Selected questions and discussion:
YK: When you plan to validate the XanaROOT emittance measurement with the OPAL simulation, don't you need some external validation?
TS: First of all, OPAL is validated (against IMPACT-T), secondly, to test XanaROOT, it does not really matter whether you use OPAL or any other code to provide you with a particle distribution from which you can compute the geometric emittance and then compare to the value found by XanaROOT. In fact we do plan to test the XanaROOT algorithm also with particle distributions obtained from simple geometrical algorithms,, but in that case you will not see the effect of space charge. Chris Gough is also working on such an algorithm (called XanaBONK).
YK: Why is there a substantial difference between measured horizontal and vertical emittances?
AO: Part of it can attributed to the asymmetry in the laser spot, but probably not all of it... Due to beam steering we almost never had a round beam.
Discussion on size of anode iris: YK thinks 2 mm is clearly too small, the beam can be seriously chopped. FLP explains that the original size of 1.5 mm was based on Kevin's calculations. The new size of 2 mm is somewhat arbitrary. TG points out that the anode cannot be made too large either since a large anode would lead to radial focusing behind the anode.
YK brings up the issue of beam chopping by the UHV laser mirror. According to him, the glowing edge of the YAG1 screen is an indication of this.
Discussion on possible sources for the observed discrepancies between simulation and measurements: precision of magnet data (0.5 Gauss), insufficient magnet cycling, solenoid misalignment, which leads to beam steering etc.
TS: Does the simulation still contain the halo observed earlier?
AO: Yes, only the sigma cut is applied to the initial distribution (laser spot), there is no collimation at the iris. But with the sigma cut, most of the halo is removed.
YK: What charge fraction are you using for the emittance evaluation?
AO: I compare the results for 100% and 95%, make sure they agree after Gauss extrapolation.
YK: Does the simulation include thermal emittance?
AO: No.
TS: This is planed for later releases of OPAL.
TG: What's the effect of the terrestrial magnet field at OBLA?
YK: It is very low.
FLP: Negligible according to the magnet group.
Some discussion on the implication for future measurements (4 MeV test stand): beam-based alignement, should we measure the tilt of the magnets, cathode geometry (size of the flat area compared to anode size)...
