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Abstract— The use of superconducting magnets in ECR ion sources has the potential for large power savings compared with resistive magnets. High temperature superconductors (HTS) offer further advantages including compactness, efficiency, and a simplification of the overall system. Space Cryomagnetics Ltd has designed and manufactured a pair of HTS magnets for an ion source for Pantechnik. In this paper we describe the design, manufacture, and test results of these magnets.

Index Terms— HTS, HTS coil, superconducting magnets, cryogen free.

I. INTRODUCTION

PACE CRYOMAGNETICS LTD has designed, manufactured, and commissioned a pair of high

temperature superconducting (HTS) coils for use in an electron cyclotron resonance (ECR) ion source. One advantage of HTS magnets is a reduction in the overall power requirement of the system compared with resistive magnets: the power required to cool the superconducting system is only about 5 percent of the total power consumption of the equivalent resistive system. The present system, in particular, is installed on a high voltage platform, so the reduction in power required at high voltage also results in a reduction in the total capital cost of the equipment.

II. MAGNETIC DESIGN

The two coils are mounted in an iron yoke together with a set of permanent magnets (Fig. 1). The combination of iron, electromagnetic coils, and permanent magnets is designed to

Manuscript received October 21, 2001 R. McMahon, S. Harrison, S. Milward, J. Ross, and R. Stafford Allen

are with Space Cryomagnetics Ltd, E1 Culham Science Centre, Abingdon, UK (telephone: +44-1865-409200, fax: +44-1865-409222, e-mail: info@ spacecryo.co.uk).

C. Bieth and S. Kantas are with Pantechnik, 12 rue Alfred Kastler, 14000 Caen, France.

G. Rodrigues is with the Nuclear Science Centre, Aruna Asaf Ali Marg, P . Box No. 10502, New Delhi-110067, India.

generate the required field profile. Because of the detail of the field profile, one of the coils (the extraction coil) has to generate slightly lower field than the other. To achieve this, two identical coils were designed, but the extraction coil is operated at a slightly lower current (see Table I). In fact, both coils were capable of generating the higher field.

Magnetic design of the system was carried out using Vector Fields OPERA-3D software to model the coils and iron in three

dimensions.

III. HTS COIL DESIGN

Each coil consists of ten pancakes of HTS wire connected in series. The BSCCO-2223 conductor was supplied by American Superconductor in the form of a tape laminated with stainless steel strip. The resulting composite has a relatively high strength and is suitably robust for winding coils. The turns were insulated and the pancakes separated by fiberglass insulation. During the design phase of the project two prototype pancakes were wound and tested to assist in the development of reliable winding and jointing techniques.

The critical current for the HTS wire is a function of temperature and magnetic field, with the direction of the field being of particular importance. In this design the maximum operating current is 181 A, and the peak radial field (perpendicular to the conductor) is 1.4 T: this means the operating temperature needs to be below 30 K.

Availability and cost of cryocoolers also has an impact on

Design and Manufacture of High Temperature Superconducting Magnets for an Electron

Cyclotron Resonance Ion Source

Richard McMahon, Stephen Harrison, Steve Milward, John Ross, Robin Stafford Allen, Claude Bieth, Saïd Kantas, and Gerry Rodrigues

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TABLE I KEY PARAMETERS OF THE HTS MAGNETS

Parameter Value

Conductor type BSCCO-2223 Operating temperature (at field) 23 K Operating current 181 A (injection coil)

145 A (extraction coil) Maximum field on the axis 1.8 T Maximum field on the conductor 3.0 T Maximum radial field on the conductor 1.4 T Total length of conductor per coil 950 m Inner coil diameter 240 mm Outer coil diameter 320 mm

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the coil design. At present, HTS conductor is relatively expensive, so in this case it is more economic to operate at relatively high current and low temperature, using a large and powerful cooler, than to operate at a higher temperature with a smaller cooler and more wire.

IV. COOLING SYSTEM

Each coil is conduction-cooled by its own single-stage Gifford-McMahon cryocooler, with a capacity of about 35 W at 25 K. The heat load is a combination of radiation, conduction through mechanical supports, dissipation in the HTS coil itself, and conduction and Joule heating in the current leads from room temperature. As this is a cryogen-free system, the heat load is dominated by the current leads.

Fig. 1. The complete HTS magnet system with its electronics rack. The end of the iron yoke has been removed to show the coil cryostat.

V. CURRENT LEADS

The current leads transfer current from room temperature terminals at the top of the cryostats to the coils. Each of the two coils has its own power supply and set of leads to allow it

to be powered independently. The leads are thermally anchored to the cold head of the cooler part way along their length. When the coil is operating at full current the leads must be able to dissipate heat from their ends faster than the heat is generated by resistive heating. The optimum lead design has just sufficient area that there is no risk of burning out.

Because the magnet operates at a rather low temperature, the design of the leads depends on the residual resistivity ratio (RRR). The RRR of the copper was measured at 78, and the leads are designed to operate at 181 A with no heat transfer from the warm end (for optimum efficiency). This means that each lead conducts about 6 W to the cryocooler cold head when the magnet is cold and at zero current, so heaters are installed near the warm ends to prevent condensation or ice build-up.

