CEA Codes forum

Good morning,

I have a question, it might be stupid, but I need to ask about some clarification on the CW beam function and Bunch Freq. (MHz) values that I can set in the main interface.
What they represent and how they work?
If I put different values for the bunch frequence the optics produces different results, but looks like the optics is not affected by ticking/unticking the CW beam function.

Can I have a little more explanation on this?

THanks
Francesco

Hello,

I made some test also using an older version (2016) of tracewin and I noticed:
1 - with the actual version it does not matter if I have CW ticked on or off, if I change the bunch freq. I have different results
2 - with the old version the change in the result only happens if CW is unticked, if it is ticked I can put any bunch freq. but the results do no change.

This is kind of confusing me...
Any clue?

Dear Francesco,

Intrinsically, TraceWIN (and its space-charge routine) is dealing with bunched beams.
Nevertheless, it considers that beam is periodic with the frequency (fbeam [Hz=/s]) given in TraceWIN.
The beam spacial period [m] is then beta*lambda, where :
- beta is the beam reduced speed,
- lambda is beam wavelength (lambda = clight[m/s]/fbeam[/s])

The space-charge routine considers :
- When the bunch length is shorter than pi/2, it is considered bunched, and no neighboor bunch is considered in the space charge force calculation (we assume that the force induced by early bunches compensates the force induced by late bunches).
- When the bunch length is shorter than pi, it is considered semi-bunched, and the SC of the 2 neighbooring bunches is considered in a simple (and faster) way.
- When the bunch length is longer than pi, it is considered as continuous, and the SC of the 2 neighbooring bunches is considered in detail (in mesh).

When you want to transport a cw beam, it is recommanded to chose a beam frequency for which ~8<beta*lambda<~40 times the rms transverse size.
- If too short (<8), some spurious longitudinal force can be observed.
- If too long (>40), the macro-particle density will be diluted and the calculated space-charge will be more noisy.

Best regards.

Nicolas.

Dear Nicolas,

thanks a lot, the explanation is clear.
If I got it right the CW option near the emittance value is not relevant and you have to set a proper bunch frequency to have a CW beam, am I right?

Thanks
F.

An I have one more question:

you write "When you want to transport a cw beam, it is recommanded to chose a beam frequency for which ~8<beta*lambda<~40 times the rms transverse size."
so, if my input beam has a transverse size of, for example 1mm rms (radius), should I set the bunch frequency to have a value 8mm < beta*lambda <~40mm?
or has it to be 16mm < beta*lambda <~80mm (beam diameter is 2mm)?

Thanks
F.

P.s.: I was really thinking that CW option override any bunch frequency...

Dear Francesco,

The frequency given in main page is the beam repetition frequency (RF is an harmonic of it).
In one sense, this frequency has no importance for a CW beam.
Nevertheless, it has just an importance for the space charge routine calculation, as it defines the longitudinal size of the mesh, and a periodicity of beam distribution.
Generally, the model of the numerical solver is better when the lattices of the mesh are almost cubic. Nevertheless, one can accept aspect ratio with a factor 1/10 to 10, so there are"margins".
for example for a 1 mm rms size beam, the mesh transverse total size would be about 7 to 10 mm (depending on your choise). I would then recommand to chose a frequency such that the total longitudinal mesh size (beta*lambda) would be not between 16 mm and 80 mm.

May I recommand you to do some tests with different frequencies (and, why not, share it to user the community) ?
The best diagnostics is the visualisation of (phi,E) (phi,x') and (phi,y') phase-space. Normaly, they should be no visible effect at beam extremities (-180° and +180°).

PS: In CW beam, a longitudinal force may appear when the beam is focused (particles are slowed-down) or defocused (particles are speed-up). I'm not sure that the periodicity of the space-charge force will reproduce perfectly this effect at beam extremities.

Best regards.

Nicolas.

Dear Nicolas,

thanks a lot for clarifications.
If I understand correctly it is more a matter of mesh and space charge calculation now.
I will make some tests and share the results for further discussion

Thanks a lot
F.

Dear Nicolas,

it is taking some time, but I am investigating and running some tests.
I am attaching a compressed folder with TW files, the txt and dst files of the particles I am using and a short resume of the results in a pdf.

What I have done here is to create a 200mm double drift and let the proton beam flying through it changing the bunch frequency and changing the bunch distribution (real and dst).
My beam has 7,36mm rms size, so according to my understanding I should consider a mesh size between 51 and 73mm correspoding to a freq between 45 and 33 MHz (or at least my assumption comes form the interpretation I gave to your statement "for example for a 1 mm rms size beam, the mesh transverse total size would be about 7 to 10 mm (depending on your choise). I would then recommand to chose a frequency such that the total longitudinal mesh size (beta*lambda) would be not between 16 mm and 80 mm. ").
I have also considered 1 and 1000MHz to see the differences.

My conclusion is that small differences are always there, in the real space distribution cases the differences in the beam parameters at the beam line output are more or less comparable for the 4 frequencies.
If I consider the dst, then I have much larger differences. these differences are smaller for 45 and 33 MHz, but still they are not negligible.

I believe there is something I am not understanding correctly.

I have done the same simulation considering my dirft made by 1mm dirfts sequences, I did not put these result in the pdf as I need to look at them more carefully, but looks like there is not difference with a geometry made by two long drifts.

Also, I have tryed to add some lens, again not repored, and I had the feeling that in this case the situation is more drammatic as the beam changes a lot with the beam frequency. But, again I need to look better into this case maybe after I have understood what I am doing wrong for this simple case

Thanks a lot for helping
Francesco

Dear Fancesco,

Interesting work. I focused my analysis on your pdf file.

In the first set of simulations (pages 1-5), the beam generated by TW is CW.
The results of the simulation are "as expected" : when frequency is low, the longitudinal mesh is bigger and each macro-particle is representing a bigger charge, giving to a more "noisy" longitudinal dynamics, leading to a bigger energy dispersion. In this sense, it seams that using a high frequency is better. Nevertheless, at high frequency, a lack of space charge force should be seen. In order to see it, I suggest to do tests with an initial parrallel beam with very little transverse emittance (when the beam dynamics will be dominated by space-charge, not emittance).

In the first set of simulations (pages 6-10), the beam is clearly not CW (see the long. phase space). You should reconsider your .dst.

Best regards.

Nicolas.

Thanks Nicolas,

I will carefully read you comment and made some more tests as you suggest.
I'll be back on the dst topic, probably, later

Cheers
F.