Malte Hildebrandt

Malte Hildebrandt

MEG Review Meeting, 17.02.2010

Malte Hildebrandt MEG Review MeetingPSI, February 2010

Drift Chamber System• hardware status in 2009

Malte Hildebrandt


Outline

• reminder: HV instability problem in 2007, 2008

• summary of tests and proof of via hypothesis

• repair work → new anode pcb→ unexpected observations

• installation 2009

• MEG Run 2009 → remaining currents

• new cathode foil

• Summary / Outlook

Malte Hildebrandt


HV Trips

• characteristics of HV trips in 2007 and 2008:

• significant deterioration of HV stability started

2007: at end of run 2008: beam time (XEC, Dalitz)

2007: after 2-3 months with 2008: after 2-3 months with gas and HV gas and HV → at beginning: same planes affected

as in 2007

• further deterioration during remaining run time even without any further beam time

2007: Sep – Dec 2008: May – Dec

• stable operation with reduced HV settings

2007: dc system off during 2008: second beam time beam time

→ deterioration due to helium environment ?

Reminder

Malte Hildebrandt


HV Tests in 2008

• tests with dc system in MEG during run 2008:

• exchange of infrastructure / hardware (HV module, HV cables)

• variation of dp_dc regulation value (pdc-pCOBRA) ↔ small leaks ?

• increase ethane fraction in dc counting gas ↔ inside sensitive volume ?

• increase air admixture to COBRA ↔ outside dc module ?

→ no clear cause and effect (on shorterm scale)

→ but: hint, that problem is connected to longterm exposure to helium

Reminder

Malte Hildebrandt


HV Tests in 2009

• tests with dc system in helium cabin • dc system inside helium environment

since 16th Jan 2009

• dc modules flushedsince 16th Jan : heliumsince 30th Jan : helium / ethan

• operated with MEG dc HV system

• goal: investigate HV status • compare with HV status

at end of last years run

• identify characteristics of weak anode channels

• observations (tests finished 11th May):

• „weak“ planes (run 2008) got worse

• „good“ planes (run 2008) started to deteriorate

• all weak anode channels showed same signal characteristics

→ further proof for assumptions: • HV problem related to exposure to helium • (most likely) same reason for HV instabilities

→ 4 ½ additional months „run conditions“ after end of run 2008

Malte Hildebrandt


HV Tests in 2008 / 09

• tests in laboratory (HV test box)

• pcb, potting material

• helium environment, cHelium > 99%

• T ≈ 40-45° C

• HV = 2 kV

• longterm test (>3 months)

→ no deterioration

• finally, only one topic remained on our list of suspicious and possible weak points concerning construction and operation of the drift chambers:

→ the bottom layer of anode HV pcb where the HV via is facing the GND layer

Reminder

Malte Hildebrandt


HV Via

top layer

bottom layer

+HVGND

7 mm

Malte Hildebrandt


PCB Cross Section

GND +HV

pcb

+HV

G10 isolator

glue glue glue

G10 isolator

glue

glue

carbon frame

air

He / C2H6

He

pcb

bottom layer

top layer

bottom layer

Malte Hildebrandt


dc01A

no glue glueno glue

Malte Hildebrandt


PCB Cross Section

GND +HV

pcb

G10 isolator

glue

+HV

glue

G10 isolator

glue

glue

carbon frame

pcb

• Why are only certain vias affected?

→ gas permeability depends on thickness of „barrier“

He / C2H6

Malte Hildebrandt


PCB Cross Section

GND +HV

pcb

G10 isolator

glue

+HV

glue

G10 isolator

glue

glue

carbon frame

He / C2H6

pcb

• Why are only certain vias affected?

