Question about having a fiducial bunch in a ring with bunches from sdds file

[GPenn]

Hi,

I am doing ring simulations for long range wakes and ion effects. I would like to include a first fiducial bunch while using n_passes_fiducial=1 in the run_control command. The bunches are from an SDDS file, so I am using the sdds_beam command. This works fine if I follow the example from the folder 'twoBunchPhasing', where each bunch is a separate page, n_steps in run_control is set to the number of pages, and track_pages_separately=1 in sdds_beam.

Is this the only way to have the fiducial bunch treated differently from the other bunches? If I put all the particle data in a single page and set use_bunched_mode=1 in sdds_beam, the particles in all the bunches are tracked in the same step and only one pass is simulated regardless of how n_passes is set. Does it make any difference for long range wake field calculations if the bunch train is simulated in a single 'step' or as multiple steps? I do set the RF cavities to use FIDUCIAL="FIRST" but I'm uncertain if that means the phases are only set the first time the cavity sees any bunch, or if the first bunch continually updates the RF phase every turn around the ring. Does specifying the fiducialization in the RF cavity elements make it unnecessary to use n_passes_fiducial=1 even in a ring?

Example files are attached, just slightly modified from twoBunchPhasing.

Thanks,
Gregg

[michael_borland]

Gregg,

You definitely need to track all the bunches at once to simulate long-term wakes. An example of how to fiducialize then track a multi-bunch beam is attached.

--Michael

[GPenn]

Thank you, that helps a lot! I was definitely veering in the wrong direction.

-Gregg

[xhs05]

Hi, Michael and all,

It's Haisheng Xu. First of all, Michael, thank you for the example scripts which are very inspiring!

I'm also trying multi-bunch simulations, and struggling with the beam preparation step...

The purpose of mine is to study the transverse high-order modes induced coupled-bunch instability.

The main problem of mine is how to set up properly the ‘n_bins’, 'bin_size' of the 'trfmode' element and the initial beam (by sdds_beam command).

In the first try, I input a single-bunch sdds file by using the 'sdds_beam' command and generated bunch train by setting 'n_duplicates' and 'duplicate_stagger'. Meanwhile, the 'n_bins' and 'bin_size' of the 'trfmode' element were set to cover a single RF bucket (e.g., 6 ns in my case). I succeeded in running the simulations.

Then, I tried the similar way as the method you used in the posted example scripts but met problems. The scripts are attached. I first generated a three-bunch beam file and input this file by the 'sdds_beam' command. The 'n_bins' and 'bin_size' of the 'trfmode' element are kept the same (covering a single RF bucket). When I tried to run Pelegant (parallel version), I got a warning message 'Warning: only -468054234323243 of 99180 particles binned (TRFMODE)' and the tracking stopped. However, I got no such warning if I run the same scripts with elegant (serial version).

Any idea on the possible problem and solution would be very helpful!

Regards,
Haisheng

Gregg,

You definitely need to track all the bunches at once to simulate long-term wakes. An example of how to fiducialize then track a multi-bunch beam is attached.

--Michael