B. Szeless

Heat transfer in electrical insulation of LHC cables cooled with superfluid helium

The electrical insulation of the Large Hadron Collider (LHC) cables constitutes a thermal barrier between the conductor and the superfluid helium bath. This can prevent removal of the heat dissipated in the cable by the current rise in the dipoles or by the beam losses. The main experimental results, obtained with stacks of insulated conductors representing a piece of the actual coil, are given. The mock-ups vary only by the material composition and the structure of the electrical insulation. Analysis of the temperature distribution measured in the conductors as a function of the dissipated heat power makes it possible to determine the dominant heat transfer mode in each type of tested insulation and to classify these according to their permeability to superfluid helium. Thermal numerical modelling of the experimental mock-ups clarifies the heat transfer path in the complex structure of the insulation and enables calculating values of the thermal quantities characteristic of each insulation. The results of these studies have led to the choice of the cable insulation of the LHC magnets.

Successful high power test of a proton linac booster (LIBO) prototype for hadrontherapy

The linac booster (LIBO) project aims to build a 3 GHz proton linac to give the beam from 50-70 MeV cyclotrons, which exist in several laboratories and hospitals, a final energy of 200 MeV. This will allow the treatment of deep-seated tumours. A prototype of the first LIBO module was designed, constructed and RF tested by a collaboration of CERN, University and INFN of Milan, University and INFN of Naples, and the TERA Foundation. Low power RF measurements have shown good field uniformity and stability along the axis of the four tanks composing the LIBO module. In December 2000, full power RF measurements at a repetition rate of 100 Hz have been performed at CERN. After a very short conditioning period, an accelerating gradient approaching 30 MV/m has been easily achieved in the tanks, well above the nominal 15.8 MV/m. The particularities of the design and the reasons for the successful performance are discussed.

Characterisation of net type thermal insulators at 1.8 K low boundary temperature

The Large Hadron Colliders superconducting magnets are cooled by superfluid helium at 1.8 K and housed in cryostats that minimise the heat inleak to this temperature level by extracting heat at 70 K and 5 K. In the first generation of prototype cryostats, the radiative heat to the 1.8 K temperature level accounted for 70% of the total heat inleak. An alternative to enhance the cryostat thermal performance incorporates a thermalised radiation screen at 5 K. In order to avoid contact between the 5 K radiation screen and the cold mass, insulators are placed between both surfaces. Sets of commercial fibre glass nets (spacers) are insulator candidates to minimise the heat inleak caused by any accidental contact between the two temperature levels. A model to estimate their performance is presented. A set-up to thermally characterise them has been designed and is also described in the paper. Finally, results as a function of the number of nets forming the spacer, the boundary temperatures and the compressive force in the spacer are presented.