Abstract
A higher-order ${\mathrm{TM}}_{02n}$ mode accelerating structure is proposed based on a novel concept of dielectric loaded rf cavities. This accelerating structure consists of ultralow-loss dielectric cylinders and disks with irises which are periodically arranged in a metallic enclosure. Unlike conventional dielectric loaded accelerating structures, most of the rf power is stored in the vacuum space near the beam axis, leading to a significant reduction of the wall loss, much lower than that of conventional normal-conducting linac structures. This allows us to realize an extremely high quality factor and a very high shunt impedance at room temperature. A simulation of a 5 cell prototype design with an existing alumina ceramic indicates an unloaded quality factor of the accelerating mode over 120 000 and a shunt impedance exceeding $650\text{ }\text{ }\mathrm{M}\mathrm{\ensuremath{\Omega}}/\mathrm{m}$ at room temperature.
Highlights
Over many years, conventional disk-loaded copper structures have been a subject of much research and implementation, whose extreme cases of performance have been reported, for instance, in [1,2] which have demonstrated a high accelerating gradient of up to 100 MV=m at the X-band frequency
The dielectric assist accelerating (DAA) structure has been proposed as a new type of dielectric loaded accelerating (DLA) structure to improve both its quality factor and shunt impedance
We have shown that the wall loss of TM02 propagation mode in this structure is extremely reduced, since the surface current on the conducting cylinder can be reduced much lower than that of conventional normal-conducting cavities
Summary
Conventional disk-loaded copper structures have been a subject of much research and implementation, whose extreme cases of performance have been reported, for instance, in [1,2] which have demonstrated a high accelerating gradient of up to 100 MV=m at the X-band frequency. In the case of the dielectric loaded waveguide shown, with a1 1⁄4 0.42λ0, b1=a1 1⁄4 1.2, c1=a1 1⁄4 1.84, ε2=ε0 1⁄4 9.6 where ε0 is permittivity of free space, and operated with the same accelerating gradient A1 1⁄4 1.0 MV=m, the wall loss of its TM02 mode can be analytically calculated as Pwall 1⁄4 190 W=m This is equivalent to approximately one-sixth of the wall loss in a C-band pillbox cavity. Since the phase velocity of the propagating mode in vacuum regions shown in Fig. 1(b) is always greater than the speed of light in vacuum, a slow wave structure is required to use this mode for beam acceleration To overcome this problem, we propose that ultralow-loss dielectric disks with iris are periodically arranged as a slow wave structure, and a standing wave excited in this structure is used for beam acceleration. The details of the design of such a DAA accelerating cavity are described
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