8 Accelerator Engineering and Technology: Accelerator Technology
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8.1.3.6 Current Leads
The powering of superconducting magnets is done via current leads from room
temperature, where the warm power cables are connected, to the cold mass at
cryogenic temperature, where the magnets are operated. Current leads are often the
dominant source of heat leak into the cryogenic environment because of thermal
conduction under a large temperature gradient, as well as ohmic loss. The goal of
a current lead design is the minimization of these losses, aiming at the optimum
geometry which enables stable operation with a minimum heat in-leak.
Conventional current leads are made from metal, and are cooled either by
conduction or by heat exchange to the cryogen (commonly helium) boil-off.
The heat leak to cryogenic temperature for optimized conduction cooled leads
at operating current is about 47 W/kA, while leads cooled by helium boil-off
have a much more efficient value of 1.1 W/kA. These values are representative
of the minimum heat leak that can be achieved. They are independent of the
properties of the material chosen because of the proportionality relation between
electrical conductivity (which governs Joule dissipation) and thermal conductivity
(which rules heat in-leak) established by the Wiedemann-Franz law, to which
most metals and alloys obey. The geometry of the lead that corresponds to the
optimum performance (length and cross section) is however strongly dependent on
the materials chosen.
The loss of a conventional lead can be further decreased replacing the cold part
with High Temperature Superconducting (HTS) material, which is characterized
by low thermal conductivity and zero electrical resistivity. The use of HTS
leads was first pioneered on large scale at the LHC machine [29], where more
than 1000 HTS leads operate at currents ranging from 600 A to 13,000 A and
power the superconducting magnet circuits. The high temperature superconductor
incorporated in the LHC leads is the Bi-2223 tape with a gold-doped silver matrix
[30]. The HTS operates in a temperature range spanning from 50 K to 4.5 K, while a
resistive heat exchanger, cooled by helium gas, provides the link between 50 K and
the room temperature. In the LHC leads, the heat load into the helium bath is reduced
by a factor 10 with respect to conventional self-cooled leads [29]. An additional heat
load appears at intermediate temperature, which can however be removed with much
better thermodynamic efficiency. The typical gain on the overall heat balance that
can be achieved by proper use of high temperature superconductor in current leads
is then a factor of 3 with respect to the optimized values quoted earlier.
8.1.3.7 Mechanics, Insulation, Cooling and Manufacturing Aspects
The performance of a superconducting magnet is invariably determined by proper
consideration of the material physics and engineering aspects discussed earlier.
Good material and magnet design, however, are not sufficient, and success relies
heavily on a sound mechanical concept and the adapted manufacturing technology.
The main issue in the mechanics of a superconducting magnet is how to support
365
8.1.3.6 Current Leads
The powering of superconducting magnets is done via current leads from room
temperature, where the warm power cables are connected, to the cold mass at
cryogenic temperature, where the magnets are operated. Current leads are often the
dominant source of heat leak into the cryogenic environment because of thermal
conduction under a large temperature gradient, as well as ohmic loss. The goal of
a current lead design is the minimization of these losses, aiming at the optimum
geometry which enables stable operation with a minimum heat in-leak.
Conventional current leads are made from metal, and are cooled either by
conduction or by heat exchange to the cryogen (commonly helium) boil-off.
The heat leak to cryogenic temperature for optimized conduction cooled leads
at operating current is about 47 W/kA, while leads cooled by helium boil-off
have a much more efficient value of 1.1 W/kA. These values are representative
of the minimum heat leak that can be achieved. They are independent of the
properties of the material chosen because of the proportionality relation between
electrical conductivity (which governs Joule dissipation) and thermal conductivity
(which rules heat in-leak) established by the Wiedemann-Franz law, to which
most metals and alloys obey. The geometry of the lead that corresponds to the
optimum performance (length and cross section) is however strongly dependent on
the materials chosen.
The loss of a conventional lead can be further decreased replacing the cold part
with High Temperature Superconducting (HTS) material, which is characterized
by low thermal conductivity and zero electrical resistivity. The use of HTS
leads was first pioneered on large scale at the LHC machine [29], where more
than 1000 HTS leads operate at currents ranging from 600 A to 13,000 A and
power the superconducting magnet circuits. The high temperature superconductor
incorporated in the LHC leads is the Bi-2223 tape with a gold-doped silver matrix
[30]. The HTS operates in a temperature range spanning from 50 K to 4.5 K, while a
resistive heat exchanger, cooled by helium gas, provides the link between 50 K and
the room temperature. In the LHC leads, the heat load into the helium bath is reduced
by a factor 10 with respect to conventional self-cooled leads [29]. An additional heat
load appears at intermediate temperature, which can however be removed with much
better thermodynamic efficiency. The typical gain on the overall heat balance that
can be achieved by proper use of high temperature superconductor in current leads
is then a factor of 3 with respect to the optimized values quoted earlier.
8.1.3.7 Mechanics, Insulation, Cooling and Manufacturing Aspects
The performance of a superconducting magnet is invariably determined by proper
consideration of the material physics and engineering aspects discussed earlier.
Good material and magnet design, however, are not sufficient, and success relies
heavily on a sound mechanical concept and the adapted manufacturing technology.
The main issue in the mechanics of a superconducting magnet is how to support