FCC-ee Collective Effects and Their Mitigation

The high luminosity foreseen in the future electron-positron circular collider (FCC-ee) necessitates very intense multi-bunch colliding beams with very small transverse beam sizes at the collision points. This requires transverse beam emittances comparable to those of modern synchrotron light sources. At the same time, the stored beam currents should be close to the best values achieved in the last generation of particle factories. This combination of demanding factors poses a major challenge, namely preserving a high beam quality, while, at the same time, avoiding machine performance degradation. In consequence, a careful study of the collective effects and identification of stabilizing mechanisms are required to mitigate the foreseen instabilities. In this contribution, we discuss the current status of these studies

Building the impedance model of a real machine

A reliable impedance model of a particle accelerator can be built by combining the beam coupling impedances of all the components. This is a necessary step to be able to evaluate the machine performance limitations, identify the main contributors in case an impedance reduction is required, and study the interaction with other mechanisms such as optics nonlinearities, transverse damper, noise, space charge, electron cloud, beam-beam (in a collider). The main phases to create a realistic impedance model, and verify it experimentally, will be reviewed, highlighting the main challenges. Some examples will be presented revealing the levels of precision of machine impedance models that have been achieved

Impedance modelling and collective effects in the Future Circular e+^{+}e−^{−} Collider with 4 IPs

The FCC-ee impedance model is being constantly updated closely following the vacuum chamber design and parameters evolution. In particular, at present, a thicker NEG coating of 150 nm (instead of previous 100 nm) has been suggested by the vacuum experts, and a more realistic impedance model of the bellows has been investigated. Moreover, also the transverse impedance has been updated by considering the same sources as for the longitudinal case. Therefore, the FCC-ee impedance database is getting more complete and the impedance model is being refined. In this paper we describe the presently available machine coupling impedance in both longitudinal and transverse planes, and study the impedance-driven single bunch instabilities (with and without beam-beam interaction) for the new FCC-ee parameter set with 4 interaction points (IPs). The results are compared with the previously obtained ones and a further possible mitigation of the beam-beam head-tail instability (X-Z instability) is proposed.The FCC-ee impedance model is being constantly updated closely following the vacuum chamber design and parameters evolution. In particular, at present, a thicker NEG coating of 150 nm (instead of previous 100 nm) has been suggested by the vacuum experts, and a more realistic impedance model of the bellows has been investigated. Moreover, also the transverse impedance has been updated by considering the same sources as for the longitudinal case. Therefore, the FCC-ee impedance database is getting more complete and the impedance model is being refined. In this paper we describe the presently available machine coupling impedance in both longitudinal and transverse planes, and study the impedance-driven single bunch instabilities (with and without beam-beam interaction) for the new FCC-ee parameter set with 4 interaction points (IPs). The results are compared with the previously obtained ones and a further possible mitigation of the beam-beam head-tail instability (X-Z instability) is proposed

An interplay between beam–beam and beam coupling impedance effects in the Future Circular e+^+e−^- Collider

In order to reach high luminosity, the Future Circular e$^+$e$^-$ Collider will need very intense beams with small emittances and small beta functions in the interaction points. This is achieved with a large Piwinski angle combined with the crab waist collision scheme. Under these conditions, the luminosity and the beam–beam tune shifts are strongly influenced by the bunch length. On the other hand, in this machine, the beamstrahlung effect is dominant too, leading to an increase of bunch length and energy spread. Moreover, due to the extreme beam parameters, new important beam–beam instabilities have been found, such as the so-called coherent X-Z instability. Finally, the bunch length and energy spread are also affected by collective effects. In this paper, we study the beam–beam interaction, by focusing on the X-Z instability for FCC-ee, in the lowest energy configuration (45.6 Gev, Z-resonance), by taking into account, self-consistently, the combined effects of beamstrahlung and the machine impedance model that has been evaluated so far. Finally, we also discuss some possible mitigation methods

An interplay between beam–beam and beam coupling impedance effects in the Future Circular e + e - Collider

In order to reach high luminosity, the Future Circular e+e- Collider will need very intense beams with small emittances and small beta functions in the interaction points. This is achieved with a large Piwinski angle combined with the crab waist collision scheme. Under these conditions, the luminosity and the beam–beam tune shifts are strongly influenced by the bunch length. On the other hand, in this machine, the beamstrahlung effect is dominant too, leading to an increase of bunch length and energy spread. Moreover, due to the extreme beam parameters, new important beam–beam instabilities have been found, such as the so-called coherent X-Z instability. Finally, the bunch length and energy spread are also affected by collective effects. In this paper, we study the beam–beam interaction, by focusing on the X-Z instability for FCC-ee, in the lowest energy configuration (45.6 Gev, Z-resonance), by taking into account, self-consistently, the combined effects of beamstrahlung and the machine impedance model that has been evaluated so far. Finally, we also discuss some possible mitigation methods

Transverse Beam Stability with Low-Impedance Collimators in the High Luminosity Large Hadron Collider: Status and Challenges

The High-Luminosity upgrade of the Large Hadron Collider (HL-LHC) will double its beam intensity for the needs of High Energy Physics frontier. In order to ensure coherent stability until the beams are put in collision, the transverse impedance has to be reduced. As the major portion of the ring impedance is supplied by its collimation system, several low resistivity jaw materials have been proposed to lower the collimator impedance and a special collimator has been built and installed in the machine to study their effect. The results show a significant reduction of the resistive wall tune shift with novel materials, in agreement with the impedance model and the bench impedance and resistivity measurements. The largest improvement is obtained with a 5 {\mu}m Molybdenum coating of a Molybdenum-Graphite jaw. This coating can lower the machine impedance by up to 30% and the stabilizing Landau octupole threshold by up to 120 A. The collimators to be upgraded have been chosen based on the improvement of the octupole threshold, as well as the tolerance to steady state losses and failure scenarios. A half of the overall improvement can be obtained by coating the jaws of a subset of 4 out of 11 collimators identified as the highest contributors to machine impedance. This subset of low-impedance collimators is being installed during the Long Shutdown 2 in 2019-2020.The High-Luminosity LHC Project aims to increase the integrated luminosity that will be collected by the Large Hadron Collider for the needs of the high energy physics frontier by the end of its Run 3 by more than a factor ten. This will require doubling the beam intensity, and in order to ensure coherent stability until the brighter beams are put in collision, the transverse impedance of the machine has to be reduced. As the major portion of the ring impedance is generated by its collimation system, several low resistivity jaw materials have been considered to lower the collimator impedance and a special collimator has been built and installed in the machine to study their effect. In order to assess the performance of each material we performed a series of tune shift measurements with LHC beams. The results show a significant reduction of the resistive wall tune shift with novel materials, in good agreement with the impedance model and the bench impedance and resistivity measurements. The largest improvement is obtained with a molybdenum coating of a molybdenum-graphite jaw. This coating, applied to the most critical collimators, is estimated to lower the machine impedance by up to 30% and the stabilizing Landau octupole threshold by up to 240 A after accounting for uncertainties of the model and other destabilising effects. A half of the overall improvement can be obtained by coating the jaws of a subset of 4 out of 11 collimators identified as the highest contributors to machine impedance. This subset of low-impedance collimators is being installed during the Long Shutdown 2 in 2019-2020