• T. Higo, T. Abe, M. Akemoto, S. Fukuda, N. Higashi, Y. Higashi, N.K. Kudo, S. Matsumoto, T. Shidara, T. Takatomi, K. Ueno, Y. Watanabe, K. Yokoyama, M. Yoshida
  KEK, Ibaraki

Under the CERN-SLAC-KEK collaboration, we have been developing the high gradient TW accelerator structures. One of the main focuses is the feasibility study of CLIC accelerator structure at X-band. A high power facility, Nextef*, was established at KEK in 2007. A few structures have been tested, including an un-damped disk-loaded structure successfully tested beyond 100 MV/m, a heavily damped structure to be tested from late 2009 and a structure made in a quadrant configuration. These structures follow the same accelerating-mode RF parameter profile, called CLIC-C**, but show different features at high gradient operation. Various observables, such as dark current, vacuum activities, light emission, breakdown rate, and so on, are measured. We discuss the high gradient phenomena related to these observables and the possible improvement for stable operation at a higher gradient.

* T. Higo et al., THP038, LINAC06,2006.
**A. Grudiev, http://indico.cern.ch/conferenceDisplay.py?confId=30911

• T. Higo, Y. Higashi, S. Matsumoto, K. Yokoyama
  KEK, Ibaraki
• C. Adolphsen, V.A. Dolgashev, A. Jensen, L. Laurent, S.G. Tantawi, F. Wang, J.W. Wang
  SLAC, Menlo Park, California
• S. Döbert, A. Grudiev, G. Riddone, W. Wuensch, R. Zennaro
  CERN, Geneva

A CERN-SLAC-KEK collaboration on high gradient X-band accelerator structure development for CLIC has been ongoing for three years. The major outcome has been the demonstration of stable 100 MV/m gradient operation of a number of CLIC prototype structures. These structures were fabricated basically using the technology developed from 1994 to 2004 for the GLC/NLC linear collider initiative. One goal has been to refine the essential parameters and fabrication procedures needed to realize such high gradient routinely. Another goal has been to develop structures with stronger dipole mode damping than those for GLC/NLC. The latter requires that surface temperature rise during the pulses be higher, which may increase the breakdown rate. Structures with heavy damping will be tested in late 2009/early 2010, and this paper will present these results together with some of the earlier results from non-damped structures and structures built with a quadrant geometry.

• V.A. Dolgashev, S.G. Tantawi, A.D. Yeremian
  SLAC, Menlo Park, California
• Y. Higashi
  KEK, Ibaraki
• B. Spataro
  INFN/LNF, Frascati (Roma)

We report results of ongoing high power tests of single cell standing wave structures. These tests are part of an experimental and theoretical study of rf breakdown in normal conducting structures at 11.4 GHz. The goal of this study is to determine the accelerating gradient capability of normal-conducting rf powered particle accelerators. The test setup consists of reusable mode-launchers and short test structures powered by SLAC's XL-4 klystron. We have tested structures of different geometries, cell joining techniques, and materials, including hard copper alloys and molybdenum. We found that the behavior of the breakdown rate is reproducible for different structures of the same geometry and material. The breakdown rate dependence on peak magnetic fields is stronger than on peak surface electric fields for structures of different geometries.