Because the cold head and the magnet operate at virtually the same temperature, there is almost nothing to gain from using HTS material in the cold section of the leads. Although there is some Joule heating in this section, it is negligible compared with all the other sources of heat to the coil.

Fig. 2. The first HTS coil after winding showing all ten pancakes.

VI. THERMAL LINKS TO THE CRYOCOOLER

As the HTS wire and the cryocooler heat lift capacity are both functions of temperature it is vital that the cooler is well-linked to the coil. This has been achieved using a flexible thermal strap made from aluminium. The flexibility of the strap is sufficient to allow the necessary relative movement between the coil and the cryocooler during cool down while still transferring heat. Soldered connections are used at both ends

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of the strap to ensure good heat transfer to the cryocooler. The entire cold assembly is wrapped in superinsulation. This

is one of the most challenging aspects of the assembly as space – especially round the neck of the cryostat and in the annular region around the coil – is very restricted. The insulation system is very effective: with the magnet energized and running at 23 K there is no noticeable reduction in temperature anywhere on the vacuum vessel. Even with the coils surrounded by the iron, there has never been any trace of condensation on the surface.

VII. TRANSIENT MODELLING

To model both the steady state and transient behaviour of the magnet a mathematical model has been generated. Each pancake is divided into a number of elements, and a heat balance equation constructed for every element. These equations account for radial and circumferential conduction (but not axial because the pancakes are insulated from each other) as well as radiation from warm components, point heat loads (from supports or current leads) and dissipation within the HTS conductor itself. The resulting non-linear differential equations may be solved using finite differencing techniques, and can then be used to study transient effects (such as cool down or quench) as well as the steady state operation of the magnet.

When this model, together with analytical techniques, was used to study the quenching behaviour of the coils, the results indicated that excessive temperatures could be reached if a coil quenched with no intervention. Each coil therefore has a quench detection system, which is able to force an external energy dump if pre-set voltage levels are exceeded.

Fig. 3. Trial suspension of the first HTS coil on its supports.

VIII. ELECTRICAL DESIGN

For monitoring, each coil has a total of eleven voltage taps and one temperature sensor. There are further temperature sensors on both cryocooler cold heads and near the warm end of every current lead.

The voltage taps are connected across each of the pancakes and across every joint in the coil. During operation, proprietary electronics compares differences between monitored voltages and an electronic model. If there is a discrepancy of more than 5 mV between the real magnet and the model the stored magnetic energy is dissipated in an external resistor.

As well as the voltage protection the system has an interlock which opens if any of the temperature limits are exceeded, and a current limit on the power supplies. There are further interlocks on the cryocoolers which discharge the coils if the coolers are switched off.

IX. STRUCTURAL DESIGN

During operation, the axial magnetic load on the coils can be as much as 10 kN. The support structure is designed to ensure that the magnet is stable under this loading without introducing a large heat leak through the supports. It also has to have sufficient stiffness to withstand the unstable loading due to displacement of the coil.

In this system each coil is supported from the vacuum vessel by six fiberglass cantilevers. These react the direct loading and have the necessary stiffness. They are aligned to allow for the thermal contraction of the coil. Fig. 3 shows the arrangement of the supports: they are located around the inner diameter of the coil. (This picture was taken before the superinsulation was applied.)

X. MAGNET SYSTEM TESTING

The magnet system was manufactured between April and July 2003. Following assembly a number of tests were carried out to prove the operation of the coils. Both coils were tested individually outside the iron yoke. These tests proved the cryogenic design and the electronics, but the coils could only be run to 150 A, rather than the 181 A maximum inside the iron. This is because the iron reduces the radial field (perpendicular to the conductor) thus increasing the allowable current.

Both coils were installed together in the iron for acceptance testing (Fig. 1). Cool down of the system is shown graphically in Fig. 4.

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0

100

200

300

0 2 4 6 8 10 12

Time (hours)

Tem

pera

ture

(K

)

Coils

Coolers

Fig. 4. Cool down of the two coils and cryocoolers inside the iron.

The cold heads rapidly dropped to below 30 K, with the coils

lagging due to thermal mass and the change in conductivity of the thermal strap.

After cooling the magnets to around 20 K both coils were run to full field, with 181 A flowing through the injection coil and 145 A through the extraction coil. Fig. 5 shows the magnetic field strength along the axis with the coils carrying these currents. Of particular importance are the size and separation of the two peaks, and the profile of the field between them.

The maximum field measured on the axis of the system

during testing was 1.8 T at the injection coil peak (Fig. 5). With this field profile the coil temperatures were stable at approximately 23 K.

At various times during testing the electronic protection was checked. The coils discharged safely when voltage, temperature and current interlocks were triggered.

0

0.5

1

1.5

2

0 100 200 300 400

Distance along central axis (mm)

Axi

al F

ield

(T

)

Fig. 5. Measured field profile along the axis.

XI. CONCLUSION

Space Cryomagnetics Ltd has designed, manufactured and tested an HTS magnet system to a commercial contract. All specifications and requirements were met or bettered. The magnetic field available exceeded the specified values and the predicted profile was closely matched. The coils operate well within the predicted temperature range and are well protected against quenching.

This system shows that, although the balance of the cost benefits between cooling power, quantity of HTS wire required, and coil protection may vary as the technology matures, the use of HTS is already practical for some applications.