IBSemittance makes the emittance smaller

[Seb_Wilkes]

I am using the IBS emittance calculator tool, but I am finding a condition where the horizontal emittance output is smaller than what the Twiss file would suggest (which itself calculates it from balancing radiation). This doesn't seem right? Surely IBS emittance is an extra diffusion term, which should force apart a bunch rather than cause it to contract?

The key functionality I am using is

Code: Select all

-fixEmity=1

. What is going on is I am testing different sized vertical emittances, but it seems that past a certain point when I look at the horizontal emittance it drops below the minimum I would expect (and what is specified in the .twi file that supplies the tool).

How much should I trust the numbers coming out?

[michael_borland]

IBS can also transfer momentum between planes, e.g., from transverse to longitudinal, so this isn't impossible. I guess it might be more likely in a ring in which most of the scattering occurs at locations with small dispersion, but I'm just hand-waving here.

Do you get similar results if you fix the coupling instead of the vertical emittance?

We've recently added diffusion-matrix-based IBS calculations (Kubo and Oide, PRST-AB 4, 124401 (2001)) to elegant. It agrees reasonably well with ibsEmittance in our test cases, but those may not cover your case. I hope to release this is in the next few weeks.

--Michael