Laboratoire National des Champs Magnétiques Pulsés CNRS – INSA

einforcement only reinforces in tangential direction, it does not reduces the axial stress in the wire). Such axial
reinforcement would be most likely metallic and could as such introduce extra insulation problems, but it is
worth to be tried.
higher fields for users
Higher fields for users make onlysense if a reasonable lifetime (>100 pulses at maximum field) can be obtained.
This is not only for reasons of coil efficiency but also because coil failures can damage the cryostat, data
acquisition equipment and the sample. A good way to increase the lifetime is to downgrade the actual user field
to 90% of the maximum field. This reduces the stresses by 20%. Moreover it turned out that every modification
of a coil design, although beneficial in the long term, normally reduces the lifetime of the first few coils that are
produced in that way. This means that modifications have to be performed in a prudent and systematic way and
preferentially tested on small coils.
rapid cooling and “high duty cycle” coils”
It is quite likely that a lot of gain (10-50 times) can be obtained in rapid cooling techniques, either by optimising
the existing technique or by using coil-construction techniques used for DC coils but now with liquid nitrogen
cooling. This latter approach sounds very interesting and could be very fruitful since the laboratory has merged
with the laboratory for DC fields in Grenoble. A minor concern might be the impedance of these coils since they
are normally designed for voltages much lower than 24 kV. But we believe that this problem can be solved.
Summary & Outlook: generator
The generators are working fine and do not need a lot of modification or maintenance. In theory the idea of
replacing the current limiting self-inductances by non-linear resistors (high-T c current limiters) seems interesting
and efficient (more energy available, less rise-time limitations), but this technique is not very mature and still
expensive, so it might be better to postpone its implementation for a few years.
14 MJ generator
The generator is now very stable and reliable. The aspects that could be improved, and should be if the number
of users increases, is the duration of the charging and specially the duration of commutation between the boxes
and the time it needs to change the polarity. This modification can be realised by installing pneumatic switches
and at the same time reducing the number of thyristor stacks and replacing them by optical thyristors.
Safety should always stay a major issue, and with the installation of more independent generators the interlock
and grounding should be redesigned.
6 MJ transportable generator
Within two years this generator should become available, either (preferred) by acquiring a turnkey solution or by
building the generator in house. This will greatly facilitate the test of medium size monolithic coils in the fieldrange
from 70 to 90 T and the operation of a dual coil system.
10