Three-cell traveling wave superconducting test structure

Use of a superconducting traveling wave accelerating (STWA) structure with a small phase advance per cell rather than a standing wave structure may provide a significant increase of the accelerating gradient in the ILC linac. For the same surface electric and magnetic fields the STWA achieves an accelerating gradient 1.2 larger than TESLA-like standing wave cavities. The STWA allows also longer acceleration cavities, reducing the number of gaps between them. However, the STWA structure requires a SC feedback waveguide to return the few hundreds of MW of circulating RF power from the structure output to the structure input. A test single-cell cavity with feedback was designed, manufactured and successfully tested demonstrating the possibility of a proper processing to achieve a high accelerating gradient. These results open way to take the next step of the TW SC cavity development: to build and test a traveling-wave three-cell cavity with a feedback waveguide. The latest results of the single-cell cavity tests are discussed as well as the design of the test 3-cell TW cavity.Comment: 3 pp. Particle Accelerator, 24th Conference (PAC'11) 28 Mar - 1 Apr 2011: New York, US

A Compact Soft X-Ray Free-Electron Laser Facility Based on a Dielectric Wakefield Accelerator

X-ray free-electron lasers (FELs) are expensive instruments with the accelerator holding the largest portion of the cost of the entire facility. Using a high-energy gain dielecric wakefield accelerator (DWA) instead of the conventional accelerator may reduce the facility size and, significantly, its cost. We show that a collinear dielectric wakefield accelerator can, in principle, accelerate low charge and high peak current electron bunches to a few GeV energy with up to 100-kHz bunch repetition rate. Several such accelerators can share the same tunnel and cw superconducting lilac (operating with a few-MHz bunch repetition rate), whose sole purpose is feeding the DWAs with wake producing low-energy, high-charge electron bunches with the desired periodicity. Then, ten or more x-ray FELs can operate independently, each using its own linac. In this paper, we present an initial case study of a single-stage 850-GHz DWA based on a quartz tube with a ~100-MV/m loaded gradient sufficient to accelerate a 50-pC main electron beam to 2.4 GeV at a 100-kHz bunch repetition rate in just under 30 meters. While the accelerated electron beam has a large energy chirp, show that FEL gain can be maintained by appropriately tapering the undulator, although other schemes may be possible