→ no breakdown in He / C2H6 (confirmed by test in laboratory)

Malte Hildebrandt


dc01 Skeleton in „Aquarium“

• dc01 skeleton: anodemiddle cathodeanode(no hood cathode)

• since 19th Feb mounted inside „aquarium“

• 19th Feb – 6th Mar : helium (30 days)

6th Mar – 9th Mar : helium / ethane (3 days)

9th Mar : HV tests („rather“ stable)

9th Mar – 20th Mar : helium (22 days)

since 20th Mar : helium / ethane (untill Mar 30th: 10 d)

since 23rd Mar : HV tests

→ 30th Mar : dc01A: first direct / optical observation of discharges between HV via and GND surface on bottom side of HV pcb

→ characteristics of signals on oscilloscope same as in Jan / Feb when complete dc01 was tested in „aquarium“but now: no hood, no G10 isolators

→ 65 days „run conditions“(up to Mar 30th)

Malte Hildebrandt


dc01A anode 3

Malte Hildebrandt


Outline








Malte Hildebrandt


HV Print 2009

HV print 2009

• traces for HV on middle layer

→ no HV traces on bottom layer

→ individual layers with „only HV“ or „only GND“ (3-layer →4-layer pcb)

• „blind vias“

→ vias have only necessary depthto connect appropriate layers(like „blind hole“)

vias for +HV

pads for resistors

+HV traces

outer edgeprint 2007

print 2009

inner edge

GND

Malte Hildebrandt


HV Print 2009

HV print 2009

• soldering pads for capacitors

→ round shape on inner sidewhere electrodes of capacitor face to each other

vias for readout connectors

soldering pads for decoupling capacitors

+HV traces in middle layer

print 2007

print 2009

Malte Hildebrandt


HV Print 2009

HV print 2009

• tracks and vias for HV on top layer

→ place HV („blind“) vias close to soldering pads

→ HV tracks are not covered bysmall G10 isolator

print 2007

print 2007

print 2009

print 2009

Malte Hildebrandt


DC Wing Test Setup

• „dc wing test setup“ in HV test box

• represents the cross section through wing of dc plane

→ anode frame – G10 isolators – pcb – G10 isolator – G10 isolator – hood frame

• goal: investigate HV stability

• discharges due to „polarisation“ effects ?

• operated in exhaust line of „aquarium“

→ flushed with He / C2H6

• observations:

→ 14 days with HV thereof: 12 days >2 kV and

7 days 2.6 kV

→ no HV trips (trip threshold 8A)

Malte Hildebrandt


Potting HV Connection

HV connection to pcb + sealing

→ weak point: potting of HV soldering spot on pcb

2006 - 2008 2009

• ThreeBond 1530 (silyl polymer) • EPO-TEK 302-3M (epoxy resin)

Malte Hildebrandt


DC Repair Work

• successful „dc wing test“ was starting signal of dc repair work↔ all materials were already prepared in advance and on spec

• all dc modules were disassembled

→ middle cathode and cathode hood were recycled without any change→ anode frames: • new anode pcb‘s were glued on anode frames

• new wires were soldered on the pcbs / frames

• assembly of „new“ dc modules

→ module #1 and #2 operated in „aquarium“ for 6 ½ months

→ 16 dc modules for MEG: • each individual chamber tested inside helium cabin with cosmic rays (HV, LV + signal)

• complete dc system tested inside helium cabin (only HV)

• operation in MEG: 4 months (Sep – Dec)

→ No deterioration of HV stability during 2009 !

Malte Hildebrandt


Outline








Malte Hildebrandt


Unexpected Phenomena

• during disassembly several unexpected phenomena were observed on a limited number of cathode foils and their corresponding anodes:

• „damage“ of cathode foil

Malte Hildebrandt


dc09A Cathode

after touching:

dc09A cathode (foil 42) 1750 V

observations:

• aluminum coating is peeling offalong Vernier pattern

• complete length of cell_0

• damaged region:• sharp edges • sligthly extended at

etched gaps

• not symmetric to anode wirebut: „rotation“ of E-field due to

B-field in other direction

Malte Hildebrandt


dc09A Cathode

after touching:

dc09A cathode (foil 42) 1750 V

observations:

