Study of the beam-cavity interaction in the PS 10 MHz RF system

The eleven main accelerating cavities of the Proton Synchrotron (PS) at CERN consist of two ferrite-loaded coaxial lambda/4 resonators each. Both resonators oscillate in phase, as their gaps are electrically connected by short bars. They are in addition magnetically coupled via the bias loop used for cavity tuning. The cavities are equipped with a wide-band feedback system, limiting the beam loading, and a further reduction of the beam induced voltage is achieved by relays which short-circuit each half-resonator gap when the cavity is not in use. Asymmetries of the beam induced voltage observed in the two half-cavities indicate that the coupling between the two resonators is not as tight as expected. The total cavity impedance coupling to the beam may be affected differently by the contributions of both resonators. A dedicated measurement campaign with high-intensity proton beam and numerical simulation have been performed to investigate the beam-cavity interaction. This paper reports the result of the study and the work aiming at the development of a model of the system, including the wide-band feedback, which reproduces this interaction

The Writing is on the Wall:Proportionality of Border Controls within the Schengen Area

On 6 October 2021, Advocate General (AG) Saugmandsgaard Øe published his Opinion in the joined cases C-368/20 NW v Landespolizeidirektion Steiermark and C-369/20 NW v Bezirkshauptmannschaft Leibnitz. As discussed in our previous blog post outlining the facts and the legal questions of the case, the cases concern the legality – and limits – of introducing border controls within the Schengen Area. Six Schengen countries (Germany, France, Austria, Denmark, Norway and Sweden) have reintroduced border controls over the past years. If the Court of Justice of the European Union (CJEU) were to follow the AG’s Opinion, they would need to seriously rethink their practices in this regard. New evidence-based procedures and serious reasons, capable of passing a proportionality test, would be necessary to introduce border controls within the Schengen Zone

Schengen Restored:The CJEU Sets Clear Limits to the Reintroduction of Internal Border Controls

On 26 April 2022, the Court of Justice of the EU (CJEU) rendered a ruling in joined cases C-368/20 NW v Landespolizeidirektion Steiermark and C-369/20 NW v Bezirkshauptmannschaft Leibnitz stating that Member States of the European Union (EU) can re-introduce border controls within the Schengen Zone only under strict conditions. The Court has stepped up as a guardian of the Treaties protecting free movement of people without controls at the internal borders of the EU as “one of the major achievements of European integration” (para 65 and 74). At the same time, the Court has left some aspects of the application of these strict new rules unclear, leaving room for the European and national executives to exercise their function and fill in the blanks

Surface Resistance Measurements for the LHC Beam Screen

A critical review of resistive losses in the LHC beam screen, taking into account anomalous skin effect and surface roughness, has triggered a programme of surface resistance measurements at different temperatures, frequencies and magnetic field intensities. The aim is to establish a realistic heating budget for the LHC cryogenic system and to optimize the fabrication process for the copper coating of the beam screen. Preliminary results at cryogenic temperatures (without magnetic field) indicate a surface resistance about a factor two larger than previously estimated: an absolute measurement precision of a few per cent is reached by comparing the quality factors of even and odd TEM modes in a cylindrical structure with two inner conductors

The PS 10 MHz High Level RF System Upgrade

In view of the upgrade of the injectors for the High Luminosity LHC, significantly higher bunch intensity is required for LHC-type beams. In this context an upgrade of the main accelerating RF system of the Proton Synchrotron (PS) is necessary, aiming at reducing the cavity impedance which is the source of longitudinal coupled-bunch oscillations. These instabilities pose as a major limitation for the increase of the beam intensity as planned after LS2. The 10 MHz RF system consists in 11 ferrite loaded cavities, driven by tube-based power amplifiers for reasons of radiation hardness. The cavity-amplifier system is equipped with a wide-band feedback that reduces the beam induced voltage. A further reduction of the beam loading is foreseen by upgrading the feedback system, which can be reasonably achieved by increasing the loop gain of the existing amplification chain. This paper describes the progress of the design of the upgraded feedback system and shows the results of the tests on the new amplifier prototype, installed in the PS during the 2015-16 technical stop. It also reports the first results of its performance with beam, observed in the beginning of the 2016 run