Elegant users forum

This is for Taiwan Photon Source 3 Gev ring. One superperiod is used for CSR tracking. It should be tracked for several turns, not just one superperiod.

Ping,

Thanks for posting your files. One recommendation I have is to use the high_frequency_cutoff feature instead of the Savitzky-Golay smoothing. This gives better control over the noise cutoff frequency.

Here's an example of how to set the cutoff frequency:

Your bunch is gaussian with rms duration St and total duration 6*St. The characteristic frequency for a gaussian beam is fg=1/(2*pi*St) and the Nyquist frequency is fN=Nbins/(12*Sz). So the ratio of the characteristic frequency to the Nyquist frequency is approximately 2/Nbins. With Nbins=600 (as in your input files), this is 0.0033, so the dominant frequencies in the beam are well below the maximum frequency. (Which is good.)

Setting a cutoff of fc=fN/5 (high_frequency_cutoff0=0.2) is a good choice because it means that the highest frequency simulated is sampled with 10 time points. In this example, it also gives fg/fc=0.017, which means you will simulate frequencies that are up to 60 times fg. If you want to simulate higher frequencies, then you would increase Nbins (and the number of particles) but keep the cutoff fixed.

I've attached a PDF file that explains this a little more. I also modified your files. (I added the frequency cutoff. I also removed the rf cavity and radiation kick---these aren't needed when the beam only goes by once.)

--Michael

I used Pelegant on one of our computer clusters at APS to perform a simulation for ~100 turns (actually only 93 as I inadvertently stopped the job). This took several days on 32 processors (I didn't keep track of exactly how long it ran.) The results are pretty interesting, as the attached animated GIF shows. This is an unshielded case, which is why it looks so dramatic.

We have a program here at APS that calculates the shielded CSR impedance. (We'll be distributing this with the next release of elegant.) An initial check using this (via the ZLONGIT element) shows no issues.

--Michael

Dear Michael,
I found your prescriptions on how to use HIGH_frequency_cutoff in CSRCSBEND in this old post.
I wonder if you have any similar guidance for the LOW_frequency_cutoff, in the same element. I could not find any discussion about this option in the forum.
Thank you in advance. Best,
Simone

Simone,

I don't particularly recommend using the low frequency cutoff feature. Although it might in some sense imitate the effects of CSR shielding, I don't think it does so in a physical way. There is also a customized filter available with the WAKE_FILTER_FILE parameter, but my tests have indicated that this is also not a physical approach.

--Michael