• aluminum coating is peeling offalong Vernier pattern

• complete length of cell_0


etched gaps

• not symmetric to anode wirebut: „rotation“ of E-field due to

B-field in other direction

→ first comment from REPIC:

• peeling off maybe due tomissing chromium underlayer

Malte Hildebrandt


dc11B, dc14A – Cathode Foil

dc11B hood (foil 22) 1850 Vdc14A cathode (foil 25) 1800 V

observations:

• “spots” / peaks along Vernier pattern

• mainly on complete length of cell 0,but also at frame edges:cell 1, cell 2, cell 3 and cell 4


etched gaps

• not symmetric to anode wire

• anode wires: • mechanical tension ok• → separate transparency

• potential wires: ok (?)

Malte Hildebrandt


dc11B, dc14A – Cathode Foil

topography contrast methode

scanning electron microscope (SEM) + energy-dispersed x-ray spectroscopy (EDX)S.Ritter (NES / LNM)

Al

O Mg

Al

O Mg

Malte Hildebrandt


dc09, dc11, dc14

dc09A cathode (foil 42) 1750 Vdc11A+B cathode (foil 28)dc14A cathode (foil 23) 1800 V

observations:

• white “shadows” along Vernier pattern

• intensity decreasing with r (rate effect?)

• not continuous in r, but separated stripes

• not symmetric to anode wire

• anode wires: • mechanical tension ok• → separate transparency

• potential wires: ok (?)

1780 V1850 V

r

Malte Hildebrandt


dc09, dc11, dc14


observations:

• on first sight: like scratches

but:

• not removable with cotton bud

1780 V1850 V

Malte Hildebrandt


dc09, dc11, dc14


observations:


but: tiny „particles“ perfectly alignedalong tracks / scratches


1780 V1850 V

Malte Hildebrandt


dc09, dc11, dc14


observations:


but: tiny „particles“ perfectly alignedalong tracks / scratches


1780 V1850 V

scanning electron microscope (SEM) S.Ritter (NES / LNM)

Malte Hildebrandt


dc09, dc11, dc14

foil

particle

remark: sample table made of aluminum


Al

(O) (Mg)

Al

O Mg

Malte Hildebrandt


dc09, dc11, dc14


observations:


→ possible reason for scratches:

1780 V1850 V

polyimide foil

aluminum

Malte Hildebrandt


dc09, dc11, dc14


observations:


→ possible reason for scratches:

1780 V1850 V

polyimide foil

sheet of paper

→ improve packaging for further delivery: soft, slightly sticking foil instead of sheet of paper

Malte Hildebrandt


Unexpected Observations



• coating on anode wires

Malte Hildebrandt


dc11B, dc14A – Anode Wires

Ni / Cr (80 / 20) 25 m

EHT = 10 kV

EHT = 20 kV


new wireNi

(C)

Cr

Si Cr Ni

remark: C maybe due to sticker on sample table

Ni

(C)

Cr

Si Cr Ni

Ni balance Cr 18 - 20 %Si 1.5 %Al 1000 ppm, Fe 2000 ppm, Mn 2000 ppm

Malte Hildebrandt


dc11B, dc14A – Anode WiresNi / Cr (80 / 20) 25 m


Ni

(C)Cr

Si Cr NiO

Ni

C

Cr

SiCr Ni

O

Malte Hildebrandt


Unexpected Observations



• coating on anode wires

• evaluation of the damage:

• effects are limited to a very small number of cathode foils / anode wire frames

• damage of cathode foil as well as coating on anode wires did not deterioratethe performance of the specific chamber↔ theses modules were operated at nominal / nearly nominal HV until

the end of the run 2008

→ check carefully for dc modules which will be disassembled this spring shutdown

Malte Hildebrandt


DC Repair Work

• summary of dc repair work:

