Elegant users forum

Hi Michael,

I am trying to see in detail how ELEGANT calculates the momentum change for a RFCW element for a particle within a bunch as I believe that the variation in transverse dimension of the beam in the RFCW elements is responsible for CSR suppression (via the R51/R52 terms).

I believe I understand the mechanism behind the "ZWAKEFIELD" file, but I am a little confused on how ELEGANT uses the "TRWAKEFIELD" to compute the momentum change for a particle, and also how the (x,y) coordinates of a particle may play a role. Could you clear this up or point to a reference that could be of assistance?

Essentially, I would like to pick a particle within the bunch (particleID) and be able to calculate the momentum change on my own and confirm my findings with ELEGANT.

Thanks in advance!

The manual page for TRWAKE describes how the transverse wakes are computed. The wake data provided by the user is used to compute a transverse "voltage" Vt, which changes the slope of the particle according to (Vt/M)/pz, where M is rest mass in electron volts and pz=betaz*gamma. Since the slope is changed (not the transverse momentum), the transverse wake does not change the particle momentum. For resistive wall wakes (according to Alex Chao, SLAC-PUB-2946), this is correct since the deflection is from magnetic fields. In general, the deflection is presumably from a combination of magnetic and electric fields, so elegant is not strictly correct. However, we (Ryan Lindberg and myself) think that the dominant change to the particle momentum is from the longitudinal wake, and that any change from the transverse wake is negligible.

--Michael