SLAC-R-754 June 2005 Iron Dominated Electromagnets Design, Fabrication, Assembly and Measurements Jack Tanabe January 6, 2005 Stanford Linear Accelerator Center, Stanford Synchrotron Radiation Laboratory, Stanford, CA 94025 Work supported in part by Department of Energy contract DE-AC02-76SF00515 2 Dedicated with Love to Sumi, for her support and devo- tion. For our beautiful grand-daughter, Sarah. 3 Abstract Medium energy electron synchrotrons (see page 15) used for the produc- tion of high energy photons from synchrotron radiation is an accelerator growth industry. Many of these accelerators have been built or are under construction to satisfy the needs of synchrotron light users throughout the world. Because of the long beam lifetimes required for these synchrotrons, these medium energy accelerators require the highest quality magnets of various types. Other accel- erators, for instance low and medium energy boosters for high energy physics machines and electron/positron colliders, require the same types of magnets. Because of these needs, magnet design lectures, originally organized by Dr. Klaus Halbach and later continued by Dr. Ross Schlueter and Jack Tanabe, were organized and presented periodically at biennual classes organized under the auspices of the US Particle Accelerator School (USPAS). These classes were divided among areas of magnet design from fundamental theoretical consider- ations, the design approaches and algorithms for permanent magnet wigglers and undulators and the design and engineering of conventional accelerator mag- nets. The conventional magnet lectures were later expanded for the internal training of magnet designers at LLNL at the request of Lou Bertolini. Because of the broad nature of magnet design, Dr. S. Y. Lee, the former Director of the Particle Accelerator School, saw the need for a specialized course covering the various aspects of the design, engineering and fabrication of conventional magnets. This section of the class was isolated and augmented using the LLNL developed material resulting in the class on conventional magnet design. Con- ventional magnets are defined (for the purposes of this publication) as magnets whose field shape is dominated by the shape of the iron magnet yoke and are excited by coils, usually wound from solid or hollow water-cooled copper or aluminum conductors. Dr. S. Y. Lee and Dr. Helmut Wiedemann, past and current Directors of USPAS, encouraged the author to write a text for the purpose of consolodat- ing the lecture notes used in the USPAS course. This publication collects the lecture notes, written for the first course in the USPAS conventional magnet design course and evolved over subsequent presentations of this same course, and organizes the material roughly divided among two parts. One part is theoretical and computational and attempts to provide a foundation for later chapters which exploit the expressions and algorithms for the engineering and design calculations required to specify magnet conceptual designs. A chap- ter is devoted to the description and use of one of many magnet codes used to characterize the two dimensional field resulting from various magnet cross- sections. A chapter is included which exploits the two-dimensional theory and applies the mathematics to techniques and systems for magnet measurement. The second part of this publication ranges to practical issues associated with the fabrication of components, assembly, installation and alignment of magnets. This section also includes fabrication practices which respond to personnel and equipment protection needs. 4 Required design calculations are supplemented by examples and prob- lems. A CD is included with tools provided to simplify the computation of some of the more tedious relationships. This CD also includes useful photographs and pictures describing the high volume production of typical magnet types, which if included in the publication will add too many pages and increase the cost of publication. Styles among those facing similar problems will result in a wide vari- ation of individual magnet designs. Designs and technologies will evolve and improve. This publication provides a snapshot of the present technology and presents as examples the magnet designs developed in response to the needs of several projects, the Advanced Light Source at LBNL, PEPII Low Energy Ring and SPEAR3 synchrotron light source at SLAC and the Australian Light Source, currently under construction in Melbourne. In each example, the rea- sons for fabrication design decisions are itemized and rationalized as much as is reasonable. The examples presented in this publication are provided as start- ing points which can be used as a design basis for magnets required for future projects. It is hoped that the listing of some design choices and the motivation for these choices will be useful. It is the intention of the author to publish a document collecting and archiving the tools and techniques learned from the past masters in the craft and to provide a useful reference for future magnet designers. 