Proposal for a test to directly measure the RRR of a number of busbars of the main circuits

1. Proposal for a test to directly measure the RRR of a number of busbars of the main circuits Mike Koratzinos, Bob Flora 11 January 2010

2. The idea • Measuring the RRR of the copper in the busbars of the LHC has been proven difficult and we do not have an accurate answer for the moment. • The value of the RRR has a direct impact at the energy that the LHC can safely be operated on. • There is also a question of if we can measure the excess resistance at very low temperatures: • we have a much better tool in the nQPS compared to the old ‘Biddling’ method, • but we do not know what kind of temperature stability we can expect. • Certain aspects of temperature stability can be clarified with this proposal (i.e. what are the temperature gradients within a bus bar, and what are the temperature gradients between bus bars and temperature sensors)

3. The implementation • The proposal consists of: • Raising the temperature of a few magnets (two cryo subsectors) to 10-20K • Monitoring the resistance of busbar segments by means of: • Voltage reading using the nQPS • Current of 10-20A injected by means of a small independent power supply. • Noting the transition from superconducting to normal • The value of the full normal resistasnce gives the RRR • The length of the transition between s/c and normal gicves the temperature gradients accross the busbar • The

4. Transition s/c to normal Longitudinal temperature variation resistance 1/RRR The precision we can obtain for the RRR using this method is better than we will ever need temperature Transverse temperature calibration

5. RRR of cable • The method can measure both the RRR of the busbar segments and the RRR of the coil cable.

6. Prerequisites • Power: • Three controllable power supplies • Dipoles (1) 10A-300V • Quads (2) 10A-10V • Switch between I_min_op (1A) and I_nominal (10A) every few minutes (simple signal generator as trigger?) • Voltage measurements: • Use nQPS in logging mode • Temperature: • Should rise to more than 10K • Use standard logging of all available temperature sensors • Duration: • The minimum duration of the test proper will be as long as Cryo needs to raise the temperature to ˜ 10K (2 days?) and back down again (2 days?)

7. Repercussions • nQPS should be connected and in logging mode • We plan to use (after consulting with the power converter group) the three power supplies connected in parallel with the existing main circuit power converters usding the existing transductors). This has the advantage of • Minimal disruption in the main circuits (no disconnections at the DFB side, etc) • No work in the tunnel • Current will be measured and logged as I_MEAS in Timber. • Any non-instrumented temperature sensors should be connected to electronics cards

8. A proposal for the time and place • We propose to perform the test in Sector 81 • Advantage: this sector is the only that has all temperature sensors connected to electronics • Disadvantage: we have made no warm measurements in this sector • The test has been scheduled for the period 21/1/10 to 28/1/10 • This is after the nQPS cables have been connected and tested • This is after the phase I HC tests of the sector.

9. Safety • The setup we are proposing here has been tested already and approved. • The energy stored in the circuit is negligible • However, big voltages (few hundred volts) are present so we need to follow a certain procedure, to be defined

10. extra • The proposal can have an increased impact by changing some of the parameters – this is by no means essential but desirable: • A warmup to 25K+ might allow us to also measure excess resistance. It would be very useful to know if this is possible or not • More current (20A) will increase precision