Figure 5.1 Schematic layout of a 1 TeV electron-positron linear collider (figure not to scale). The overall length is about 30 km. Linear colliders can be built and operated in stages of increasing energy. When extended to 1.5 TeV, this machine would reach mass scales roughly comparable to the LHC and provide a complementary approach to addressing physics issues, with relatively simple backgrounds. Its technology development is mature, and it is ready to enter the conceptual design phase. POSSIBLE MAJOR FUTURE FACILITIES approach being pursued is based on a room-temperature, X-band (11 GHz) accelerating structure. Such an implementation represents a direct extrapolation of SLC experience. Major development efforts are also centered at KEK in Japan, and DESY in Germany. The KEK approach also uses room-temperature technology, DESY is pursuing a design based on superconducting accelerating structures operating at 1.3 GHz. Extrapolation from current experience to a second-generation linear collider is significant, representing a factor of ten to fifteen in energy and nearly four orders of magnitude in luminosity. Critical technology issues associated with the development of a credible design include rf power systems, accelerating structures, final-focus optics, beam alignment, stability, emittance control, beam scraping and cleanup, and reliability. The SLAC/KEK X-band designs require very small beam sizes (a few nanometers high). Component fabrication and alignment tolerances, precision control of beam trajectories, and removal of optical aberrations to high order are especially critical in these designs. Many of the requirements can be relaxed if a superconducting accelerating structure is used, allowing an increase in the bunch train length. The trade-off is that superconducting accelerating structures are inherently more expensive and provide a lower accelerating gradient. The required facility is nearly twice as long as a room-temperature-based facility, which potentially limits energy expandability. As important as technical issues is the cost. It is known that a 1 TeV linear collider will be a multi-billion dollar project. Given the possible resources that might be available for construction of such a facility, optimization of design parameters and configurations is extremely important throughout the early design stages. SLAC issued a "zeroth-order” design report (ZDR) for a machine called the NLC (for "Next Linear Collider") in the spring of 1996. The NLC ZDR represents a relatively welldeveloped concept for a 0.5 TeV linear collider, intended as the initial phase of a 1 TeV facility. As conceived, the 0.5 TeV facility would be constructed with the accelerator configured to allow for doubling the energy by doubling the number of rf power sources. The final focus geometry is designed to accommodate an upgrade to 1.5 TeV. At present, the concept for producing a 1.5 TeV accelerator is to lengthen the facility, while keeping the accelerating gradient fixed. A number of research and development initiatives have been directed towards validation of performance requirements.in several important underlying systems. The Final Focus Test Beam facility has produced a 70 nm spot size, demonstrating the required demagnification, although the spot size required for the NLC cannot be achieved due to the higher emittance of the SLAC linac. The Accelerator Structure Setup facility has demonstrated the viability of the damped/detuned structure concept for controlling wakefields. CHAPTER 5 The klystron development program has yielded a solenoid focused klystron capable of producing a 75 MW pulse and generating a gradient of 70 MV per meter in an unloaded accelerating structure. The corresponding requirements for 0.5(1) TeV operations are 50 (75) MW and 50 (85) MV per meter. These klystrons are being used to support the Next Linear Collider Test Accelerator facility, a 350 MeV prototype section of the NLC, currently in operation. However, these tubes would not be cost effective in the NLC because of the large power consumption in the solenoids. A first-generation periodic permanent magnet klystron has been constructed that overcomes this problem with a demonstrated output of 55 MW at 60% efficiency. A second-generation unit is being developed, and it appears likely that within a year a periodic permanent magnet klystron will exist that is capable of meeting the performance requirements of a 1 TeV NLC. For 0.5 TeV operation, 3300 such klystrons are required, and double that number for 1 TeV operation. In parallel with efforts on the SLAC site, SLAC has entered into a collaboration with KEK to develop an advanced test facility in Japan for studying damping ring performance requirements. Commissioning of this facility began in January 1997. The NLC ZDR was reviewed in the spring of 1996 by an international team of accelerator physicists. This panel concluded that a technical basis had been established to support performance goals for most major subsystems, and that primary outstanding issues were related to systems integration, operational stability, reliability, and reduction of costs. This assessment is still valid today. Since that review, significant effort has been invested in cost-reduction and integration of “design for manufacture" concepts into the design. Significant progress has been made in several areas; for example, the number of power systems required was reduced by 30% over the last year. The scope of a second-generation linear collider appears to require an international approach to design, construction, and operation. To this end, SLAC has played a leading role in the creation of a world-wide effort to coordinate linear collider research and development activities. Recently, SLAC and KEK have negotiated an inter-laboratory Memorandum of Understanding that would form the basis of a research and development program towards a common design. The