Pipetron at KEK

Pipetron is a 100 TeV p-p collider based on the new concept of transmission line magnet. A 75 kA sigle superconducting current produces 2T of magnetic field in the iron gaps in the both side. A very simple structure provides affordable construction of Very Large Hadron Collider (VLHC). KEK is collaborating with Fermilab in the effort of pipetron development.
Nb3Al conductor for pipetron
Nb3Al is usually used for high field but pipetron field at conductor is only 1T. However, if the temperature is high, the current density of Nb3Al become much higher than that of NbTi. By use of Nb3Al, cryogenic system of pipetron can be greatly simplified.
Nb3Al conductor structure
Although Nb3Al is stronger than Nb3Sn, it is still weak against stress on the conductor. One way of the conductor fabrication method is to mount superconductor on the copper spiral. Container of the conductor is an invar pipe. Thermal contraction of the cable is not a problem because the invar pipe does not shrink when the conductor is cooled by the liquid helium flow.
Nb3Al conductor production
A test cable was produced at Sumitomo Eleectric. It is going to be tested at Fermilab in the transformer current loop. Test production showed that invar is very stable in the high temperature heat treatment. The bonding among strands was not tight as expected. There could be problems associated with mechanical instabilities.
The copper RRR of the spiral after the heat treatment was measured at KEK.
Alternative conductor design
It was found that the diffusion bonding among strands is not strong even the conductor is processed at high temperature. Then a woven sleeve of conductor/copper can be flexible even after the heat treatment. Having superconducting sleeve at the outside of the invar pipe makes construction of the cabe and joint simpler. Superconductor, in this case, is cooled indirctly through the invar pipe by liquid helium. Better symmetry and complete transposition as well as better mechanical stability are the advantages. (Click the picture to blow-up.)
The brading machine available near KEK carries 32 bobbins and each bobbin can have up to 11 strands. The test braid with 6x32 strands was fabricated.
Welding Joint of the Invar Pipe
For the direct cooled conductor which is contained in an invar pipe, welding of the inver pipe at a joint could be a problem. Use of LASER welding is a promissing method for this. Photo shows the welding of stainles steel flange on to the invar pipe. 65W YAG LASER pulse of 65pps width 1.2 msec was given in air. The beam size was 0.6 mm. The welding bead was beautifully formed within the spot size and there was no apparent damage to the superconductor. The maximum temperature of the conductor may be less than 100C.
Transition Joint between Direct and Indirect Cooling
Since the present test loop is made by direct cooled cable, testing of indirect cooled conductor neeed a transition joint. Invar can be brazed to copper. We can make soldering joint strong enough for the pressure operation in the copper part of the pipe.