TE-MPE-CP, RD, LHC risk review 06-Mar R. Denz TE-MPE-CP Radiation Hardness of Cold By-pass Diodes Acknowledgements: D. Hagedorn (former project engineer – cold diodes) Reference: LHC Project Report 688

TE-MPE-CP, RD, LHC risk review 06-Mar Radiation hardness of cold bypass diodes  Cold diodes act as by-pass elements in case of a main magnet quench. –Installed inside the magnet cryostat relatively close to the beam tubes and exposed to radiation resulting from beam-gas interactions and proton losses.  Radiation induced damage affects several diode parameters –Turn-on voltage  late turn-on may cause damage to protected magnet –On-state resistance  burn-out of diode Mechanical support structure will prevent an opening of the diode circuit –Reverse breakdown voltage (also external breakdown)

TE-MPE-CP, RD, LHC risk review 06-Mar Expected dose Fynbo, A.C. and Stevenson, G. “Annual doses in the standard LHC ARC sections, ” Engineering Specification LHC-S-ES-0001, Fynbo, A.C. and Stevenson G., “Radiation environment in the dispersion suppressor regions of IR1 and IR5 of the LHC,” LHC-Project Note 296, Critical areas will be the dispersion suppressor regions after some years of LHC operation.

TE-MPE-CP, RD, LHC risk review 06-Mar Diode type selection  LHC cold by-pass diode is a specially developed high current diode of the diffusion type –Used for the protection of all MQ and MB type magnets in LHC Turn-on voltage at V TO (T = 1.8 K) ≈ 6 V Reverse blocking voltage V BR (T = 1.8 K) ≈ 250 V –The development of the diffusion type diode is based on type testing of numerous prototype and pre-series diodes. –Final design is a compromise between the required radiation resistance, the highest possible reverse blocking voltage and a reasonable yield for mass production in industry. –General use of more radiation hard epitaxial diodes has been discarded Low production yield for 75 mm wafer Low reverse blocking voltage  high risk of damaging the diode during assembly and test 80 spares available for as replacement of quad diodes in dispersion suppressor areas

TE-MPE-CP, RD, LHC risk review 06-Mar Radiation tests  Small sample tests at T = 4.6 K at the low temperature irradiation facility of the research reactor FRM I in Munich –Irradiation position inside reactor core –Turn-on voltage –Annealing effects (warm-up to room temperature) –1 kGy, 2 x n cm -2 –Nuclear reactor radiation spectrum  Sample test at T = 77 K and T = 300 K in the CERN radiation test facility in the north target area TCC2 –On-state resistance –Reverse blocking voltage –Annealing effects (warm-up to room temperature) –2 kGy, 3 x n cm -2 –Mixed, more LHC like radiation spectrum  Both test facilities are de-commissioned since several years

TE-MPE-CP, RD, LHC risk review 06-Mar Radiation tests – results I  Development of the turn-on voltage as a function of the radiation load depends strongly on the diode design (= doping levels) = close to series device

TE-MPE-CP, RD, LHC risk review 06-Mar Radiation tests – results II  Increase of forward bias voltage  Significant recovery after partial annealing

TE-MPE-CP, RD, LHC risk review 06-Mar Supervision of cold by-diodes in the LHC  Only online accessible device parameter is the voltage drop across the diode –Measured by several quench detection systems (magnet, bus-bar, symmetric) using all available voltage taps –Sampling frequencies 5 Hz (normal operation) and 200 Hz (magnet quench) –Data acquired during magnet quenches allow determination of turn-on voltage and on-state resistance  Radiation monitoring –RADMON system Ionising dose, neutron and hadron fluence –Data from BLM

TE-MPE-CP, RD, LHC risk review 06-Mar Summary  Cold by-pass diode used for the protection of LHC main magnets based on special radiation tolerant design  A failure of a by-pass diode will cause significant accelerator down-time (weeks)  Annealing (even partial) will prolong the lifetime of the by-pass diodes  Post mortem data recorded during magnet quenches carefully to be evaluated  Radiation monitoring essential to identify hot spots in due time  Pre-emptive maintenance during LHC shutdown periods  Additional spares to be ordered now as knowledge about production risks to get lost