Cold contact
Applying power to the coils of an electromagnet whose temperature must
be kept within a few degrees of Absolute Zero relies on some pretty
special electrical contact technology, as Les Hunt found out during a
recent visit to Oxford Magnet Technology
Oxford Magnet Technology (OMT) builds super-conducting magnets for the
MRI (magnetic resonance imaging) scanner manufacturing industry. The
company supplies around 40% of the world's needs for MRI magnets,
delivering some 800 units annually to the medical equipment divisions of
key manufacturers such as Siemens, Philips and Toshiba. The magnet is a
very important component of the MRI imaging system, accounting for around
a third of its total cost.
MRI magnet coils become super-conducting at extremely low temperatures,
so they need to be cooled using liquid helium, the temperature of which
is maintained by a cryostat at between 4.2 and 4.7K. In order to energise
the coils and produce the required magnetic field, a low-voltage current
is passed through them, ramping up to a maximum of 600A over 25 minutes.
As long as the temperature of the coils does not rise above 5.5K during
the energising process, the magnet becomes super-conducting and will
remain in this 'charged' state for about eighteen months. Power
connection to the coils is thus only a temporary arrangement, but due to
the critical temperatures involved and the vital requirement for very low
contact resistance, this is far from being a conventional application of
electrical contact technology.
Since 1992, OMT has been working closely with Multi-Contact to develop
the ideal power connector solution. Multi-Contact has a novel contact
interface called 'Multilam', the basic form of which comprises a strip of
beryllium copper with raised parallel louvers that form multiple,
mechanically sprung electrical contacts. OMT's present power connector is
a coaxial device (see photograph) consisting of a brass outer shell, a
PCTFE insulating medium and an inner 9.6mm diameter, copper-alloy
conductor, upon which the Multilam bands are mounted. The inner conductor
has a concentric duct through which liquid helium can flow to maintain
the coil's super-conducting temperature while the power connector is
attached.
According to OMT project consultant engineer, Adrian Thomas, the Multilam
contact interface introduces a number of advantages compared with the
screw-type connector arrangement. Principal among these is greater
flexibility in tackling the contact resistance problem, which has a
direct effect on the maintenance of super-conductivity. Several design
iterations have seen multi-band configurations, plus thicker Multilam
bands offering greater contact pressure - all of which have seen the
contact voltage drop reduced to just 60mV. A further reduction in the
voltage drop across the contacts remains a goal, and the current thinking
is to increase the inner conductor diameter, though this would introduce
compatibility problems. Coincidentally, Multilam's louvers are also
useful in scraping ice away from the contact surface on disconnection!
Multi-Contact
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Oxford Magnet Technology
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