FHCT SEB cross-section measurements at H4IRRAD and consequences for MKD reliability

1. FHCT SEB cross-section measurements at H4IRRAD and consequences for MKD reliability Viliam Senaj LIBD meeting 24/01/2012

2. Motivation • Crucial importance of reliable operation of the beam abort system for LHC security • Generators are installed in UA63/67 and will see up to 106 HEH.cm-2.year-1 in axis of cable ducts according to simulations • 30 MKD and 8 MKBH generators operate up to maximum voltage of ~ 29 kV at 7 TeV; 12 MKBV up to 16 kV at 7 TeV • 680 FHCTs in MKD and MKBH exposed to an average voltage od 2.9 kV; up to ~ 3 kV on several devices expected (due to spread of voltage sharing resistors value and FHCT leakage currents) • FHCT producers (Dynex & ABB) provide only basic SEB failure rate due to cosmic rays at sea level to be 100 FITat 2.8 kV (1 FIT = 1 failure in 109 device hours); failure rate at 2.9 kV and 3 kV is unknown • Failure of a FHCT will provoke an asynchronous dump with associated beam loses and machine down time necessary for generator replacement and system re-calibration (~ 1 day)

3. FHCTs used • ABB - 5STH20H4502 • DYNEX - DG648BH45-185 • Similar specifications from both producers: • Umax: 4.5 kV • Udc: 2.8 kV (100 FIT) • Imax: 80 kA • dI/dt: 20 kA/µs • Ileak: 10 µA @3kV • Load integral ~1.106 A2s • wafer diameter ~ 60 mm

4. SEB failure rate estimation • 680 FHCT with up to 6000 hours per year at maximum average voltage of 2.9 kV • According to simulations the maximum fluence of 106HEH.cm-2.year-1in cable duct axis • Estimation of number of failures per year for 1/3rd of FHCT at 3 kV, 1/3rd at 2.9 kV and 1/3rd at 2.8 kV and an average HEH fluence equivalent to ¼ of the maximum simulated value gives 4 failures per year • Cost estimation of hardware modification (adding 2 FHCT into a stack) : • 160 pcs of additional FHCTs ~150 kCHF • 80 pcs new trigger transformers with 12 secondaries ~300 kCHF • Up-grade of stack mechanical and electrical parts (snubber capacitors, voltage sharing resistors, longer stack return rods) ~150 kCHF • 160 pcs of PTU and 72 pcs of HV PS with increased voltage (if improvement with new trigger transformer is insufficient) ~500 kCHF • Total ~1.1MCHF • More accurate failure rate evaluation is needed in order to decide if hardware modifications are necessary

5. SEB measurement results slot 3 • H4IRRAD test done with 2 setups of 5 FHCT each: • External zone: 3 FHCTs Dynex + 2 FHCTs ABB type I (production batch- KV.xxx) • Measurement within 2.7 kV- 2.9 kV; cumulated fluence of 2.67e9 HEH/cm2; 294 SEBs for Dynex; 0 SEBs for ABB I: • 2.7 kV: 7.45e8 HEH/cm2 – 198 SEBs Dynex; • 2.8 kV: 1.82e8 HEH/cm2 – 93 SEBs Dynex; • 2.9 kV: 1.6e6 HEH/cm2 – 3 SEB Dynex • Internal zone: 3FHCTs ABB type II (batch GV.xxx) + 2 FHCTs ABBI (batch KV.XXX) • Measurement within 2.7 kV- 3.1 kV; cumulated fluence of 1.32e10 HEH/cm2, 110 SEBs for ABB I, 30 SEBs for ABB II: • 2.7 kV: 1.51e9 HEH/cm2 – 0 SEBs ABB I, 0 SEBs ABB II; • 2.8 kV: 8.36e9 HEH/cm2 – 18 SEBs ABB I; 3 SEBs ABB II; • 2.9 kV: 2.74e9 HEH/cm2 – 42 SEBs ABB I, 7 SEBs ABB II; • 3 kV : 4.7e8 HEH/cm2 – 35 SEBs ABB I, 8 SEBs ABB II; • 3.1 kV: 1.2e8 HEH/cm2 – 15 SEBs ABB I, 12 SEBs ABB II; • In total: • 294 SEBs for 3 Dynex – leakage current increased significantly • 140 SEBs for 4 ABB I and 3 ABB II – no damage

6. SEB cross-section slot 1 + slot 3 + cosmic Insufficient statistics

7. Radiation levels measured last year • Efficiency of cable duct shielding surprisingly high: ratio RA/gen = 2E+5 (fluence in RA/fluence on generator); • HEH fluence in front of duct (UA67 only): 3E+4 HEH/cm2 • Estimation for full intensity and energy: 3E+5 HEH/cm2 • !Duct next to shielded one has 2.6E+6 HEH/cm2 (in duct UA side); at generator level fluence is unknown; TLDs reqested to be installed; necessity of adding the shielding might result from measurement • Estimation of average fluence of 2.5E+5 HEH/cm2 kept

8. Abort system MTBF re-evaluation • Due to insufficient statistics of Dynex SEB data at 2.9 kV and non – existing data at 3 kV cross-section values are extrapolated to 5x10-7 cm2 at 2.9 kV and to 1x10-6 cm2 at 3 kV; • In present, 300 Dynex and 300 ABB made FHCT are installed (ratio of type II/type I is unknown; type II is old technology so the estimation is > 50 % for type II); • Number of SEB related failures estimated under the same conditions as before (1/3rd of FHCTs at 2.8, 2.9 and 3 kV), with an average HEHfluence of 2.5x105 HEH.cm-2.year-1 (¼ of the simulated maximum) and with 300 Dynex, 150 ABB type I and 150 ABB type II is 25.6 per year (25 Dynex) • If Dynex are replaced by ABB type I (the only produced in present) the number of failures will be 1.6 per year. • Placing of generators populated by ABB type II in front of ducts with the highest fluence should reduce number of failures down to less than 1 per year

9. Conclusions - Recommendations • Dynex cross-section much higher than expected • ABB type I cross section measured in internal zone ~ 3x higher compared to results obtained during slot1 (external zone) • ABB type II – confirmed lower cross-section compared to type I (at least 5x). Producer claims the only difference in passivation process. • FHCT SEB C-S sufficiently low to not require modyfication of the number of FHCT in a stack • Real neutron fluence in UA galleries close to non shielded ducts to be evaluated • Generators at positions with higher radiation to be populated with ABB FHCT type II • Shielding of other ducts if necessary

10. New MKD trigger transformer • ABB recommendation for reliable FHCT gate triggering: 2 kA peak, 5 kA/us commutation speed • In time of MKD design the recommendations were ~ 10x lower • PTU operational voltage limited to 3 kV; higher voltage would require new design • New trigger transformer with co-axial design (reduced stray inductance) was developed • Mechanical and electrical contacts compatible with present design • Increasing of FHCT gate current (2x) and its commutation speed (3x) without need to modify PTU voltage • Further improvement possible (replacement of triax by multiwire twisted shielded cables)

11. Comparison old vs. new transformer