Here's the calculation for the #RF field model derived from behaviour near the axis.  The RF multipole case got quite complicated, maybe there's a simpler way but I wanted to see if this method, which I'd used for magnetostatics (magnet field models), could be generalised. 
<br /><a href='https://stephenbrooks.org/ap/report/2026-9/rffield.pdf
'>https://stephenbrooks.org/ap/report/2026-9/rffield.pdf
</a><br />  #physics #electromagnetism #accelerators