A TeV-range e e ÿ linear collider has emerged as one of the most promising candidates to extend the high energy frontier of experimental elementary particle physics. A high accelerating gradient for such a collider is desirable to limit its overall length. Accelerating gradient is mainly limited by electrical breakdown, and it has been generally assumed that this limit increases with increasing frequency for normal-conducting accelerating structures. Since the choice of frequency has a profound influence on the design of a linear collider, the frequency dependence of breakdown has been measured using six exactly scaled single-cell cavities at 21, 30, and 39 GHz. The influence of temperature on breakdown behavior was also investigated. The maximum obtainable surface fields were found to be in the range of 300 to 400 MV=m for copper, with no significant dependence on either frequency or temperature. Introduction.-The feasibility of a compact e e ÿ linear collider (CLIC) [1] which aims for a center-ofmass energy in the TeV range is studied at CERN within an international collaboration. CLIC is characterized by the choice of a very high accelerating gradient of 150 MV=m, a high operating frequency of 30 GHz, and a two-beam accelerator scheme to produce the necessary rf power. The high-power testing of rf structures is currently being carried out in the CLIC Test Facility (CTF II) [2], a two-beam accelerator providing up to 280 MWof 30 GHz power at a pulse length of 16 ns. The discovery two years ago of substantial damage due to electrical breakdowns in prototype 30 GHz structures at accelerating fields of about 60 MV=m obliged the CLIC team to undertake a more systematic study of the phenomenology of rf breakdown. To complement the rather expensive development and testing of traveling wave CLIC-type accelerating structures, a series of experiments using simple single-cell standing-wave cavities, directly driven by a high-charge electron beam, were performed. This test setup enables very high surface fields to be obtained in the cavity at a well defined location. These tests assume that the maximum electrical field on the surface is the key parameter for breakdown initiation, and enable fundamental questions such as frequency and temperature dependence of the breakdown behavior to be investigated in a relatively simple way.Cavity design and fabrication.-A total of six cavities, two at each of three different frequencies (21, 30, 39 GHz) were made. These high-gradient single-cell cavities have a pillbox-type geometry exactly scaled for the different frequencies for operation with the transverse-magnetic (TM 010 ) mode (see Fig. 1). The scaling factor s 30 GHz =f 0 GHz was applied to the beam-pipe diameter, the cavity length, the cavity diameter, and the radius at the beam-pipe opening. The location of the maximum surface fieldÊ E S in these cavities is also indicated in the figure. A small coupling aperture (about 1 mm wide) was

During the past five years, there has been an concerted program at SLAC and KEK to develop accelerator structures that meet the high gradient (65 MV/m) performance requirements for the Next Linear Collider (NLC) and Global Linear Collider (GLC) initiatives. The design that resulted is a 60-cm-long, traveling-wave structure with low group velocity and 150 degree per cell phase advance. It has an average iris size that produces an acceptable short-range wakefield, and dipole mode damping and detuning that adequately suppresses the long-range wakefield. More than eight such structures have operated at a 60 Hz repetition rate over 1000 hours at 65 MV/m with 400 ns long pulses, and have reached breakdown rate levels below the limit for the linear collider. Moreover, the structures are robust in that the rates continue to decrease over time, and if the structures are briefly exposed to air, the rates recover to their low levels within a few days. This paper presents a summary of the results from this program, which effectively ended last August with the selection of 'cold' technology for an International Linear Collider (ILC).

One priority of the CLIC (Compact Linear Collider) accelerating-structure development program has been to investigate ways to achieve accelerating gradients above 150 MV/m. Two main concepts to achieve such high gradients have emerged: reduced surface field geometries and the use of alternative materials. An experimental demonstration of these two concepts has been made in CTFII (CLIC Test Facility) using three 30 GHz accelerating structures: one made entirely from copper, one with copper cavity walls and tungsten irises and one with copper cavity walls and molybdenum irises. A peak accelerating gradient of over 190 MV/m was achieved using the molybdenum-iris structure. The effect of pulse length on achievable gradient was investigated using a novel 'pulse stretcher'. EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CERN -AB Division AbstractOne priority of the CLIC (Compact Linear Collider) accelerating-structure development program has been to investigate ways to achieve accelerating gradients above 150 MV/m. Two main concepts to achieve such high gradients have emerged: reduced surface field geometries and the use of alternative materials. An experimental demonstration of these two concepts has been made in CTFII (CLIC Test Facility) using three 30 GHz accelerating structures: one made entirely from copper, one with copper cavity walls and tungsten irises and one with copper cavity walls and molybdenum irises. A peak accelerating gradient of over 190 MV/m was achieved using the molybdenum-iris structure. The effect of pulse length on achievable gradient was investigated using a novel 'pulse stretcher'.

The proposed Compact Linear Collider (CLIC) is a multi-TeV electron-positron collider for particle physics based on an innovative two-beam acceleration concept. A highintensity drive beam powers the main beam of a high-frequency (30 GHz) linac with a gradient of 150 MV/m, by means of transfer structure sections. The aim of the CLIC Test Facility (CTF3) is to make exhaustive tests of the main CLIC parameters and to prove the technical feasibility. One of the points of particular interest is the demonstration of bunch train compression and combination in the Delay Loop and in the Combiner Ring. Thus, detailed knowledge about the longitudinal beam structure is of utmost importance and puts high demands on the diagnostic equipment. Among others, measurements with a streak camera have been performed on the linac part of the CTF3 as well as on the newly installed Delay Loop. This allowed e.g. monitoring of the longitudinal structure of individual bunches, the RF combination of the beam, the behavior during phase shifts and the influence of the installed wiggler. This article first gives an overview of the CTF3 facility, then describes in detail the layout of the long optical lines required for observation of either optical transition radiation or synchrotron radiation, and finally shows first results obtained during the last machine run this year.

The recent RF structure testing program carried out in the CLIC Test Facility, CTFII, is described. The main objectives of the testing program have been to gain an insight into the physical processes involved in breakdown and damage, to isolate parameters that influence breakdown and damage, and to determine gradient limits for 30 GHz structures. The layout of CTFII in the new 'Test Stand' configuration, the instrumentation used to study breakdown and the experimental results are summarised. The new results are compared to previously published results at 11, 30 and 33 GHz produced in the context of the CLIC study.