Centroid Offsets with Multi-bunch Operation and FRFMODE

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england
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Joined: 08 Sep 2020, 09:27

Centroid Offsets with Multi-bunch Operation and FRFMODE

Post by england » 15 Oct 2020, 13:42

I am observing a numerical effect in ELEGANT when using multi-bunch trains on the LCLS2-HE linac for BBU studies, which I was hoping you might have some insight about. Analysis of the watch data for the bunches reveals an offset of the bunch centroids in the X coordinate that affects all bunches in the train and seems to be magnified when the number of bunches is increased. The effect is correlated with the use of the FRFMODE element (monopole wakes), but is absent when using only the FTRFMODE element (dipole wakes). Monopole wakes may produce offsets, since their energy kicks can be translated into X offsets by dispersive elements. However, I would not expect the magnitude of these offsets to change for bunches at the head of the train when additional bunches are added to the tail of the train.

I show below a simple example (X offset vs bunch number), where I run 10 (blue dots) and then 20 (orange dots) bunches through a single linac section (consisting only of cavities and quadrupoles). Even the first bunch in the train shows a relative offset when more bunches are added to the simulation. The only difference between the two runs is the number of bunches. I plot only up to bunch M = 10. When I turn off the monopole wakes, the two data sets below overlap exactly.
Screen Shot 2020-10-15 at 11.09.42 AM.jpg
For reference, I am using n_duplicate to make the bunches and I verified that the phase spaces of the input bunches are all identical except for their z-positions. Although the difference noted above appears small, it is magnified when more bunches are used. When I run 500 bunches through the full linac, for example, I get an offset of 10 microns at the head of the train that is not present when only a single bunch is used. I’m wondering if perhaps it is some accumulation of numerical errors? For small number of bunches and a single cavity I do not see the effect… it becomes pronounced only with longer linacs and/or more bunches.

Any thoughts you may have on the cause of this would be much appreciated.

michael_borland
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Re: Centroid Offsets with Multi-bunch Operation and FRFMODE

Post by michael_borland » 28 Oct 2020, 13:51

Joel,

As we discussed off-line, the problem seems to be that as you add more bunches, elegant adjusts the reference momentum (via CHANGE_P0=1 on the RFCWs) to the average momentum of the bunch train. As bunches are added, the reference momentum gets lower because of beam loading. This means that with more bunches, the quadrupole fields are different.

Using your files, I've worked up an example to show how to get elegant to keep the reference momentum profile fixed to the one obtained for tracking a single bunch. If this is used, the problem is resolved. Please see attached.

--Michael
Attachments
linacWithHOMs.zip
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michael_borland
Posts: 1796
Joined: 19 May 2008, 09:33
Location: Argonne National Laboratory
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Re: Centroid Offsets with Multi-bunch Operation and FRFMODE

Post by michael_borland » 28 Oct 2020, 13:56

Here's an example of the results. You'll see a bit of deviation for <delta>, but it is close to machine precision.

--Michael
example.png

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