Good day.
The task is to calculate impedance of resonator in case of Gaussian beam circulating. The beam is bunched. Wake potential, frequency spectrum, longitudinal and transvers impedances are interesting. Solver - HFSS, ANSYS Electronics Desktop - never used by me previously. Can somebody give me an advice for starting with HFSS? Which type of solver? Eigenmode? HFSS? Other solver? How to assign beam parameters (current, length)? Which option is used for that? I can see that I can use "Assign Excitation->Incident Wave->Gaussian Beam". But here I cannot see settings for beam current and length. If somebody calculated wake fields from beam (for any structural element), please, give some starting information how to do it in HFSS, ANSYS Electronics Desktop. References? Examples? Any information will be helpful.
Thanks a lot...
Hi Victor,
The answer depends on what kind impedance you are looking for, resonant or broadband? For finding the resonant (long-range) impedance the preferred method is calculation in Frequency Domain (FD), ie using HFSS Eigenmode solver (Article Resonant excitation of high order modes in the 3.9 GHz cavit...
), while for broadband (short-range) - Time Domain is the best choice. Particularly for short Gaussian bunches (< 1mm), HFSS is not the best tool for calculating wake potential, I would recommend the ECHO code (echo4d.de) instead. For a symmetric rotation structure, ECHO runs very fast and allows to approximate the Green's function of a point charge. You can then convolute it with an arbitrary bunch profile and find the total wake potential (SLAC-PUB-14536, TESLA Report 2003-19).Regards,
Andrei
Thanks, Andrei.
I thought about ECHO, but the structure (set of structures) is complicated without rotation symmetry. Beam length is 120 cm, and no problem with short impulses.
Both broadband and long-time impedances should be investigated.
So, if I will use eigenmode solver, how to represent Gaussian beam? Do I need to use Ports? If I use Gaussian beam I cannot use port.
Time domain solver you mean Transient? How to set the beam? I cannot use Gaussian beam for an excitation. I can use only wave port. In transient solution type I can set input impulse TDR with vary length. But how to introduce beam current? ( For Gaussian beam I also cannot see current settings.) Does TDR represent a beam?
A asked a lot of questions. Sorry for that. Any answer is useful. I never used ANSYS.
Do you know where to find completed examples for to look at the settings?
Thanks again.
For the eigenmode solver, you do not need a Gaussian beam in your model, the electromagnetic field will be the same for calculating the integral over the particle trajectory. The exponential form factor exp(-w02σ2) can be applied later to take into account the spectrum of the bunch. Have you looked at the first article I linked to?
By TD option I mean the Wakefield solver, unfortunately it is not available in ANSYS, but you can try CST Particle Studio if your bunch length is that long (>4ns?)
I guess you are talking about the beam coupling impedance as used in particle accelerators and that you assume a Gaussian beam to mimic the TEM like field of a highly relativistic particle beam. I know that certain people artificially introduce a TEM wave (not Gaussian beam) into the empty beampipe which (under waveguide cutoff ) would not allow to propagate such wave in reality. Now the catch in this case is that the gaussian beam if its waist is small enough has no image currents on the wall of the beampipe and can (physically) only propagate far above waveguide cutoff.And if there are no image current on the wall you will not measure any beam coupling impedance this way. I agree with the comment already given by Andrei and others. Of course you still have the option to apply numerically the coaxial wire method by introducing a wire into the beampipe which then allow for the propagation of a TEM wave starting already at DC. This method may be numerically easier and can give useful hints but we must keep in mind that the presence of the wire may represent a serious perturbation and the results have to be interpreted with caution.
Andrei and Fritz, thanks for the answers. Let me think a bit about the new information provided by you.
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