• M. Meddahi, S. Bart Pedersen, A. Boccardi, A.C. Butterworth, B. Goddard, G.H. Hemelsoet, W. Höfle, D. Jacquet, M. Jaussi, V. Kain, T. Lefèvre, E.N. Shaposhnikova, J.A. Uythoven, D. Valuch
  CERN, Geneva
• A.S. Fisher
  SLAC, Menlo Park, California
• E. Gianfelice-Wendt
  Fermilab, Batavia

Unbunched beam is a potentially serious issue in the LHC as it may quench the superconducting magnets during a beam abort. Unbunched particles, either not captured by the RF system at injection or leaking out of the RF bucket, will be removed by using the existing damper kickers to excite resonantly the particles in the abort gap. Following beam simulations, a strategy for cleaning the abort gap at different energies was proposed. The plans for the commissioning of the beam abort gap cleaning are described, and the first results from the beam commissioning are presented.

• M. E. Angoletta, A. Blas, A.C. Butterworth, A. Findlay, P.M. Leinonen, J.C. Molendijk, F. Pedersen, J. Sanchez-Quesada, M. Schokker
  CERN, Geneva

In 2008 a project was started to renovate the CERN's PS Booster (PSB) low-level RF (LLRF). Its aim is to equip all four PSB rings with modern LLRF systems by 2013 at the latest. Required capabilities for the new LLRF include frequency program, beam phase, radial and synchronization loops. The new LLRF will control the signals feeding the three RF cavities present in each ring; it will also shape the beam in a dual harmonic mode, operate a bunch splitting and create a longitudinal blow-up. The main benefits of this new LLRF are its full remote and cycle-to-cycle controllability, built-in observation capability and flexibility. The overall aim is to improve the robustness, maintainability and reliability of the PSB operation and to make it compatible with the injection from the future LINAC4. The chosen technology is an evolution of that successfully deployed in CERN's ion accumulator ring LEIR and it is based upon modular VME 64X hardware and extensive digital signal processing. This paper outlines the main characteristics of the software and hardware building blocks. Promising initial beam tests are shown and hints are included on the main milestones and future work.

• M. E. Angoletta, J. Bento, A. Blas, E. Bracke, A.C. Butterworth, F. Dubouchet, A. Findlay, F. Pedersen, J. Sanchez-Quesada
  CERN, Geneva

The Low Energy Ion Ring (LEIR) is an accumulation ring in the Large Hadron Collider ion injector chain. After its successful start in 2005, it has been running in three operational campaigns. The LEIR LLRF system is the first all-digital low-level RF (LLRF) system to be made operational in a CERN circular machine. Composed of modular VME 64X hardware, it carries out extensive digital signal processing via Field Programmable Gate Arrays and Digital Signal Processors. System capabilities include beam control tasks, such as frequency program, beam phase, radial and synchronization loops, as well as cavity voltage/phase loops. All the system's control parameters are fully configurable, remotely and in-between cycles; extensive built-in diagnostics and signal observation features are available. The system has proven to be not only flexible and powerful but also extremely reliable. This is very important as the LEIR LLRF system is the pilot project for the LLRF renovation of other CERN's machines. This paper gives an overview of the main system building blocks and outlines their capabilities and operational features, along with results obtained during the first years of beam operation.

• T. Mastorides, J.D. Fox, C.H. Rivetta, D. Van Winkle
  SLAC, Menlo Park, California
• P. Baudrenghien, A.C. Butterworth, J.C. Molendijk
  CERN, Geneva

Radio Frequency (RF) accelerating system noise and non-idealities can have detrimental impact on the LHC performance through longitudinal motion and longitudinal emittance growth. A theoretical formalism has been developed to relate the beam and RF loop dynamics with the bunch length growth [1]. Measurements were conducted at LHC to validate the formalism, determine the performance limiting RF components, and provide the foundation for beam diffusion estimates for higher energies and intensities. A brief summary of these results is presented in this work.

[1] T. Mastorides et. al., "RF system models for the LHC with Application to
Longitudinal Dynamics", prepared for submission to Physical Review ST-AB.

• D. Van Winkle, J.D. Fox, T. Mastorides, C.H. Rivetta
  SLAC, Menlo Park, California
• P. Baudrenghien, A.C. Butterworth, J.C. Molendijk
  CERN, Geneva

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.

[1] D. Van Winkle et. al. 'Feedback Configuration Tools for LHC Low Level RF System,' PAC'09, Vancouver, Canada, May 2009, THZCH03, p. 249 (2009); http://www. JACoW.org.

• L. Arnaudon, P. Baudrenghien, O. Brunner, A.C. Butterworth
  CERN, Geneva

The LHC ACS RF system is composed of 16 superconducting cavities, eight per ring, housed in a total of four cryomodules each containing four cavities. Each cavity is powered by a 300 kW klystron. The ACS RF power control system is based on industrial Programmable Logic Controllers (PLCs), but with additional fast RF interlock protection systems. Operational performance and reliability are described. A full set of user interfaces, both for experts and operators has been developed, with user feedback and maintenance issues as key points. Operational experience with the full RF chain, including the low level system, the beam control, the synchronisation system and optical fibres distribution is presented. Last but not least overall performance and reliability based on experience with beam are reviewed and perspectives for future improvement outlined.

• R.J. Steinhagen, A. Boccardi, A.C. Butterworth, E. Calvo Giraldo, R. Denz, M. Gasior, J.L. Gonzalez, S. Jackson, L.K. Jensen, O.R. Jones, Q. King, G. Kruk, M. Lamont, S.T. Page, J. Wenninger
  CERN, Geneva

The LHC deploys a comprehensive suite of beam-based feedbacks for safe and reliable machine operation. This contribution summarises the commissioning and early results of the LHC feedback control systems on orbit, tune, chromaticity, and energy. Their performance – strongly linked to the associated beam instrumentation, external beam perturbation sources and optics uncertainties – is evaluated and compared with the feedback design assumptions.