The LHC has demonstrated an unprecedented optics control for high energy colliders, down to the 1% level in β-beating and |C−| = 2×10−4 in coupling. The high luminosity upgrade of the LHC (HL-LHC) will challenge the efficiency and accuracy of optics measurement and correction algorithms in many ways. As a consequence:
- About 50 different optics will need to be finely commissioned for physics production
- β* measurement accuracy is limited by tune ripple when applying the K-modulation technique
- Local corrections in the Interaction Regions will be significantly more constrained in HL-LHC than in LHC as the β functions at the crab cavities should be controlled with similar accuracy as the β*
- Sensitivity to transverse coupling will be highly increased, requiring frequent adjustments. In HL-LHC, |C−| will have to be
The potential for significant distortion of tune footprints and Landau damping as a function of the squeeze or the crossing scheme makes local compensation of high-order IR errors an operational concern for the HL-LHC. It will be very important to establish procedures for the linear and non-linear optics commissioning in the HL-LHC and identify possible limitations coming from increased non-linearity. Having the facility to compensate such errors will be essential but may require a serious revision to the linear optics correction strategy.
Different correction methods
The HL-LHC will require robust methods for beam-based correction of the non-linear errors in its experimental insertions. Two methods exist which may apply generally to a wide range of multipole errors. The measurement of resonance driving terms has the potential to allow for direct compensation of nonlinear sources in the IRs. So far only octupolar resonance driving terms have been used to validate corrections in the LHC.
Direct measurement of dynamic aperture (DA) may also be a viable option. Conventional measurements based on single kicks are not possible at top energy due to the high rigidity of the beams and destructive nature of the measurement. The study of short-term and long-term DA as observables has been validated at injection in the LHC, where a clear impact on beam lifetime was observed, indicating direct DA measurement should be viable in the HL-LHC. DA and lifetime are significant figures of merit for the IR correction quality which makes direct optimisation of DA an appealing option for compensation of the IR errors, but given the global nature of such a measurement the challenge in its application will be separating contributions from different IRs and multipole orders. To this end DA must be complemented by methods based on study of the optics.
Regarding the sextupole-order errors, direct beam-based minimisation of observed feed-down is a viable option, which should in principle compensate the operational impact arising from linear optics and coupling perturbations. On the downside, minimising feed-down is unlikely to provide an optimal compensation of sextupole resonances, and may give less gains in dynamic aperture. It will be necessary to study this option further in simulation to help define correction priorities in the HL-LHC: it may be that the operational impact of β∗ imbalance and coupling induced instabilities outweighs the need for an optimal resonance driving term correction. In particular, it may be possible to adapt the segment-by-segment technique normally used for commissioning of local quadrupole corrections in LHC insertions, to the study of sextupole feed-down.
The measurement of Resonance driving terms (RDTs) is another effective method to probe machine nonlinearities and can thus be used to confirm the effectiveness of specific corrections and will be important to validate nonlinear correction schemes in the HL-LHC.
Summary
HL-LHC represents a challenge for optics measurement and correction in both the linear and non-linear regimes. Fast and flexible tools will be required to efficiently commission the large amount of optics foreseen for the β∗ levelling process. Algorithms are being developed and tested in the LHC to address these challenges. Further developments are still required to guarantee a 2% accuracy in β∗.
The non-linear errors will pose severe challenges even for the linear optics commissioning via their feed down to β-beating and coupling, and by reducing the available DA for optics measurements with the AC dipole. Iterative corrections alternating the target between linear and non-linear orders will be required. A broad spectrum of techniques to measure and correct IR non-linear errors are emerging but a substantial effort is required to demonstrate their feasibility. A strategy based on these techniques should be defined and verified with simulations of realistic scenarios for optics commissioning in HL-LHC.
The PhD project’s work will be focused on the computation of optics for High Luminosity LHC, and the demonstration of measurement and correction techniques’ feasibility. The project will cover exploration of new parameter space, optimization of nominal running scenarios, reduction of optics transition time, and improvement of simulation software as well as beam-based tools for automatic optics generation.
Student: Félix Soubelet
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