1. Field of the Invention
The invention in general relates to superconducting magnets, and more particularly to an improved cooling arrangement therefor.
2. Description of Related Art
Superconducting magnets operate at extremely low temperatures and find utility in a variety of applications such as magnetic resonance imaging, ore separation and magnetic influence minesweeping, to name a few.
Superconducting magnets operated at cryogenic temperatures make use of the fact that the electrical resistivity of certain metals drops with decreasing temperature, thus lowering the power consumed by the magnet itself. The operation requires cooling at cryogenic temperatures near absolute zero and such cooling typically is accomplished with liquid helium at a temperature of around 4.degree. Kelvin in a forced flow or pool boiled convection mode. The use of liquid helium and the requirement for constant replenishment contributes to the high cost of operation of various types of superconducting equipment. Further, liquid helium storage and handling are a logistic impediment to the use of superconducting coils for magnetic influence mine sweeping, particularly when the carrying platform must operate reliably under harsh conditions in the marine environment.
In an effort to eliminate the requirement for liquid helium to maintain superconductivity, another type of cooling arrangement, conduction cooling, may be utilized. In conduction cooling of a magnet, the superconducting coil is cooled by conduction heat transfer to a nominally isothermal heat sink maintained at a sufficiently cold temperature by one or more cryocoolers employing closed cycle refrigeration. For conduction cooled magnets proper operation requires that the maximum heat dissipation rate via conduction exceed the net heat generation rate.
Typical sources of heat input which may significantly reduce efficiency or destroy superconductive operation, include AC losses, losses in joints, cold mass support heat losses, and heat conduction along unventilated current leads. Additional sources of heat may, depending upon the application, include friction and mechanical hysteresis due to vibration and/or transient stress wave propagation. Accordingly, the efficiency of conduction heat transfer within the superconducting magnet must be maximized in order to minimize the temperature difference between the heat sink and peak conductor temperature of the coil.
The present invention provides for a design which will meet the required objective of maximizing conduction heat transfer between a superconducting coil and a cryocooler.