5 Aknowledgements This work would not have been possible without the creativity, guid- ance and patience of Dr. Klaus Halbach. Klaus mentored, taught and guided the efforts of many individuals at Lawrence Berkeley National Laboratory (LBNL) as well as making major contributions to numerous fields including the field of synchrotron light. His generosity and wisdom in supplying hints rather than answers made it possible for the author of this work to own knowl- edge rather than merely learn it. Two examples of Klaus’ creative work among a much broader spectrum are relevant to the field of particle accelerators. His development of permanent magnet circular arrays made it possible to design and install compact per- manent magnet quadrupoles in linac drift tubes as well as multipole magnet arrays for circular storage rings. His work on the use of permanent magnets in longitudinal arrays has resulted in the proliferation of numerous wigglers and undulators at electron synchrotron light facilities throughout the world. Indeed, this means of exploiting electrons to produce high energy and high intensity synchrotron light has stimulated the construction of numerous elec- tron synchrotrons throughout the world. In addition to contributions in the area of permanent magnets, Klaus formulated and documented relationships invaluable to the design and fabrication of electromagnets. The accelerator community misses his presence and remains in his debt. I am grateful for the opportunities to participate in important accel- erator projects at LBNL. Alan Jackson, Ron Yourd and Dr. Alan Paterson trusted me to lead the magnet design, fabrication and measurement program for the Advanced Light Source (ALS). Dr. Ross Schlueter and Dr. Steve Marks encouraged an academic approach and rigor in my written work. Dr. Michael I. Green at LBNL and Dr. Zack Wolf at SLAC helped me become familiar with the intricacies of magnetic measurements. John Meneghetti and Tom Henderson taught me about capabilities and limitations of machine shops and fabrication techniques and prevented the design of unbuildable hardware. Someone once said, “the toughest boss you have is the one who works for you”. In that regard, I am indebted to many engineers and designers whose creativity reflected any success I have achieved. John Milburn’s successful ALS designs have been mirrored at Pohang in Korea and scaled for numerous other projects. Collaborative design work with James Osborn, Johanna Swann and Dr. Matt Kendall resulted in the design and fabrication of important final focus magnets for PEPII. Conversations and notes from contacts with Jack Jagger at FNAL and Hank Hsieh when he was at BNL were important in the development of specifications for the manufacture of large numbers of magnets. Frequently, during the course of designing magnets, one comes across especially difficult problems. This was the case for the design of rapidly cycling correctors mag- nets for feedback of the beam orbits for SPEAR3. I am indebted to Glen Lambertson of LBNL for his help in simplifying my understanding of transient fields providing insight into eddy current effects on magnetic fields. Finally, the author has gained enormously from contacts with scientists 6 and engineers, especially at the Stanford Synchrotron Radiation Laboratory (SSRL), for their patience in sharing knowledge of the physics of accelerators. Dr. Tom Elioff and Richard Boyce were instrumental in my participation in SLAC projects providing an opportunity to contribute to the design of mag- nets for PEPII and SPEAR3. It was a pleasure working with Nanyang Li in collaborations with IHEP in China during the manufacture of the PEPII and SPEAR3 magnets. During trips to China, contacts with Mrs. Rui Hou of IHEP were especially profitable and memorable. Dr. Max Cornacchia’s and Dr. Hermann Winnick’s enthusiasm for physics was infectious. The generosity of Dr. Jeff Corbett and Dr. Bob Hettel is especially appreciated. Their patient and generous explanations provided new insight into the physics of accelerators and greatly expanded my understanding and appreciation of the performance specifications that drive magnet design. Jim Sebek helped me through difficult mathematics and helped minimize my frustration working with difficult com- puter codes. I was also fortunate in sharing an office during the last months of Dr. Moohyun Yoon’s sabbatical from the Pohang Light Source in Korea while he spent a year participating in the SPEAR3 commissioning. He made it possi- ble to understand the fundamental physics which are the genesis of the simple differential equations whose solutions form the basis for magnet theory. In addition, Dr. Yoon, Dr. S. Y. Lee at the University of Indiana and Dr. Helmut Wiedemann at SLAC generously consented to provide their time and knowledge to edit early versions of this work. Their suggestions and recommendations have been incorporated into the final version of this text. Undoubtedly, many more colleagues and friends not named in this ac- knowledgement have influenced me and have contributed in a real way to this text. A work such as this is indeed built on the shoulders of others.