pCentral and average of p

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jan
Posts: 13
Joined: 02 Oct 2013, 07:20
Location: ELSA, Bonn, Germany

pCentral and average of p

Post by jan » 14 Jun 2016, 09:42

Hello everybody,

I am confused about an observation concerning p and pCentral for an electron bunch tracked in a ring including synchrotron radiation and rf cavities.
I looked at the p column of WATCH elements in "coordinate" mode and recognized that its average over all turns is not equal to pCentral (e.g. read from the same file). Do I misinterpret the meaning of the p column?

The deviation is about 0.6 at pCentral about 6000 (about 0.3MeV at 3GeV). The rf cavities are set up with rf_setup and the beam is stable. The deviation does not depend on the number of turns, which is large compared to the synchrotron oscillation period.

Many thanks in advance!

michael_borland
Posts: 1795
Joined: 19 May 2008, 09:33
Location: Argonne National Laboratory
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Re: pCentral and average of p

Post by michael_borland » 14 Jun 2016, 11:04

Jan,

Depending on where the WATCH element is placed relative to the rf cavities and the elements that emit synchrotron radiation, the beam may have a small observed momentum centroid error. This is a real effect. Also, you may have a path-length error if you are using CSBEND elements with N_KICKS set to too small a value; this can be resolved by increasing the number of kicks or by setting REFERENCE_CORRECTION=1 on the CSBEND elements.

If these steps don't address the issue, please post an example and I'll take a more detailed look.

--Michael

jan
Posts: 13
Joined: 02 Oct 2013, 07:20
Location: ELSA, Bonn, Germany

Re: pCentral and average of p

Post by jan » 15 Jun 2016, 03:24

Dear Michael,

the momentum offset was indeed caused by N_KICKS of CSBEND set to a too small value. Now the deviation is more than one order of magnitude smaller.
It was no real effect, because it occurred everywhere around the ring with about the same value.

If anybody else wants to check: I calculated & printed the average p using
sddscombine <watchfile> -pipe=out -merge | sddsprocess -pipe -process=p,average,"pAverage" | sddsprintout -pipe=in -par=pCentral -par=pAverage

Thank you very much!
Jan

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