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The Delay Line Distribution System (DLDS) is an alternative to conventional pulse compression, which enhances the peak power of rf sources while matching the long pulse of those sources to the shorter filling time of accelerator structures. We present an implementation of this scheme that combines pairs of parallel delay lines of the system into single lines. The power of several sources is combined into a single waveguide delay line using a multi-mode launcher. The output mode of the launcher is determined by the phase coding of the input signals. The combined power is extracted from the delay line using mode-selective extractors, each of which extracts a single mode. Hence, the phase coding of the sources controls the output port of the combined power. The power is then fed to the local accelerator structures. We present a detailed design of such a system, including several implementation methods for the launchers, extractors, and ancillary high power rf components. The system is designed so that it can handle the 600 MW peak power required by the NLC design while maintaining high efficiency., Comment: 25 pages, 11 figures

Earlier RDDS (Rounded Damped Detuned Structures) [1,2], designed, fabricated and tested at SLAC, in collaboration with KEK, have been shown to damp wakefields successfully. However, electrical breakdown has been found to occur in these structures and this makes them inoperable at the desired gradient. Recent results [3] indicate that lowering the group velocity of the accelerating mode reduces electrical breakdown events. In order to preserve the filling time of each structure a high synchronous phase advance (150 degrees as opposed to 120 used in previous NLC designs) has been chosen. Here, damping of the wakefield is analyzed. Manifold damping and interleaving of structure cell frequencies is discussed. These wakefields impose alignment tolerances on the cells and on the structure as a whole. Tolerance calculations are performed and these are compared with analytic estimations., Comment: Paper presented at the 2002 8th European Particle Accelerator Conference (EPAC 2002) Paris, France, June 3rd -June 7th, 2002

Dipole wakefields in NLC (Next Linear Collider) structures have been measured with ASSET [1] and well predicted by a circuit model [2]. However, the experimental technique is both time-consuming and expensive. Here, we report on kick factor and synchronous frequency determination for 1st and higher order dipole bands for TW (Traveling Wave) accelerators via a wire measurement technique. This stand-alone technique is relatively inexpensive and may lead to an efficient determination of wakefield parameters. The perturbative effect of the wire on the dipole band is pointed out and a two-wire scheme with a limited perturbative effect is also discussed. [1] C. Adolphsen, 1997, SLAC-PUB-7519 [2] R.M. Jones et al, LINAC96, SLAC-PUB-7287, Comment: Paper presented at the 2002 8th European Particle Accelerator Conference (EPAC 2002) Paris, France, June 3rd -June 7th, 2002

Recent experiments at SLAC [1,2] and CERN [3] have revealed evidence of significant deformation in the form of "pitting" of the cells of the 1.8m series of structures DDS/RDDS (Damped Detuned Structure/Rounded Damped Detuned Structure). This pitting occurs in the high group velocity (vg /c = 0.012) end of the accelerating structure and little evidence of breakdown has been found in the lower group velocity end of the structure. Additional, albeit preliminary experimental evidence, suggests that shorter and lower group velocity structures have reduced breakdown events with increasing accelerating field strengths. Two designs are presented here, firstly a 90cm structure consisting of 83 cells with an initial vg/c = 0.0506 (known as H90VG5) and secondly, an even shorter structure of length 60cm consisting of 55 cells with an initial vg /c = 0.03 (known as H60VG3). The feasibility of using these structures to accelerate a charged beam over 10km is investigated. The particular issue focussed upon is suppression of the dipole wakefields via detuning of the cell frequencies and by locally damping individual cells in order to avoid BBU (Beam Break Up). Results are presented on beam-induced dipole wakefields and on the beam dynamics encountered on tracking the progress of the beam through several thousand accelerating structures. [1] C. Adolphsen, ROAA003, this conf. [2] R.H. Miller et al, FPAH062, this conf. [3] L. Groening et al, MPPH039, this conf, Comment: PAC2001 conf paper

