360
F. Bordry et al.
is decreased by backing the superconductor with a matrix of a material with good
conductivity properties (e.g. copper, aluminium or silver). The discharge can be
made faster by reducing the inductance of the magnet, which is the reason why
large scale magnets are wound using cables with large operating current in the place
of single wires. There are various possibilities to dump the magnetic energy, based
on one or more of the following standard strategies:
• Energy extraction, in which the magnetic energy is extracted from the magnet
and dissipated in an external circuit (e.g. a dump resistor or diode);
• Coupling to a secondary circuit, in which the magnet is coupled inductively to a
secondary that absorbs and dissipates a part of the magnetic energy;
• Subdivision, a partition of the magnet in sections, with each section shunted by
an alternative current path (resistance or diode) in case of quench;
• Active quench initiation, that relies most commonly on heater embedded in the
winding pack, fired at the moment a quench is detected to spread the normal zone
over the whole magnet mass and thus reduce the peak temperature. Alternative
means to actively initiate quench have been revisited, based on over-current or
fast field changes induced by capacitive discharge in the coil.
8.1.3.4 Magnetization, Coupling and AC Loss
An ideal superconductor (type-I) tends to exclude field variations from its bulk,
i.e. a perfect diamagnetic behaviour. In practice, in the superconducting materials
listed above (type-II) the magnetic field can penetrate the bulk, still resulting
in partial diamagnetism. Macroscopically seen, a field change induces shielding
currents, which, in a superconductor, do not decay. For this reason these currents
are referred to as persistent. The magnetic moment per unit volume M associated
with persistent currents is proportional to the current density J C of the shielding
currents, and the characteristic size D of the superconductor, i.e. M ≈ J C D. This
magnetic moment can attain large values, perturb the field generated by the magnet,
and lead to instabilities in case the magnetic energy inside the superconductor is
dissipated in a process referred to as flux-jump. For this reason, the superconductor
in wires and tapes is subdivided in fine filaments that have characteristic dimension
in the range of 10 to 100 μm. Persistent currents and the associated magnetization
produce a significant field perturbation in accelerator magnets, and must be subject
to optimization and tight control.
Similar to the bulk behaviour described above, field variations also induce
shielding currents between the superconducting filaments. These currents couple
the filaments electromagnetically by finding a return path crossing the wire matrix.
The amount of filament coupling depends on the resistivity of the matrix, which
has to be low for good protection, and the geometry of the current loop. In the
extreme case of wires and tapes with untwisted filaments, coupling currents could
travel along long lengths (e.g. the km length in a magnet) and find a low crossresistance. The net effect would be that the multi-filamentary matrix would respond
F. Bordry et al.
is decreased by backing the superconductor with a matrix of a material with good
conductivity properties (e.g. copper, aluminium or silver). The discharge can be
made faster by reducing the inductance of the magnet, which is the reason why
large scale magnets are wound using cables with large operating current in the place
of single wires. There are various possibilities to dump the magnetic energy, based
on one or more of the following standard strategies:
• Energy extraction, in which the magnetic energy is extracted from the magnet
and dissipated in an external circuit (e.g. a dump resistor or diode);
• Coupling to a secondary circuit, in which the magnet is coupled inductively to a
secondary that absorbs and dissipates a part of the magnetic energy;
• Subdivision, a partition of the magnet in sections, with each section shunted by
an alternative current path (resistance or diode) in case of quench;
• Active quench initiation, that relies most commonly on heater embedded in the
winding pack, fired at the moment a quench is detected to spread the normal zone
over the whole magnet mass and thus reduce the peak temperature. Alternative
means to actively initiate quench have been revisited, based on over-current or
fast field changes induced by capacitive discharge in the coil.
8.1.3.4 Magnetization, Coupling and AC Loss
An ideal superconductor (type-I) tends to exclude field variations from its bulk,
i.e. a perfect diamagnetic behaviour. In practice, in the superconducting materials
listed above (type-II) the magnetic field can penetrate the bulk, still resulting
in partial diamagnetism. Macroscopically seen, a field change induces shielding
currents, which, in a superconductor, do not decay. For this reason these currents
are referred to as persistent. The magnetic moment per unit volume M associated
with persistent currents is proportional to the current density J C of the shielding
currents, and the characteristic size D of the superconductor, i.e. M ≈ J C D. This
magnetic moment can attain large values, perturb the field generated by the magnet,
and lead to instabilities in case the magnetic energy inside the superconductor is
dissipated in a process referred to as flux-jump. For this reason, the superconductor
in wires and tapes is subdivided in fine filaments that have characteristic dimension
in the range of 10 to 100 μm. Persistent currents and the associated magnetization
produce a significant field perturbation in accelerator magnets, and must be subject
to optimization and tight control.
Similar to the bulk behaviour described above, field variations also induce
shielding currents between the superconducting filaments. These currents couple
the filaments electromagnetically by finding a return path crossing the wire matrix.
The amount of filament coupling depends on the resistivity of the matrix, which
has to be low for good protection, and the geometry of the current loop. In the
extreme case of wires and tapes with untwisted filaments, coupling currents could
travel along long lengths (e.g. the km length in a magnet) and find a low crossresistance. The net effect would be that the multi-filamentary matrix would respond