1. November 8th, 201218th International Workshop on Neutrino Beams and Instrumentation

2. Just for Reminder Specification – Three horns – Aluminum alloy A6061-T6 – 320kA (rated)  250kA (200kA) operation – Water cooling – Operated in He atmosphere History – Installed in 2008- 2009 – Operation for physics from Jan. 2010. – 8 million pulses operated. November 8th, 201228th International Workshop on Neutrino Beams and Instrumentation

3. What We struggle with High Power Beam Design beam power = 750kW. – 200kW = 1x10 14 protons per pulse at this moment. – Radioactivity: Be7 and Tritium in horn cooling water. => Oyama’s talk – Hydrogen production by water radiolysis. => This talk Horns are operated in Helium atmosphere to reduce production of Tritium and NOx. – Air contamination problems sometimes happened. => This talk In order to achieve 750kW beam, accelerator cycle will decrease to 1.28sec. – Input load to power supply should be low. => Striplines modification. – Power supplies for high rep. rate operation. => This talk November 8th, 201238th International Workshop on Neutrino Beams and Instrumentation

4. Hydrogen Production Horn inner volume filled with He gas. Hydrogen gas produced by water radiolysis. – Estimated production rate at 750kW = 40L/day (0.7%/day). – Explosion limit in air = 4% What for He atmosphere? => Try to keep below 1%. Countermeasures for H 2 removal – He flushing once a week at early stage ( < 50kW ). He gas line Exhaust He supply Water supply Water return Buffer tank November 8th, 201248th International Workshop on Neutrino Beams and Instrumentation

5. Hydrogen Recombination For beam power > 50kW, weekly flushing was not sufficient. Hydrogen recombination system – He gas circulated through catalyst during beam : ~50L/min – At every maintenance day, He gas sampled and H 2 contamination measured by gas chromatography. Circulation pump Cooling water Buffer tank for horn cooling water Cover gas (Helium) Catalyst canister (Catalyst = Alumina pellet with 0.5%Pd ) November 8th, 201258th International Workshop on Neutrino Beams and Instrumentation

6. Problem in Current System Small flow rate (~50L/min) – At beam power > 150kW, H 2 density gradually increased to 5000ppm. – Another pump and catalyst added => kept around 2000ppm. H 2 diffusion from horns to buffer tank is unknown. – H 2 density in horns may be higher than buffer tank. – Frequent He flushing was needed, but prohibited during beam. He gas line Exhaust He supply Water supply Water return Buffer tank H 2 recomb. diffusion November 8th, 201268th International Workshop on Neutrino Beams and Instrumentation

7. Upgrade for High Power Beam He buffer tank (~6m 3 ) – Exhaust gas stored for several hours for short-lifetime radioactivity to decay. – He flushing during beam is allowed. => Everyday flushing He gas line Exhaust He supply Water supply Water return Buffer tank diffusion 6m 3 He buffer tank November 8th, 201278th International Workshop on Neutrino Beams and Instrumentation

8. He buffer tank (~6m 3 ) – Exhaust gas stored for several hours for short-lifetime radioactivity to decay. – He flushing during beam is allowed. => Everyday flushing Forced circulation – He gas plumbing outside He vessel was modified to allow forced He circulation. (800L/min flow rate) Upgrade for High Power Beam He gas line He supply Water supply Water return Buffer tank diffusion H 2 recomb. Blower Exhaust 6m 3 He buffer tank November 8th, 201288th International Workshop on Neutrino Beams and Instrumentation

9. November 8th, 201298th International Workshop on Neutrino Beams and Instrumentation He buffer tank (~6m 3 ) – Exhaust gas stored for several hours for short-lifetime radioactivity to decay. – He flushing during beam is allowed. => Everyday flushing Forced circulation – He gas plumbing outside He vessel was modified to allow forced He circulation. (800L/min flow rate) – New horns have two He gas plumbing to allow forced circulation inside horns. Upgrade for High Power Beam He gas line He supply Water supply Water return Buffer tank H 2 recomb. Blower Exhaust 6m 3 He buffer tank

