Dielectric-loaded Accelerating Structure Research Articles

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

It has been technically challenging to efficiently couple external radio frequency (RF) power to cylindrical dielectric-loaded accelerating (DLA) structures, especially when the DLA structure has a high dielectric constant. This article presents a novel design of matching section for coupling the RF power from a circular waveguide to an <inline-formula> <tex-math notation="LaTeX">$X$ </tex-math></inline-formula>-band DLA structure with a dielectric constant <inline-formula> <tex-math notation="LaTeX">$\varepsilon _{\text {r}}=16.66$ </tex-math></inline-formula> and a loss tangent <inline-formula> <tex-math notation="LaTeX">$\tan \delta =3.43\times {10}^{-5}$ </tex-math></inline-formula>. It consists of a compact dielectric disk with a width of 2.035 mm and a base angle of 60&#x00B0;, 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&#x00B0; 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.

Design, fabrication, and low-power rf measurement of an X -band dielectric-loaded accelerating structure

Dielectric-loaded accelerating (DLA) structures are being studied as an alternative to conventional disk-loaded copper structures to produce the high accelerating gradient. This paper presents the design, fabrication and low-power RF measurement of an externally-powered X-band DLA structure with a dielectric constant epsilon_r=16.66 and a loss tangent tan_delta=0.0000343. A dielectric matching section for coupling the RF power from a circular waveguide to an X-band DLA structure consists of a very compact dielectric disk with a width of 2.035 mm and a tilt angle of 60 degree, resulting in a broadband coupling at a low RF field which has the potential to survive in the high-power environment. Based on simulation studies, a prototype of the DLA structure was fabricated. Results from bench measurements and their comparison with design values are presented. The detailed analysis on the fabrication error which may cause the discrepancy between the RF measurements and simulations is also discussed.

Complete multipactor suppression in an X-band dielectric-loaded accelerating structure

Multipactor is a major issue limiting the gradient of rf-driven Dielectric-Loaded Accelerating (DLA) structures. Theoretical models have predicted that an axial magnetic field applied to DLA structures may completely block the multipactor discharge. However, previous attempts to demonstrate this magnetic field effect in an X-band traveling-wave DLA structure were inconclusive, due to the axial variation of the applied magnetic field, and showed only partial suppression of the multipactor loading [Jing et al., Appl. Phys. Lett. 103, 213503 (2013)]. The present experiment has been performed under improved conditions with a uniform axial magnetic field extending along the length of an X-band standing-wave DLA structure. Multipactor loading began to be continuously reduced starting from 3.5 kG applied magnetic field and was completely suppressed at ∼8 kG. Dependence of multipactor suppression on the rf gradient inside the DLA structure was also measured.

Observation of multipactor suppression in a dielectric-loaded accelerating structure using an applied axial magnetic field

Efforts by a number of institutions to develop a Dielectric-Loaded Accelerating (DLA) structure capable of supporting high gradient acceleration when driven by an external radio frequency source have been ongoing over the past decade. Single surface resonant multipactor has been previously identified as one of the major limitations on the practical application of DLA structures in electron accelerators. In this paper, we report the results of an experiment that demonstrated suppression of multipactor growth in an X-band DLA structure through the use of an applied axial magnetic field. This represents an advance toward the practical use of DLA structures in many accelerator applications.

Progress Toward Externally Powered X-Band Dielectric-Loaded Accelerating Structures

We summarize recent progress in a program to develop externally powered dielectric-loaded accelerating (DLA) structures that can sustain high accelerating gradients. High-power RF tests of earlier structures showed strong multipactor loading. In addition, arcing at dielectric joints between the uniform DLA structure and matching sections at either end limited the achievable gradient. In this paper, we study the onset of multipactor in a DLA structure. We also study the effect of thin-film TiN coatings applied by atomic layer deposition and the effect of a reduction in the inner diameter of the structure. Test results of these structures show significant decreases in multipactor loading. We also test new structure designs that eliminate separate dielectric matching sections and, thus, the requirement for dielectric joints, including a DLA structure using a coaxial coupler and a clamped DLA structure. The clamped structure demonstrated a significantly improved gradient without breakdown.

Self-consistent nonstationary two-dimensional model of multipactor in dielectric-loaded accelerator structures

In this paper, a self-consistent nonstationary two-dimensional model of multipactor in dielectric-loaded accelerating structures is proposed. In comparison with existing models, this model calculates dc electric field produced by secondary electrons self-consistently and it also includes effects of cylindricity. Results of our calculations are compared with the ones obtained for the 11.424 GHz alumina-based dielectric-loaded accelerating structure designed and experimentally tested by Argonne National and Naval Research Laboratories.

Development of a dual-layered dielectric-loaded accelerating structure

rf Power attenuation is a critical problem in the development of dielectric-loaded structures for particle acceleration. In a previous paper [C. Jing, W. Liu, W. Gai, J. Power, T. Wong, Nucl. Instr. Meth. A 539 (2005) 445] we suggested the use of a Multilayer Dielectric-Loaded Accelerating Structure (MDLA) as a possible approach for reducing the rf losses in a single layer device. The MDLA is based on the principle of Bragg reflection familiar from optics that is used to partially confine the fields inside the dielectric layers and reduce the wall current losses at the outer boundary. We report here on the design, construction and testing of a prototype X-band double-layer structure (2DLA). The measurements show an rf power attenuation for the 2DLA more than ten times smaller than that of a comparable single-layer structure, in good agreement with the analytic results. Testing and operation of MDLAs also requires efficient power coupling from test equipment or rf power systems to the device. We describe the design and construction of two novel structures: a TM 03 mode launcher for cold testing and a power coupler for planned high-gradient experiments.

Multipactor discharge in a dielectric-loaded accelerating structure

This paper presents a Monte-Carlo model to explain the multipactor discharge and its high-power absorption in a dielectric-loaded accelerating (DLA) structure reported recently [J. G. Power et al., Phys. Rev. Lett. 92, 164801 (2004)]. Susceptibility diagrams are constructed. Dynamic calculations for beam loading and its power absorption by the multipactor discharge are performed. It is found that the fraction of power absorbed by multipactor discharge at saturation is much larger than the case of a simple rf window, and it is sensitive to the incident power, which confirms the prior experimental results. This enhanced power absorption is due to the fact that the length of a DLA structure is much larger than the radius of the structure. A resonant condition of a maximum growth region has also been determined numerically and analytically. The difference between the resonant condition and saturation (due to beam loading) is clarified.