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Megagauss fields at the LNCMI

The LNCMI Megagauss generator has been designed and constructed in the mid-90’s at the Humboldt University Berlin from where it was transferred to Toulouse in 2006. It features a 200 kJ modular capacitor bank that can be charged up to 60 kV. Its record field amounts to 331 T obtained in a STC with 3 mm inner diameter.

For scientific experiments, fields between 150 and 260 T with a pulse duration of 6 μs can be generated in coils featuring 8 to 15 mm diameter. Low temperatures are available depending on coil diameter: measurements at liquid Helium temperature require a STC with at least 12 mm diameter, capable of producing close to 190 T. A coil with 8 mm diameter still permits measurements at liquid Nitrogen temperature while producing a field of 260 T. Experiments can in principle be performed once every half hour.

Performance, reproducibility and operating conditions — relevant information for users

The magnetic fields that are routinely available at the LNCMI are listed below together with relevant parameters that determine their suitability for scientific experiments. Further information on each column as well as additional specifications are given below.

Peak field Pulse duration Bore diameter Temperature Damage risk
261 ± 4 T 5.6 ± 0.1 μs 8 mm 77 K 10 %
214 ± 3 T 6.3 ± 0.1 μs 10 mm 10 K 5 %
188 ± 2 T 6.4 ± 0.1 μs 12 mm 4.2 K 2 %
154 ± 2 T 6.4 ± 0.1 μs 15 mm 4.2 K < 1 %
114 ± 2 T 6.4 ± 0.1 μs 20 mm 4.2 K < 1 %

Peak field: The given values refer to the usual operating voltage of 55 kV. Fluctuations are mainly caused by the charging supply’s operating mode in conjunction with losses via resistance bridges and spray discharge. Once the nominal charging voltage is reached, the charging supply switches on and off in constant-current mode in order to keep the charging voltage within predefined margins. The latter give rise to variations of the peak field. In practice, this rarely represents a handicap as the magnetic field pulse is always recorded with a calibrated pick-up coil. For more information on field measurements see experimental techniques.

Pulse duration: The fact that the pulse duration is almost independent of the coil diameter is related to the design of the LNCMI generator, which does not resemble a simple oscillatory circuit. The pulse shape, however, changes considerably as shown below. Deviations indicated in the table represent a strict upper limit.

Bore size: The choice of a suitable bore diameter represents a trade-off between peak field and the possibility to accommodate the experimental set-up. Smaller coils are also more prone to damage. For more precise information on the available sample space see experimental techniques.

Temperature: The final temperature depends on the cryostat that can be accommodated by the bore. All cryostats are flow-type.

Damage risk: Damage can occur either accidentally or systematically. Accidental damage is mostly related to insulation failures near the coil that give rise to electric arcs. While causing considerable damage, such arcs occur rarely. Systematic damage is caused when both the rise time and intensity of the current exceed a critical threshold where considerable amounts of conductor material evaporate before the coil starts to expand. In this case, the metallic vapor trapped inside the bore tends to destroy the protecting tube of the cryostat and sometimes the cryostat itself. This phenomenon occurs primarily in smaller coils featuring higher current densities and slightly smaller rise times.

Synchronization and jitter: As a consequence of the microsecond timescale, the delay between the dispatching of a trigger signal from the control console and the onset of the actual discharge becomes non-negligible in Megagauss generators. The necessary synchronization between field pulse, data acquisition systems and other scientific equipment is achieved with the aid of a delay generator that issues a sequence of trigger pulses addressing different components of the set-up according to their respective response times. A crucial parameter in this context is the jitter that characterizes fluctuations in the response time of each component. The LNCMI generator is triggered with a jitter of less than 5 ns, which is largely sufficient for experimental applications.

Electromagnetic noise protection: Electromagnetic perturbations are primarily caused by the trigger signal that starts the arc discharge in the generator’s main switches. It consists of a 60 kV pulse with less than 10 ns risetime. As a general protection of electronic devices as well as for security reasons the generator is enclosed in a Faraday cage. To guarantee an optimum shielding efficiency, all remote control and data transfer lines communicating across this boundary are based on optical fibers and pneumatic tubes. The mains lines entering the cage are secured by an adequate isolation transformer and filter. In addition, detectors and data recording systems are battery-driven and separately mounted in Faraday cages so as to avoid any unnecessary exposure.

Preparation time: Typically 30 to 40 minutes are necessary after each shot to replace the STC and to dismount and reassemble cryogenic equipment and sample holders. Additional time may be necessary for adjusting experimental parameters such as temperature or optical alignment.

Security: The explosion of a STC gives rise to particle speeds well in excess of those obtained with explosive-driven projectiles. The coil is therefore enclosed in a solid fragment catching box featuring 5 mm steel plates on the outside and absorbers in all relevant areas on the inside. As a protection against high voltage accidents, the entire generator is enclosed in a Faraday cage as described above.

Technical implementation of the generator

to appear soon