Abstract: Quasistatic magnetic fields

Abstract

A prototype switching system has been developed which can switch 20 kA at 230 V for short periods of time through inductive loads. High power silicon controlled rectifiers are used to switch the National Magnet Laboratory dc generators on and off into a liquid N2 cooled, low impedance high field magnet so that high fields can be generated for a period of time approaching 1 sec. These transient ’’quasistatic fields’’ involve times which are intermediate between the usual pulsed field operation of milliseconds or less and the more limited field range produced by dc techniques. Each of the four specially designed low internal impedance dc generators can produce as much as 8 MJ/sec on a 5 sec transient mode of operation, which is sufficient to project development of magnets producing quasistatic fields approaching 40 tesla. The present switching arrangement can furnish between 0.5 and 10 megajoules for intervals between 0.1 and 2 seconds using 2 generators operating in parallel and drawing energy from the 84 ton flywheel attached to the generator set. Details of the switching hardware can be found in Ref. 1. Several liquid nitrogen precooled magnets have been tested at various field levels. The inner bores generally were 2 cm i.d. The copper conductor cross section and coil dimension determined the current, field and time at high fields. Square cross-section wire, multilayer coils were made as single sections for the lower current, lower field regime. Pancakes connected in parallel provide lower impedance and allow higher current magnets and higher fields. Other winding configurations use tape-wound, rectangular cross-section wire, or Bitter-plate type magnets. Fields above 30 T were generated on a quasistatic basis with such magnets. A detailed description of this work has been submitted for publication elsewhere.