Elegant users forum

Dear Michael,

I have a question about horizontal deflection.

At the end of a linac section I have a deflecting cavity (RFDF-element, used in the same manner as in the elegant example folder) and depending on the tilt, I can deflect the beam in either x or y direction (so far it’s fine).
I’m interested in the kick caused by transverse wakes (same in x and y), so if I introduce a misalignment in the cavity section before the deflector and then deflect the beam in the other direction I should be able to see the kick from the wakes on the beam tail.
This works perfectly fine for cavity misalignment in x (dx=0.001) and a vertical deflection (tilt=1.57).

But, when I misalignment cavities in y (dy=0.001) and deflect the beam horizontally (tilt=0) I can’t see the kick from the wakes anymore!? The beam looks the same as if I had no cavity misalignment except that it has moved slightly (mm scale) in the y direction on the screen.

What did I do wrong in the horizontal deflection case?
Why can’t I see the effect form the wakes after the horizontal deflection (when it works so nicely for the vertical case) and how come it moves on the screen?

Thanks in advance,
Best Regards,
Olivia

Olivia,

First, a clarification: the effect you see in the momentum distribution is not a wake, but rather the result of the longitudinal field that is required to satisfy Maxwell's equations when a tranverse deflection is imposed. There's also a smaller effect due to the fact that the deflection is from electric fields and hence changes the total momentum of the particles. If you really want the wake from a deflecting mode, you need to use the TRFMODE element. The RFDF element is only for the externally imposed field.

I'm not able to duplicate this problem. I compared horizontal offset combined with vertical deflection to vertical offset combined with horizontal deflection, getting identical results. I also compared horizontal offset combined with horizontal deflection to vertical offset combined with vertical deflection, with identical results. (I'm comparing the (t, p) phase space.)

Can you post a set of example input files that exhibit the problem?

--Michael

Hi Michael,

Thank you so much for your reply.

Please, find attached to this post a simplified lattice and a beam file (it is not completely optimized yet but it works…). I also tried to make a small sketch (jpeg-file).

I’m interested in the effect from the transverse wakefields in a RFCW- cavity section (L04) that is placed before the deflecting cavity. (In this lattice I have two deflecting cavities placed after each other, one horizontal and one vertical.)
I misalign the RFCW-cavities to have a stronger effect from the wakes when the beam passes of axis.
Then I would like to deflect the beam to see the head and the tail on a screen (and see how the tail has suffered from wakes).

Using vertical deflection (and a dx= 0.002 in the RFCW-cavities) I can see the how the bunch tail have received a kick from wakes in the previous cavity section. But if I put dy= 0.002 in the RFCW-cavities and do a horizontal deflection I can just see change in position on the screen?

Best regards,
Olivia

And some more files =)

Olivia,

I think it is just a copy-and-paste error. You have, for example, Note that you give WxColumn twice, but never give WyColumn. If you change the definition to I think it will work as expected.

Let me know if not and I'll investigate further.

--Michael

Hi Michael,

I’m sincerely sorry for taking up your time with this copy-and-paste error.
Thank you very much for the help and time, it works perfectly fine now.

Best regards,
Olivia

Olivia,

No need to apologize. I'm glad I could help resolve the problem.

--Michael