Development of Dielectric Disk Accelerators For Argonne 500MeV Short Pulse Two Beam Wakefield Accelerator Demonstrator

In the last two decades, the theoretical and experimental investigations of dielectric accelerating structures for application to wakefield acceleration have predominantly used a dielectric-lined waveguide, due to its simple geometry (i.e. low fabrication cost). However, in comparison with the prevailing metallic disk-loaded accelerators, the dielectric-lined waveguide suffers from a lower Q-factor and lower shunt impedance. Taking the opportunity of DoE SBIR Program, we plan to develop a new dielectric disk-loaded accelerator (DDA). The preliminary simulation shows that, even with 5e-4 of loss tangent dielectric material (eps=50), we can achieve ~200Mohm/m shunt impedance at 26GHz traveling wave operation (the beam aperture keep the same in the comparison), which is 4 times higher than those of the conventional dielectric loaded accelerator. The main objective of Phase I was to develop an 11.7 GHz DDA prototype structure. We have accomplished all tasks listed in the Phase I work plan, including an optional high power rf test (thanks to the full support from the AWA facility). A gradient of ~88 MV/m was reached on the surface of the ceramic iris without any indication of breakdown in that region. During the test, breakdowns around the ceramic-copper braze joint were observed. Results were thoroughly analyzed. The issue was identified in the fabrication process. The solution was found, which will be demonstrated in Phase II. The main deliverables in Phase II will be a practically operational high gradient DDA structure with no breakdown below the real upper bound of dielectric material breakdown limit (the targeted value is >500MV/m on the iris surface). In order to achieve this goal, multiple iterations of improved structure designs and rf breakdown tests will be carried out. In order to validate the dielectric-based short-pulse wakefield Two Beam Accelerator concept on a relatively large scale without a significant budget increase, the High Energy Physics Division of ANL plans to demonstrate a ~500- MeV module in the current Argonne Wakefield Accelerator facility within the next 5 years. A new high shunt impedance dielectric wakefield accelerator is the key element for this experiment. Meanwhile, development of the X- band DDA structure will lead to a substantial reduction in length and weight, which is extremely important to particle accelerators for medical, security and industrial applications as well as for scientific research.