1. The fault occurred at the level of 400V-500V. Investigation of the fault to ground at the RCS.A34B2 circuit Between 3 and 12 September, 2014 we carried out the investigation of the short to ground which demonstrated itself as a voltage break in the RCS.A34B2 magnet circuits People involved : G. D’Angelo, P. Jurkiewicz, M.Martinez, Z. Natkaniec, W. Ostrowicz, D. Wojas

2. Measurement setup (Fig. 1) Investigation of the fault to ground at the RCS.A34B2 circuit

3. HV source - connected to spool no. 10 and to ground. Two 25m coax cables connected to two oscilloscope channels get signal from Vtaps D20_C1. Each end of the coax cables terminated with a special type of AC 50 ohm TERMINATOR CH1 connected to the right side of the fragment and used as a trigger CH2 connected between Vtaps D20_C1 and ground CH3 = CH1-CH2. Investigation of the fault to ground at the RCS.A34B2 circuit

4. It is based on measuring the time delay of the signal passing through the selected sections of the magnets. According to Figure 1, if the HV breakdown occurs on the right side of the sections, the falling edge of the signal first triggers CH1, and then after a time DELAY it appears in CH2. It results in a negative pulse in channel CH3. Investigation of the fault to ground at the RCS.A34B2 circuit Principles of measurements

5. If the HV breakdown occurs to the left of the sections, it will result in the falling edge of the signal which will first appear in CH2, and the subtractions in CH3 will give a positive pulse. Investigation of the fault to ground at the RCS.A34B2 circuit Principles of measurements In this way, by performing measurements at various positions along the tunnel, we have been able to find the location of the damage.

6. If the HV breakdown occurs to the left of the sections, it will result in the falling edge of the signal which will first appear in CH2, and the subtractions in CH3 will give a positive pulse. Investigation of the fault to ground at the RCS.A34B2 circuit Principles of measurements In this way, by performing measurements at various positions along the tunnel, we have been able to find the location of the damage.

7. Investigation of the fault to ground at the RCS.A34B2 circuit Results of the measurement in first investigation on spool no. 10 (between 3 and 5 September) Conditions of measurements: P He =1 bar in all half-cells, except 15L4/14L4 where P air =1bar

8. Test1: A16-CH2(blue) B15-CH1(yellow). M1(brown) = CH1-CH2. Breakdown to GND @450V. Conclusion: HV-break is on the left side of the measured section (the differential pulse is positive – pulse in CH2-blue is first - ). HV-break is not too close to the measured position. We could estimate the distance between the HV-break and the measured point on the basis of the sharpness of the edges of the pulses.

9. Test 2: C34-CH2(blue) A34-CH1(yellow). M1(brown) = CH1-CH2. Real breakdown to GND@450V. Conclusion: HV-break on the right side of the measured section (differential pulse is negative – pulse in CH1-yellow is first). HV-break is far from the measured position (the slope is mild). The delay between the pulses is about 228ns.

10. Test 3: B23-CH2(blue) C22-CH1(yellow).. M1(brown) = CH1-CH2. Breakdown to GND @500V. Conclusion: HV-break on the right side very close to the measured position (delay is still about 228ns).

11. Test 4: A22-CH2(blue) B21-CH1(yellow). M1(brown) = CH1-CH2. Real breakdown to GND@ 500V. Conclusion: HV-break on the left side very close to the measured position.

12. Test 5: C22-CH2(blue) A22-CH1(yellow). M1(brown) = CH1-CH2. Real breakdown to GND@ 500V. Conclusion: HV-break is between the probes (between CH1 and CH2) exactly on the position (delay is shortened dT~150ns)

13. Investigation of the fault to ground at the RCS.A34B2 circuit Final conclusion: HV-break occurs in half-cell 22L4 between magnets A22L4 and B22L4. The exact location of the damage can be estimated from the structural drawings of the corresponding magnets. For safety reasons it is recommended to apply a bypass to both magnets B22L4 and A22L4

14. Results of the measurement in second investigation on spool no. 9&10 Results of the measurement in second investigation on spool no. 9&10 after opening A22L4-B22L4 Previous Conditions of measurements: P He =1 bar in all half-cells, except 15L4/14L4 where P air =1bar Current Conditions of measurements: P He =1 bar in all half-cells, except 15L4/14L4 and 22L4 opened @QBBI.A22L4 where P air =1bar

15. Test 1: C22-CH2(blue) A22-CH1(yellow).. Conclusion: When we monitored spools #10, we observed HV-break on spool # 9 far from 22L4. The pulse does not approach zero ​​ in a specified time. After opening interconnection in the position 22L4 HV-break disappeared, and it appeared on the spool no. 9 Measurement at the -22L4 position of the detected HV-break. Initially, only spool #10 was cut here. The CH1 connected directly to spool #10

16. Test 1a: A19-CH2(blue) B18-CH1(yellow). M1(brown) = CH1-CH2 Real breakdown to GND@ 450V. Conclusion: HV-break on the right side very close to the measured position. Signal in CH1 (yellow) is first, subtraction pulse is negative. We examined the magnet chain, performing measurements in different locations using the method discussed above. The test results given below are confined to the last tests on the location close to the located failure. CH 2CH 1 QC19B19A19Q18C18B18AQ17C17B17A MCS _9

17. Test 2a: B18-CH2 (blue) C17-CH1 (yellow). M1(brown) = CH1-CH2 Real breakdown to GND@ 450V. Conclusion: HV-break on the left side very close to the measured position. Signal in CH2 (blue) is first, subtraction pulse is positive. CH 2CH 1 QC19B19A19Q18C18B18AQ17C17B17A MCS _9

18. Test 3a: A19-CH2(blue) C17-CH1(yellow). M1(brown) = CH1-CH2 Real breakdown to GND@ 450V. Conclusion: HV-break point is inside the measured section of the magnets, close to the centre of the measured section. Pulses CH1 and CH2 practically overlap. A) Real Pulse (from real break) CH 2CH 1 QC19B19A19Q18C18B18AQ17C17B17A MCS _9

19. Conclusion: HV-break occurs in the half-cell 18L4, most likely in the magnet B18L4. An accurate short position can be calculated based on the actual mechanical dipole drawing. B) Artificial Pulse (from artificial break - HV=200V). Artificial short on B18L4. In order to determine the exact place of the break, we made an artificial short on the position B18L4. The resulting signal on the oscilloscope is almost identical to the real short-circuit. Test 3a: A19-CH2(blue) C17-CH1(yellow). M1(brown) = CH1-CH2 Real breakdown to GND@ 450V.

20. After opening the interconnection, the short on position 22 L4 disappeared permanently and it was highly probable that the situation would happen again in position 18L4. Accordingly, we tried to overtake this problem and force the same conditions as those existing in the cell 22L4 (that is with the pressure equal to 1 bar of air), but before opening the interconnection. After opening the cryogenic valves in half-cell 18L4 and providing (air -1 bar) the proper conditions, we performed an HV test. This time the HV test passed, which means that the short disappeared. After restoring conditions of 1 bar helium the short circuit occurred again in half-cell 18L4 at about HV = 400V. Activity after establishing a HV short circuit @ spool 9:

21. Conclusions: Similar phenomena occurred in both cells 22L4 and 18L4. A short circuit appears at cryogenic conditions (helium = 1 bar) and disappears in the air conditions. Recommendations: In the 18L4 we suggest repeating tests at conditions: helium 6 bar. Perform bypass on both half-cells 18L4 and 22L4. Activity after establishing a HV short circuit @ spool 9: