The Generation and Use of Pulsed Magnetic Fields

Abstract

The generation and use of pulsed magnetic fields are surveyed from the very beginning with the work of P. Kapitza. The survey is focused on fields in the megagauss range, including non-destructive fields above half a megagauss where the development is now aimed at approaching the 100-tesla limit. In the open literature, megagauss fields were first reported from an experiment with a capacitor-driven single turn coil. This was followed by explosive-driven magnetic flux compression that consistently generates the highest fields. Electromagnetic flux compression was developed from a modest beginning into a research instrument. By far the largest part of scientific research in megagauss fields was accomplished with the single turn coil after it had been developed into a reliable and practical research instrument. The potential for future development of the different techniques and of some novel techniques is indicated. SURVEY AND ORDERS OF MAGNITUDEThis is a review on all aspects of generating pulsed magnetic fields. Since the author has participated in the development of most of the different techniques almost from the beginning, the review will reflect his preferences that are characterized by the design of uncomplicated, reliable devices and by a straightforward theoretical treatment that is in balance with experimental uncertainties. Only key references are given, more details and references can be found in a recent review 1 and a series of books 2 , and of course in the proceedings of all megagauss conferences (see table 1 below for topics).The highest steady-state magnetic field available at present is 45 T, generated by a large hybrid magnet. It is unlikely that this will be extended beyond 50 T in the foreseeable future. Pulsed magnets can generate much higher fields at a fraction of the cost. With a view to experiments, a pulsed field is clearly defined by the fact that no adjustments of the experimental apparatus can be made during the pulse -this also applies to the socalled "quasi-stationary" magnets. With the many different pulse shapes that can be generated, the overall pulse duration is only a rough criterion; more specifically the rise time, a flat top or the time the field is above a given percentage of the peak value, and the decay time (or the time constant in case of exponential decay) ought to be quoted. For flux compression experiments, it is the slope of the final steep field increase that counts. An ultra-strong magnetic field is defined by a field strength that inevitably destroys the current-carrying structure needed for its confinement. Considering the mechanical strength of currently available materials, the limit between destructive and nondestructive fields with feasible dimensions is in the vicinity of 100 T, corresponding to a magnetic stress of 4 GPa. The term "semi-destructive" can be applied to the single turn coil technique where the coil is destroyed but the sample and cryogenic equipment survives. Non-destructive magnets are now mostly made according to the Leuven design with opti...