Active lower order mode damping for the four rod LHC crab cavity

The high luminosity upgrade planned for the LHC requires crab cavities to rotate bunches into alignment at the interaction points. They compensate for a crossing angle near to 500~$\mu$Rad. It is anticipated that four crab cavities in succession will be utilized to achieve this rotation either side of each IP in a local crossing scheme. A crab cavity operates in a dipole mode but always has an accelerating mode that may be above or below the frequency of the operating mode. Crab cavities are given couplers to ensure that unwanted acceleration modes are strongly damped however employing standard practice these unwanted modes will always have some level of excitation. Where this excitation has a random phase it might promote bunch growth and limit beam lifetime. This paper sets out a method for active control of the phase and amplitude of the unwanted lowest accelerating mode in the crab cavities. The paper investigates the level of suppression that can be achieved as a function cavity quality factor and proximity to resonance

Design and simulation studies of the novel beam arrival monitor pickup at Daresbury Laboratory

We present the novel beam arrival monitor pickup design currently under construction at Daresbury Laboratory, Warrington, UK. The pickup consists of four flat electrodes in a transverse gap. CST Particle Studio simulations have been undertaken for the new pickup design as well as a pickup design from DESY, which is used as a reference for comparison. Simulation results have highlighted two advantages of the new pickup design over the DESY design; the signal bandwidth is 25 GHZ, which is half that of the DESY design and the response slope is a factor of 1.6 greater. We discuss optimisation studies of the design parameters in order to maximise the response slope for bandwidths up to 50 GHz and present the final design of the pickup

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

High gradient testing of an X-band crab cavity at XBOX2

CERN’s Compact linear collider (CLIC) will require crab cavities to align the bunches to provide effective head-on collisions. An X-band quasi-TM11 deflecting cavity has been designed and manufactured for testing at CERN’s Xbox-2 high power standalone test stand. The cavity is currently under test and has reached an input power level in excess of 40MW, with a measured breakdown rate of better than 10-5 breakdowns per pulse. This paper also describes surface field quantities which are important in assessing the expected BDR when designing high gradient structures

Control and performance improvements of a pulse compressor in use for testing accelerating structures at high power

New developments relating to compact X-band, SLED-I type pulse compressors being developed at CERN for testing high gradient structures are described. Pulse compressors of interest take rf pulses from one or more high power klystrons with duration typically >1.5 μs and deliver up to 5 times the input power for a shorter duration <250 ns. Time domain models for pulse compressor operation with low level rf (LLRF) control have been developed. Input drive amplitude and phase for each pulse is evolved with a control algorithm from the pulse compressor output for previous pulses. The goal is to deliver precise amplitude for pulses to test stands and precise amplitude and phase for pulses to accelerator systems. Control algorithms have been developed and validated experimentally

Phase locked magnetrons for accelerators

Magnetrons offer lower capital costs and higher efficiencies than klystrons, however are natural oscillators rather than amplifiers. This paper reviews techniques and issues for applying high efficiency L band magnetrons to long pulse, high intensity proton linacs. Reference is made to a proof of principle experiment whereby the phase of an SRF cavity was accurately controlled when energised by a magnetron

Electronic and ionic processes and ionic bombardment of the cathode in a DC hydrogen glow discharge

The ionic and electronic reactions occurring in the cathode dark space (CDS) and negative glow (NG) of a hydrogen glow discharge with operating parameters typical of those used in surface treatments are modelled to calculate the angular and energy distributions of the ions incident on the cathode. The self-consistent model comprises three interactive sub-models, two of which are based on Monte Carlo simulations of electrons and positive ions, the third on conservation and mobility equations and is used for the determination of the ions' diffusion current from the NG to the CDS. Where available, reaction cross sections are taken from the literature with estimates being made for those such as H3+ to H+ which are not documented. An experimental determination of the energy of H+, H2+ and H3+ ions arriving at the cathode of the discharge is described and it is shown that the hydrogen ion currents predicted by the model are in accordance with those measured experimentally

Femto-second synchronisation with a waveguide interferometer

CERN’s compact linear collider CLIC requires crab cavities on opposing linacs to rotate bunches of particles into alignment at the interaction point (IP). These cavities are located approximately 25 metres either side of the IP. The luminosity target requires synchronisation of their RF phases to better than 5 fs r.m.s. This is to be achieved by powering both cavities from one high power RF source, splitting the power and delivering it along two waveguide paths that are controlled to be identical in length to within a micrometre. The waveguide will be operated as an interferometer. A high power phase shifter for adjusting path lengths has been successfully developed and operated in an interferometer. The synchronisation target has been achieved in a low power prototype system

Phase space analysis of multipactor saturation in rectangular waveguide

In certain high power RF systems multipactor cannot be avoided for all operating points, but its existence places limits on performance, efficiency, lifetime, and reliability. As an example multipactor in the input couplers of superconducting RF cavities can be a major limitation to the maximum RF power. Several studies have concentrated on rectangular waveguide input couplers which are used in many light sources. Most of these studies neglect space charge assuming that the effect of space charge is simply to defocus the electron bunches. Modelling multipactor to saturation is of interest in determining the performance of waveguide under a range of conditions. Particle-in-cell modelling including space charge has been performed for 500 MHz half-height rectangular waveguide. Phase plots of electron trajectories can aid understanding the processes taking place in the multipactor. Results strongly suggest that the multipacting trajectories are strongly perturbed by space charge causing the electrons to transition from two-surface to single-surface trajectories as the multipactor approaches saturation

Prototype Development of the CLIC Crab Cavities

CLIC will require two crab cavities to align the beams to provide an effective head-on collision with a 20 mdeg crossing angle at the interaction point. An X-band system has been chosen for the crab cavities. Three prototype cavities have been developed in order to test the high power characteristics of these cavities. One cavity has been made by UK industry and one has been made using the same process as the CLIC main linac in order to gain understanding of breakdown behaviour in X-band deflecting cavities. The final cavity incorporates mode-damping waveguides on each cell which will eventually contain SiC dampers. This paper details the design, manufacture and preparation of these cavities for testing and a report on their status