1. COBRA Status W. Ootani MEG Review Meeting Jul. 18th, 2007

2. Quench Problem (slide from last meeting) We had two quenches during the beam time in last December. Dec.16th, 21:30: COBRA ON Dec.19th, 19:05: First quench (cooling water problem?) Dec.21st, 12:00: Second quench Dec.22nd, 7:00: COBRA OFF No (fake) quench for the past 8 months! External noise was observed in the quench detector signal line. Much faster than the previous one New noise source? inside the MEG system? What measures can be taken? Modify shielding and grounding scheme Different method to detect quench? SQD? A few good things to know... Nothing was damaged by the quench. Quick recovery (~2.5hrs) thanks to newly installed TMP 200msec 100mV

3. The shielding and grounding scheme was improved after the December run. There hasn’t been no unexpected shutdown since then. No shutdown for ~10 days with SC+NC+Bypass during shutdown period. No shutdown for ~10 days with SC + (BTS) after accelerator started to run. The problem by the external noise seems fixed. Quench Problem Status

4. Another Measure for External Noise Quench test to investigate a possibility to loosen the threshold for the quench detection. Current threshold: 100mV x 50ms Idea: Loosen the voltage threshold and compensate the response delay by tightening the time threshold a little. Test procedure Induce quench by firing a heater attached to the coil. Check the quench propagation in the coil (the induced voltage, temperature rise,...) Start the test at partial current and increase the current step by step. The test was success! No noise could have fired the quench detector with 500mV x 40ms. 100mV x 50ms200mV x 40ms500mV x 40ms Max. Voltage1261V1188V1237V Max. Temperature110K109K110K Original

5. Overheat problem of compensation coil power supply Caused by a blockage in cooling water tube(s) inside the power supply Electrical decomposition of Cu pipe. The tubes were cleaned up by using chemicals (H 2 O 2 ). Instability of compensation coil power supply New Problems... ✔

6. Instability of Compensation Coil Power Supply A new problem happened during CW accelerator testing. COBRA was unexpectedly shutdown after 40 hrs operation. Still under investigation, but it seems there was a problem with NC power supply according to the measured waveform. Problem of primary power line? Malfunction of control circuit? Failure of electrical parts in the main circuit? Thyristor rectifier, transistor, free-wheeling diode, filter capacitor...