Search

Design studies of a future TeV e+e− Linear Collider (TLC) are presently being made by five major laboratories within the framework of a world-wide collaboration. A figure of merit is defined which enables an objective comparison of these different designs. This figure of merit is shown to depend only on a small number of parameters. General scaling laws for the main beam parameters and linac parameters are derived and prove to be very effective when used as guidelines to optimize the linear collider design. By adopting appropriate parameters for beam stability, the figure of merit becomes nearly independent of accelerating gradient and RF frequency of the accelerating structures. In spite of the strong dependence of the wake fields with frequency, the single-bunch emittance blow-up during acceleration along the linac is also shown to be independent of the RF frequency when using equivalent trajectory correction schemes. In this situation, beam acceleration using high-frequency structures becomes very advantageous because it enables high accelerating fields to be obtained, which reduces the overall length and consequently the total cost of the linac.

One of the major objectives of CTF3 (CLIC Test Facility) is the production of 30 GHz power for the high-gradient testing of CLIC accelerating structures. To this end a dedicated beam line, power generating structure and power transfer line have been designed, installed and commissioned. 52 MW of 30 GHz power with a pulse length of 74 ns and a repetition rate of 16 Hz were delivered to the high-gradient test area. This will allow operation of test accelerating structures in the first CTF3 run of 2005 up to the nominal CLIC accelerating gradient of 150 MV/m and beyond the nominal pulse length. The system is described and the performances of the CTF3 linac, beam line and the rf components are reviewed.

In order to achieve accelerating gradients above 150 MV/m, alternative materials to copper are being investigated by the CLIC study. The potential of refractory metals has already been demonstrated in tests in which a tungsten-iris and a molybdenum-iris structure reached 150 and 193 MV/m respectively (30 GHz and a pulse length of 15 ns). In order to extend the investigation to the pulse lengths required for a linear collider, a molybdenum-iris structure scaled to X-band was tested at the Next Linear Collider Test Accelerator (NLCTA). The structure conditioned to only 65 MV/m (100 ns pulse length) in the available testing time and much more slowly than is typical of a copper structure. However the structure showed no sign of saturation and a microscopic inspection of the rf surfaces corroborated that the structure was still at an early stage of conditioning. The X-band and 30 GHz results are compared and what has been learned about material quality, surface preparation and conditioning strategy is discussed.

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'.

A TeV-range ${e}^{+}{e}^{\ensuremath{-}}$ 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 $400\text{ }\text{ }\mathrm{M}\mathrm{V}/\mathrm{m}$ for copper, with no significant dependence on either frequency or temperature.

Each linac proposed for the CERN Linear Collider (CLIC) is composed of 50000 25-cm-long-accelerator sections operating at 29 GHz with gradients of 80 MV/m to produce beam energies of 1 TeV. Basic structure parameters have been established, and fabrication of these sections by the electroforming and machine-and-braze techniques is being investigated. Results obtained from various prototype test pieces are given and discussed. >

High frequency linear collider schemes envisage the use of rather high accelerating gradients: 50 to 100 MV/m for X-band and 80 MV/m for CLIC. Because these gradients are well above those commonly used in accelerators, high gradient studies of high frequency structures have been initiated and test facilities have been constructed at KEK, SLAC and CERN. The studies seek to demonstrate that the above mentioned gradients are both achievable and practical. There is no well-defined criterion for the maximum acceptable level of dark current but it must be low enough not to generate unacceptable transverse wakefields, disturb beam position monitor readings or cause RF power losses. Because there are of the order of 10,000 accelerating sections in a high frequency linear collider, the conditioning process should not be too long or difficult. The test facilities have been instrumented to allow investigation of field emission and RF breakdown mechanisms. With an understanding of these effects, the high gradient performance of accelerating sections may be improved through modifications in geometry, fabrication methods and surface finish. These high gradient test facilities also allow the ultimate performance of high frequency/short pulse length accelerating structures to be probed. This report describes the high gradient test at SLAC of an X-band accelerating section built at CERN using technology developed for CLIC.

The objectives of the CLIC Test Facility (CTF) are to study the generation of short intense electron bunches using a laser driven photocathode in an RF gun, to generate 30 GHz RF power for high gradient tests of prototype CLIC components, and to test beam position monitors. The performance of the CTF has improved dramatically in the course of the past year and highlights are presented.

We discuss the RF system, the drive linac, drive beam generation, the isochronous ring drive beam scheme, the main linac injector system, machine parameters, beam dynamics and final focus studies and the alignment test facility and beam monitor test results.

The present parameters of the CLIC study require the collision of small emittance beams with a vertical spot size of 1 nm. The tolerances on vertical quadrupole vi-bration (above a few Hz) are as small as a few nm in the linac and most of the Final Focus. The final focusing quadrupole has a stability requirement of 4 nm in the horizontal and 0.2 nm in the vertical direction. Those tol-erances can only be achieved with the use of damped support structures for CLIC. A study has been set-up at CERN to explore the application of stabilization devices from specialized industry and to predict the time-dependent luminosity performance for CLIC. The results will guide the specification of required technological im-provements and will help to verify the feasibility of the present CLIC parameters.

The recent RF structure testing program carried out in the CLIC Test Facility, CTF II, 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.

Probing beyond the established picture of particle physics will require some radical re-thinking of accelerator designs. If accelerators are to reach the ever-higher energies that theorists would dearly like to see explored, the technological spin-offs of this engineering feat could be as surprising as the new subatomic physics.

Experimental studies on a CERN-CLIC 32.98 GHz 26-cell High Gradient Accelerating (HGA) section were carried out with input powers from 0.1 MW to 50 MW. MIT's Free Electron Laser (FEL) amplifier driven by a mildly relativistic electron beam (750 kV, 300 A, 30 ns) served as the RF power source for the structure. The maximum power in the transmitted pulse was measured to be 15 MW for an input pulse of /spl sim/35 MW. The theoretically calculated shunt impedance of 116 M/spl Omega//m predicts a field gradient of 65 MeV/m inside the HGA. For power levels >3 MW the pulse transmitted through the HGA was observed to be shorter than the input pulse and pulse shortening became more serious with increasing power input. At the highest power levels the output pulse length (about 5 nsec) was about one quarter of the input pulse length. Various tests suggest that these undesirable effects occur in the input coupler to the HGA. Light and X-ray production inside the HGA have been observed. A feasibility study suggests that conditioning is a likely candidate for improving the performance of the HGA.

The LEP (Large Electron Positron colliding beam accelerator) uses a dual-frequency acceleration system consisting of coupled accelerating and storage cavities. Basic design features of this system with its accessories and tuning system are described. One hundred and thirty cavities have been built by industry; the assembly and conditioning with RF power were done at CERN. All cavities are now installed in the LEP tunnel. The procedures used and first experiences are presented. >

The merits of strategic planning as a marketing tool are discussed in this article which takes the view that although marketers claim to be future-oriented, they focus too little attention on long-term planning and forecasting. Strategic planning, as defined by these authors, usually encompasses periods of between five and twenty-five years and places less emphasis on the past as an absolute predictor of the future. It takes a more probabilistic view of the future than conventional marketing strategy and looks at the corporation as but one component interacting with the total environment. Inputs are examined in terms of environmental, social, political, technological and economic importance. Because of its futuristic orientation, an important tenant of strategic planning is the preparation of several alternative scenarios ranging from most to least likely. By planning for a wide-range of future market conditions, a corporation is more able to be flexible by anticipating the course of future events, and is less likely to become a captive reactor--as the authors believe is now the case. An example of strategic planning at General Elecric is cited.

Basic formulae governing thermal flow in water-cooled solids are qiven. By calculating the heat dissipation at room temperature of the LEP accelerating cell with the program SUPERFISH and then using this as input to the thermal analysis program DOT, the surface temperatures and losses at full power have been estimated. Good agreement is obtained when these LEP results are scaled and compared with measurements made on similar accelerating structures in PEP and PETRA. The calculated decrease in shunt impedance of 4.2% with respect to the room temperature value has been stubstantiated by experimental measurements on a prototype LEP cavity.

The Compact Linear Collider (CLIC) aims at colliding e/sup +/e/sup -/ beams at 1.5 TeV with effective transverse spot sizes of 60 nm (horizontal) times 0.7 nm (vertical). Strict stability tolerances must be respected in order to achieve a sufficient overlap of the two colliding beams. A stability test stand has been set up at CERN, bringing latest stabilization technology to the accelerator field. Using this technology, a CLIC prototype magnet was stabilized in a normal CERN working environment to less than 1-nm vertical RMS motion above 4 Hz. Detailed simulations of the time-dependent luminosity performance of CLIC are discussed. They include the beam-beam interaction, the beam-based feedbacks and the measured data on magnet stability.