Various Observables of TW Accelerator Structures Operating 100MV/m or Higher at X-band Facility, Nextef of KEK

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 diskloaded structure successfully tested beyond 100 MV/m, a heavily damped structure being tested now and a structure made in a quadrant configuration. These structures follow the same accelerating-mode RF parameter profile of the CLIC design, called CLIC-C, but showed very different features at high gradient operation. Various observables, such as dark current, vacuum activity, light emission, breakdown rate, and so on, were measured. We discuss the high gradient phenomena related to these observables. INTRODUCTION The collaboration program of the feasibility study on CLIC [1] accelerator structure was started since 2007 mainly lead by CERN, SLAC and KEK. At test facility Nextef of KEK, we have been testing three prototype structures to date. Last year, we have proved the operation of 100 MV/m [2]. Since then, we have tested two more structures, both equipped with heavy damping feature with magnetically coupled damping waveguides facing the accelerator cell. The surface temperature rise within a pulse has been recognized as a possible cause of breakdown trigger, though the mechanism not understood. The two structures tested last year are classified in this group with high surface pulse temperature rise. In the present paper are described those structure test results in various aspects of the high gradient performance to understand the physics behind. TEST ACCELERATOR STRUCTURES The test structures were electrically designed by CERN [3]. All the structures are equipped with 18 accelerating cells. The information on fabrication is listed in the Table 1. The un-damped structure (T18) and damped structure (TD18) was made each as one of the twins through SLAC –KEK collaboration based on GLC/NLC technology [4]. Another structure, called Quad_#5, was made by KEK with CuZr material [5]. The structures were tested with 60Hz at NLCTA of SLAC and with 50Hz at Nextef of KEK. Each one of the twins (T18_Disk and TD18_Disk) was tested at each laboratory, while the quadrant Quad_#5 described in this paper was tested at KEK. HIGH GRADIENT TEST RESULTS The test results compiling all those from SLAC and KEK are discussed [6].