The Tuning Algorithm of the LHC 400 MHz Superconducting Cavities

The LHC Superconducting 400 MHz cavities are of the single-cell type, equipped with a movable coupler allowing for a range of loaded Q from 20000 to 180000. Their mechanical tuner has a range of 250 kHz with a 25 Hz resolution. The LHC bunch spacing is 25 ns but the filling pattern exhibits various gaps free of particles and of different length. Consequently the RF beam current at 400 MHz is also modulated. The largest gap has a length of 3ï­s to accommodate for the rise-time of the beam dump kicker. It will always be kept free of particles. With the strong RF feedback the decay time of the cavity field is between one and two ï­s, much smaller than the revolution period (90 ï­s). Hence two steady state situations are encountered during one turn: The klystron power toggles between beam and no-beam values as the RF feedback keeps the field constant. The tuner position is set to keep the klystron power equal in both cases (Half Detuning). This note presents the tuning algorithm that sets and keeps the tuner at this optimal position

Low-level RF - Part I: Longitudinal dynamics and beam-based loops in synchrotrons

The low-level RF system (LLRF) generates the drive sent to the high-power equipment. In synchrotrons, it uses signals from beam pick-ups (radial and longitudinal) to minimize the beam losses and provide a beam with reproducible parameters (intensity, bunch length, average momentum and momentum spread) for either the next accelerator or the physicists. This presentation is the first of three: it considers synchrotrons in the lowintensity regime where the voltage in the RF cavity is not influenced by the beam. As the author is in charge of the LHC LLRF and currently commissioning it, much material is particularly relevant to hadron machines. A section is concerned with radiation damping in lepton machines.Comment: 27 pages, contribution to the CAS - CERN Accelerator School: Specialised Course on RF for Accelerators; 8 - 17 Jun 2010, Ebeltoft, Denmar

Double batch injection into LEP

LEP is now routinely filled from two elementary injector cycles, one positron and one electron. During each of these cycles, 8 bunches are accelerated in the PS accelerator, then the SPS and then injected into 4 bunches in LEP. Two SPS bunches being injected into the same LEP bucket. This mode of operation has become possible as a result of synchrotron injection. The time between successive injections into the same LEP bucket is presently 6 LEP turns. This value is optimum for the present synchrotron tune used at injection in LEP. The LEP injectors normally run with a 14.4 second cycle repetition, of which four 1.2 second elementary cycles were traditionally dedicated to LEP filling. However with the new scheme only two elementary cycles (each lasting 1.2 seconds) are needed for LEP. The time saved, amounting to one sixth of the total injector cycle time has become available for other users. During the last year these have included extra physics cycles in the PS and heavy ion and LHC machine development cycles in the SPS. No loss in performance has been observed with the accumulation rates in LEP. The scheme as put into place will be described in detail

Performances and future plans of the LHC RF

The ramp-up of the LHC operation has been exceptionally fast: from the first acceleration of a single bunch at nominal intensity (1.1E11 p) to 3.5 TeV/c on May 2010, to the accumulation of 11 fb-1 integrated luminosity two years later (June 2012). On the RF side this was made possible by a few key design choices and several developments, that allow reliable LHC operation with 0.35 A DC beam at 4 TeV/c (1380 bunches at 50 ns spacing, 1.5E11 p per bunch). This paper reviews the RF design and presents its performance. Plans are also outlined that would allow operation with 25 ns bunch spacing (doubling the beam current) and even increased bunch intensity with the target of above 1A DC current per beam, without big modification to the existing RF power system

Proposal for an RF Roadmap Towards Ultimate Intensity in the LHC

The LHC currently operates with 1380 bunches at 50 ns spacing and 1.4 1011 p per bunch (0.35A DC). In this paper the RF operation with ultimate bunch intensity (1.7 1011 p per bunch) and 25 ns spacing (2808 bunches per beam) summing up to 0.86A DC is presented. With the higher beam current, the demanded klystron power will be increased and the longitudinal stability margin reduced. One must also consider the impact of a klystron trip (voltage and power transients in the three turns latency before the beam is actually dumped). In this work a scheme is proposed that can deal with ultimate bunch intensity. Only a minor upgrade of the Low Level RF is necessary: the field set point will be modulated according to the phase shift produced by the transient beam loading, thus minimizing the RF power while keeping the strong feedback for stability and reduction of the RF noise

LHC One-Turn Delay Feedback Commissioning

The LHC One-Turn delay FeedBack (OTFB) is an FPGA based feedback system part of the LHC cavity controller, which produces gain only around the revolution frequency (frev = 11.245 kHz) harmonics. As such, it helps reduce the transient beam loading and effective cavity impedance. Consequently, it increases the stability margin for Longitudinal Coupled Bunch Instabilities driven by the cavity impedance at the fundamental and allows reliable operation at higher beam currents. The OTFB was commissioned on all sixteen cavities in mid-October 2011 and has been used in operation since. The commissioning procedure and algorithms for setting-up are presented. The resulting improvements in transient beam loading, beam stability, and required klystron power are analyzed. The commissioning of the OTFB reduced the cavity voltage phase modulation from approximately six degrees peak-to-peak to below one degree at 400 MHz with nominal bunch intensity of 1.1e11 protons

Commissioning of the I-LHC RF low level with beam

During the machine development session of 2007-10-03, the energy matching between PS and SPS was completed for the 208Pb82+ beam. The I-LHC RF low level, including the phase loop and the synchronisation loop, was commissioned to capture a single bunch of 208Pb82+. After commissioning, the beam was used for several non RF related machine developments