2 dc modules in „aquarium“ for longterm test

16 dc modules in MEG experiment

3 sets of spare frames (modules with damaged cathode foil) → order new foil

February – April • „dc skeleton“ in aquarium

• discussion / design / tests of new anode pcb

May – July • construction of new dc‘s → test 2 new dc‘s in aquarium→ test of mounted dc modules in support structur

July • middle of July: 16 dc modules in support structure→ close helium cabin

• end of July: repair / construction work finished (2 + 16 )

August • further tests, prepare support structure and reserve

1st September → installation of dc system in MEG experiment

Malte Hildebrandt


Outline








Malte Hildebrandt


Strain Relief at Inside PatchPanel

• location: signal / LV cables inside patch panel

• problem: missing dc signal channels / LV channel (2006, 2007)

→ shutdown 2008: improve strain relief of cables on pcbwith aluminum clamps

→ no LV lost, reduced number of missing signal channels

→ but: weak point shiftet to the connector / socket on patch panel pcb

• installation 2009: 1 signal cable pcb completely disconnected 1 partially disconnected

→ intensive repair work to fix and to recover ~24 signals (endoscope, sawing, sealing, …)

→ shutdown 2010: improve strain relief of signal cable pcb on patch panel pcb with bracket

Malte Hildebrandt


Outline








Malte Hildebrandt


DC Performance 2009

• dc04B: • cosmic test in laboratory: • nominal HV, normal puls height

• during MEG run: • nominal HV, but: low gas gain (I ≈ 0.2·Inormal)

• cosmic run in MEG (Jan2010) • nominal HV, normal puls height

→ not yet understood

• dc08B: • cosmic test in laboratory: • nominal HV, no HV trips

• at beginning of MEG run: • nominal HV, periodic HV trips: 1 per 1-2 d

• during MEG run • 3 weeks stable, then again: 1 per 1-2 d

• cosmic run in MEG (Jan2010) • nominal HV, no HV trips

→ charging up effect ?

• dc04, dc05, dc06, dc11: MEG run: • increasing dark / remaining currents→ reduced HV, but still >1800 V

→ replace during spring shutdown

• all other dc modules on nominal HV

→ 30 / 32 planes on HV >1800 V ! • 4 months operation with flushing gas• 3 ½ months operation with HV and rate

Malte Hildebrandt


Dark / Remaining Currents

• observations: • some dc planes show dark / remaining currents (up to several A)

• current starts during high rate irradiation

• remaining current stays, even when irradiation has finished

• only if HV is reduced to 1300 V, remaining current dies awayremark: no gas gain below ~1300 V

Malte Hildebrandt



dc14A

Malte Hildebrandt







→ hint: • self-sustaining discharge (not surface current, …)

→ certain primary charge density necessary to start gaseous discharge

→ once started it remains even without source of primary charge

• Malter effect: e- multiplication at anode wire (1st Townsend coefficient)

e- emission at cathode due to field emission

Malte Hildebrandt


Malter Effect

cathode

anode wire

isolating film / layer

e- →← +

cathode

anode wire


e- →

++++

+

++

• insulating film / layer on cathode

• primary charge due to irradiation

• motion of charge due to electrical field

• small surface conductivity

→ rate of charge build up higher than its removal rate

1 2

Louis Malter, Phys.Rev. 50 (1936) 48-58: Thin Film Field Emission

Malte Hildebrandt


Malter Effect

cathode

anode wire


e- →

cathode

anode wire


e- →

++++

+

++

• hugh electrical field strengthbetween surface of isolating filmand cathode

→ electron emission from cathode

• electron emission / current remains even after stop of „primary charge“ due to irradiation with beam

→ reduce HV until current dies away

++++

+++

e-

e-

3 4

Louis Malter, Phys.Rev. 50 (1936) 48-58: Thin Film Field Emission

Malte Hildebrandt







→ hint: • self-sustaining discharge (and surface current, …)

→ certain primary charge density necessary to start gaseous discharge

→ once started it remains even without source of primary charge

• Malter effect: e- multiplication at anode wire (1st Townsend coefficient)

e- emission at cathode due to field emission

→ remark from REPIC: maybe remaining photoresist on cathode foil…

→ improved and intensified cleaning procedure for new foils !

