Andreas Adelmann: Opal Validaton Status Room: WBGB 019 Selected questions and discussion: YK wants to know whether the OPAL simulation now includes thermal emittance at the emission, incoherent synchrotron radiation and coherent synchrotron radiation in drift sections. The answer is no on all accounts (AA). YK insists in particular on a timely implementation of the thermal emittance. AA: Give me a model, and I can implement it! YK: ASTRA has a model. In addition, there are many papers on the subject. TS: With the simulation we are now reaching the actual number of electrons. Does it make any sense to go further, i.e. to oversample? AA: In astrophysics this is being done (since galaxies are extended objects); in our case (electrons), I do not expect any gain from it. TS: Once you reach the real number of particles, what is the ideal grid size? AA: For hadron machines the best value is believed to be between 10 and 20 particles per grid cell, but of course a thinly populated halo can give rise to problems. A better understanding of this issue is the very goal of our ongoing convergence studies! This is of particular importance for slice emittance and undulator studies. SR: I am puzzled about the last plot: when you go from 128 to 256 grid size, the emittance becomes lower? What is the physics behind that? AA: You have a different number of particles per grid cell, therefore different field etc.; more grid points means smoother field... SR: But wouldn't you expect stronger kicks from a shorter grid spacing? AA: The general assumption is that the fields are smoother with smaller grid size, but this is still under study.

Bolko Beutner: Summary of 'Microbunching and COTR Workshop', Berkeley, 2008 Room: WBGB 019 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.