Kenneth Gordon Herd

Vibration isolation of superconducting magnets

In order to improve image resolution and magnetic field uniformity in a magnetic resonance imaging device, the vibrations and loads produced by the cryocooler are isolated from the superconducting magnet and cryostat. The vibrations and loads are isolated through the use of flexible, laminated copper connectors and rubber mounts.

Advanced Cryocooler Cooling for MRI Systems

Advances in cryocooler technology during the past several years have enabled the design of new cooling methods for magnetic resonant imaging (MRI) systems. Open cycle operation of MRI systems using a Gifford-McMahon (GM) cycle cryocooler to cool two thermal shields, one at 20 K and the second at 80 K, has been the standard approach used to minimize helium usage in these systems. The concept has worked very well and enabled the development of an important medical imaging modality. However, the 12 K temperature limit of these cryocoolers has limited the design flexibility of the MRI magnet system by requiring a cylindrical design with two thermal shields and a large helium container to extend the operating time. The development of Gifford-McMahon cycle cryocoolers capable of cooling below liquid helium temperature, or providing larger cooling capacities between 4.2 and 10 K, has removed this design barrier and provided greater overall system design flexibility. The paper describes the impact that new GM cryocooler developments, based on rare earth intermetallic compounds in the second-stage regenerator, has had on MRI designs. By extending the cooling capacity of these units to below 4.2 K with rare earth materials, new MRI products have been developed that operate as closed cycle systems without the need for replenishing liquid helium to maintain the magnet at temperature for long periods of time. The paper describes the evolution of MRI systems at the General Electric Company from open cycle systems to two new developments using conduction cooling and helium recondensing to eliminate the need for refilling with helium. The paper reviews the design of a conductively cooled system developed for an open MRI magnet used for interventional therapy and a helium recondensing system that was incorporated into GE’s product line. In addition to a description of these systems, the operational reliability of cryocooled systems will also be reviewed.

A compact 0.8T superconducting MRI magnet

Abstract A compact, cryogen-free 0.8T superconducting MRI magnet has been woundusing Nb 3 Sn tape conductor and is maintained at 8K using a two-stage Gifford-McMahon refrigerator. The magnet is housed in a low eddy-current cryostat which incorporates a fiberglass-reinforced epoxy (FRE) vacuum enclosure with an integral stainless-steel permeation barrier, a composite copper/fiberglass thermal shield heat stationed to the first stage of the cryocooler, and a FRE magnet support structure heat stationed to the second stage of the cryocooler. The low eddy-current components eliminate the need to shield the gradient coils. The magnet components and the measured performance characteristics of the system are described in detail.

A dual refrigerator assembly for cryogen-free superconducting magnet applications

Reliable refrigeration is critical to the viability of cryogen-free superconducting magnets. The servicing of a refrigerator coldhead must not interrupt the magnet operation. A dual refrigerator coldhead assembly is described in this paper which enables the disconnection of a coldhead for servicing while a second coldhead provides continuous cooling. The two-stage Balzers UCH-130 coldheads are thermally connected to the cryostat thermal shield at the first stage and to the superconducting magnet at the second stage. The assembly includes high-T/sub c/ superconducting current leads heat stationed between the first and second stages of the coldheads in order to minimize the heat leak to the magnet. Vibration isolation components minimize the transmission of the coldhead vibration to the magnet and the thermal shield. This paper describes the design, fabrication, and testing of the dual refrigerator assembly.<<ETX>>

A cryogen-free superconducting magnet for Maglev applications: design and test results

A novel cryogen-free superconducting magnet for use in levitating and propelling Maglev vehicles has been developed and tested. The magnet is conduction-cooled by a two-stage CTI Cryodyne-1020 Gifford-McMahon cryogenic refrigerator. YBCO high-Tc superconducting current leads are incorporated in order to minimize the heat leak to the magnet. A low eddy-current cryostat has been fabricated using fiberglass-reinforced epoxy structures with subdivided stainless steel permeation barriers. The magnet has been cooled to approximately 10 K and ramped to the design current of 100 amps DC. The magnet has also been operated with a 20 Hz, +/-15 amp sinusoidal excitation. Test results for the thermal and magnetic performance of the magnet are presented.<<ETX>>