[en] A novel cryogenic interferometric fiber optics sensor for the measurement of strain and stress in the coil windings of superconducting accelerator magnets is described. The sensor can operate with two different readout sources, monochromatic laser light and white light respectively. The sensor head is built up as an extrinsic Fabry-Perot interferometer formed with two cleaved fiber surfaces, and can be mounted in several configurations. When read with laser light, the sensor is an extremely sensitive relative strain or temperature detector. When read with white light the absolute strain and pressure can be measured. Results are presented of tests in several configurations at 77 K and 4.2 K, both for the relative and absolute readout method. Finally, the possible use for quench localization using the temperature sensitivity is described

[en] The degradation of the critical current of several impregnated and insulated Rutherford type Nb₃Sn cables is investigated under an applied transverse load and magnetic field. The cables are made of a new TWCA modified jelly-roll strand material which has been optimized for an extremely high critical current density. The cables have various keystone angles and compaction factors and the dependency of the degradation on these parameters is described. The voltage-current characteristics are determined for the magnetic field ranging from 5 to 11 T and pressure up to 250 MPa. A micro-analysis of the strands is performed before and after the tests to determine possible damage to the filaments. It is found that the initial degradation and subsequent sensitivity to transverse pressure show a strong dependence on the compaction at the narrow edge during cabling. For a cable in which the strands are deformed by 33%, an initial degradation due to cabling is found to be 67%, with a relative degradation of 25% at 150 MPa. For a rectangular cable with a deformation of only 11%, the initial damage is less than 2%, with 3% degradation at 150 MPa. Moreover, the heavily compacted cables tend to show irreversible damage, whereas the less compacted cables show a full recovery

[en] The thermal conduction of glass-mica tape, used as the insulation material for superconducting Nb₃Sn cables in fully impregnated windings for accelerator type of dipole magnets, has been investigated. Extremely low values for the thermal conductivity coefficient, ranging from 5 to 20 mWm^-1K^-1 at 4.2 K are found. The bad bonding properties of the mica flakes in the tape is the main reason for this behaviour. Moreover the temperature rise in the windings due to beam losses is studied. Beam-loss calculations show a maximum heat deposit of 10 mWcm^-3 which can lead, assuming the low thermal conductivity found, to an intolerable temperature increase of 1.5 K. Therefore, the thermal properties of the Nb₃Sn windings have to be improved