Abstract

1. Abstract Role of Magnetic Measurements in Magnet Production  Animesh Jain, NSLS-II Project The role played by magnetic measurements in any large scale magnet production is discussed in this talk. It is shown that magnetic measurements play a role in nearly all aspects of magnet production, starting from formulation of specifications, continuing through magnetic and engineering design, and final installation in an accelerator. For magnets that must meet very stringent field quality requirements, magnetic measurements become an integral part of manufacturing because most magnets may need to shimmed. In such cases, the quality of magnets produced is directly linked to the quality of magnetic measurements. Making good measurements continues to be a challenging task for various reasons discussed in this talk. One particularly challenging aspect is poor agreement between some measurement systems and to ensure that a measurement system is performing well. Some suggestions are presented in this talk to help with monitoring the quality and consistency of measurements over a long production phase, and some unique contributions made by the NSLS-II project in this area are highlighted. *Work performed under auspices of the United States Department of Energy, under contract DE-AC02-98CH10886 NSLS-II Magnet Workshop: April 11-12, 2012

2. Role of Magnetic Measurements in Magnet Production Animesh Jain for the NSLS-II magnet team Magnet WorkshopApril 11-12, 2012

3. Outline • Role of magnetic measurements • Interface with different aspects of magnet production • Challenges faced by any measurements group • Measurement hardware (rotating coils only) • Data analysis (Raw data) • Data archival and magnet evaluation • NSLS-II actions that were particularly useful • What could have been done better? • Unique contributions of NSLS-II • General remarks • Conclusions NSLS-II Magnet Workshop: April 11-12, 2012

4. General Role of Magnetic Measurements • The contributions of magnetic measurements start at very early stage, and continue throughout the magnet production. • Measurements play a role in • Defining specifications • Evaluating prototypes and/or first articles • Studying potential issues during use (e.g. cross-talk between magnets) • Pinpointing production issues affecting field quality • Validating magnetic design calculations (e.g. chamfering or shimming) • Field tuning to meet stringent field quality requirements • Identifying any long term trends in production (timely feedback) • Provide data needed for acceptance and use of the magnets • The primary role of magnetic measurements should be to validate the field quality in magnets, once regular production starts. But it is not always limited to this. NSLS-II Magnet Workshop: April 11-12, 2012

5. Interface with Magnet Specifications • Ultimately, the quality of a magnet is determined by the quality of the field it produces, although other electro-mechanical aspects are important as well. • The field quality specifications come primarily from physics considerations, but a specification that can not be measured is impossible to enforce (e.g. 5 micron reassembly specs for NSLS-II). • Preferably, the specifications should conform to measurement capabilities (in addition to manufacturing capabilities) of “state of the art” systems to keep the costs to a minimum. • If difficult specs are really essential, new technology may have to be developed (e.g. alignment for NSLS-II). NSLS-II Magnet Workshop: April 11-12, 2012

6. Interface with Magnetic Design • Magnetic design is carried out with the help of finite element codes, with inherent limitations (finite mesh size, unknown material properties, hysteresis effects, …). • Once a prototype is available, the magnetic design can be validated in terms of strengths of allowed harmonics. • Finite element calculations of perturbation effects, such as chamfering and shimming can be verified for accuracy. • Any unusual effects overlooked during magnetic design (e.g. base plate effects at high fields) can be uncovered. • Cross-talk between magnets is relatively difficult to compute. More reliable results are obtained by measurements. NSLS-II Magnet Workshop: April 11-12, 2012

7. Interface with Engineering Design • Although engineering design precedes a final magnet available for testing, magnetic measurements can help with design improvements based on data in the prototypes (or first articles). • Some examples from NSLS-II include: • Elimination of pole clamps in some magnet types to improve field quality reproducibility under magnet reassembly • Change of pole clamp material in some magnet types to non-ferrous metal due to slight magnetic properties of stainless steel • Use of carbon fiber (instead of invar) for vacuum chamber stands based on detrimental effects on the field quality of magnets nearby. • Standardization of torques to be used to get better reproducibility. NSLS-II Magnet Workshop: April 11-12, 2012

8. Interface with Magnet Manufacturing • Role of magnetic measurements in magnet manufacturing is two-fold: • To detect problems (or even worse, mistakes) in the production process (e.g. packing factors, incorrect procedure used for cutting chamfers, ...). An appropriate solution can then be found once the problem is known. • To improve the field quality of magnets to achieve specifications that are otherwise hard to achieve by manufacturing tolerances alone. • Most multipoles for NSLS-II require adjustments using shims (or any such chosen device) based on magnetic measurements. • Measurements become an integral part of the manufacturing process. Magnet quality is directly linked to the quality of magnetic measurements available to the manufacturer. NSLS-II Magnet Workshop: April 11-12, 2012

