Centroid Shift without any magnet alignment error

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ranamonikaa
Posts: 12
Joined: 07 Jul 2022, 01:07

Centroid Shift without any magnet alignment error

Post by ranamonikaa » 15 Dec 2022, 02:19

Hi all. I am simulating a transfer line. I expect zero centroid shift as there's no magnet errors in the input file. But, I see centroid shift when I give non-zero emittance with n_particles_per_bunch >1 in the bunched beam command, however if I set the emittance to zero or don't mention it at all or set n_particles_per_bunch =1 with non-zero emittance, then I see zero centroid shift. Can someone please tell where am I going wrong. I have attached ele file for reference.
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TL2n3pr.ele
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michael_borland
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Location: Argonne National Laboratory
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Re: Centroid Shift without any magnet alignment error

Post by michael_borland » 30 Jan 2023, 10:01

Without a lattice file it isn't possible to say what the problem might be.

--Michael

ranamonikaa
Posts: 12
Joined: 07 Jul 2022, 01:07

Re: Centroid Shift without any magnet alignment error

Post by ranamonikaa » 31 Jan 2023, 00:56

I have attached the lattice file below.
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TL2n3pr.lte
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michael_borland
Posts: 1927
Joined: 19 May 2008, 09:33
Location: Argonne National Laboratory
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Re: Centroid Shift without any magnet alignment error

Post by michael_borland » 06 Feb 2023, 18:47

The reason for this is that your beam line contains dipole magnets with small bending radii. There are sextupole-like terms present in such magnets due to the small radius of curvature of the coordinate system. This results in distortion of the phase space, which shows up as a small centroid error.

You can demonstrate this to yourself by setting ORDER=1 on the RBEND and SBEND elements.

To understand the theory, see equation (27) in https://cds.cern.ch/record/283218/files/SLAC-75.pdf
Setting n=0 and beta=0 gives your case (magnets with no explicit gradient or sextupole).

Simulating such magnets is challenging. I suggest switching from RBEND and SBEND to CSBEND and CCBEND, which have more accurate models.

--Michael

ranamonikaa
Posts: 12
Joined: 07 Jul 2022, 01:07

Re: Centroid Shift without any magnet alignment error

Post by ranamonikaa » 08 Feb 2023, 02:05

Thank you Michael. :)

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