W.B. Fowkes

Bio: W.B. Fowkes is an academic researcher. The author has contributed to research in topics: Rat-race coupler & Collider. The author has an hindex of 3, co-authored 3 publications receiving 38 citations.

Accelerator structure R&D for linear colliders

Abstract: For more than ten years, we have been working on R&D for X-band accelerator structures for the JLC/NLC linear collider. Several types of Detuned (DS) and Damped Detuned Structures (DDS) have been successfully designed and fabricated. They have been experimentally tested at both low power and high power to characterize their mechanical and electrical properties. Recently we started developing a new type of damped detuned structure with optimized round-shaped cavities (RDDS). This paper discusses the special specifications, design methods, fabrication procedures, measurement technologies, and anticipated future improvements for all these structures.

A compact RF power coupler for the NLC linac

Abstract: A high power RF coupler connecting WR90 rectangular waveguide to the disc loaded waveguide accelerating structure of NLC is described. The coupler design is symmetric and free of beam deflecting field components. It makes an efficient electrical match between the rectangular waveguide power feed and the accelerator structure while keeping electrical surface field enhancement low. Electrical and mechanical design of the coupler are presented as well as quantitative comparisons between numerical simulation and low power cold test models.

Design, fabrication and measurement of the first rounded damped detuned accelerator structure (RDDS1)

Abstract: As a joint effort in the JLC/NLC research program, we have developed a new type of damped detuned accelerator structure with optimized round-shaped cavities (RDDS). This paper discusses some important R&D aspects of the first structure in this series (RDDS1). The design aspects covered are the cell design with sub-MHz precision, HOM detuning, coupling and damping technique and wakefield simulation. The fabrication issues covered are ultra-precision cell machining with micron accuracy, assembly and diffusion bonding technologies to satisfactorily meet bookshelf, straightness and cell rotational alignment requirements. The measurements described are the RF properties of single cavities and complete accelerator section, as well as wakefields from the ASSET tests at SLAC. Finally, future improvements are also discussed.

Low-Field Accelerator Structure Couplers and Design Techniques

Abstract: Recent experience with $X$-band accelerator structure development has shown the rf input coupler to be the region most prone to rf breakdown and degradation, effectively limiting the operating gradient. A major factor in this appears to be high magnetic fields at the sharp edges of the coupling irises. As a first response to this problem, couplers with rounded and thickened iris horns have been employed and successfully tested at high power. To further reduce fields for higher power flow, conceptually new coupler designs have been developed, in which power is coupled through the broad wall of the feed waveguide, rather than through terminating irises. A ``mode-launcher'' coupler, which launches the ${\mathrm{TM}}_{01}$ mode in circular waveguide before coupling through a matching cell into the main structure, has been tested with great success. With peak surface fields below those in the body of the structure, this coupler represented a breakthrough in the Next Linear Collider structure program. The design of this coupler and of variations which use beam line space more efficiently are described here. The latter include a coupler in which power passes directly through an iris in the broad wall of the rectangular waveguide into a matching cell, also successfully implemented, and a variation which makes the waveguide itself an accelerating cell. We also discuss in some detail a couple of techniques for matching such couplers to traveling-wave structures using a field solver. The first exploits the cell number independence of a traveling-wave match, and the second optimizes using the fields of an internally driven structure.

Applications of X-band technology in medical accelerators

Abstract: Most radiation therapy machines are based on microwave linear accelerators. The majority of medical accelerators use frequencies in the S-band range. Having a compact accelerator allows for a wide range of treatments. The size and weight of the accelerator is substantially reduced if a higher frequency is used. X-band frequencies are suitable for such applications. The X-band accelerator technology has been used in high-energy as well as industrial applications. In the radiation therapy field, it is already implemented in some machines. The Mobetron an Intra-Operative Radiation Therapy (IORT) treatment system is one example. Another example is the Stereotactic Radiosurgery machine, the CyberKnife. The compactness of these machines required the use of an X-band accelerator. The basis for choosing the X-band technology in some of the medical machines is analysed. A review of the exiting medical applications is included. We also discuss the availability of other X-band components in the machine, including high-power RF sources.

Coupler studies for clic accelerating structures

Abstract: Due to the high input power required to feed the accelerating structures of the Compact Linear Collider, the RF input and output couplers are critical components. Four different types of double-feed cavity-based couplers as well as a mode launcher have been investigated. Three of them are based on magnetic coupling between the input waveguides and the cavity while the fourth is based on electric coupling. The different designs have been optimized to minimize surface electric field as well as field asymmetry and to reduce the pulse surface heating and the sensitivity to mechanical errors.

Progress in the Next Linear Collider Design

Abstract: An electron/positron linear collider with a center-of-mass energy between 0.5 and 1 TeV would be an important complement to the physics program of the LHC. The Next Linear Collider (NLC) is being designed by a US collaboration (FNAL, LBNL, LLNL, and SLAC) which is working closely with the Japanese collaboration that is designing the Japanese Linear Collider (JLC). The NLC main linacs are based on normal conducting 11 GHz rf. This paper will discuss the technical difficulties encountered as well as the many changes that have been made to the NLC design over the last year. These changes include improvements to the X-band rf system as well as modifications to the injector and the beam delivery system. They are based on new conceptual solutions as well as results from the R&D programs which have exceeded initial specifications. The net effect has been to reduce the length of the collider from about 32 km to 25 km and to reduce the number of klystrons and modulators by a factor of two. Together these lead to s...

Mode launcher as an alternative approach to the cavity-based rf coupler of periodic structures

Abstract: Recent studies of the accelerating structures at a high gradient showed problems associated with high surface electric fields. In particular it was observed that the input RF coupler of the structure suffered more severely from the surface damage caused by local RF breakdowns than regular accelerating cells. A new design of the RF coupler with reduced surface electric and magnetic fields is presented.