Mitigation of Beam Impedance for Machine Elements with Interferences

The performance of the CERN accelerators, and in particular the desired doubling of beam intensity in the injector chain to reach the HL-LHC target is critically depending on beam coupling impedance contributions of machine components. Several campaigns for beam impedance reduction have been carried out in the recent past which let to mitigation measures such as the installation of RF-shields or design changes in newly developed elements. In these cases, the beam impedance contribution was determined from calculation of individual elements.

Recent simulations of beam coupling impedance and wake fields for different machine components have however shown that in a number of cases, it is not sufficient to only calculate the individual contribution of an isolated element and then multiply with the total number of installed elements of that type to obtain the contribution to the beam impedance budget of a machine. Other factors, as the position in the machine element, its vicinity to the next neighbour element, and the question whether the element is sufficiently separated from its neighbours, e.g. by a separating beam pipe has to be taken into account. Examples for machine elements where no clear separation is expected are kickers and septa in the PS Booster, where the requirement of 4 beam lines in many cases lead to a compact design of which the EM-fields can be interfering. Equally, in the PS and the SPS machines, interferences are to be expected in the case of two machine elements which are installed in close vicinity and are coupling via a large beam pipe diameter (non-separation of unwanted cavity resonances).

The main objectives for the PhD candidate will be to determine the beam coupling impedance of such machine elements by detailed modelling and RF-simulation with the goal to develop mitigation methods such as RF-shields or geometric adaptations as well as a systematic approach to obtain criteria to understand when element interference takes place. Further, a contribution to beam impedance as indicated from beam dynamics studies is to be investigated. He/she will also contribute to routine RF-measurements in the framework of the Impedance Working Group as requested by equipment owners and to impedance mitigations studies depending on requests, e.g. from LHC experimental side.

Subject to the usual funding approval, the successful candidate will spend the first and final year of their PhD studies based at the Cockcroft Institute as part of the University of Liverpool’s QUASAR group. The 2^nd and 3^rd years will be spent at CERN as a member of the Beams and RF section in the RF group before returning to the UK to complete the PhD.

For any enquiries please contact: Prof Carsten P Welsch, c.p.welsch@liverpool.ac.uk 

To apply for this opportunity, please visit: https://www.liverpool.ac.uk/study/postgraduate-research/how-to-apply/  Please quote studentship reference PPPR006 in the funding section of the application form.