High Temperature Superconducting

High Temperature Superconducting Current Leads

Our High Temperature Superconducting Current (HTSC) Leads are designed to bring high power currents between ambient temperatures and/or liquid nitrogen temperatures and liquid helium superconducting temperatures with minimal heating.

Refractory Metals - Composites -Special Alloys - Ceramics -Tungsten - Titanium

Conventional vapor cooled leads and direct metal conductors used to power magnets in cryostats provide a direct heat leak to ambient, thereby increasing the helium consumption or load on a cryocooler. A dramatic reduction in this heat leak is accomplished by placing a low thermally conductive ceramic directly in the heat path of the electrical conductor. Also, since the ceramic is superconducting, there is no resistive heating factor to add to the heat load.

The use of Bi-2223 HTSC material brings not only the highest critical temperatures of Bi based superconductors but also, when properly treated, a high critical current density with a lowered sensitivity to the external magnetic field.

The specific advantages of our HTSC leads are:
  • Low Heat Conduction
  • Low Heat Dissipation
  • Reduced Helium Consumption
  • Remarkably Low Coolant Costs
  • Useful Cold End Temperature 25 - 4°K or below
The remarkable reduction in helium loss and dissipation of heat into the helium reservoir is accomplished by a multi--stage design:
  • Power is first carried from ambient temperature to about 77°K via conventional copper leads or via vapor cooled leads.
  • The conventional lead is connected to our specially designed high temperature superconducting composite leads.
  • The cold end of the lead is then connected either to a copper wire or to a low temperature superconducting wire that is joined to the magnet.

NOTE: The critical current is the amount of current a HTSC lead can carry before loosing its superconducting properties.

Key Applications for HTSC leads

  • MRI Magnet Systems
  • Superconducting Magnetic Separators
  • High Energy Particle Accelerators
  • SMES Systems
  • Large Superconducting Magnet Systems
  • Superconducting Generators and Motors
For nearly any application where high currents are being conducted from a region of 77°K to colder regions, MCCC leads can reduce coolant costs.

Superconducting magnet systems requiring current from 100 to more than 1000 amps can benefit from the significantly reduced helium consumption provided by our HTSC leads.

MCCC can help you design a complete current lead system to accommodate your design and performance requirements for low temperature superconducting applications.

Fabrication of Leads

In order to ensure a quality lead, we begin by fabricating our own BSCCO ceramic powders under carefully controlled laboratory conditions. The powder is then isopressed around a mandrel and fired under strict time, temperature procedures. Following the firing of a silver conductive band, each tube is checked for critical current, temperature characteristics. These tubes are produced in several standard diameters and lengths. In addition, we offer three material grades, which are selected based on the final design specifications.

Bare leads with silvered ends are available for the customer to attach their own metal conductors and final fabrication design, however, most prefer that MCCC supply a complete package, ready for installation. We have three standard composite HTSC lead options.

Composite Current Leads
We begin by soldering flexible copper braid to the warm end of the tube and either a copper braid or a low temperature SC wire to the cold end. This is normally encased in either a G-10 Fiberglass tube or a NiCu or stainless steel tube.
Superconducting Current Leads in G-10 Fiberglass Casing
The entire ceramic lead and a short portion of the metal leads are secured in a G-10 fiberglass tube with epoxy and sealed with fiberglass end caps. This ensures the completed assembly is protected environmentally as well.
Superconducting Safety Current Leads in Metal Casing
Superconducting Bi based 2223-phase tubes are encased in either a SS or a CuNi casing for protection against mechanical strains and any accidental operating temperature increase.

Design Criteria for HTSC Leads to Minimize Heat Leak

Electrical Load The first criteria to consider is the maximum expected electrical load. Increasing the load requires a larger ceramic tube to be used. The heat leak is directly proportional to the cross section of the ceramic.

Length The heat leak is inversely proportional to the length of the ceramic tube. The longer the tube, the lower the heat leak. The design should accommodate the longest length of tube possible.

Temperature The critical current for a given lead is significantly changed by the warmest temperature the lead will see. The critical current at 50K will be as much as 4.4 times the 77K value and just 50% the 77K value at 90K. The operating temperature will affect the size of ceramic tube required.

Also, the temperature drop from warm to cold end will also affect the amount of heat leak. The heat leak from 64K to 4K is about 70% that of the heat leak from 77K to 4K.

Magnetic Field The critical current of all ceramic superconductors is affected by both the self field and external fields generated by the magnet and whether the external field is parallel or perpendicular to the lead. Knowing this information is important in the proper design of the size of the lead. For example, a 50-mT field parallel to the lead will reduce the critical current by 80% at 77K. The effect of perpendicular or transverse fields will be about 20% lower.

This effect is normally at least partially off set by the fact that the regions of higher magnetic fields closer to the magnet are generally also colder, which raises the critical current.
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