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 10 6 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 FIT at 2.8 kV (1 FIT = 1 failure in 10 9 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 ~1.10 6 A 2 s – 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 10 6 HEH.cm -2.year -1 in cable duct axis Estimation of number of failures per year for 1/3 rd of FHCT at 3 kV, 1/3 rd at 2.9 kV and 1/3 rd 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 cm 2 at 2.9 kV and to 1x10 -6 cm 2 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 (1/3 rd of FHCTs at 2.8, 2.9 and 3 kV), with an average HEH fluence of 2.5x10 5 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