Magnet and Magnetic Fields Phase III Flight Safety Review January 12, 2010
AMS-02 Magnet • Experiment core is a 0.8T superconducting magnet used to bend the trajectories of incoming particles. • Observation of the direction of the bend allows the sign of the particle charge to be determined. • AMS-01 used a permanent magnet with a similar field structure.
Magnetic Field Design • Majority of the field strength comes from two large dipole coils. • Twelve smaller racetrack coils used to minimize external field levels while maintaining high strength in the magnet bore. • External field drops rapidly as distance from Vacuum Case increases. Racetrack Coils (2x6) Dipole Coils (2x)
EVA/EVR Effects from AMS-02 Field • AMS-02 reviewed magnetic field limits on EVA/EVR hardware. • US EVA hardware (SAFER, PGT, etc) verified by test to function in a 300 G magnetic field. • Orlan suit requirements allow operation in a 175 G magnetic field. • SPDM requirements allow operation in a 10 G magnetic field. • Field maps developed for each of these limits and projected field measurements provided to MAGIK team for assessment.
MAGIK Magnetic Field Assessments • MAGIK reviewed AMS-02 magnetic field map against all nearby planned and contingency operations. • AI #1321 confirmed that field did not impact MT translation, even with SPDM on board. • AI #1608 confirmed that all S3 H-fixtures could be reached without SPDM entering KOZ. • AI #1808 confirmed that all S1/S3 ORUs could be reached by SPDM without entering KOZ. • AI #2373 confirmed that all outboard ORUs on ELC2 could be removed and GF operated by SPDM without entering KOZ.
AMS-02 Effect on Plasma Environment • AMS-02 provided magnetic field and operational data to the ISS to assess potential effects on plasma field. • AMS-02 magnetic field will adjust potential levels around the S3 truss. • AMS-02 gives off small amounts of Xenon, which is an ionizable gas. • ISS assessment showed none of AMS-02’s effects on the plasma field led to hazardous conditions.
Other External Field Assessments • AMS-02 Project Office has provided data for several additional assessments. • Magnetic torque provided to ISS GN&C for review and determined to be well within CMG capabilities. • Potential field effects on magnetic switches in MT shown to be negligible. • Field shown to have no effect on visiting vehicles and US Lab. • Overall ISS Program approval documented in TIA #310. • Waiver to SSP 57003 in signature cycle.
Field Strength Changes Since Phase II • All field strength maps used in the previous assessments are based on a magnet current of 460A. • Based on operational experience, nominal operating current is now 400A, and magnet avionics prevent system from being charged above 424A. • Dipole moment is a function of the current: • 424A current means 8% reduction in field strength. • 400A current means 13% reduction in field strength. • Field strength maps have been verified by measurement during magnet commissioning at CERN.
Superconductive Quench • “Quenching” is the sudden loss of superconductivity in part or all of the magnet leading to a collapse of the magnetic field. • Energy in magnetic field is converted to heat. • Quenching is off-nominal behavior and not expected to occur, but most likely during charging or discharging. • Since Phase II, several modifications have been made to charging process to reduce the risk of AMS-02 quenching.
Safety Assessment of Quenching • Three potential concerns associated with quenching. • Rapidly collapsing field could generate eddy currents in nearby metallic structure with physical loops, creating additional forces on the structure. • If coils ramp down unevenly, magnetic field could develop localized lobes of higher-than-nominal strength. • Dispersing field energy in single location could lead to localized overheating and damage. • AMS-02 system has been assessed for all three issues and shown to present no safety hazard.
Eddy Currents • Several items within AMS-02 were identified which could structurally support eddy currents. • Magnet developer calculated potential currents which could be induced in structures exposed to the high field in the bore of the magnet. • Maximum induced load was found to be ~60 lb, far less than the same structure sees during launch. • Field levels outside the bore are multiple orders of magnitude lower, so additional assessments are not required.
Magnet Discharge Possibilities • During a commanded ramp-down, the magnet circuit is connected to nine large dump diodes mounted outside the VC. Heat is dispersed via the diodes into the USS-02. • If the onset of a quench is detected, the magnet avionics will automatically initiate an “assisted quench” by turning on heaters buried inside the magnet coils to quench all of them simultaneously. Heat is dispersed evenly across the magnet. • Superconducting wire is clad in a pure aluminum sheath and all coils are connected in series. Should quench detection system fail to operate, coils will still discharge simultaneously. Heat will be dispersed across the overheated coil.
Magnet Avionics Changes Since Phase II • During commissioning, it was determined that risk of quench was reduced by reducing rate of current change. • Magnet avionics redesigned to limit the rate of current rise during charging. • One bank of dump diodes removed to limit the rate of current fall during discharging. • Team also observed occasional voltage irregularities during charging/discharging. Cause was determined to be flawed joint in the current leads.
Original Current Lead Configuration Joint between flat section and disconnect
Flat Section to Disconnect Joint Cu Al Cu Al Al Al
Current Lead Design Review • Current leads had pure copper blocks soldered directly to copper-clad aluminum. At cryogenic temperatures, differential thermal contraction caused the soldered joint to crack. • Joint redesigned to reduce loads on soldered connection. • Copper blocks replaced by copper-clad aluminum. • Aluminum bolts added to take structural loads from relative motion. • Soldering process requalified to ensure nearby items not overheated during repair. • Entire system was requalified after repair and everything has performed as intended.
Conclusion • Full details of the magnet systems are included in the Safety Data Package • Magnet structure and operations described in sections 5.1.1 and 5.1.2. • Cryomagnet Avionics Box (CAB) described in section 5.12.4. • Torques caused by dipole field included in Hazard Report #6. • Magnetic field issues included in Hazard Report #7