Pulse high power rf generationwith an x-band dielectric power extractor

Short pulse high power rf generation is one of the key technologies for the Argonne Flexible Linear Collider (AFLC), a proposed 3 TeV electron-positron linear collider based on two-beam acceleration (TBA) scheme. Compared with metallic power extractors, dielectric structures have the potential to achieve lower fabrication cost and to withstand higher gradient. Recently, an X-band dielectric power extractor (a.k.a, DPETS) has been developed at the Argonne Wakeield Accelerator (AWA) facility and achieved 105 MW output power when driven by a high charge 8bunch train separated by 770 ps. The design, the cold test measurement, the preliminary high power test results, and the structure inspection will be presented in this paper. INTRODUCTION TBA is an approach to the structure-based wakeield acceleration which may meet the luminosity, eiciency, and cost requirements of future linear colliders and has been selected as the baseline design of CLIC [1] and AFLC [2]. Due to the strong dependence of rf breakdown rate on pulse length [3], AFLC applies a much shorter rf pulse (∼20 ns) than CLIC (∼240 ns) to obtain a higher loaded accelerating gradient (267 MV/m vs. 100 MV/m). Rather than metallic power extractors and accelerators, AFLC adopts dielectric structures which may potentially reduce the fabrication cost with their simple geometries and withstand high gradient as there is no surface electric ield enhancement [2, 4]. The AWA facility is a lexible, state-of-art linear collider testbed with two parallel beam lines, which has been devoting much efort to the short pulse dielectric TBA research [2, 5, 6]. Demonstrating short pulse high power rf generation with DPETS is one of the key technologies of AFLC and one main mission of AWA. In a proof-ofprinciple dielectric TBA experiment, 55 MW rf power has been obtained by a K-band 26 GHz DPETS, the prototype of AFLC [6]. The power extractor, however, was damaged during the experiment. Although the mechanism is still under investigation, beam irradiation might be ascribed because the transmission of the high charge drive beam through the structure was less than 70% and more than 50 nC beam were lost within the structure. The low transmission was a result of the high aspect ratio of the structure (30 cm long ∗ jshao@anl.gov with 7 mm inner diameter) and the relatively low energy of the drive beam (∼70 MeV), which could be well-solved in AFLC with GeV drive beam. In order to achieve higher rf power by improving the transmission with the current AWA coniguration, an X-band 11.7 GHz DPETS with a larger inner diameter has been developed recently.