Commercial electromagnet components

Lead End Material Selection Aid

Thermal conductivity

Electrical resistivity

Cu 110 Grade: ETP

ETP copper is at least 99.9% pure and is therefore a suitable choice of meterial for general electrical applications.

Cu 101 Grade: OFHC

OFHC copper is at least 99.99% pure and is therefore a suitable choice of meterial for applications at low temperatures.

At low temperatures impurities and lattice defects dominate the behaviour of the electrical resistivity and the thermal conductivity. Different grades of copper are classed according to their residual resistivity ratio (RRR) which is the ratio of the resistivity at 273K to that at 4K. In the plots presented below the ETP copper has RRR=30 which is chosen to represent the lowest grades. ETP copper is usually supplied with higher RRR. Similarly the OFHC copper has conservative value of RRR=200 and most commercial suppliers will exceed this.

When selecting the copper grade for the lead ends it should be noted that above about 50K there is very little performance difference between the grades. If a lead end is to be stationed at a temperature above this then it may be more cost effective to make that end from ETP copper.

Nb (99.99% pure)

Niobium is one of the few Type II elemental superconductors and has the highest critical temperature for any element (9.26K). Niobium may be an appropriate material for a lead end stationed at a temperature below 9K. This may assist in reducing the low temperature heat load.

Niobium also has high critical fields. These are very dependent on temperature and purity with Hc1 varying from 0-0.2T and Hc2 varying from 0-0.8T. This makes Niobium an appropriate choice for cold lead ends that may be exposed to high fringing fields.

A customer is designing a current injection point for a superconducting beam-line element. A two-stage pulse tube cryocooler (PT810 from Cryomech Inc. (New York)) is being used to maintain the element below 12K and it is required that 500A be delivered to the element from a room temperature power supply. During operation the 1st stage of the cryocooler is maintained below a maximum of 65K while the 2nd stage is maintained below a maximum of 12K. The cooling budget has been set at 50W extracted through the 1st stage and 5W extracted through the 2nd stage and hence it is critical that as little heat as possible be conducted to the element through the current injection point. The customer has specified metric components.

Worked Solution

Under these circumstances NaeTec would recommend that the current injection be performed in close proximity to the cryocooler or even integrated with the cryocooler. It is important that the cooling power of the cryocooler be used as efficiently as possible. To achieve this, the current would be conducted from the room temperature injection point, via an optimized copper lead, to an intermediate stage well thermally connected to the 1st stage of the cryocooler. Nearly all of the heat conducted down the copper lead will be intercepted by the 1st stage.

In order to minimize the heat that arrives at the beam-line element a High Temperature Superconducting current lead will be used to carry the current between the intermediate stage and a cold stage which is well thermally connected to the 2nd stage of the cryocooler. The selection of current lead is made using the following considerations:

1. A flexible HLX current lead body is chosen so that the 1st and 2nd stages of the cryocooler are as mechanically decoupled as possible. This choice will also reduce the cross section of the body of lead which will assist in reducing the thermal conductance of the lead.
2. The choice of superconductor is critical in reducing the heat load on the 2nd stage of the cryocooler. Sumitomo DI-BSCCO Type G wire is selected as it has a much reduced thermal conductivity as compared with other available wire types. Type G is available with a current carrying capacity of 100A at 77K and hence 6 wires would be used for this lead to accommodate the 500A and an operating margin.
3. Each lead is labeled with a warm and a cold end. For the warm end (70K) cooling is provided via good thermal connection to the 1st stage of the cryocooler. The 1st stage of the cryocooler has considerable heat lift and contact resistance to the HTS lead is not significant. A Type B1M lead end is appropriate. Also, at 70K, the thermal conductance and electrical resistance properties of various copper grades do not vary significantly and hence the material choice is 110 copper which provides a cost saving.
4. The cold end (4K) of the lead is cooled via connection to the 2nd stage of the cryocooler and every effort is taken to reduce the load on this stage. Contact resistance to this end of the lead is reduced by choosing the large area B3M lead and the material is chosen as OFHC grade copper.

From these considerations the work order placed on NaeTec is for:

Field	Code	Description
Lead Type	HLX6	6 Helical conductor grooves
Body Type	F	Flexible body
Wire Type	G	Sumitomo Type G (100A)
Number of Wires	6	-
Warm End Type	B1	Type B1
Warm End Material	110	Multipurpose grade copper
Cold End Type	B3	Type B3
Cold End Material	101	OFHC grade copper

HLX6-F-6-G-B1-110-B3-101

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Figure : Example of a 500A current injection point. The cryocooler is the PT810 from Cryomech.

A customer has designed a superconducting magnet using HTS conductor for the windings. The magnet is conductively cooled using a single two-stage cryocooler. The magnet is in the form of a split pair of coils and each coil is housed in a separate cryostat. The maximum operating current for the magnet is 150A and the maximum operating temperature of the superconducting coils is 20K. The customer would like to minimize the heat load on the cryocooler due to current transport so that the cryocooler performance is being efficiently used to cool the superconducting coils. The customer has specified metric components.

Worked Solution

If each coil is energized using a pair of leads running from room temperature then four leads in total are required, each presenting approximately 15W (60W total) of heat to the cryocooler. This heat load can be halved by connecting the two cryostats through a bellows hose and interconnecting the coils via an HTS lead that remains cold along its length.

The current lead selection is made according to the following considerations:

1. A solid LNX type body is chosen. During operation the coils experience considerable attractive force and mechanical support of the coils is essential. The solid body type allows for the option of using the lead as a structural element. Also, in the event that the current lead length needs to be altered from the standard 200mm then it is cheaper to customize an LNX style of lead rather than an HLX type lead.
2. The superconducting wire chosen for this lead is Sumitomo Type HTCA. This wire is laminated on both sides with 50m of copper. In this application the lead runs between two contact points at nominally the same temperature and so there is no concern regarding the flow of heat from one end to the other. However, there will always be a radiative heat load along the length of the lead and the copper laminations assist the conductive cooling of the lead. The critical current of this wire is typically 160A at 77K and so a single wire is sufficient to carry the 150A for this application.
3. This lead is symmetrical and hence both lead ends are the same. Heating due to contact resistance is not significant and so Type AM lead ends are used and are manufactured from OFHC grade copper.

From these considerations the work order placed on NaeTec is for:

Field	Code	Description
Lead Type	LNX6	6 Linear conductor grooves
Body Type	S	Solid body
Wire Type	HTCA	Sumitomo Type HT-CA (160A)
Number of Wires	1	-
Warm End Type	AE	Type A with English dimensions
Warm End Material	101	OFHC grade copper
Cold End Type	AE	Type A with English dimensions
Cold End Material	101	OFHC grade copper

LNX6-S-1-HTCA-AE-101-AE-101