A study of multipacting in rectangular waveguide geometries

Electron multipacting is a resonant process by which electrons build-up under the influence of a radio-frequency (RF) field. This process can occur in evacuated RF equipment such as the input coupler of accelerator cavities. The superconducting RF cavities designed by Cornell University, that are used in a number of synchrotron light sources including the DIAMOND Light Source, have had a history of vacuum breakdown in the CESR ring at Cornell with frequencies that would be inappropriate for a reliable synchrotron facility. This work aims to understand and correct the problem and ensure smooth operation of the cavities. The cause of the vacuum breakdown hindering the cavities’ operation at Cornell was identified as being multipactor in the rectangular input coupler waveguide. Prior studies carried out by R.L. Geng at Cornell University identified a number of solutions which he proposed to verify experimentally. Two series of experiments were carried out at Cornell University on short waveguide sections. The first session allowed us to observe, measure and attempt to suppress multipactor using techniques such as a longitudinal static magnetic bias field and a groove cut along the waveguide centreline. While the first technique was found to be quite effective, since a relatively weak 10G field was found to be sufficient to achieve complete multipactor suppression, the groove did not allow such total suppression of the multipactor though it did mitigate its effects. The second experimental session was designed to complement the first. The waveguide allowed the testing of other methods such as multiple grooves, a ridge in place of the groove, or surface coatings. The ridge proved to be as effective as a groove with regards to multipactor suppression, while multiple grooves proved to increase rather than reduce the total multipacting current. The waveguide could also be heated or cooled to study the effect of baking the surface as well as that of condensed gases. Surface coatings were tested and found to have the expected effect of lowering the multipactor current, but the surface areas covered as well as the vacuum quality achieved were insufficient to conclusively validate the use of coatings as a means of achieving multipactor-free operation of the coupler. Both series of experiments provided extensive measurements of electron currents at various locations on the waveguide and at a range of RF power levels; these were compared to simulations of multipactor developed using the MAGIC PIC code. The code results and the simulations were found to agree closely when using a secondary electron model including backscattered low energy electrons. The code was able to predict the effectiveness of a ridge, as well as agreeing with experimental observations. Instead of sharply defined multipactor bands as predicted by simple multipactor models, the multipactor current (above a certain power level) does not disappear completely even though it may show peaks and troughs for various values of the RF power. In conclusion, the magnetic bias is the only proven method to ensure multipactor-free operation of the CESR-type cavities. A ridge or a groove cut along the centreline of the waveguide could be a simple, passive way of limiting and retarding the effects of multipactor in the input coupler, while coatings should certainly be considered, though more research is needed to fully validate the concept

RF surface resistance study of non-evaporable Getter coatings

In many particle accelerators the beam parameters could be affected by the beam pipe wakefield impedance. It is vital to understand how the wakefield impedance might vary due to various coatings on the surface of the vacuum chamber, and this can be derived from surface resistance measurements. The bulk conductivity of two types of NEG films (dense and columnar) is determined. This is achieved by measuring the surface resistance of NEG-coated samples using an RF test cavity and fitting the experimental data to a standard theoretical model. The conductivity values obtained are then used to compare resistive wall wakefield effects in beam pipes coated with either of the two types of film

Design and testing of a four rod crab cavity for High Luminosity LHC

A 4-rod deflecting structure is proposed as a possible crab cavity design for the LHC high luminosity upgrade. Crab cavities are required for the LHC luminosity upgrade to provide a greater bunch overlap in the presence of a crossing angle, but must fit in the existing limited space. The structure has two parallel sections consisting of two longitudinally opposing quarter-wave rods, where each rod has the opposite charge from each of its nearest neighbors. The structure is transversely compact because the frequency is dependent on the rod lengths rather than the cavity radius. Simulations were undertaken to investigate the effect of rod shape on surface fields, higher order multipole terms and induced wakefields in order to obtain the optimal rod shape. The simulation results presented show that the addition of focus electrodes or by shaping the rods the sextupole contribution of the cavity voltage can be negated; the sextupole contribution is 321.57 mTm/m2, Epeak=27.7 MV/m and Bpeak=63.9 mT at the design voltage of 3 MV. The damping requirements for the LHC are critical and suitable couplers to damp all modes but the operating mode are presented. The results of various testing cycles of the first SRF 4 rod prototype cavity are presented and show that the cavity has reached the required transverse voltage of 3 MV

Status of the CASCADE microwave cavity experiment

The CASCADE experiment is a "light shining through a wall" (LSW) experiment consisting of microwave cavities. It is dedicated to search for photon oscillations into hidden sector photons (HSP). The main measurement setup consists of two normal conducting TM010 pillbox cavities at 1.3 GHz. In this paper we present the planned measurement campaign that is divided in four main phases

CASCADE:a cavity based dark matter experiment

An experiment is proposed that uses a pair of RF cavities as a source and detector of hidden sector photons (HSP). HSP's are hypothetical low-mass dark matter candidates with coupling to ordinary photons. SRF cavities are favoured in this experiment as they are able to store a high number of photons for a given input power due to the high Q available. When powered, such a cavity will act as a source of HSPs, while an empty cavity will be able to capture any HSP's decaying back into RF photons. Such an experiment (CASCADE) is being developed at the Cockcroft Institute using single cell 1.3 GHz cavities previously utilised for manufacturing and BCP studies. The aims of the CASCADE project are detailed, along with the system specification

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

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