MQXFS1e – PH-to-Coil hipot tests S. Stoynev and G. Chlachidze. AUP 20 March 2019

MQXFS1 at FNAL MQXFS1 - the first 150 mm aperture, 1.5 m long Nb3Sn quadrupole within the CERN and LARP collaboration Coils fabricated by CERN (#103 and #104) and LARP (#3 and #5) Five tests performed since 2016 MQXFS1a: Training, mag. measurements, PH tests, energy extraction MQXFS1b: (high azimuthal pre-stress) Training, mag. measurements, CLIQ and heater tests MQXFS1c: (high axial pre-stress) Training, MQXFS1d: (welded SS shell test ) Training, mag. measurements MQXFS1e: (Through busbar test) Magnet training, mag. measurements, spot heater tests MQXFS1d test preparations

MQXFS1 quench/trip history 119 spontaneous quenches 230 provoked quenches and trips (above or equal to 1 kA) 48 unintended trips (above or equal to 1 kA) Total of 8 thermal cycles (not counting the latest one for the hi-potting tests). Training history (20 A/s ramps)

MQXFS1 Outer Layer Heaters Heaters were fired in about 397 quenches and trips Only in some of them (~30) heaters were delayed up to 1 s Due to limited number of HFUs (power) not all heater strips were included in the magnet protection Peak power density in OL heaters varied between up to 200 W/cm2 PH to coil Insulation: 50 um of Polyimide, plus Cable insulation - 150 um of fiber glass, all impregnated in CTD-101k CERN coils LARP coils

Goals of the hi-pot test Verify heater to coil insulation soundness after the series of cold tests OL PH-to-coil hi-potting tests at 75 K, 150 K and room temperature in the helium gas Gradually decreasing the breakdown voltage in GHe Providing different safety margins (see V. Marinozzi’s presentation) If the polyimide insulation (Kapton layer) under the heater is cracked, the minimum distance between the coil and heaters could be as low as 0.2 mm 75K GHe 1 bar 104 275K GHe 1 bar 103 102 102

Test Preparations/Connections Hi-potting tests in GHe never have been done above 1 kV at VMTF All “weak” points identified, main concern was the current limiting resistors All voltage tap/SG/heater wirings were disconnected Eliminating the current limiting resistors All connectors, terminals were insulated Magnet leads also were disconnected from the main power leads Condensation on copper flags makes hi-potting difficult CLIQ leads (3) allow to bypass the resistor boards Part of the feedthrough in the dewar not potted CLIQ leads used for heater and coil connections The CLIQ lead from the magnet connected to CL2 OL heaters (8 strips) in coils 3 & 103 connected to CL3 OL heaters (8 strips) in coils 5 & 104 connected to CL1

Hi-pot Tests Cool-down to 1.9 K Warm up to room temperature Cold capacitance measurements (coil-heater and both to ground) Warm up to room temperature Heater-coil hi-pot at 75 K up to 800 V OK* Heater-coil hi-pot at 150 K up to 800 V OK Heater-coil hi-pot at 300 K up to 800 V OK Heater-coil hi-pot up to 2300 V OK Coil-ground hi-pot up to 1800 V FAILED at 1250 V Heater-coil hi-pot at 75 K up to 1500 V OK Heater-coil hi-pot at 150 K up to 1500 V CL1** OK, CL3 reached 1.5 kV but failed on arc at flattop Heater-coil hi-pot at 300 K up to 1200 V CL1 OK, CL3 failed at 970 V Heater-coil hi-pot at 300 K up to 1500 V CL1 failed at 1360 V * HipotUltra 7804, threshold was 10 uA, flattop 5-30 s **CL1: PH in coils 5/104, CL3: PH in 3/103

Backup Slides

1.9 K Measurements Capacitance measurements at 1.9 K Coil-to-heater at 1 kHz and 100 Hz: consistently 20.0 nF for CL1 and 20.5 nF for CL3 Coil-to-ground at 2 kHz: 13 nF with few nF fluctuations (worse for lower frequency) Heater-to-ground at 2 kHz: 10 nF with few nF fluctuations (worse for lower frequency)