Contact Resistances in the Cold Bypass Diode Leads of the Main LHC Magnets

The main superconducting dipole and (de)focusing quadrupole magnets of the LHC are equipped with cold bypass diodes to assure the passage of the current outside the magnets in case of a magnet quench. Each magnet has a parallel bus bar circuit with a diode that is connected via resistive bolted connections. Heat sinks are pressed to the wafers under a constant load to assure a good thermal and electrical contact in order to avoid overheating of the wafers. During the series production, all the diodes of the LHC have passed the reception tests in industry. However, during high current testing in the LHC, some of the diode leads showed an anomalous resistance, triggering an off-line verification of the design and assembly procedures supported by cold powering tests. This paper summarizes the measurement results obtained during the series production of the diodes before their installation in the magnets, complemented by a smaller number of specific tests performed at CERN in the past and recently.