Elegant users forum

I think I have a misunderstanding on how first_is_fiducial and p_central work together. I have a bunch with high energy (<p>=4e4) feeding into a lattice with just a bend, where p_central=2e4 in run_setup. With first_is_fiducial=1, the resulting x-centroid is -2.3e-7, which doesn't make sense. It seems to imply that elegant is somehow reading the input's momentum, even though I've specified a different momentum. With first_is_fiducial=0, the x-centroid is 6.0e-3, which makes more sense - it should be way off in x since the bend is designed for a bunch with half the energy.

I would expect that p_central would override first_is_fiducial at least in terms of momentum expansion. What is the correct way to look at this?

Also, with first_is_fiducial=1, I changed p_central, and the centroid never changed.

Joel,

When you set first_is_fiducial=1, elegant redefines the reference momentum (p_central) to match the first beam that it sees. That means the beam has Cdelta=0.

What are you trying to achieve by using first_is_fiducial? The intended use is when you are going to send in a series of beams and want the first one to establish a reference momentum and arrival-time profile. If you don't have acceleration or rf systems, it really doesn't serve much function.

--Michael

Michael,

Thanks for the info!

I'm actually trying to split a jitter simulation into parts. The first part works as expected. But then I dump the particles and notch collimate them, changing their average momentum. I want to reload the particles and simulate the second part of the beamline, without changing the beamline, so that all the fiducialization would be exactly as if I had never collimated. So I thought, use first_is_fiducial=1 and define momentum in p_central, and load in all of the particles. I didn't think it would make a difference to do it that way. It was just easier to not change first_is_fiducial from the initial run, since I use the same ele file and comment sections of it to turn things on and off. And then when I did that, the R56 of the beamline was off, since the fiducialization was done with a momentum that the beamline wasn't designed for.

My solution, I think, will be to use sddscombine to add on an untouched version of the fiducial bunch for the first part to the collimated bunches I load in the second part. That way, the second part will fiducialize correctly to the first part and the following collimated cases with "incorrect" energy will be simulated correctly.

Joel

Joel,

Yes, your solution is the right thing to do.

--Michael