Accelerator Physics

High-energy physics is an experimental science whose reach is limited by one of its major instruments of investigation, particle accelerators. To further our understanding, accelerators are being designed and constructed to expand both the energy frontier and the event rate necessary for precision measurements. For example, to understand the origins of electroweak symmetry breaking, many physicists are looking toward multi-TeV lepton collisions. However, the highest energy e + e – collider (LEP at CERN) operates in the 0.2 TeV energy range. Similarly, to understand the origins of CP violation, physicists are working to produce collisions roughly 100 times faster than has been previously possible. This disparity reflects the status of accelerator physics as it is understood at this writing. While the ultimate reach into the sub-atomic world has yet to be decided, it is clear that it will greatly depend on invention and discovery in the physics of beams and the technology of accelerators.

At SLAC, accelerator physics innovation has led to break-through discoveries in high-energy physics. The saga began with the Two-Mile Electron Accelerator, a bold and ambitious project for the time, which resulted in the first direct evidence for quarks in the nucleon. Next was the development and construction of the electron-positron collider, SPEAR, resulting in the discovery of the psi meson and the tau lepton and leading to a new field of research using beam-based synchrotron radiation which has wide-ranging applications in applied physics, biology, chemistry, and material science. More recently, the transformation of the SLAC Two-Mile Electron Accelerator into the 100-GeV Stanford Linear Collider (SLC), resulted in the first accelerator-measured limit of three quark generations and the best single test of electroweak theory, as well as paving the way for much higher energy e + e – colliders.
 

International Linear Collider (ILC)

Using the knowledge gained from the SLC design and operation, physicists around the world have been studying the next-generation of linear colliders. These facilities would have energies in the center-of-mass of roughly 1 TeV, about ten times that of the SLC. Presently, the US high-energy physics community has endorsed a full design study at SLAC for a future collider, referred to as the ILC, having energy ranging from 0.5 to 1.5 TeV in the center-of-mass. Because of the magnitude of the project, this future collider will be an international endeavour, and SLAC is performing the design study in close collaboration with the KEK laboratory in Japan.

To obtain a higher acceleration gradient, the ILC is based on an rf frequency four times that of the SLC and, to obtain the desired luminosity, the ILC must collide long trains of bunches that have transverse dimensions of roughly 300nm by 5nm. The ongoing research for the ILC includes: the development of high-power rf systems and components such as klystrons, methods of rf-pulse compression, the study of field-emission and rf breakdown, and the design of accelerator structures; the study of the beam dynamics in the very low-emittance damping rings and the linacs; the development of final focus systems where the beams are focused to the nanometer-sized beam spot sizes; the design of instrumentation to measure the position and sizes of the beams; and the complex feedback and control systems needed to control the beams. Finally, to verify the physics and
technology supporting the design, a number of test facilities will be constructed and commissioned over the next few years.
 

Advanced Accelerator Research

To reach much higher energies, to 5 TeV and beyond, the collider scalings indicate that the linac and final focus based on current ideas would be of enormous size, probably incurring expenses greater than society would care to support. At these energies, there is no technology adequate to the task and inventions are required. The focus of advanced accelerator research at SLAC is the invention, design, and commissioning of a new machine, 1 meter in length, capable of producing a beam of energy 1 GeV or higher. Research in this area includes design, fabrication and testing of mm-wave accelerator structures and power sources, construction and commissioning of a 1 GeV beam-driven plas-ma accelerator in the Final Focus Test Beam Facility, and commissioning of a laser-driven linac. Other topics for accelerator research for linear colliders include field-emission and breakdown, periodic permanent magnet-focused klystrons at cm and mm wave-lengths, overmoded and quasi-optical rf systems, active pulse compression based on laser-triggered silicon and diamond switches, advanced pulsed-power systems, structure design and precision manufacture, collective and non-linear beam dynam-ics, and more.
 

B Factory

Another aspect of high-energy physics at SLAC is the PEP-II B Factory where two storage rings with asymmetric energies have beams that will collide in a single interaction region to study CP violation in the BABAR detector. In PEP-II, the lumi-nosity limits of a positron-electron collider are being significantly extended beyond the present world limits. The B Factory employs many state of the art accelerator physics techniques which have recently been developed but not all put to experimental test. Commissioning this accelerator through 1999 will offer many opportunities for study. Furthermore, additional studies investigate significant future lumi-nosity improvements.

The ongoing research areas include a broad range of topics. Beam-beam interactions will ultimately limit PEP-II perfor-mance, and studies to increase the limits are underway. The beam currents stored in PEP-II are in the one- to two-ampere range, and the vacuum system can handle up to three amperes. Thus, studies of high-current beam loading in the rf systems are important. These high beam currents are distributed over 1650 bunches introducing multi-bunch instabilities cured by digital feedback systems which work on the nanosecond time frame. Future improvements will need better feedbacks. The very small spot sizes at the collision point force the magnetic lattice design to delicately correct the chromatic beam optics in PEP-II. Improvements in the chromatic corrections will lead to increased luminosity. Far-term future improvements will likely involve a redesign of the interaction region which is presently one of the best in the world. This area needs new and innova-tive ideas.
 

Graduate Studies in Beam Physics

As an academic discipline, beam physics is interdisciplinary, calling on the physics and techniques often found in the area of applied physics, electrical and mechanical engineering, high-energy physics, and materials science. Thesis work has included subjects as diverse as the design, construction, and commissioning of electron guns, and beam dynamics from Lie algebraic or quantum mechanical perspectives. From the realm of advanced metallurgy and materials science, to the first-principles problems of quantum electrodynamics in media, beam physics includes problems and challenges from every area of the physical sciences. Some glimpse of the breadth of interests can be seen on the Web, under the Division of Physics of Beams, of the American Physical Society.

The research topics at SLAC run the gamut of beam physics for contemporary and advanced accelerators: nonlinear dynamics, collective effects, studies of multi-bunch instabilities, advanced beam feedback systems, real-time machine modeling and operational analysis, high-power rf systems, mm-wave accelerator fabrication, assembly, bench and high-power test, and plasma and laser acceleration techniques.

Graduate beam physics research at SLAC is for those who are taken with the idea of exploration into the most remote realms of the universe, who have the desire to invent the machines of the next millenium. For such physicists, SLAC is a unique and challenging research environment. SLAC has the largest beam physics faculty in the world with 10 members who, along with approximately 50 staff physicists and roughly 15 graduate students from Stanford and elsewhere, pursue beam physics research of some kind at SSRL, the SLC, the B Factory, and for ILC and advanced accelerator
research.

SLAC facilities available to provide general support for beam physics research include departments devoted to vacuum, metrology and precision assembly, power conversion, control systems, software, and klystrons, as well as special purpose facilities for hydrogen brazing, plasma deposition, surface studies, digital electronics, and rf, microwave and mm-wave bench measurements. Specific beam lines available for experimentation include the SLAC Two-Mile Accelerator, a 500 MeV test accelerator, two 2200 m circumference storage rings, one storage ring employed as a light source with two 100 MeV accelerators housed in its injector vault. Two additional accelerators are housed in End Station B.
 
 
 

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