Commissioning of the LCH Low Level RF System Remote Configuration Tools

The LHC Low Level RF system (LLRF) is a complex multi-loop system used to regulate the superconductive cavity gap voltage as well as to reduce the impedance presented by RF stations to the beam. The RF system can have a profound impact on the stability of the beam; a mis-configured RF system has the potential of causing longitudinal instabilities, beam diffusion and beam loss. To configure the RF station for operation, a set of parameters in the LLRF multi-loop system have to be defined. Initial system commissioning as well as ongoing operation requires a consistent method of computer based remote measurement and model-based design of each RF station feedback system. This paper describes the suite of Matlab tools used for configuring the LHC RF system during the start up in Nov2009-Feb2010. We present a brief overview of the tool, examples of commissioning results, and basics of the model-based design algorithms. This work complements our previous presentation [1], where the algorithms and methodology followed in the tools were described

The LHC Low Level RF

The LHC RF consists of eight 400 MHz superconducting cavities per ring, with each cavity independently powered by a 300 kW klystron, via a circulator. The challenge for the Low Level is to cope with very high beam current (more than 1 A RF component) and achieve excellent beam lifetime (emittance growth time in excess of 25 hours). Each cavity has an associated Cavity Controller rack consisting of two VME crates which implement high gain RF Feedback, a Tuner Loop with a new algorithm, a Klystron Ripple Loop and a Conditioning system. In addition each ring has a Beam Control system (four VME crates) which includes a Frequency Program, Phase Loop, Radial Loop and Synchronization Loop. A Longitudinal Damper (dipole and quadrupole mode) acting via the 400 MHz cavities is included to reduce emittance blow-up due to filamentation from phase and energy errors at injection. Finally an RF Synchronization system implements the bunch into bucket transfer from the SPS into each LHC ring. When fully installed in 2007, the whole system will count over three hundred home-designed VME cards of twenty-three different models installed in forty-five VME crates. The paper presents the various loops: it outlines the expected performances, summarizes the algorithms and the implementation. Thanks to a full scale test set-up including klystron and cavity we have measured the response of the RF Feedback and Tuner Loop; and these will be presented and compared to the expectations

Shifting markers of identity in East London's diasporic religious spaces

This article discusses the historical and geographical contexts of diasporic religious buildings in East London, revealing – contrary both to conventional narratives of immigrant integration, mobility, and succession and to identitarian understandings of belonging – that in such spaces and in the concrete devotional practices enacted in them, markers and boundaries of identity (ritual, spatial, and political) are contested, renegotiated, erased, and rewritten. It draws on a series of case-studies: Fieldgate Street Synagogue in its interrelationship with the East London Mosque; St Antony's Catholic Church in Forest Gate where Hindus and Christians worship together; and the intertwined histories of Methodism and Anglicanism in Bow Road. Exploration of the intersections between ethnicity, religiosity, and class illuminates the ambiguity and instability of identity-formation and expression within East London's diasporic faith spaces

Initial longitudinal diagnostics for ELENA’s commissioning

CERN’s Extra Low ENergy Antiproton (ELENA) ring underwent its second full year of beam commissioning in 2018 and succeeded in decelerating more than 1 E7 antiprotons to extraction energy. Traditional DC beam transformers do not work at ELENA’s low intensity hence alternative measurements methods, described in this note, were embedded within ELENA’s Low-Level RF (LLRF) system. These longitudinal diagnostics have provided to the users the measured bunched beam intensity and bunch length as Oasis signals. Derived from longitudinal and from transverse pick-up signals, the longitudinal diagnostics were used to ascertain and estimate progress with the machine transmission efficiency. This note describes the digital signal processing implementing the longitudinal diagnostics deployed in the 2018 ELENA run. Some challenges encountered and the beam results obtained are shown. Initial tests for de-bunched beam measurements based upon Schottky scans are also mentioned. Finally, details on the system upgrade that will take place during Long Shutdown 2 (LS2) are given

The new digital Low-Level RF system for CERN's Extra Low ENergy Antiproton machine

CERN’s new Extra Low ENergy Antiproton accelerator/decelerator (ELENA) completed its initial commissioning in 2018. This machine is equipped with a new digital Low-Level RF (LLRF) system that implements beam and cavity loops as well as longitudinal diagnostics. ELENA’s LLRF was instrumental for machine commissioning by decelerating some 1 E7 antiprotons from 5.3 MeV to 100 keV. Commissioning with H$^-$ ions took also place. Challenges faced included coping with low beam intensity and the wide frequency swing. This paper gives an overview of the LLRF system capabilities and operation. Beam results achieved with both H$^-$ ions and antiprotons are also shown

