Radiation Hardness of Cold By-pass Diodes

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

2. 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)

3. Expected dose Critical areas will be the dispersion suppressor regions after some years of LHC operation. Fynbo, A.C. and Stevenson, G. “Annual doses in the standard LHC ARC sections, ” Engineering Specification LHC-S-ES-0001, 6.12.2001. Fynbo, A.C. and Stevenson G., “Radiation environment in the dispersion suppressor regions of IR1 and IR5 of the LHC,” LHC-Project Note 296, 27.2.2002.

4. 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 VTO(T = 1.8 K) ≈ 6 V • Reverse blocking voltage VBR(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

5. 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 1012 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 1013 n cm-2 • Mixed, more LHC like radiation spectrum • Both test facilities are de-commissioned since several years

6. 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

7. Radiation tests – results II • Increase of forward bias voltage • Significant recovery after partial annealing

8. 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

9. 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