Elegant users forum

Hi! I'm trying to do slow extraction simulations from a synchrotron using elegant and got a little confused by the behaviour of the limit_in_4d flag to the bunched_beam command. What I actually want to do is to create something like a KV beam in the transverse planes. To this end, I choose "uniform-ellipse" as distribution type for x and y. The transverse hard edge emittances are asymmetric, ex=50um, ey=8um (well, it's a hadron machine). When I set limit_in_4d=0, I get the expected ellipses in (x,xp) and (y,yp), and sddsanalyzebeam reproduces these values. However, the beam distribution in (x,y) is a rectangle, where I would want a flat ellipse. Therefore, I tentatively limit_in_4d=1. But this changes the emittances of the distribution: sddsanalyzebeam now reports ex=32um and ey=14um. I tried to figure out how the limit_in_4d works from the documentation unsuccessfully.
So here are my questions:

• Am I missing something essential?
• What exactly does limit_in_4d=1 do?
• Is it possible to create a KV beam using the bunched_beam command after all?

Best regards, David

David,

Try adding enforce_rms_values[0] = 2*1, to your &bunched_beam command. This will give you the desired emittances.

--Michael

Michael,
what you suggest works, so thanks for the answer. However, having looked into the manual again, it is not clear to me why this works. Would it be possible to add a few lines to the manual explaining the way limit_in_4d=1 works in mathematical terms?
Cheers, David