Methods for full coupling operation in a synchrotron light source

Several synchrotron light sources are currently designing a lattice upgrade to reach low (sub-nm) emittances, which inevitably entails a significant beam lifetime reduction. In view of the ALBA lattice upgrade, it was decided to evaluate different methods using the current ALBA storage ring to reach large betatron coupling as a way to increase the lifetime. In a first attempt, coupling was introduced by means of static skew quadrupolar magnets and by tuning the working point onto the resonance <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>Q</a:mi><a:mi>x</a:mi></a:msub><a:mo>=</a:mo><a:msub><a:mi>Q</a:mi><a:mi>y</a:mi></a:msub></a:math>, but in a realistic scenario, some kind of tune feedback is required to counteract the unavoidable tune drifts and fluctuations that would drive the system out of resonance. Furthermore, the condition <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msub><c:mi>Q</c:mi><c:mi>x</c:mi></c:msub><c:mo>=</c:mo><c:msub><c:mi>Q</c:mi><c:mi>y</c:mi></c:msub></c:math> constrains the linear optics resulting in an important lack of flexibility. Therefore a second method is proposed, based on the excitation of the coupling resonance with an ac skew quadrupole driven at the frequency <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msub><e:mi>f</e:mi><e:mi>rev</e:mi></e:msub><e:mo stretchy="false">(</e:mo><e:msub><e:mi>Q</e:mi><e:mi>x</e:mi></e:msub><e:mo>−</e:mo><e:msub><e:mi>Q</e:mi><e:mi>y</e:mi></e:msub><e:mo stretchy="false">)</e:mo></e:math>. In this case, we used the existing four-electrode tune excitation stripline recabled as a skew quadrupole. A fast tune tracking system was implemented to drive the skew quadrupole exactly on the resonance despite the tune fluctuations. This work goes over the collected results and experiences, aiming to put into light pitfalls and limits of the application of coupling to achieve round beams in a synchrotron light source.