10. 8th International Workshop on Neutrino Beams and Instrumentation Air Contamination Horn inner volume filled with He <= He purge. – He purge is not perfect, so a few % air exists. NOx produced by beam dissolves in water. – Acidification of water occurs by HNO 3. – pH=5 expected with 3 months beam (200kW) and 1% air contamination. Furthermore, large amount of air have been contaminated twice. – Plumbing and pump itself have broken in both cases. Cooling (pure) water were getting white. – pH measurement: pH=4.3 – Analysis of components in cooling water Al : ~51 mg/L NO 3 - : ~100 mg/L November 8th, 201210

11. Aluminum Corrosion by Acid Aluminum Corrosion – Aluminum corrosion rate gets large w/ pH<4. – Water whitened due to Al 2 O 3 or Al(OH) 3 – Amount of corroded aluminum: ~51mg/L => 138 g in 2700L ~6  m-thick aluminum corroded  3mm total thickness. – Not serious problem at this moment. Countermeasures – Online He purity monitor Air contamination is monitored. – Remote pH measurement pH is frequently measured. – Remote ion-exchange Done if pH is getting lower. pH 0 2 4 6 8 10 12 14 0.5 1.0 1.5 2.0 2.5 Corrosion rate (mm/year) November 8th, 2012118th International Workshop on Neutrino Beams and Instrumentation

12. Horn Power Supply Motivation – Since K2K power supplies (more than 15years old) were refurbished for T2K from 2006, a new power supply was manufactured in 2010. New power supply – New power supply drives all three horns connected in series. – Energy recovery (60% energy reused for next pulse). – Symmetric circuit is adopted to reduce earth current. November 8th, 2012128th International Workshop on Neutrino Beams and Instrumentation

13. Horn Power Supply Circuit Charger (10kV, 10A) Capacitor Bank (7.5mF) SCR Switch Polarity control by IGBT Output Return Earth line 40 capacitors (750  F) 2 in series and 20 in parallel. Neutral point is grounded. Bridge layout Bridge layout for reversing current direction. November 8th, 2012138th International Workshop on Neutrino Beams and Instrumentation

14. Current Flow (Charging) Charger (10kV, 10A) Capacitor Bank (7.5mF) SCR Switch Polarity control by IGBT Output Return Earth line + _ Two IGBT switches turned on. November 8th, 2012148th International Workshop on Neutrino Beams and Instrumentation

15. Current Flow (Discharging) Charger (10kV, 10A) Capacitor Bank (7.5mF) SCR Switch Polarity control by IGBT Output Return Earth line + _ Two SCR switches turned on. November 8th, 2012158th International Workshop on Neutrino Beams and Instrumentation

16. Current Flow (Reverse Charging) Charger (10kV, 10A) Capacitor Bank (7.5mF) SCR Switch Polarity control by IGBT Output Return Earth line + _ Other two IGBT switches turned on. November 8th, 2012168th International Workshop on Neutrino Beams and Instrumentation

17. Current Flow (Discharging) Charger (10kV, 10A) Capacitor Bank (7.5mF) SCR Switch Polarity control by IGBT Output Return Earth line + _ Two SCR switches turned on. Polarity not changed November 8th, 2012178th International Workshop on Neutrino Beams and Instrumentation

18. Charging Sequence November 8th, 2012188th International Workshop on Neutrino Beams and Instrumentation 75% (spec.) of energy recovered.  60% due to uncounted joint resistance => Motivation for low resistance (inductance) striplines

19. 19November 8th, 20128th International Workshop on Neutrino Beams and Instrumentation Earth current flows at discharging timing due to stray capacitance. – The earth current will affect the control circuit through ground line, which will cause a malfunction of power supply. Grounding neutral point of capacitors. – Earth currents from P-side and N-side flow opposite side and cancel at earth line. Symmetric Circuit Shielded power cables Stray capacitance Cancel Symmetric circuitNot symmetric circuit

