Dielectric-loaded Accelerating Structures 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.

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.

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.

Decreasing power losses in multilayer dielectric-loaded accelerating structures

We have studied the possibility of increasing the efficiency of dielectric-loaded accelerating (DLA) structures. It is shown that the level of losses in DLA structures can be significantly decreased using the Bragg principle of waveguide design for microwave radiation focusing along the axis. The waveguide lining comprises a set of layers with alternating high and low values of permittivity, which decreases the magnetic field intensity in the external metal shell of the accelerating structure. The results of calculations are presented for the TM 03 mode with a frequency of 11.4 GHz in DLA structures of the two types, with two-and four-layer lining, which show a decrease in the level of losses from −14 to −2 dB/m in the case of multilayer dielectric lining.

High-power RF tests on X-band dielectric-loaded accelerating structures

A joint Argonne National Laboratory (ANL)/Naval Research Laboratory (NRL) program is under way to investigate X-band dielectric-loaded accelerating (DLA) structures, using high-power 11.424-GHz radiation from the NRL Magnicon Facility. DLA structures offer the potential of a simple, inexpensive alternative to copper disk-loaded structures for use in high-gradient radio-frequency (RF) linear accelerators. A central purpose of our high-power test program is to find the RF breakdown limits of these structures. In this paper, we summarize the most recent tests results for two DLA structures loaded with different ceramics: alumina and Mg/sub x/Ca/sub 1-x/TiO/sub 3/ (MCT). No RF breakdown has been observed up to 5 MW of drive power (equivalent to 8 MV/m accelerating gradient), but multipactor was observed to absorb a large fraction of the incident microwave power. The latest experimental results on suppression of multipactor using a TiN coating on the inner surface of the dielectric are reported. Although we did not observe dielectric breakdown in the structure, breakdown did occur at the ceramic joint, where the electric field is greatly enhanced. Lastly, the MCT structure showed significantly less multipactor for the same level RF field.