Malte Hildebrandt


Outline








Malte Hildebrandt


New Cathode Foil

• situation July 2009 after observation of damaged cathode foils

• need of new foils to prepare spare modules for Run 2010

• improve adhesion of aluminum sputtering on polyimid filmto avoid peeling off of aluminum from polyimid

• improve and intensify cleaning procedure to remove photo resist

• improve packaging to avoid micro-scratches on aluminum

→ 3 options: • option A: 1 nm Ni-Cr underlayer on polyimid film→ very good adhesion of aluminum→ but: double-etching process

→ Al may be affected during Ni-Cr etching

• option B: 1 nm SiO2 underlayer on polyimid film

→ very good adhesion of aluminum→ advantage: SiO2 is not conductive, remains in gaps

→ only single-etching process

• option C: samed design as 2005 production series (no underlayer)

→ just „backup solution“, in case A and B fail

Malte Hildebrandt


New Cathode Foil

• option A: • 1 nm Ni-Cr underlayer on polyimid film

→ very good adhesion of aluminum

→ but: double-etching process, Al may be affected during Ni-Cr etching

→ result: 2nd etching removes partially Aluminum layer

→ option A failed !

cross markers nearly lost jaggy edges

Malte Hildebrandt


New Cathode Foil

• option B: • 1 nm SiO2 underlayer on polyimid film


→ advantage: SiO2 is not conductive and may remain in gaps → only single-etching process

→ result: very good quality (adhesion of aluminum, shape of pattern)

• but several concerns:

• SiO2 is isolator → charging up in high rate environment ?

• Si may lead to aging in gaseous detector

• SiO2 is electron supplier (e.g. in muonium production)

→ starting point of discharges ?

→ option B was rejected !

Malte Hildebrandt


New Cathode Foil

• option C: • same design as 2005 production series (no underlayer)

→ „backup solution“, in case A and B fail

→ result: very poor adhesion of aluminum layer, worse than in 2005

(different polyimid ?)

→ option C failed !

aluminum removed with sticky tape

Malte Hildebrandt


New Cathode Foil

• option A: • 1 nm Ni-Cr underlayer on polyimid film


→ but: double-etching process, Al may be affected during Ni-Cr etching

→ result: 2nd etching removes partially Aluminum layer

→ option A failed !

• option A’: • 0.5 nm Ni-Cr underlayer on polyimid film

→ after adjusting (nearly) all sputtering and etching parameters: excellent

→ foil production finally started middle of December 2009 (order in July)

→ first delivery to PSI: beginning of January 2010

cross markers nearly lost jaggy edges

Malte Hildebrandt


Outline








Malte Hildebrandt


Summary / Outlook

• The reason for the HV instability problem in 2007 and 2008 was identified.

• The new anode pcb design eliminates this weak point. → There was no „system-wide“ HV instability problem during run 2009.

• During the repair work several unexpected phenomena were discovered:limited number of damaged cathode foils and coating on anode wires→ for 2009: anode wire were exchanged (due to new anode pcb anyway)

→ for 2010: 20 new cathode foils with Ni-Cr underlayer are produced

→ We have to check very carefully the dc modules which will be disassembled during this spring shutdown.

• A few dc planes suffered from dark / remaining currents during the run 2009.→ New cathode foils underwent an improved and intensified cleaning procedure

to avoid possible starting points of Malter effect.

• The dc construction tools need to be modified and adapted to the pitch of the etched gaps of the new cathode foils.→ construction of new dc modules will start end of February (min. 4 modules)

→ dc system will be ready for installation middle of April

Malte Hildebrandt

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