9. Challenges Faced by Measurement Group • Magnetic measurement systems are not commercially available. • Considerable lead time is needed to put a new system together. • Experienced manpower is difficult to find. Newcomers hired “just in time” may not always prove useful in developing a new system. • Commercial organizations can hardly afford to keep dedicated measurement professionals on staff, let alone for a given project. • Assuming all hardware, software, and manpower issues are taken care of (a big IF in itself),the biggest challenge comes from the fact that a magnet can not be measured until it is produced. • Magnet production is invariably delayed, causing a big rush in end. • Magnetic measurements are then seen as an obstacle to installing the magnets, despite all the nice things mentioned earlier. NSLS-II Magnet Workshop: April 11-12, 2012

10. Measurement Hardware • Nearly all measurement systems in use around the world (except at BNL) use radial coils with analog bucking and digital integrators. • For measuring multipoles, both the main terms and the feed down terms must be bucked for accurate measurements. This requires specific coil design for each multipole type. Also, the bucking ratios may not be satisfactory for both terms due to coil construction errors. • The mechanics of rotating coil does affect the quality of harmonic measurements, even with bucking. • BNL uses tangential coils with digital bucking and high resolution voltmeters. This allows measurement of all magnet types using a common coil design. This also allows perfect bucking of any combination of harmonics needed for a given magnet type. • Inconsistencies exist between different systems - Who is right? NSLS-II Magnet Workshop: April 11-12, 2012

11. Measurement Hardware (Contd.) • The type of coil design, the type of bucking, or the type of data acquisition device, in principle, should not influence the final measurement results. • The differences between systems arise primarily from bucking quality (ratio) and mechanics of rotation. Poor bucking and poor mechanics lead to spurious harmonics, mainly for the lower order terms. • Systems that require the rotating coil to be disconnected from the drive for every use are more prone to develop rotation problems or calibration changes with time. It is essential to keep a close watch! • Precise measurement of roll angle demands systems equipped with precision level sensors and frequent system calibration (at least once a day). • Thorough evaluation of systems to be used is very essential. NSLS-II Magnet Workshop: April 11-12, 2012

12. Measurement Hardware (Contd.) • Discrepancies between manufacturer and in-house measurements are extremely difficult to resolve during an ongoing production. • Magnets of similar size and roughly similar field characteristics should be procured (build in-house, borrow, old spares, …) and measured at various production sites, well before regular production starts. (Requires measurement systems to be available early.) • Measurement differences at ~20% of the specification (or > 0.1 unit for higher harmonics) should not be ignored and need investigation. • There is no guarantee that the differences will be resolved. In such cases, one can at least have a realistic estimate of what to expect for measurement accuracy. • At BNL, considerable time and effort has been invested in validating measurements made by BNL rotating coil systems. NSLS-II Magnet Workshop: April 11-12, 2012

13. Measurement Hardware (Contd.) • Apart from the basic rotating coil mechanical system, attention needs to be paid to other parts of the system, such as the data acquisition system, signal cable shielding and routing, etc. • Periodic checks must be performed to ensure that the entire system is working properly. • It is immensely helpful if one of the early magnets can be held in reserve by both the manufacturer and the customer to serve as a “reference”. (Very difficult to do when magnets are late!) • Periodically repeating measurements in the “reference” magnet can tell if the system is stable over the duration of the production. • The time needed to install and prepare a magnet for test can be more than the measurement time – good fixtures should be designed to make this process as efficient as possible. NSLS-II Magnet Workshop: April 11-12, 2012

14. Analysis of the Raw Data • Data analysis is needed to convert the raw voltage (or integrated voltage) signals from the rotating coil to the final field harmonics. • Data analysis involves: • Proper calibration of rotating coil geometric parameters • Correcting raw signals for any inherent errors (e.g. integrator drift) • Establishing coordinate system for data representation • Establishing harmonics definitions and sign conventions • Applying corrections for rotating coil offsets from the magnetic axis • Applying corrections for angular rotation of the measuring coil frame • Applying corrections for any background fields (e.g. measurement of dipole terms in a sextupole magnet are severely affected by earth’s field.) • It is difficult to enforce a common convention across many suppliers. Most places do not have the manpower needed to modify existing analysis software. Customer must do the necessary transformations. NSLS-II Magnet Workshop: April 11-12, 2012

