Numerical Studies on Locally Damped Structures
In the JLC/NLC X-band linear collider, it is essential to reduce the long-range dipole wakefields in the accelerator structure to prevent beam break up (BBU) and emittance degradation. The two methods of reducing the long-range wakefields are detuning and damping. Detuning reduces the wakefields rapidly as the dipole modes de-cohere but, with a finite number of modes, the wakefield will grow again as the modes re-cohere. In contrast, damping suppresses the wakefields at a longer distance. There are two principal damping schemes: synchronous damping using HOM manifolds such as that used in the RDDS1 structure and local damping similar to that used in the CLIC structure. In a locally damped scheme, one can obtain almost any Q value, however, the damping can have significant effects on the accelerating mode. In this paper, we present a medium local-damping scheme where the wakefields are controlled to meet the BBU requirement while minimizing the degradations of the fundamental rf parameters. We will address the load design and pulse heating issues associated with the medium damping scheme.
Upgrade of the Drive LINAC for the AWA Facility Dielectric Two-Beam Accelerator
We report on the design of a seven-cell, standing-wave, 1.3-GHz rf cavity and the associated beam dynamics studies for the upgrade of the drive beamline LINAC at the Argonne Wakefield Accelerator (AWA) facility. The LINAC design is a compromise between single-bunch operation (100 nC @ 75 MeV) and minimization of the energy droop along the bunch train during bunch-train operation. The 1.3-GHz drive bunch-train target parameters are 75 MeV, 10−20-ns macropulse duration, and 16 × 60 nC microbunches; this is equivalent to a macropulse current and beam power of 80 A and 6 GW, respectively. Each LINAC structure accelerates approximately 1000 nC in 10 ns by a voltage of 11 MV at an rf power of 10 MW. Due to the short bunch-train duration desired (~10 ns) and the existing frequency (1.3 GHz), compensation of the energy droop along the bunch train is difficult to accomplish by means of the two standard techniques: time-domain or frequency-domain beam loading compensation. Therefore, to minimize the energy droop, our design is based on a large stored energy rf cavity. In this paper, we present our rf cavity optimization method, detailed rf cavity design, and beam dynamics studies of the drive beamline. INTRODUCTION The Argonne Wakefield Accelerator (AWA) facility is dedicated to the development of new rf accelerating structures capable of producing gradients in excess of 100 MV/m, on the basis of electron-beam-driven wakefield acceleration [1]. The current facility uses a 1.3-GHz rf photocathode gun and rf cavity to produce 15-MeV single bunches of 100 nC or bunch trains of up to 4 × 30 nC. The maximum gradient generated at the current facility, 100 MV/m, was achieved in a short dielectric structure. In order to reach higher acceleration gradients (up to 300 MV/m), a facility upgrade is under way to increase the drive beam kinetic energy to 75 MeV and the total charge in the drive bunch train to 1000 nC. The format of the drive bunch train is dictated by our current understanding of high-gradient breakdown. While breakdown is not well understood, it is clear that structures powered by short rf pulses sustain higher fields. On this basis, the AWA is now pursuing a short-pulse wakefield acceleration scheme [2]. Two modes of operation are envisioned. In single-bunch mode, a highcharge (~100-nC) drive bunch excites wakefields in a dielectric or metallic structure. In bunch-train operation, a 1.3-GHz drive bunch train will deliver approximately 1000 nC of total charge to the structure, distributed in bunch trains ranging from 8 × 100 nC to 32 × 30 nC. We describe the design procedure for the upgraded rf cavity and present preliminary simulations of a drive beamline. DRIVE LINAC DESIGN OBJECTIVES The drive beam required for the AWA upgrade is an extremely high-charge, short-pulse beam. Approximately 1000 nC of charge located in a 10-ns bunch train must be accelerated up to 75 MeV. This means that the rf cavities must deliver 75 J in 10 ns, or 7.5 GW of beam power. Due to limited space, we only explain the design choices that were most relevant to our application. 2D Design Procedure The design procedure was to tune the first six parameters of Table 1 to optimize those of Table 2, while minimizing cost. Optimization was subject to the usual constraints that the rf cavity (Fig. 1) be tuned and balanced. Table 1: Optimized Iris-Loaded RF Cavity Parameters Parameter (unit) Value iris radius, a (mm) 46.000 iris thickness, t (mm) 32.755 iris tip length to t/2 ratio 1.16 number of cells, N 7
Discourse Forms and Processes in Indigenous Lowland South America: An Areal-Typological Perspective
▪ Abstract In indigenous lowland South America there are several discourse forms and processes that are shared by groups of people of distinct genetic linguistic affiliations; this leads us to posit this large region, which we label greater Amazonia, as a discourse area, a concept that parallels the notion of linguistic area. The discourse forms and processes we examine are ceremonial dialogue, dialogical performance, templatic ratifying, echo speech, ceremonial greeting, ritual wailing, evidentiality, speech reporting practices, parallelism, special languages, and shamanistic language use. We hypothesize that in lowland South America, discourse is the matrix for linguistic diffusion, i.e., that linguistic areas emerge within discourse areas. What we propose then is a discourse-centered approach to language change and history, parallel to a discourse-centered approach to language structure and use. Our survey includes a plea for a careful archiving of recorded and written materials dealing with lowland So...