Selected questions and discussion:
YK (commenting on solenoid-based BC): A solenoid-based bunch compressor would require an enormous magnetic field, and therefore unrealistic requirements on alignment!
YK (commenting on the option to have only one BC): SPARC want to use only one BC, but they have a much lower peak current (1 kA); in our case with a larger peak current, the timing jitter tolerances become much too tight!
AA: Can you summarize the importance of the microbunch instability on our CTF3 and LEG designs, where do we have to watch out?
BB: Our beam parameters are not that different from LCLS: we have a low energy spread (the most important figure). Therefore, we should adapt the LCLS model and estimate the gain curves to see in which wavelength regime we will end up. If it is in the optical, we will have a problem with COTR and slice emittance!
AA: What can we learn from the 250 MeV injector?
BB: We will have to learn from it.
YK: The LCLS design has dog leg, at SDL they have a spectrometer at low energy; in our case, we have no such chicanes at low energy, therefore better prospects. But our beam size is around 60 μm, close to that of LCLS--according to COTR theory, a small beam size gives a large effect. The current situation is not clear.
RG: What is at the origin of density modulations of the electron beam?
BB: There are different models, the most accepted version is noise on the beam profile which then becomes amplified.
RG: What is the role of the laser in this?
BB: The laser alone cannot generate the small wavelength observed in the microbunch instability...
RG: Is the effect purely longitudinal or also transverse?
BB: Most studies focus on longitudinal effects, but there are also transverse effects.
Some discussion on the source of the instability and its potential effects in our design. The source is probably shot noise in the gun, which is then amplified by momentum compaction in beamline chicane elements such as dog legs or bunch compressors. But the exact mechanisms remain unknown as there is still no satisfactory agreement between observations and simulations.
YK: Next year there will be the third workshop on microbunch instabilities. Both PSI and SPARC are interested in hosting it. If PSI were hosting it, we could profit a lot!
Much more details on the status of microbunching studies, both experimentally and theoretically, will be given by Yujong's series on the topic in the framework of the beam dynamics palaver.