15. Validation of Measurement Data • It is very difficult to prove beyond doubt that a given set of measurements are accurate enough. However, certain tests may be performed to look for signs of problems (or good performance). • Measurements could be made with the coil in the normal orientation, but with the magnet installed in both the normal, as well as after flipping end for end. Systematic errors in harmonics that should change sign with respect to the main field (e.g. b3, a4, b5, .. in a quad, or b4, a5, b6, .. in a sextupole) can be detected by comparing the two sets of measurements. • If available, measurements could be made using two independent systems, preferably with windings of different designs. • Best approach is to build redundancy in the measuring coil such that several independent measurements are made simultaneously. NSLS-II Magnet Workshop: April 11-12, 2012

16. Data Archival and Magnet Evaluation • Production measurements for a large accelerator project results in huge amounts of data. • These data must be authenticated, and then made accessible, on demand, to the final user in a convenient and flexible way. • The data archive should be designed such that • All quantities of interest for every magnet are included. • It is easy to create queries to filter out data needed to address any questions or concerns regarding any specific group of magnets. • Ability to integrate seamlessly with operational requirements (e.g. for providing strengths of the main field term to set the power supplies) is also essential. Any unit conversions should be transparent to the user. • This archive can be used for magnet evaluation, trend studies, etc. • The effort required to design, set up, populate, and maintain such an archive should not be underestimated. Early start is needed. NSLS-II Magnet Workshop: April 11-12, 2012

17. NSLS-II actions that were particularly useful • Early in the project, several multipoles were borrowed from the Swiss Light Source, ALBA and SPEAR3/IHEP. This allowed us to: • Get a glimpse of field quality in recent, commercially produced, magnets • Study field quality impacts of correctors integrated into multipoles • Study interference effects between neighboring magnets, etc. • Develop vibrating wire alignment technique to required precision • 1st magnet borrowed from SLS measured at BNL: May, 2007;1st NSLS-II Production magnet measured at BNL: June, 2010 • Sincere attempt was made to fully understand and evaluate field measurement systems of all magnet suppliers for NSLS-II • Site visits to witness measurements in the first articles (or similar) magnets • Independent analysis of suppliers’raw data • Suggestions for improvements, where applicable • Measurements of “reference magnets” at some suppliers’ locations. • Only partially successful (late commissioning, unresolved discrepancies, …) NSLS-II Magnet Workshop: April 11-12, 2012

18. What could have been done better? • A database structure should have been worked out much earlier. A relational database (IRMIS) is in place now, but is still missing auser interface. • A common data reporting structure could not be prescribed to the suppliers early in the project. All suppliers chose to report data in their own format, which made data handling quite tedious (not to mention different coordinate and sign conventions). • Putting together new measurement systems took much longer than anticipated. Test fixtures were not built until well into production. • Modernizing of measurement systems could not be completed. • Calibration of power supply current readout occurred after well into the production. This required renormalization of all the old data. NSLS-II Magnet Workshop: April 11-12, 2012

19. Unique Contributions of NSLS-II • The NSLS-II testing requirements have led to the development of a novel “universal” 9-winding rotating coil that fully exploits the power of digital bucking: • Can measure any type of magnet (dipole to 12-pole, perhaps more) • Provides numerous, simultaneous measurements of the same harmonic (several dozens in principle, only ~10 best ones are used) • Each harmonic is measured with winding combinations that have the most sensitivity for that harmonic, thus ensuring least noise. • For details: http://immw16.web.psi.ch/Presentations/2_03_IMMW16_Coils.pdf • Data generated for the NSLS-II project can be used for the most extensive comparison ever between completely different measurement systems in large number of magnets of each type (BNL Vs. each supplier) • Advances in the vibrating wire alignment technique (next part of talk). NSLS-II Magnet Workshop: April 11-12, 2012

20. General Remarks • In-house capability of making good field measurements is essential to ensure that all magnets truly meet field quality specifications. • Deficiencies in suppliers’ data • Any changes during shipping and handling • An organization with no prior measurement experience may have to rely solely on suppliers’ measurements (which may be improving with time). • BNL is home to one of the finest collections of measurement systems, with extensive experience in doing large scale, high throughput production measurements (RHIC and SNS in therecent past). • BNL measurement systems have repeatedly proven their reliability and consistency in the course of a long history. NSLS-II Magnet Workshop: April 11-12, 2012

21. Conclusions • Magnetic measurements have played an important role in all aspects of the NSLS-II magnet production since the start of the program. • Good in-house measurement capability, coupled with vast in-house magnet experience, has helped in continued improvement of the field quality in NSLS-II magnets. • Disagreement between in-house and manufacturer’s data in some cases has led to further evaluation of (and renewed faith in) the in-house measurement systems. • Lack of a common format (as well as conventions) for suppliers’ data has made it difficult to routinely evaluate those data. This will be an interesting task for the future. There is a wealth of interesting data. • All multipole magnets are currently being measured in-house. The measurements are now keeping up with practically no backlog. NSLS-II Magnet Workshop: April 11-12, 2012