Most realistic transverse deflecting cavity element?

[Björklund]

Hi,

I'm trying to model a transverse deflecting cavity and I'm wondering which element you regard as being the most realistic? I'm currently using an RFDF element which is implemented along the same lines as in the example file, but I would like to be as precise as I can be with respect to fields and such. We will be using a very long setup of 2x3 m (see http://accelconf.web.cern.ch/AccelConf/ ... pal027.pdf), so we are likely far from a thin-lens case. I'm using magnetic_deflection = 1, 1 kick/cm and a peak voltage of 61.4 MV.

Another question: when I use a chirped beam (sigma_s = 1.273e-5, momentum_chirp = -80), I get a smearing of the slice energy spread (increase of ~2-3 orders of magnitude) after the element. Any idea what could be causing this?

Best regards
Jonas

[michael_borland]

Jonas,

I think the RFDF element is the best choice at present. It is built on a very simple field model, which is intended to provide deflection that is independent of transverse position. My understanding is that this is what one gets in real cavities when all the end fields are included. You can model the length-dependent aspects using the L parameter and N_KICKS parameters.

In the near future we hope to provide a new element that will read fields from CST or use space harmonics for modeling deflecting elements.

--Michael

[michael_borland]

Jonas,

As for the increased slice energy spread, my guess is that it comes from the x- or y-dependent longitudinal field.

--Michael

[Björklund]

Hi Michael,

All right, I will stay with the RFDF then. I will talk to our RF guy who works on this and see if transverse independence is a reasonable assumption also for our design. I have the L-parameter to the geometric length of the two units. Is there any sort of general rule-of-thumb to use when setting the number of kicks? Also, regarding the voltage, is that defined as the peak integrated electric field over the structure, or in some other way?

And regarding the slice energy spread; for the *slice* spread to change that much, the field gradient would have to be absolutely massive, the slice length is about 0.5 fs. I get the same sort of effect from the dipole in the setup, but it's not CSR (which would anyway look different), since it's there when I turn that off. I have tried a bunch of different settings for both elements, but the only thing that decreases the effect is just lowering the strength of either element. When both are off, the beam just propagates through the optics without much change in any parameter. I will keep looking, but I can't really figure out what happens here, it doesn't look physical.

Best regards
Jonas

[michael_borland]

Jonas,

There's no rule of thumb for setting the number of kicks. I just increase it until things don't change significantly.

The voltage is the effective average over the cavity if STANDING_WAVE=0, otherwise, it's the peak.

Please keep me informed about the slice energy spread anomaly. That shouldn't happen in a bending magnet unless you have ISR=1.

--Michael

[Björklund]

Hi again!

I e-mailed you some files yesterday which should show this behavior for both the RFDF and the dipole. There is no ISR enabled, so it can't be that either. The only way that I can reduce the effect from either element is to turn the respective "strength" down.

We don't have a standing wave structure, and so I left that flag at 0, which I think gives me the value that is defined in the RF design.

Thanks!
Jonas

[Björklund]

Hi,

So, a small update on our case. I talked to our RF guy yesterday and we had a look at some field plots. The electric field kick is a rather large fraction of the magnetic field kick in our cavities. These should have a different effect on the beam, as the electric field changes the overall momentum, while the magnetic field doesn't, right? Is this somehow included, or are the choices between purely magnetic and purely electric kicks?

Also, there is a longitudinal variation in the field amplitude inside the cavity, is this somehow included with the standing_wave option? If so, which direction is the assumed attenuation (i.e. the RF feeding direction) going, backward or forward?

I assume that I *could* model both of these effects by stacking RFDF elements width different strengths and field kicks. If that's the only solution, I'll look into doing that, but it is a bit of a hassle. Are there any entrance/exit edge effects, like on rfcw elements, that I would have to worry about?

Best regards
Jonas

[michael_borland]

Jonas,

The choice in RFDF is between electric and magnetic deflection for the entire device. The standing-wave and traveling wave options differ only in whether a relativistic beam sees a varying or constant phase as it travels through the device.

There's no longitudinal variation in the fields within the device, nor are end effects included. My understanding is that for single-cell cavities, the end fields combine with the body fields to create a radius-independent deflection, but I'm hardly an expert on this.

As you mentioned, if you want to go beyond this simple model, splitting into many RFDF elements is certainly an option. If you go this route, the PHASE_REFERENCE feature may be helpful in that it will allow you to synchronize the fields in many devices (mostly useful for standing wave mode).

We are planning a more detailed model, using fields from CST or space harmonics, but that isn't likely to be ready for several months.

--Michael