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We are exploring a new parameter space of the two-beam acceleration (TBA) scheme based on an ultra-short (~20 ns) rf pulse in a dielectric TBA. All two-beam accelerators (TBAs) use an electron drive beam to generate high-power rf in a decelerator and extract this power to drive an accelerating structure to high gradient. Typically, the rf pulse is on the order of hundreds of ns or greater in order to maintain good rf-to-beam efficiency. However, recent scaling arguments show that the rf breakdown threshold improves with decreasing rf pulse length, so it desirable to find a way to run at short-pulse length with good efficiency. In this paper, we discuss how we chose the design parameters of a short-pulse TBA for a TeV linear collider module. We then present plans for an experimental program to demonstrate TBA at Argonne wakefield accelerator (AWA) facility including high-power rf generation, high-gradient acceleration, and staging. [ABSTRACT FROM PUBLISHER]

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 MgxCa1-xTiO3 (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. [ABSTRACT FROM AUTHOR]

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 ε. We have designed and constructed an X-band structure loaded with a permittivity ε=20 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. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Dielectric-loaded accelerating structures (DLAs) for wakefield applications are an alternative to conventional corrugated metal structures. These structures have simple geometry, scale up to THz frequencies, and feature higher breakdown threshold. Diamond has been proposed as a loading material for DLAs due to a number of its unique thermal, microwave, and electrical properties. In this paper we report results of a wakefield breakdown test of a single crystal diamond resonator. A large, 72 nC, charge beam with 2.5 mm length, 8.6 kA peak current was transmitted through a diamond-loaded parallel plate resonator, and induced a standing wave with a 70 MV/m electric field strength. One of the diamond plates had a laser cut, 20 μm wide and 200 μm deep groove, that provided an additional ~ 5-fold field enhancement at edges of the groove resulting in an electric gradient as high as 0.3 GV/m on the surface of the groove. SEM and x-ray topography analyses before and after the test did not reveal any structural damage to the diamond resonator plates. This corroborates further promotion of diamond for high gradient wakefield acceleration at high repetition rates. [ABSTRACT FROM AUTHOR]