8. Development of Very Strong Magnet by New Techniques
Masayuki Kumada
Keywords: permanent magnets, magnetic circuit
The author has been developing a very strong magnet based on a new type of magnetic circuit of permanent magnet material. The strongest magnetic material was invented by Dr. Sagawa of Sumitomo Special Metal(SMMC) many years ago. It was an ingenious material. Although its patent is going to expire this year, no new material has been invented since then. Instead its performance has been improved by the continuing efforts of Dr. Sagawa's successors at SMMC. The permanent magnet has been used to produce a relatively low field magnet in many applications as volume is less. The applications go from hard disk in computers to MRI (Magnetic Resonance Imaging). The weight of MRI magnet is more than one metric ton. The market share of MRI magnets is 1/3 where that of superconducting magnets is 2/3. Electric MRI magnets are almost unused. This is a field which requires extremely high uniformity and stability. Another high tech application is in the filed of synchrotron radiation of light by a high energy electron beam. Permanent magnet wiggler/undulator is an example of this. Main development of this kind of advanced magnet was exploited by Halbach of Laurence Berkley laboratory(LBL). He developed the magnetic circuit of the strongest PM undulator magnet. The strongest undulator magnet was constructed by a Japanese researcher based on Halbach's idea. It was about 3 Tesla and represented a great achievement, as it is stronger that the residual field of a material itself which is about 1.3 Tesla
The author produced a 4.45Tesla dipole filed last year using a permanent magnet which was only possible by a superconducting magnet or pulsed field in collaboration with SSMC http://www.cerncourier.com/main/article/41 /7/5/1CERN Courier September 2001 p9). This is shown in Figure 10. This is the world's strongest PM magnet ever manufactured. The key point was to use saturated iron as part of the pole and cool it down to -50 degree C. This world record was broken by other researchers this year at ESRF(European Synchrotron Radiation Facility) in Grenoble, France(http://cerncourier.com/main/article/42/3/5/1). They generated 5 Tesla in a tiny submicron gap. The author uses a 6 mm gap.
The author has started to invent a variable field magnet, using PM material. It is named MiM, Magnet -in-Magnet, and is a combination of PM magnet and electro magnet. The author made a proof of principle demonstration and manufactured a 3 Tesla variable filed model magnet. There are many variations of this concept. After a successful model experiment, funding was granted from the Japan Science and Technology Corporation(JST) as a 3 years project. The author is collaborating with Prof.Iwashita of Kyoto Univ. for application of a quadrupole magnet system to get a nano-size beam. This is the application for a linear collider for high energy physics in future. Other projects for the MiM or PM cyclotron are going to be started.
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