New LLRF capabilities and beam results for the second year of ELENA’s commissioning

CERN’s Extra Low ENergy Antiproton (ELENA) ring underwent its second full year of beam commissioning in 2018. The commissioning was very successful, although delayed by recurrent lack of beam availability from the H- source as well as from the Antiproton Decelerator (AD) ring. Major achievements of the ELENA commissioning in 2018 included the deployment of all required features in its Low-Level RF (LLRF) system, the commissioning of the electron cooler and the delivery of extracted beam to the Gravitational Behaviour of Antihydrogen at Rest (GBAR) experiment. This note describes the LLRF functionalities deployed during the 2018 ELENA run together with the main beam results obtained and some problems encountered. Remaining tasks to be integrated and validated are also summarised. This note is not only a general documentation of what has been achieved. First and foremost this note is intended to contribute in the training of LLRF experts and OP / ABP colleagues, in view of the future ELENA LLRF operation. Owing to the similarities between ELENA and the AD machine, this note will also be useful for the development and commissioning of the post-Long Shutdown 2 (LS2) AD LLRF, based upon the same system

Cavity Voltage Phase Modulation MD

The LHC RF/LLRF system is currently configured for extremely stable RF voltage to minimize transient beam loading effects. The present scheme cannot be extended beyond nominal beam current since the demanded power would exceed the peak klystron power and lead to saturation. A new scheme has therefore been proposed: for beam currents above nominal (and possibly earlier), the cavity phase modulation by the beam will not be corrected (transient beam loading), but the strong RF feedback and One-Turn Delay feedback will still be active for loop and beam stability in physics. To achieve this, the voltage set point will be adapted for each bunch. The goal of this MD was to test a new algorithm that would adjust the voltage set point to achieve the cavity phase modulation that would minimize klystron forward power

LLRF beam results on the first year of ELENA’s commissioning with beam

CERN’s Extra Low ENergy Antiproton (ELENA) ring’s commissioning with beam started in earnest in March 2017. Ions from an H- source were injected in ELENA at low energy, although in a degraded way (lower voltage and intensity than planned) and with not constant reproducibility of the injection. From August 2017 onwards antiprotons from the AD were also injected in ELENA at high energy for up to three, weekly MD session. The 2017 ELENA commissioning run stopped on December 1st to allow the installation of the electron cooler. This note gives an overview of the successful operation carried out by the ELENA Low-Level RF (LLRF) during the first year of ELENA commissioning and of the main beam results obtained. Operation with H- ions and with antiprotons are considered, together with different operational settings and problems encountered. Hints on future deployment and commissioning steps are also provided

Commissioning the New LLRF System of the CERN PS Booster

The PS Booster (PSB) is the first synchrotron in the injection chain for protons. The beams produced for the LHC and various fixed target experiments cover a very large parameter space. Over the Long Shutdown 2 (LS2), the PSB was heavily upgraded as part of the LHC Injectors Upgrade (LIU) project. The low-level RF systems now drive the new Finemet-loaded cavities, control RF synchronisation for the new injection mechanism, and cope with the increased injection and extraction energies. The Finemet cavities provide exceptional flexibility, allowing an arbitrary distribution of voltage at different revolution frequency harmonics, but at the cost of significant broadband impedance. The new injection mechanism allows bunch-to-bucket multi-turn injection, which significantly reduces the amount of beam loss at the start of the cycle. The longitudinal beam production schema for each beam-type was developed based on simulations during LS2, and then adapted during the setting-up phase to suit the final operational configuration. This paper discusses the commissioning of the new LLRF, and the consequences of the LIU upgrades on the production of various beams

Coupled-bunch instabilities due to fundamental cavity impedance

The purpose of this MD was to estimate the stability threshold due to longitudinal coupled-bunch instabilities (CBIs) driven by the RF cavities' fundamental impedance. These instabilities are not a concern for the LHC. A model was developed to study the stability threshold for the High-Luminosity LHC (HL-LHC). The goal of this MD was to validate this model and thus its predictions for the HL-LHC stability margins