Laboratoire National des Champs Magnétiques Pulsés CNRS – INSA

Scientific projects involving the Toulouse STC (also called MegaGauss) include the infrared
spectroscopy of carbon-based nanostructures. Key issues for this class of materials are manifestations
of the Aharonov-Bohm effect with and without quantum confinement, the breakdown of the quasirelativistic
regime in graphene and electron-hole asymmetries in the same material. We also envisage
measurements on diamond whose low mobility forestalls the resolution of resonant transitions unless
very high magnetic fields are applied. Another important class of materials, whose low mobility has
foiled most attempts to investigate their electronic bandstructure, is that of organic conductors. We
plan to investigate these systems either in-house or in collaboration with various national and
international groups who have already expressed their interest in using the Toulouse STC.
High magnetic fields for neutron and X-ray scattering (ESRF/ILL collaboration)
Neutron and X ray scattering are often used to characterize magnetic structures of materials, in
particular at high flux large facility sources like the Institut Laue Langevin (ILL) and the European
Synchrotron Radiation Facility (ESRF). The maximum magnetic field available at such facilities is
typically around 15 T, provided by dedicated superconducting magnets, but a strong scientific need
exists to extend this value to much higher fields, in order to study high field phenomena that have been
identified in high field facilities. The LNCMI has been engaged for several years already to provide
much higher fields, 30 T or more, for experiments at these two facilities.
The LNCMI-T has developed and constructed mobile high voltage capacitor banks (initially 150 kJ,
since 2009, 1 MJ) and the corresponding pulsed field magnets to operate on beam lines at the ESRF
and the ILL. Both large scattering angle solenoidal magnets and a radial access split coil magnet have
been constructed and operated, together with the corresponding cryogenics, and the first scientific
results obtained with this approach validate its usefulness and confirm the scientific case for high
magnetic fields on neutron and X ray beam lines. The ESRF has now partly dedicated one beam line
(ID6) to operate with the LNCMI mobile pulsed field installation. The mobile generator has also been
used at the kilojoule laser source at LULI (Palaiseau) and its use at SOLEIL and at the SLS
(Switzerland) is being considered. This strategy will be further pursued in the future, aiming for even
higher fields and larger duty cycles in order to more efficiently use the beam time at such facilities that
has a very limited availability. Part of the upgrade of the LNCMI-T, funded by the CPER Midi
Pyrenees 2007-2013, will provide fast capacitor bank modules totalling 6 MJ. These will be
constructed and operated in sea containers, so that they can also be moved to other facilities, like ILL
and ESRF, to perform pulsed field measurements. With such a configuration, fields in excess of 60 T
are feasible at such facilities, opening entirely new possibilities for experiments.
However, not all scattering experiments provide sufficient signal to noise to give meaningful results
with a pulsed field installation and a beam time of typically one week. Therefore, there is a clear need
to install very high static magnetic fields on these beam lines. To define the technical basis of such an
installation, a design study has started in 2007, in the framework of a work package in the FP7 design
study ‘ESRFUpgrade’, uniting ILL, ESRF and the LNCMI. The conclusion of this design study is that
the electrical power and the corresponding cooling capacity (40 MW) can be made available at the
ILL-ESRF site. In the same work package two DC resistive magnets designs have been considered in
detail:
1 - a horizontal field magnet suitable for scattering and absorption experiments. The design is
derived from the actual 35 T/34 mm vertical field magnet operating at the LNCMI-G using
longitudinally cooled helix technology
2 - a split magnet design. An innovative design has been proposed and studied that takes
benefits from the experience of the LNCMI in operating radially cooled magnets. In this configuration,
the high heat transfer coefficients required to cool high field magnets are obtained in radial channels
arranged between the magnet turns. Consequently, the innermost windings can be cooled more
efficiently than traditional longitudinally cooled windings. It is then possible to increase current
densities in the innermost windings resulting in compact magnet designs. This is of primary concern,
to be able to implant the magnet in the limited space available for instrumentation on neutron or X ray
beam lines. Moreover, the main cooling water stream flows in the direction parallel to the mid plane
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