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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

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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)

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Expected dose

Fynbo, A.C. and Stevenson, G. “Annual doses in the standard LHC ARC sections, ” EngineeringSpecification LHC-S-ES-0001, 6.12.2001.Fynbo, A.C. and Stevenson G., “Radiation environment in the dispersion suppressor regions of IR1 andIR5 of the LHC,” LHC-Project Note 296, 27.2.2002.

Critical areas will be the dispersion suppressor regions after some years of LHC operation.

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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

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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

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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

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Radiation tests – results II

Increase of forward bias voltage Significant recovery after partial annealing

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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

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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