John Matheson and Tony Weidberg

HV IR Summary

Draft 1 21/10/04

John Matheson and Tony Weidberg

This note gives a short summary of the investigations of the HV IR problems on the barrel LMTs. The problem was first found on B5 harnesses and it turns out that all the harnesses with high values of HV IR came from Elgoline batches numbers 5 and 6.

The high values of HV IR can be reduced to a low value by either performing a bake out at 80C for one hour or by operating the HV at 500V for several hours (it was also checked that operation of the HV at 150V with a temperature of 0C to simulate ATLAS conditions is also satisfactory). For B5 harnesses with high HV IR values, bake out was used at RAL. It was discovered on some tapes that when the bake out was done with a hot air gun, then only a short length of the LMT was responsible for the HV leakage. This implied that there was some local contamination on the LMT. However when the HV IR values were checked for the harnesses some months after the first test, there were found to be 24 LMTs with low values of HV IR (< 1 GOhm). During this period the relative humidity in the RAL clean room had become very high. The HV leakages were all cured by operation of the HV at 500V.

Vlado checked the measured values of HV leakage at 500V performed by Elgoline and discovered that for batches 5 and 6, there were many more tapes which were rejected because they had a higher than normal HV leakage, although it was always less than the ATLAS spec of 1 GOhm. Further investigation by Vlado confirmed that there was contamination over short lengths of tapes from the bad batch. The contamination was on the back of the kapton on the upper LMT. Tests in Ljubljana showed that this contamination is not Ferric Chloride (used in the etching) but does contain potassium. The large values of HV leakage are then due to the ionic contamination becoming conducting in the presence of moisture. Therefore the HV leakage disappears on bake out which removes the moisture or HV operation which uses up the available ions.

All the harnesses can be operated in ATLAS conditions with very low values of HV leakage so the one remaining concern is if the contamination could lead to any auto-catalytic chemical reactions which would corrode the Al tracks. In order to test this hypothesis it was decided to do an accelerated aging test with LMTs operated at 500V in a moisture chamber at an elevated temperature. The first attempts to perform such tests with a B5 harness (one which showed a large HV leakage on B5) failed because the moisture created too much leakage current so that the HV supply current limited at a low voltage. Further unsuccessful attempts were made with this harness and in the end the Al was badly damaged by the moisture. This is of course not a surprising result as it is well known that water can corrode Al.

A better test was done with new LMTs from Elgoline. One was from the bad batch which had been rejected because it had larger than normal HV leakage (26 nA) and one was from a good batch. This time the ends of the LMTs with bare Al were potted to avoid moisture directly attacking the Al. The tapes were operated at 500V for one week at 80C and 100% R.H. This time we were able to maintain 500V over the full period. The leakage current increased to about 100 microA during the test but the leakage current decreased to ~ 0.1 microA after 1 hour HV operation after the LMTs were removed from the moisture chamber.

Inspection of the two LMTs showed that there was bad corrosion over a few mm length at the far end of the LMT to that which the HV was applied to. However the HV lines were probed and found to have a good connection from one end of the tape to the region next to the corrosion. Presumably moisture was able to reach the ends of the tapes which were corroded but could not reach the middle of the tapes. The HV line was examined under a microscope for both tapes. Apart from this one corroded area, there was no evidence for any corrosion on either the tape from the bad batch or the good batch. Some microscope photos are shown in Figure 1 to Figure 4.

Figure 1 Corrosion of HV line near the end of the LMT for the LMT from the bad batch.

Figure 2 Corrosion of HV line near the end of the LMT for the LMT from the good batch

Figure 3 HV line in the middle of the LMT from the bad batch.

Figure 4 HV line in the middle of the LMT from the bad batch.

Assuming a conservative value for the activation energy of 0.5 eV, that the LMTs will operate at close to 0C in the barrel and using the Arrhenius equation gives an acceleration factor of 123. This test is then equivalent to 7.4 years of ATLAS operation (with the usual assumption that ATLAS operates for 107 seconds/year).

Conclusions

There is contamination including potassium on two of the six batches of LMTs used for the barrel harnesses. If the LMTs are exposed to moisture there will be a significant increase in HV leakage. However this should not create any problems for ATLAS operation as this leakage is quickly cured by HV operation.There is no doubt that water vapour can corrode any exposed Al tracks and we must ensure that the SCT is kept in reasonably dry conditions throughout the operation. However there is no evidence for any additional sensitivity to corrosion caused by the contamination on the two batches of LMTs. Therefore there is no real motivation to remake these LMTs.

1