20. 8th International Workshop on Neutrino Beams and Instrumentation Series Connection of Horns Horn operation – Until 2010 summer: 1 PS for horn1, 1 PS for horn2&3 (series). – From 2010 fall : 1 PS for 3 horns in series. Three-horn operation by 1 PS – Simple connection of horn1 and horn2&3 caused 15% current unbalance. – Different inductance for inner and outer striplines? – Modified striplines to reduce current unbalance. => 2% ~15% Current at inner striplines Current at outer striplines Total current 169kA Striplines Stripline configuration at Service Pit Transformer Horn1Horn2Horn3 November 8th, 201220

21. Power Supply Failure New horn power supply was broken just before beam resumed after earthquake recovery (2 days before beam came back). – Two IGBTs turned on during discharging due to malfunction. – ~70kA flowed to IGBTs and burned out. November 8th, 2012218th International Workshop on Neutrino Beams and Instrumentation ON! Broken IGBTs Burned choke coils

22. November 8th, 20128th International Workshop on Neutrino Beams and Instrumentation22 Refurbish Old Power Supply Old power supply – Moved to J-PARC on Jan. 4, 2012. – 250kA operation on Feb. 9, 2012. Concerns – First operation with 3 horns by old PS Charging voltage 6.7kV at 250kA  8.5kV(rated) Spike voltage 12kV => 8kV by filters. – Large inrush current at SCRs We started physics run from Mar 2. 12.3kV 8.2kV 260kA

23. November 8th, 20128th International Workshop on Neutrino Beams and Instrumentation23 SCR Failure SCR failure on Mar. 6 – Switched to a spare SCR (4kVx3) but reduced to 200kA for 2 weeks. – Acc. failure happened and 2 week downtime => investigation work. Investigation – Modified earth line configuration. – Replaced with higher rated voltage SCRs (7kVx3). Inrush current finally reduced by half (2020A -> 1180A) – No problem happened during physics run. 260kA 2020A 3.0  s 1180A 260kA 4.9  s Broken SCRs

24. November 8th, 20128th International Workshop on Neutrino Beams and Instrumentation24 Stripline Reconfiguration for Dual PS After new PS is recovered, dual PS operation is preferable. – Low voltage operation greatly reduces risks of failure. – 4.4kV for horn1, 5.5kV for horn2&3  6.7kV for 3 horns Stripline reconfiguration for dual PS operation. – Small change => easily go back to 1 PS operation if one PS fails. Current condition – Unfortunately, new PS not recovered yet. Hopefully recovered by Jan. 2013. – 3 horn operation by old PS until Dec. 2012. => currently working well. – One more new PS will be produced by the end of FY2013 (Mar. 2014). Horn1 Horn2 Horn3 Old configuration New configuration Horn1 Horn2 Horn3

25. November 8th, 20128th International Workshop on Neutrino Beams and Instrumentation25 High Rep. Rate Operation with 320kA For 320kA operation, we really need to reduce operation voltage. To achieve 1 Hz operation, dual (or preferably triple) PS operation is necessary. – More than two power supplies to reduce input load for each PS. One more new power supply – Specification should be optimized for 1 Hz. – New low inductance and low resistance striplines. Shorter, wider and narrower gap. Improved stripline cooling also applied to survive >750kW (current limit is 400kW). Existing horns must be replaced with new horns with new striplines. – 1 Hz operation => 2x10 7 pulses/year  Designed to survive 10 8 pulses Time scale – Production of new horns and new striplines will be completed in 2013. – Replacement of horns will be completed within three years.

26. November 8th, 20128th International Workshop on Neutrino Beams and Instrumentation26 Summary T2K horn operational experiences presented. Hydrogen recombination applied. – Not efficient to remove H 2 inside horns. – Need to replace with upgraded horns in near future. Air contamination causes acidification and corrosion. – Some countermeasures applied. Horn power supply and stripline system are being modified. – New horn PS: energy recovery, symmetric circuit. – New PS IGBT failure => Refurbish old PS. – Dual PS operation for safety and redundancy. 1 Hz operation is discussed. – Production of one more PS and new striplines by 2013. – Replacement of horns and striplines within three years.