Operating the SLAC/KEK DDS (Damped Detuned Structure) X-band linacs at high gradients (in excess of 70MV/m) has recently been found to be limited by the accelerator structures breaking down and as a consequence severe damage occurs to the cells which makes the structures inoperable. A series of recent experiments at SLAC indicates that arcing in the structures is significantly reduced if the group velocity of the accelerating mode is reduced and additionally it has been discovered that reducing the length of the accelerating structure also limits the number and intensity of breakdown events [1]. However, in designing new accelerating structures care must be taken to ensure that the beam-induced transverse wakefields do not cause the beam to become unstable. Here, we report on damping transverse wakefields in two different short structures: a 90cm traveling wave structure in which the wakefield is coupled out to four attached manifolds and secondly, in a standing wave structure in which a limited number of cells heavily damp down the wakefield. [1] C. Adolphsen, ROAA003, this conf., Comment: PAC2001 Conf Paper

Two basic designs have been developed for waveguide hybrids, or 3-dB couplers, capable of handling hundreds of megawatts at X-band. Coupling is provided by one or two connecting waveguides with h-plane junctions and matching elements. In the former case, the connecting waveguide supports two modes. Small apertures and field-enhancing e-bends are avoided to reduce the risk of rf breakdown. The h-plane symmetry also allows the use of over-moded rectangular waveguide in which the height has been increased to reduce field amplitudes without affecting the scattering matrix. The theory and designs are presented, along with the results of prototype tests of functionality and power-handling capability. Such a device is integral to the rf pulse compression or power distribution system of the Next Linear Collider (NLC) for combining, splitting, and directing power. This work was motivated by the observation of rf breakdown at power levels above 200 MW in conventional and modified magic-T's., Comment: presented at the 1999 IEEE Particle Accelerator Conference

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., Comment: For LINAC2000 (TUA03)

In damping the wakefield generated by an electron beam traversing several thousand X-band linacs in the NLC we utilise a Gaussian frequency distribution of dipole modes to force the modes to deconstructively interfere, supplemented with moderate damping achieved by coupling these modes to four attached manifolds. Most of these modes are adequately damped by the manifolds. However, the modes towards the high frequency end of the lower dipole band are not adequately damped because the last few cells are, due to mechanical fabrication requirements, not coupled to the manifolds. To mitigate this problem in the present RDDS1 design, the output coupler for the accelerating mode has been designed so as to also couple out those dipole modes which reach the output coupler cell. In order to couple out both dipole mode polarizations, the output coupler has four ports. We also report on the results of a study of the benefits which can be achieved by supplementing manifold damping with local damping for a limited number of cells at the downstream end of the structure., Comment: For Linac2000 (TUA09)

In the NLC (Next Linear Collider) small misalignments in each of the individual accelerator structures (or the accelerator cells) will give rise to wakefields which kick the beam from its electrical axis. This wakefield can cause BBU (Beam Break Up) or at the very least it will dilute the emittance of the beam. Several Gaussian detuned structures have been designed and tested [1] at SLAC and in this paper we explore new distributions with possibly better damping properties. The progress of the beam through approximately 5,000 structures is monitored in phase space and results on this are presented. [1] J.W. Wang et al, TUA03, LINAC2000, Comment: Conceptual clarification and additional references: Replaces paper TUA10 in LINAC2000 conference

In fabricating the first X-Band RDDS (Rounded Damped Detuned Structure) accelerator structure, microwave measurements are made on short groups of discs prior to bonding the discs of the entire structure. The design dispersion curves are compared with the frequency measurements. The theory utilised is based on a circuit model adapted to a short stack of slowly varying non-uniform discs. The model reveals the nature of the modes in the structure and may also be used to refit the experimental data to the parameters in a model of the wakefield given earlier [1]. This method allows a more faithful determination of the wakefield that a beam will experience as it traverses the structure. Results obtained on the frequencies are compared to the original design. [1] R.M.Jones, et al, EPAC96 (also SLAC-PUB-7187), Comment: Paper for LINAC2000 (TUA08)

We present numerical procedures for analyzing the properties of periodic structures and associated couplers based upon time domain simulation. Simple post processing procedures are given for determining Brillouin diagrams and complex field distributions of the traveling wave solutions, and the reflection coefficient of the traveling waves by the input and output. The availability of the reflection coefficient information facilitates a systematic and efficient procedure for matching the input and output. The method has been extensively applied to coupler design for a wide variety of structures and to a study directed towards elimination of the surface field enhancement commonly experienced in coupler cells., Comment: 3 pages, 4 figures, LINAC Conference 2000