T2K Horn Operational Experience

8th International Workshop on Neutrino Beams and Instrumentation 1

T2K Horn Operational Experience

Tetsuro Sekiguchi (KEK)for T2K Beamline Group

2012/11/8

November 8th, 2012


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, 2012


What We struggle with High Power Beam

• Design beam power = 750kW.– 200kW = 1x1014 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, 2012


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 H2 removal– He flushing once a week at early stage ( < 50kW ).

He gas line

Exhaust H

e su

pply

Water supply

Water returnBuffer tank

November 8th, 2012


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 H2 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, 2012


Problem in Current System• Small flow rate (~50L/min)

– At beam power > 150kW, H2 density gradually increased to 5000ppm.– Another pump and catalyst added => kept around 2000ppm.

• H2 diffusion from horns to buffer tank is unknown.– H2 density in horns may be higher than buffer tank.– Frequent He flushing was needed, but prohibited during beam.

He gas line

Exhaust H

e su

pply

Water supply


H2 recomb.

diffusion

diff

usio

n

diff

usio

n

diff

usio

n

November 8th, 2012


Upgrade for High Power Beam• He buffer tank (~6m3)

– 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

supp

ly

Water supply

Water returnBuffer tankdi

ffus

ion

diff

usio

n

diff

usio

n

6m3

He buffer tankNovember 8th, 2012


• He buffer tank (~6m3)– 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

supp

ly

Water supply

Water returnBuffer tankdi

ffus

ion

diff

usio

n

diff

usio

n

H2 recomb.

Blower

Exhaust

6m3

He buffer tankNovember 8th, 2012

8th International Workshop on Neutrino Beams and Instrumentation 9November 8th, 2012

• He buffer tank (~6m3)– 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

supp

ly

Water supply


H2 recomb.

Blower

Exhaust

6m3

He buffer tank


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 HNO3.– 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• NO3

- : ~100 mg/L

November 8th, 2012


Aluminum Corrosion by Acid

• Aluminum Corrosion– Aluminum corrosion rate gets large w/ pH<4.– Water whitened due to Al2O3 or Al(OH)3

– Amount of corroded aluminum: ~51mg/L => 138 g in 2700L• ~6mm-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

Cor

rosi

on ra

te (m

m/y

ear)

November 8th, 2012


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, 2012


Horn Power Supply CircuitCharger

(10kV, 10A)Capacitor

Bank(7.5mF)

SCRSwitch

Polarity control by IGBT

Output

Return

Earth line

40 capacitors (750mF)2 in series and 20 in parallel.Neutral point is grounded.

Bridge layout

Bridge layout for reversing current direction.

November 8th, 2012


Current Flow (Charging)Charger


Bank(7.5mF)

SCRSwitch


Output

Return

Earth line

+

_

Two IGBT switches turned on.

November 8th, 2012


Current Flow (Discharging)Charger


Bank(7.5mF)

SCRSwitch


Output

Return

Earth line

+

_

Two SCR switches turned on.

November 8th, 2012


Current Flow (Reverse Charging)Charger


Bank(7.5mF)

SCRSwitch


Output

Return

Earth line

+

_

Other two IGBT switches turned on.

November 8th, 2012


Current Flow (Discharging)Charger


Bank(7.5mF)

SCRSwitch


Output

Return

Earth line

+

_

Two SCR switches turned on.

Polarity not changed

November 8th, 2012


Charging Sequence

November 8th, 2012

75% (spec.) of energy recovered. 60% due to uncounted joint resistance

=> Motivation for low resistance (inductance) striplines


• 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 Stray capacitance

CancelSymmetric circuitNot symmetric circuit


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

Hor

n1

Hor

n2

Hor

n3

November 8th, 2012


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, 2012

ON!

Broken IGBTs

Burned choke coils


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 8.2kV260kA 260kA


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.

260kA2020A

3.0ms

1180A

260kA

4.9ms

Broken SCRs

November 8th, 2012 8th International Workshop on Neutrino Beams and Instrumentation 24

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


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 => 2x107 pulses/year Designed to survive 108 pulses • Time scale

– Production of new horns and new striplines will be completed in 2013.– Replacement of horns will be completed within three years.


Summary

• T2K horn operational experiences presented.• Hydrogen recombination applied.

– Not efficient to remove H2 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.

T2K Horn Operational Experience

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