A Compact, Low-Field, Broadband Matching Section for Externally Powered X-Band Dielectric-Loaded Accelerating Structures

Abstract

It has been technically challenging to efficiently couple external radiofrequency (RF) power to cylindrical dielectric-loaded accelerating (DLA) structures, especially when the DLA structure has a high dielectric constant. This paper presents a novel design of matching section for coupling the RF power from a circular waveguide to an X-band DLA structure with a dielectric constant 𝜺 𝐫 = 𝟏𝟔. 𝟔𝟔 and a loss tangent 𝐭𝐚𝐧𝜹 = 𝟑. 𝟒𝟑 × 𝟏𝟎 −𝟓 . It consists of a compact dielectric disk with a width of 2.035mm and a base angle of 60°, resulting in a broadband coupling at a low RF field which has the potential to survive in the high-power environment. To prevent a sharp dielectric corner break, a 45 o chamfer is also added. A microscale vacuum gap, caused by metallic clamping between the thin coating and the outer thick copper jacket, is also studied in detail. Through optimization, most of RF power is coupled into the DLA structure, with no enhancement of peak electromagnetic fields above those of the structure itself. Tolerance studies on the geometrical parameters and mechanical design of the full-assembly structure are also carried out as a reference for fabrication.

Enhancement of transmission line model to analyze the matching section of a dielectric-loaded accelerator with sharp discontinuities

“…Therefore, the results of 𝑆 11 and 𝑆 21 of the full-wave simulation and analytical method are compared in figures 15 and 16, respectively. The investigated matching section is introduced in [14] and a traveling-wave DLA structure consisting of a uniform dielectric-loaded tube and two matching sections at both ends is constructed and tested by low power RF in [15]. Moreover, two TE 10 -TM 01 mode converters and choke structures are placed at the beginning and end of the DLA structure for power injection.…”

“…The most important one is a thin vacuum gap between the copper coating of the flared section and the metal jacket covering the whole DLA structure. In [14] a matching section with the same structural parameters as considered in this paper without vacuum gap is simulated by HFSS and the minimum of 𝑆 11 is about −51.5 dB at 𝑓 0 = 11.994 GHz. However, in [15] a significant discrepancy can be seen between the simulation and test results of the setup including the converters, chokes, and DLA at very low values of 𝑆 11 around the resonant frequency.…”

“…Different types of matching sections are used for standing-wave and traveling-wave DLAs to convert TM 01 mode in a conventional circular waveguide to TM 0𝑛 mode in a dielectric-loaded one [12][13][14][15]. However, because of more power in traveling-wave DLAs, proper S-parameters and power handling are more essential in coupler design for these structures.…”

“…Research on designing more efficient couplers is continued. For example, in [14] a 𝜆/4 dielectric-loaded matching section with a tilt angle is proposed for X-band DLA which has a much shorter length relative to the previous dielectric-loaded couplers occupying lower space for the coupler and its solenoid. This structure is fabricated and tested by low power RF in [15].…”

Dielectric assist accelerating structures for compact linear accelerators of low energy particles in hadrontherapy treatments

“…A DLA structure consists of a dielectric tube surrounded by a conducting cylinder. The dielectric decreases the phase velocity as well as the ratio of the peak electric field to the average accelerating gradient, which is about unity [11,12]. In dielectric breakdown studies, a surface field threshold of 13.8 GV/m was observed for 30-330 fs pulse length at THz frequencies [13].…”