Is this a Bug?

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CCH
Posts: 16
Joined: 16 Apr 2010, 02:04

Is this a Bug?

Post by CCH » 01 Jul 2010, 04:36

I did a tracking of a simple lattice ,but the normalized emittance changed.Is this a bug? Can you have a look
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twi.png
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CCH
Posts: 16
Joined: 16 Apr 2010, 02:04

Re: Is this a Bug?

Post by CCH » 01 Jul 2010, 04:37

These are the files
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erl.lte
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erl.ele
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michael_borland
Posts: 1927
Joined: 19 May 2008, 09:33
Location: Argonne National Laboratory
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Re: Is this a Bug?

Post by michael_borland » 01 Jul 2010, 08:25

It's not a bug, but rather a result of the fact that you plotted the uncorrected emittance and have dispersion in the system. The uncorrected emittance is defined as
ex = sqrt{ <x^2> <xp^2> - <x*xp>^2 }

If there is dispersion (e.g., following a dipole magnet), then this quantity can grow.

If you plot the corrected emittance (ecx or ecnx), you get a different picture. For the corrected emittance, we make the transformation
x -> x - delta*etax
xp -> xp - delta*etaxp
prior to computing the emittance.

As you can see in the attached plot, ecnx is nearly constant. It isn't exactly constant because of chromatic aberrations from the quadrupoles.

--Michael
Attachments
plot.png

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