Construction and testing of an 11.4 GHz dielectric structure based traveling wave accelerator

Abstract

We report on the design, numerical modeling, and experimental testing of a cylindrical dielectric loaded traveling wave structure for charged particle beam acceleration. This type of structure has similar accelerating properties to disk-loaded metal slow wave structures but with some distinct advantages in terms of simplicity of fabrication and suppression of parasitic wakefield effects. Efficient coupling of external rf power to the cylindrical dielectric waveguide is a technical challenge, particularly with structures of very high dielectric constant ⑀. We have designed and constructed an X-band structure loaded with a permittivity ⑀ϭ20 dielectric to be powered by an external rf power source. We have attained high efficiency broadband rf coupling by using a combination of a tapered dielectric end section and a carefully adjusted coupling slot. Bench testing using a network analyzer has demonstrated a power coupling efficiency in excess of 95% with bandwidth of 30 MHz, thus providing a necessary basis for construction of an accelerator using this device. We have also simulated the parameters of this structure using a finite difference time domain electromagnetic solver. Within the limits of the approximations used, the results are in reasonable agreement with the bench measurements.

Design of a Power Extractor Using a Rectangular Dielectric Loaded Waveguide

“…Dielectric-loaded (DL) accelerating structures have been studied for decades as a potential alternative for future high-gradient particle acceleration [1][2][3][4], because of the simple geometry and the ability to maintain accelerating gradients similar to conventional structures. Recently, there has been some interest in microwave power generation and extraction using DL waveguides [5,6] based on the two-beam-acceleration concept.…”

High-power radio frequency pulse generation and extration based on wakefield excited by an intense charged particle beam in dielectric-loaded waveguides.

“…By changing the inner and outer radii, and the permittivity of the dielectric, the structure can be tuned so that the phase velocity is set to the particle velocity at the desired frequency. Compared with the traditional, disc-loaded metallic waveguide with similar RF properties, the dielectric- loaded waveguide is potentially simpler to fabricate due to its uniformity in the longitudinal direction, especially when high precision is needed at high frequencies (Zou 2000). Furthermore, a simple scheme is available for damping the undesired deflection modes (Chojnacki 1991).…”

“…Although there are two main types of DL accelerators, one based on rectangular waveguides (Zhang 1997) and one based on circular waveguides (Zou 2000), in this thesis we concentrate on the latter.…”

“…Some traditional cavity-based accelerators encounter breakdown problems for RF pulse lengths on the order of hundreds of nanoseconds when the gradient reaches ~100MV/m (Aldophsen 2007). In order to push the acceleration gradient even higher, alternative acceleration techniques have been extensively investigated, such as DL acceleration (Zhang 1997;Zou 2000), laser acceleration (Plettner 2005) and laser-plasma acceleration (Leemans 2006).…”

Hybrid dielectric and iris-loaded periodic accelerating structure

“…The use of uniform dielectric-lined circular waveguides as accelerating structures has been discussed in many previous studies [3,4]. One distinct advantage is that the axial accelerating electric field is the maximum field in this class of structure.…”