P. Goudket

Suppression of multipacting in rectangular coupler waveguides.

Although a rectangular waveguide coupler has the conceptual advantages of simplicity and capability of withstanding higher power, builders of modern superconducting accelerators are routinely choosing instead a coaxial coupler for its proven performance. Multipacting induced discharge has been found to be an operating mechanism that prevents a rectangular waveguide coupler from reaching its full potential. Earlier calculations predicted the existence of two-sided multipacting in a rectangular waveguide geometry. In the present study, special waveguide sections of CESR type were built and tested. Multipacting characteristics of the waveguide were identified. Two multipacting suppression methods, the slotted waveguide method and the DC magnetic bias method, were experimentally evaluated. The multipacting current is suppressed by a factor of more than 2 by opening a slot on the broad wall. Complete multipacting suppression can be realized by using the DC magnetic bias method.

Design of the ILC crab cavity system.

The International Linear Collider (ILC) has a 14 mrad crossing angle in order to aid extraction of spent bunches. As a result of the bunch shape at the interaction point, this crossing angle at the collision causes a large luminosity loss which can be recovered by rotating the bunches prior to collision using a crab cavity. The ILC baseline crab cavity is a 9-cell superconducting dipole cavity operating at a frequency of 3.9 GHz. In this paper the design of the ILC crab cavity and its phase control system, as selected for the RDR 1 in February 2007 is described in fuller detail.

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.

Power coupler for the ILC crab cavity

The ILC crab cavity will require the design of an appropriate power coupler. The beam-loading in dipole - mode cavities is considerably more variable than accelerating cavities, hence simulations have been performed to establish the required external Q. Simulations of a suitable coupler were then performed and were verified using a normal conducting prototype with variable coupler tips.

Design approach for the development of a cryomodule for compact crab cavities for Hi-Lumi LHC

A prototype Superconducting RF (SRF) cryomodule, comprising multiple compact crab cavities is foreseen to realise a local crab crossing scheme for the “Hi-Lumi LHC”, a project launched by CERN to increase the luminosity performance of LHC. A cryomodule with two cavities will be initially installed and tested on the SPS drive accelerator at CERN to evaluate performance with high-intensity proton beams. A series of boundary conditions influence the design of the cryomodule prototype, arising from; the complexity of the cavity design, the requirement for multiple RF couplers, the close proximity to the second LHC beam pipe and the tight space constraints in the SPS and LHC tunnels. As a result, the design of the helium vessel and the cryomodule has become extremely challenging. This paper assesses some of the critical cryogenic and engineering design requirements and describes an optimised cryomodule solution for the evaluation tests on SPS.

HOM and LOM coupler optimizations for the ILC crab cavity

The FNAL 9-cell 3.9 GHz deflecting cavity designed for the CKM experiment was chosen as the baseline design for the ILC BDS crab cavity. Effective damping is required for the lower-order TM010 modes (LOM), the same-order TM110 pi-mode (SOM) as well as the higher-order modes (HOM) to minimize the beam loading and beam centroid steering due to wakefields. Simulation results of the original CKM design using the eigensolver Omega3P showed that both the notch filters of the HOM/LOM couplers are too sensitive to the notch gap, and the damping of the SOM is insufficient for the ILC. To meet the ILC requirements, the couplers were redesigned to improve the damping and tuning sensitivity. With the new design, the damping of the LOM/SOM/HOM modes is significantly improved, the sensitivity of the notch filter for the HOM coupler is reduced by one order of magnitude and mechanically feasible, and the LOM coupler is simplified by aligning it on the same plane as the SOM coupler and by eliminating the notch filter. In this paper, we will present the coupler optimization, tolerance studies and multipacting analysis for the crab cavity.

Progress on Superconducting RF Cavity Development With UK Industry

As part of a STFC Industrial Programme Support (IPS) Scheme grant, Daresbury Laboratory and Shakespeare Engineering Ltd have been developing the capability to fabricate, process, and test a 9-cell, 1.3 GHz superconducting RF cavity. The objective of the programme of work is to achieve an accelerating gradient of greater than 20 MV/m at an unloaded quality factor of 1.0 x 10 10 or better. Processes such as the high pressure rinsing and the buffer chemical polishing are being developed at Daresbury Laboratory and the manufacturing of the cavity half-cells and beam-pipes are being optimised by Shakespeare Engineering to enable this target to be achieved. These are discussed in this paper.

Saturation of multipactor in rectangular waveguide

Summary form only given. Multipactor is a limiting factor in a many RF systems, restricting performance, efficiency, lifetime and reliability. In many situations it is possible to avoid multipactor by various suppression methods or the avoidance of specific operating points. However this cannot always be achieved. When multipactor cannot be avoided, the saturation level becomes critical. In for instance, superconducting applications, multipactor which does not saturate at a low level can limit peak power.

Development of circuits and system models for the synchronization of the ILC crab cavities

The ILC reference design report (RDR) recommends a 14 mrad crossing angle for the positron and electron beams at the IP. A matched pair of crab cavity systems are required in the beam delivery system to align both bunches at the IP. The use of a multi-cell, 3.9 GHz dipole mode superconducting cavity is proposed, derived from the Fermilab CKM cavity being developed as a beam slice diagnostic [1]. Dipole-mode cavities phased for crab rotation are shifted by 90deg with respect to similar cavities phased for deflection. Uncorrelated phase errors of 0.086deg (equivalent to 61 fs) for the two cavity systems, gives an average of 180 nm for the relative deflection of the bunch centers. For a horizontal bunch size sigmax = 655 nm, a deflection of 180nm reduces the ILC luminosity by 2%. The crab cavity systems are to be placed ~30 m apart and synchronization to within 61 fs is required; this is on the limit of what is presently achievable. This paper describes LLRF circuits under development at the Cockcroft Institute for proof of principle experiments planned on the ERLP at Daresbury and on the ILCTA test beamline at FNAL. Simulation results for stabilisation performance are also given.