrix-based tracking (SBEN, QUAD, SEXT), then set default_order=1 in run_setup, the beta functions agree between max4.twi and max4.sig. However, if I set default_order=2, the discrepancy appears. If I set the energy spread to zero, there's agreement even with default_order=2.

The twiss parameter computation can't include such effects, whereas determination of twiss parameters from particle distributions necessarily has them (if the tracking is so configured).

--Michael

Thanks for the help Michael!