Wakefield damping for the CLIC crab cavity

A crab cavity is required in the CLIC to allow effective head-on collision of bunches at the IP. A high operating frequency is preferred as the deflection voltage required for a given rotation angle and the RF phase tolerance for a crab cavity are inversely proportional to the operating frequency. The short bunch spacing of the CLIC scheme and the high sensitivity of the crab cavity to dipole kicks demand very high damping of the inter-bunch wakes, the major contributor to the luminosity loss of colliding bunches. This paper investigates the nature of the wakefields in the CLIC crab cavity and the possibility of using various damping schemes to suppress them effectively

Initial study on the shape optimisation of the CLIC crab cavity

The compact linear collider (CLIC) requires a crab cavity to align bunches prior to collision. The bunch structure demands tight amplitude and phase tolerances of the RF fields inside the cavity, for the minimal luminosity loss. Beam loading effects require special attention as it is one potential sources of field errors in the cavity. In order to assist the amplitude and phase control, we propose a travelling wave (TW) structure with a high group velocity allowing rapid propagation of errors out of the system. Such a design makes the cavity structure significantly different from previous ones. This paper will look at the implications of this on other cavity parameters and the optimisation of the cavity geometry.Comment: 3 pages. To be published in proceedings of LINAC 2008, Victoria, Canad

COPPER PROTOTYPE MEASUREMENTS OF THE HOM, LOM AND SOM COUPLERS FOR THE ILC CRAB CAVITY

Abstract The ILC Crab Cavity is positioned close to the IP and delivered luminosity is very sensitive to the wakefields induced in it by the beam. A set of couplers were designed to couple to and damp the spurious modes of the crab cavity. As the crab cavity operates using a dipole mode, it has different damping requirements from an accelerating cavity. A separate coupler is required for the monopole modes below the operating frequency of 3.9 GHz (known as the LOMs), the opposite polarization of the operating mode (the SOM), and the modes above the operating frequency (the HOMs). Prototypes of each of these couplers have been manufactured out of copper and measured attached to an aluminum nine cell prototype of the cavity and their external Q factors were measured. The results were found to agree well with numerical simulations

X-band crab cavities for the CLIC beam delivery system

The CLIC machine incorporates a 20 mrad crossing angle at the IP to aid the extraction of spent beams. In order to recover the luminosity lost through the crossing angle a crab cavity is proposed to rotate the bunches prior to collision. The crab cavity is chosen to have the same frequency as the main linac (11.9942 GHz) as a compromise between size, phase stability requirements and beam loading. It is proposed to use a HE11 mode travelling wave structure as the CLIC crab cavity in order to minimise beam loading and mode separation. The position of the crab cavity close to the final focus enhances the effect of transverse wake-fields so effective wake-field damping is required. A damped detuned structure is proposed to suppress and de-cohere the wake-field hence reducing their effect. Design considerations for the CLIC crab cavity will be discussed as well as the proposed high power testing of these structures at SLAC.Comment: Proceedings of X-Band Structures and Beam Dynamics Workshop (XB08), 44th ICFA beam dynamics workshop, Cockcroft Institute, UK, 1-4 dec. 200

Crab cavities for linear colliders

Crab cavities have been proposed for a wide number of accelerators and interest in crab cavities has recently increased after the successful operation of a pair of crab cavities in KEK-B. In particular crab cavities are required for both the ILC and CLIC linear colliders for bunch alignment. Consideration of bunch structure and size constraints favour a 3.9 GHz superconducting, multi-cell cavity as the solution for ILC, whilst bunch structure and beam-loading considerations suggest an X-band copper travelling wave structure for CLIC. These two cavity solutions are very different in design but share complex design issues. Phase stabilisation, beam loading, wakefields and mode damping are fundamental issues for these crab cavities. Requirements and potential design solutions will be discussed for both colliders.Comment: 3 pages. To be published in proceedings of LINAC 2008, Victoria, Canad

Monoculture vs mixed-species plantation impact on the soil quality of an ecologically sensitive area

