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Superconductivity leaves the lab

01 Oct 2000
The cryostat

Superconducting magnets are a common tool in many physics and chemistry laboratories, and are used in a host of research applications, including solid-state physics, nuclear-magnetic-resonance chemistry and particle physics. Outside the research lab, the only truly widespread use of superconducting magnets is in magnetic resonance imaging (MRI) in medicine.

However, applications of superconducting magnets are becoming more diverse. And nowadays, physicists and engineers might equally find themselves in remote mining sites in the Amazonian rainforest or in state- of-the-art hospitals. While such environments represent the extremes, they illustrate the movement of superconducting magnets out of the laboratory and into the wider world (see “Industry warms to superconductors” by Jeffery Tallon Physics World March 2000 pp27-31).

Mineral processing

Magnetic separation in the minerals industry is the largest application of superconducting magnets in industrial processing. Since the early 1990s Oxford Instruments has worked with the Carpco division of Outokumpu Technology Inc., developing magnets for laboratory and full industrial-scale use. Over 30 systems are now in operation across the world, in locations ranging from Cornwall in the UK to Australia, India and Brazil.

A wide variety of materials can be purified using a magnetic-separation system with a high magnetic-field gradient. The technique relies on the fact that the magnetic-separation force is a product of the material susceptibility, the base magnetic field and the field gradient. The main application is in the improvement of kaolin (china clay), which is used in the ceramics and paper industries. Dark-coloured impurities, like iron oxide, tend to be paramagnetic and can be removed from the non-magnetic kaolin using high-gradient magnetic fields. This process increases the “whiteness” of the material, producing a higher grade, and therefore higher value, product.

To separate the magnetic impurities, the raw material is suspended in a wet slurry, and passed through a filter canister in the magnetic field. The filter is typically a fine ferromagnetic matrix, which in its most simple form is similar to steel wool. This matrix creates the high localized magnetic-field gradients that attract and hold the impurities. The system contains a so-called reciprocating canister to avoid downtime while the filter is flushed once it is saturated with impurities. This works by building two filter matrixes into a single long canister such that the filters may be alternated in the magnetic field – one filter processes the slurry while the other is being flushed.

The superconducting magnets used produce a field of 5 tesla and are cooled to 4.2 K using liquid helium. A magnet with a 1 m bore, coupled with a reciprocating canister developed by Carpco, can process over 100 tonnes of material per hour.

In general, magnetic-separation systems operate in demanding environmental conditions, so there is a clear need to have fully robust magnets and to save helium. For example, it can take three weeks to transport liquid helium by sea and river from Sao Paulo to a Brazilian mining site, losing over 30% of the liquid along the way. To avoid further losses, the system uses low-loss cryogenic techniques developed originally for MRI magnets. Specifically, so-called cryocoolers use a fixed volume of helium gas to cool the radiation shields between the room-temperature outer cryostat and the liquid- helium vessel.

To allow the operating costs to be recouped, the magnet must be able to operate for at least 8000 hours per year (i.e. 92% of the time) without interruption. Modern magnets easily meet this standard – even in the most hostile environments.

While high-gradient magnetic separation can be applied to materials that can be suspended in slurry, this is not possible – or desirable – for many materials that could be purified using magnetic fields. An alternative method, known as open-gradient magnetic separation, segregates a stream of dry material falling from a conveyor belt through a field gradient in front of a magnet.

Physicists at Oxford Instruments have designed a cryogen-free “racetrack-coil” magnet to produce the highest possible field and field gradient outside the system, with a geometry that matches the material feed. By removing the liquid-cryogen volume and using a closed-cycle refrigerator operating at 4 K to cool the system, the magnet coil – and thus the region where the field gradient is highest – can be brought close to the outer wall of the cryostat body and the flow of material.

The high field gradients generated by the superconducting magnets can be used to separate weakly magnetic granular materials, such as sand, soda ash, marble and diamonds. In contrast, permanent-magnet separators do not provide a sufficient field and field gradient to separate these dry materials effectively.

Magnets for surgery

An exciting new application for superconducting magnets is a magnetic navigation system that could potentially allow surgeons to steer catheters through the brain and the vascular system. The device that is being developed by Stereotaxis Inc. in St Louis, USA, may allow instruments to be fed through these pathways to deliver drugs, carry out biopsies, close aneurysms and make electrical maps of the heart wall.

The anticipated possible speed and ease of these procedures could potentially offer many benefits to patients and doctors, including more rapid surgical response, a lower level of surgical support and consequent cost savings. Faster procedures could potentially improve patient recovery times. Meanwhile, the number of procedures abandoned due to an inability to properly position the catheter may be greatly reduced.

Oxford Instruments is building a series of magnets for these systems, the first of which was installed earlier this year at the Barnes-Jewish Hospital in St Louis (see figure). Meanwhile, Stereotaxis has developed a range of catheters, each with a small magnetic tip, designed to be oriented by changing the relative magnetic fields of three orthogonal superconducting coils positioned around the patient’s head or chest. Each coil generates a magnetic field of up to 5 tesla, giving a projected field of 0.3 tesla at the centre of the set of the three magnets.

Very high stresses are generated in the magnet coils and surrounding structure due to the high rates at which the magnets are ramped in order to rapidly, and frequently, change the direction of the magnetic field. Intensive finite-element modelling played a key part in the successful magnet design. The magnets have also been designed so that patients may be positioned within the system easily, and so that X-rays can be taken to monitor the position of the catheter tip in real time.

A hemispherical cryostat, quite unlike any conventional superconducting magnet, is used to provide a compact system (see figure). The current leads in the magnet are made from high- temperature superconductors so that the magnetic field can be increased quickly without generating high heating losses. The system also uses a closed-cycle cryocooler operating at 4 K that condenses the liquid-helium vapour so that it falls back into the liquid-helium vessel. This so-called recondensing system reduces the amount of liquid helium required and eliminates the need for a large helium reservoir in the operating theatre. When in use, the magnet should require only 30 litres of helium per month.

Like many technologies, elements of the new superconducting-magnet designs draw heavily on previous experience. For example, the heavy iron shielding on the Carpco magnets helped us to develop stress-modelling and engineering techniques that are now being applied to the Stereotaxis magnets. Similarly, one of the first liquid-helium recondensing systems was used in a Carpco magnet, and this technology is similarly being extended to the Stereotaxis and other systems. As our engineering experience continues to grow, it is likely to enable more and more superconducting-magnet applications in the future.

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