natural next step is the production of a Conceptual Design Report with a complete technical design and associated cost and schedule for specific sites. DESY is also aggressively pursuing the development of a technological base for a 500 GeV linear collider, called TESLA (for "TeV Electron Superconducting Linear Accelerator"). A design study, based on superconducting accelerating cavities, has been released and reviewed. POSSIBLE MAJOR FUTURE FACILITIES The critical issue in the TESLA approach is the development of low-cost superconducting rf cavities capable of supporting an accelerating gradient of 25 MV per meter with a quality factor in excess of 5x10°. Because of the relaxed tolerances characteristic of the superconducting design, issues related to alignment tolerances, wakefield suppression, and beam orbit control are less severe than in the roomtemperature approach. The concept for extending the energy to 800 GeV is based on improved (to 40 MV per meter) cavity performance, predicated on as-yet-unidentified methods of improving the purity of the Nb superconductor. Extension to 1.6 TeV would then be achieved by doubling the length. The major activity at DESY is construction and operation of the TESLA Test Facility, a 500 MeV demonstration test representing a complete integrated accelerator system. Electrons have been accelerated to 120 MeV in the first (eight cell) acceleration module. Goals for the next year include installation of two more accelerating modules, leading to the demonstration of full energy, full bunch current operations. The superconducting cavity development program is currently meeting the 15 MV per meter gradient and quality factor specification for the TESLA Test Facility. Considerable progress has been made in understanding the limits to production of high gradients, and at least one cavity has exceeded 25 MV per meter, at high quality factor, as is required for TESLA Achievement of this performance has required stringent process control during fabrication and sophisticated surface processing techniques. Current effort is concentrated on raising the yield of acceptable cavities to 95% and on development of less expensive fabrication techniques. As a result of the extensive research and development described above, both the SLAC/KEK and DESY efforts appear capable of developing complete conceptual designs and cost estimates for a 1 TeV electron-positron linear collider, extendible to 1.5 TeV. These designs could be completed early in the next decade, if given sufficient support. C. MUON COLLIDERS The concept of a muon collider was first discussed in the 1960s, but only recently (since 1994) has it received a significant degree of attention. In the past three years, numerous workshops and conferences have been held; during and between these meetings, considerable progress has been made in the study of the formidable technical issues involved. At the workshop that took place in the summer of 1997, a collaboration devoted to the design of a muon collider was formally established, comprising about CHAPTER 5 ninety scientists and engineers (about fifty-five are from the national laboratories; the rest are from universities). The focus prior to 1997 was on a muon collider with an extensive physics capability: a center-of-mass energy of 4 TeV, and a luminosity of 1035 cm2 s1. The accelerator systems in this machine (figure 5.2) are a proton source, an ionization cooling channel, a series of muon accelerators, and a collider ring. To provide some context for the research and development issues involved in developing this machine, each of these systems is described briefly in the following paragraphs. 13 The proton source is a 15-30 GeV, high-intensity (5-10x013 protons/pulse), rapid cycling (15 Hz) proton synchrotron, which serves as the driver for the muon source. The extracted proton bunch is required to be quite short in length (1 nsec rms) in order to allow subsequent momentum-spread reduction of the muons through rf phase rotation. The proton beam impinges on a heavy-metal target in the form of a liquid jet; the pions produced are collected in a strong (20 T) solenoidal field and enter a decay channel formed by a periodic array of superconducting solenoids. The resulting muons produced in the channel are phase-rotated using a 30-60 MHz linac. Momentum selection of the muons at the end of the channel allows some control over the polarization of the beam, at the price of a reduction in muon flux. The muons then enter a 750 m ionization cooling channel. The basic structure of the cooling channel is a focusing lattice formed from alternating superconducting solenoids, containing LiH absorbers for transverse ionization cooling. Simultaneous momentum cooling is accomplished by the use of LiH wedge absorbers in dispersive regions. Linacs within the solenoids restore the energy lost in the absorbers. The entire cooling system is designed to reduce the transverse emittance by three orders of magnitude in both planes. In the final sections of the cooling channel, where very short focal lengths are required, current-carrying liquid Li lenses replace the solenoids as the focusing elements. The muon beam energy at the end of the channel is roughly 15 MeV. The muons emerging from the channel must be rapidly accelerated to their final energy before they decay. This acceleration is accomplished in several stages: a conventional linac to 700 MeV; followed by a recirculating linac (with warm rf at low energy, superconducting rf at higher energies), to 100-200 GeV; followed by a series of rapidcycling (0.2-1 msec period) pulsed synchrotrons, with hybrid (alternating resistive/superconducting) magnet systems and pulsed superconducting rf, to the final beam energy of the collider. Typically, 35% to 40% of the muons collected in the capture channel survive to be injected into the collider. Each muon bunch has an intensity of roughly 2x1012 |