Over the past four decades Western Ghats, one of the eight hottest hotspots of biological diversity in the world, has witnessed the transformation of its prime forests into other land-use types mainly monoculture plantations. The present study evaluated the impact of conversion of natural forests to mixed-species (teak) and monoculture (rubber) plantations on the soil quality of the Typic Plinthohumults soil series in the Southern Western Ghats region of Kerala, India. The baseline physicochemical and biological parameters of the different locations were analyzed using standard methods. To comprehend the impact of plantations on the overall soil quality, the soil quality index of the different land-uses was quantified using the forest as the reference land-use Significant variations in different soil physical, chemical, and biological properties of plantation and forest soils were observed in the present study. The overall soil quality index was found to follow the order: forest (1.0) > teak plantations (0.9) > rubber plantations (0.6), thus signifying the negative impact, monoculture rubber plantations had on the soil quality of the study area. The results emphasize the need for the development of better land management practices and mixed-species plantation systems such as the teak plantations in the present study which did not deteriorate the soil quality

X-Band Crab Cavities for the CLIC Beam Delivery System

The CLIC machine incorporates a 20 mrad crossing angle at the IP to aid the extraction of spent beams. In order to recover the luminosity lost through the crossing angle a crab cavity is proposed to rotate the bunches prior to collision. The crab cavity is chosen to have the same frequency as the main linac (11.9942 GHz) as a compromise between size, phase stability requirements and beam loading. It is proposed to use a HE11 mode travelling wave structure as the CLIC crab cavity in order to minimise beam loading and mode separation. The position of the crab cavity close to the final focus enhances the effect of transverse wake-fields so effective wake-field damping is required. A damped detuned structure is proposed to suppress and de-cohere the wake-field hence reducing their effect. Design considerations for the CLIC crab cavity will be discussed as well as the proposed high power testing of these structures at SLAC. Design of a crab cavity for CLIC is underway at the Cockcroft Institute in collaboration with SLAC. This effort draws on a large degree of synergy with the ILC crab cavity developed at the Cockcroft Institute and other deflecting structure development at SLAC. A study of phase and amplitude variations in the cavity suggests that the tolerances are very tight and require a 'beyond state of the art' LLRF control system. A study of cavity geometry and its effect on the cavity fields has been performed using Microwave studio. This study has suggested that for our cavity an iris radius between 4-5 mm is optimum with an iris thickness of 2-3 mm based on group velocity and peak fields. A study of the cavity wakefields show that the single bunch wakes are unlikely to be a problem but the short bunch spacing may cause the multi-bunch wakefields to be an issue. This will require some of the modes to be damped strongly so that the wake is damped significantly before any following bunch arrives. Various methods of damping have been investigated and suggest that waveguide damping in the cells should provide sufficient damping in the vertical plane, which is the most sensitive

The groupoid approach to Leavitt path algebras

When the theory of Leavitt path algebras was already quite advanced, it was discovered that some of the more difficult questions were susceptible to a new approach using topological groupoids. The main result that makes this possible is that the Leavitt path algebra of a graph is graded isomorphic to the Steinberg algebra of the graph’s boundary path groupoid. This expository paper has three parts: Part 1 is on the Steinberg algebra of a groupoid, Part 2 is on the path space and boundary path groupoid of a graph, and Part 3 is on the Leavitt path algebra of a graph. It is a self-contained reference on these topics, intended to be useful to beginners and experts alike. While revisiting the fundamentals, we prove some results in greater generality than can be found elsewhere, including the uniqueness theorems for Leavitt path algebras

LHC and CLIC LLRF final reports

Crab cavities rotate bunches from opposing beams to achieve effective head-on collision in CLIC or collisions at an adjustable angle in LHC. Without crab cavities 90% of achievable luminosity at CLIC would be lost. In the LHC, the crab cavities allow the same or larger integrated luminosity while reducing significantly the requested dynamic range of physics detectors. The focus for CLIC is accurate phase synchronisation of the cavities, adequate damping of wakefields and modest amplitude stability. For the LHC, the main LLRF issues are related to imperfections: beam offsets in cavities, RF noise, measurement noise in feedback loops, failure modes and mitigations. This report develops issues associated with synchronising the CLIC cavities. It defines an RF system and experiments to validate the approach. It reports on the development of hardware for measuring the phase performance of the RF distributions system and cavities. For the LHC, the hardware being very close to the existing LLRF, the report focuses on the requirements on the LLRF to mitigate anticipated imperfections