Compensation of transient beam loading in ramping synchrotrons using a fixed frequency processing clock

Transient beam loading compensation schemes, such as One-Turn-FeedBack (OTFB), require beam synchronous processing (BSP). Swept clocks derived from the RF, and therefore harmonic to the revolution frequency, are widely used in CERN synchrotrons; this simplifies implementation with energy ramping, where the revolution frequency changes. It is however not optimal for state-of-the-art digital hardware that prefers fixed frequency clocks. An alternative to the swept clocking is the use of a deterministic protocol, for example White Rabbit (WR): a fixed reference clock can be extracted from its data stream, while enabling digital distribution of the RF frequency among other data. New algorithms must be developed for BSP using this fixed clock and the digital data transmitted on the WR link. This is the strategy adopted for the SPS Low Level RF (LLRF) upgrade. The paper gives an overview of the technical, technological and historical motivations for such a paradigm evolution. It lists the problems of fixed clock BSP, and presents an innovative solution based on a real-time variable ratio re-sampler for implementing an OTFB with the new fixed clock scheme.Peer ReviewedPostprint (published version

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

Luminosity reduction caused by phase modulations at the HL-LHC crab cavities

The design of the High-Luminosity Large Hadron Collider (HL-LHC) requires two pairs of crab cavities to be installed either side of Interaction Points (IPs) 1 (ATLAS) and 5 (CMS) to compensate for the geometric reduction in luminosity due to the beam crossing angle at the IP. The HL-LHC beam current is a factor of two larger than the LHC design value. The existing RF system has insufficient power to use the existing low level RF (LLRF) scheme for HL-LHC and therefore a new scheme is proposed which results in an irregular bunch pattern in the ring; here in referred to as a phase modulation. In this paper we study the effect of this phase modulation on the crab cavity scheme and the resulting impact on peak luminosity. We have developed an analytical model to calculate the luminosity and its dependence on the related beam and RF parameters. We compare this model to tracking simulations in PYTRACK and show a good agreement between the model and simulations. In the case of a coherent phase error between the counter-rotating bunch trains, having the maximum expected time shift of 100 ps (0.25 radians at the RF frequency), the reduction of analytical peak luminosity is found to be 1.89% when the crabbing voltage is 6.8 MV. For incoherent phase errors, the luminosity reduction for a 100 ps phase error is 5.67%; however the expected incoherent phase error is significantly less than 100 ps. These reductions are not foreseen as an issue when the crabbing scheme is used for luminosity levelling during physics experiments

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

Prediction of Beam Losses during Crab Cavity Quenches at the HL-LHC

Studies of the crab cavities at KEKB revealed that the RF phase could shift by up to 50o within ~50 us during a quench; while the cavity voltage is still at approximately 75% of its nominal amplitude. If such a failure were to occur on the HL-LHC crab cavities, it is likely that the machine would sustain substantial damage to the beam line and surrounding infrastructure due to uncontrolled beam loss before the machine protection system could dump the beam. We have developed a low-level RF system model, including detuning mechanisms and beam loading, and use this to simulate the behaviour of a crab cavity during a quench, modeling the low-level RF system, detuning mechanisms and beam loading. We supplement this with measurement data of the actual RF response of the proof of principle Double-Quarter Wave Crab Cravity during a quench. Extrapolating these measurements to the HL-LHC, we show that Lorentz Force detuning is the dominant effect leading to phase shifts in the crab cavity during quenches; rather than pressure detuning which is expected to be dominant for the KEKB crab cavities. The total frequency shift for the HL-LHC crab cavities during quenches is expected to be about 460 Hz, leading to a phase shift of no more than 3o. The results of the quench model are read into a particle tracking simulation, SixTrack, and used to determine the effect of quenches on the HL-LHC beam. The quench model has been benchmarked against the KEKB experimental measurements. In this paper we present the results of the simulations on a crab cavity failure for HL-LHC as well as for the SPS and show that beam loss is negligible when using a realistic low-level RF response.Comment: 21 Pages, 22 figures, Submitted to PRA

Beam Loading and Longitudinal Stability Evaluation for the FCC-ee Rings

In high-current accelerators, interaction of the beam with the fundamental impedance of the accelerating cavities can limit machine performance. It can result in a significant variation of bunch-by-bunch parameters (bunch length, synchronous phase, etc.) and lead to longitudinal coupled-bunch instability. In this work, these limitations are analysed together with possible cures for the high-current option (Z machine) of the future circular electron-positron collider (FCC-ee). The time-domain calculations of steady-state beam loading are presented and compared with frequency-domain analysis

Transient beam loading and rf power evaluation for future circular colliders

Interaction of the beam with the fundamental impedance of the accelerating cavities can limit the performance of high-current accelerators. It can result in a significant variation of bunch-by-bunch parameters (bunch length, synchronous phase, etc.) if filling patterns contain gaps that are not negligible compared with the cavity filling time. In the present work, this limitation is analyzed using the steady-state time-domain approach for the high-current option (Z) of the future circular electron-positron collider and for the future circular hadron-hadron collider. Mitigation of transient beam loading by direct rf feedback is addressed with evaluation of additional required generator power

LLRF Studies for HL-LHC Crab Cavities

The HL-LHC upgrade includes sixteen Crab Cavities (CC) to be installed on both sides of the high luminosity experiments, ATLAS and CMS. Two issues have been highlighted for the Low Level RF: transverse emittance growth (and associated luminosity drop) caused by CC RF noise, and large collimator losses following a CC trip. A prototype cryomodule with two CCs has been installed in the SPS, and tests have started in May 2018 with beam. This paper briefly reports on preliminary results from the SPS tests. It then presents emittance growth calculations from cavity field phase and amplitude noise, deduces the maximum RF noise compatible with the specifications and presents a possible cure consisting of a feedback on CC phase and amplitude. To reduce the losses following a CC trip we propose to implement transverse tail cleaning via the injection of CC noise with an optimized spectrum, which selectively excites the particles of large transverse oscillation amplitudes

Beam loading compensation for the Future Circular hadron-hadron Collider (FCC-hh)

The power consumption of the rf system can be minimised by optimising the cavity detuning and the loaded quality factor. In high-current accelerators, the presence of gaps in the filling results in a modulation of the cavity voltage along the ring (transient beam loading) and as a consequence a spread in the bunch parameters. In addition longitudinal coupled-bunch instabilities can appear, caused by the cavity impedance at the fundamental. Both issues can be mitigated by using an rf feedback around the amplifier and cavity, a technique used in many high intensity machines including the Large Hadron Collider (LHC). Compared to the LHC machine, the energy increase and the radiation loss for the Future Circular hadron-hadron Collider (FCC-hh) will be larger, resulting in a synchronous phase deviating significantly from 180 degrees. The solutions adopted for the LHC must therefore be revisited. This paper evaluates several beam loading compensation schemes for this machine