Field Stabilization of an MRI Magnet Operating in Driven Mode

In the framework of the Iseult/Inumac project, the development of a 500 MHz whole body MRI magnet has been launched in 2006. This magnet has outstanding specifications compared to standard MRI systems in that the central field is 11.7 T with a warm bore of 900 mm. This magnet will operate in driven mode, i.e. the magnet is permanently connected to a power supply. As the field stability needed for MRI imaging requires a field drift of less than 0.05 ppm/h, it is hardly feasible to directly transpose this requirements in the power supply specification. As a first step, existing solutions for other applications have been found in literature. Two of them have been selected as potentially applicable to our project: one using a semi-persistent mode, and the other one using a short-circuited superconducting coil in the inner bore. For each solution, an experimental assessment has been done on a very small 7 T magnet. The objectives of these tests are to get some experimental inputs for the achievable stability in order to design and to build reduced scale prototype of components required to apply these stabilization methods on the full scale magnet. We will present the results of the two stabilization methods and their extrapolations.