Conceptual design of a magnet system to generate 20 T in a 0.15 mdiameter bore, employing an inductor precooled by liquid nitrogen

摘要

The research program for an eventual neutrino factory or muon collider needs a magnet of ~0.15 m diameter bore to generate ~20 T over a length of ~0.3 m. Downstream for ~3 m the held should fall gradually to ~1.25 T, while the bore increases fourfold inversely as the square root of the field. A conventional magnet would require ~40 MW; a superconducting or hybrid magnet might cost tens of millions of dollars. An economically feasible system employs a pulse magnet precooled by liquid nitrogen, with two sets of coils energized sequentially. An outer set of coils of ~12 tons, energized in ~20 s by a 16 kA, 250 V supply available at Brookhaven National Laboratory, generates a peak field of ~9 T and stores ~20 MJ. A resistor of ~? Ω inserted across the terminals of the set introduces a voltage drop, initially ~4 kV, to energize an inner set of coils to ~10 kA in ~? s. This set adds ~13 T to the ~7 T remaining from the enter set, whose current has decayed to ~12 kA. Complicating the design is a superconducting coil, once part of the PEP-4 detector at the Stanford Linear Accelerator Center, that begins only ~2.2 m downstream and is sensitive to eddy-current heating by rapid flux changes. Therefore the proposed magnet system includes a conventional DC coil of~0.7 MW to distance the pulse magnet from the PEP-4 coil. Also, a bucking coil in series with the outer set reduces by an order of magnitude the pulsed flux seen by the PEP-4 coil. The bucking coil serves also to reduce the axial force on the PEP-4 cryostat to below its limit of 200 kN