A Prototype for Assessing the Field Homogeneity of the Iseult MRI Magnet

As part of the Iseult/Inumac project, the development of a 500 MHz whole body MRI magnet has been launched in 2006. This magnet with a central field of 11.7 T in a warm bore of 900 mm has outstanding specifications with respect to usual MRI systems. During the design process of this magnet, an innovative winding method using double pancakes has been made up which allows to reach the MRI required field homogeneity, i.e. 0.5 ppm peak-to-peak in a 22 cm diameter sphere. As the field homogeneity is a key issue of this project, a reduced scale prototype magnet has been designed in order to demonstrate the magnet manufacturing feasibility at the required accuracy, and to analyse the field quality of such a design taking into account the manufacturing errors. Dedicated tooling machines with on line measurement controls have been built with the objective of having a full geometrical description of each turn of each pancake. In addition to these toolings, an algorithm has been implemented in order to estimate the effect of the winding deformation, compared to the ideal pancake, on the field homogeneity. These results will be used to estimate the prototype bare magnet homogeneity before its cryogenic test. (authors)