Updated coupling study from 12 March measurements.
Beam size measurements were performed at MW3X wire scanner position for quadrupole scan QF5X
and skew quadrupole scan QK3X. From those measurements were reconstructed parabola, providing the
twiss parameters at QF5X.The parabola reconstructed from QK3X showed an explicit coupling since it
was not centered to zero. Using simulation based on real machine status, twiss parameters were back
propagated to the entrance of extraction line.Then we search to reproduce the coupling measurements,
supposing a source of coupling at QM7. Do to that, we added in the simulation a skew quadrupole
(with the same properties than QK3X) at QM7, for which we make varied the strength until to fit the
QK3X parabola.The strength needed to attempt to reproduced measurement were found to be 0.018m-1,
corresponding to a current of 3.5 A. This would correspond to a bump of 0.5mm at QM7.
This virtual skew added at the exit of QM7 reproduces the factor 3 emittance growth we observed experimentally, comparing the emittance we measured in the damping ring to the one reconstructed from
QF5X scan. But simulation does not perfectly fit with the experimental QK3X parabola.
Experimental bump studies didn’t show such a growth for a bump of 0.5mm, but only a factor 1.2.
This leads to the conclusion that QM7 maybe not the only source of coupling.
Next study will consist to fit better the data playing with twiss parameters at the entrance of extraction line and to check for the uniqueness of coupling mimics at QM7, or if it can be reproduce as well supposing other coupling sources.