The Eight Tesla Test Station (SEHT) has been installed in Building 128 as part of the Iseult high field MRI magnet project. The purpose of the test station is to evaluate and confirm developments in 1.8 K cryogenics, the electrical supply circuit, control and instrumentation, and the protection of the magnet in the event of a transition. The test station now provides SACM with a wide diameter test area with a field of 8 T at ambient temperature.
The SEHT station uses a solenoid winding with an NbTi conductor assembled from double pancake coils and cooled in a superfluid helium bath at 1.8 K and 1.2 bar. The SEHT solenoid is a reconditioned superconducting coil previously used in the 35 T hybrid magnet at LCMI in Grenoble.
The station has been installed in Building 198 close to the CELLO helium refrigerator already in use for the SCHEMA station. The liquid helium and nitrogen supplies to the two stations, which are capable of operating simultaneously, are provided from a cryogenic valve box adjacent to the refrigerator. Using the same cryogenic concepts as the Iseult project, SEHT includes a transfer line feeding a cryogenic satellite containing all the components and instrumentation needed during initial cooling and maintenance at 1.8 K, together with the electrical supply to the 8 T coil.
The magnet cryostat has a single connection to the satellite via a heat pipe which provides an almost perfect superfluid helium heat drain between the pumped helium bath cold source in the satellite and the helium tank enclosing the superconducting winding. Initial cooling is provided by a dedicated loop carrying cold helium at high pressure from the CELLO refrigerator to the cold masses. The magnet tank is finally filled with superfluid helium by condensation followed by subcooling of the helium in the satellite.
Following verification of the cryogenic performance (initial cooling period from 4.2 K to 1.8 K = 26 hours; measured losses = 9.5 W at 1.8 K) in mid 2008, live tests were carried out to demonstrate the correct operation of the magnet and its protection system. At the end of 2008, tests were carried out to trigger a transition in the superconductor by local heating in the winding. These tests were used to characterize the mechanisms governing the propagation of the normal zone, and to measure the resulting pressure rises throughout the system.
Examples of the pressure rises measured in the magnet during triggered quenches. The various pressure rises were triggered under different operating conditions of temperature and current.
In 2009, the station was used (in collaboration with IRAMIS for the NMR measurements) to carry out successful tests on two systems enabling improvements to be made to the stability of the magnetic field under steady state supply conditions. The first of these consisted of a superconducting coil placed inside the main solenoid and inducing an opposing current to cancel out any fluctuations in the main electrical supply. The second system was installed in the satellite and consisted of a resistor in parallel with the main winding together with a current limiter limiting the current through the superconductor. This combination damps out any variations in the supply current.
The station is now available for experiments requiring a high magnetic field and will be used during 2010 and 2011 for taking measurements of a reduced scale prototype of the Iseult magnet.
Last update : 12/20 2013 (3059)