Abstract
Measurements of length change in glass-cloth laminates FRP insulator of superconducting magnets for fusion reactors were made after reactor irradiation at 5K in directions both parallel and perpendicular to the fibre axis and during subsequent annealings in the temperature range of 10-320K.
Measurements of the length change were carried out by changes in capacity using a ac ratio bridge. The fractional changes of length induced from the irradiation in the direction perpendicular to the fibre axis Δl/l was estimated to be 2.3×10-3 for a dose of 4.5×108 Rad and this corresponds to about 40 times larger than that of pure Al after reactor irradiation at 5K and also to 70% magnitude of thermal contraction of pure copper between room temperature and 4.2K. A small recovery of length change in the direction parallel to the fibre axis below 100K was followed by a large recovery around 200K. Reverse recovery of the length change in the direction perpendicular to the fibre axis was found above 200K.
When those insulators are exposed by the irradiation under the operating condition of the superconducting magnet for fusion reactor, they experience two kinds of stress, one is due to the magnetic field itself and the other to the effect of the irradiation. The length change introduced by the irradiation will give rise to mechanical disturbances, for example, slip, debond and crack, and to deleterious changes in the engineering properties of the magnet.
These disturbances due to length change in the insulator and the reverse recovery of length change are discussed.
