The LHC’s Mike Lamont has just posted a preprint here based on a recent Moriond talk describing the current status of the LHC commissioning. Here is his discussion of the on-going campaign to identify bad splices and figure out what to do about them. From it, I gather that the big unanswered question is what to do about possible bad splices in sectors that have been cooled down, since warming up the sectors would significantly delay the scheduled startup.
One additional danger that has recently surfaced is a bad electrical contact between the copper of the busbar and the U-profile of the splice insert on at least one side of the joint. Combined with a bad contact between the cable and the copper this leaves the splice without an alternate route for the current in the case of a busbar quench – in a good splice the current can flow in the copper removing the danger of excessive resistive heating in the quenched superconductor. A good contact between the Rutherford cable joint is assumed (i.e. less that 2 nano-ohm).
Such situation can be detected by measurements at warm using low current and a nanovoltmeter across short segments of the machine. Under such circumstances the current flows in the copper and the resistance of a good joint is around 12 micro-ohm. Extensive measurements of the four warm sectors (May 2009) have revealed 16 segments with excess resistance of over 30 micro-ohm. The relevant interconnects have been opened. Individual splice measurements have revealed resistances of 30 – 50 micro-ohms. All such splices have been re-done and re-measured.
Warm quadrupole measurements started in May 2009. Measurements at at 80 K in sector 23 are also ongoing at this time. The measurements at 80 K are more difficult and show a lot more signal variation – the resistivity of copper falls by a factor of 7.5 at this temperature. The question of what to do if suspect splices are found at this stage of re-commissioning is to be addressed.
