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Der elegant users and experts,

We use elegant to match the beam in our FEL injector using the following procedure: The beam sizes are measured at a position downstream the first accelerating module (which increases the beam energy from 5 MeV to 160 MeV). We use then elegant to calculate the actual Twiss functions at the gun in order to have the correct start values for the matching. The result of the matching can be checked again using the screens downstream the first module. The problems we had occurred when we tried to track the particles backwards through the cavity. I think that the cavity focusing, which is strong at this position, makes the problems. We bypassed this problem and run a match for the Twiss functions at the gun. Nevertheless, I was wondering if there is maybe a possibility to track backward through cavities obtaining the correct Twiss functions.

Thanks for your help!
Best, Matthias

Matthias,

This should work as long as you flip the sign of the voltage on all the rf cavities. Please see attached example.

--Michael

Hi Michael,

thanks for your help! It works perfectly.

Best,
Matthias