Malte Hildebrandt MEG Review Meeting, 17.02.2010 Malte Hildebrandt MEG Review Meeting PSI, February...

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


Outline








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

pcb



He / C2H6

Malte Hildebrandt


PCB Cross Section

GND +HV

pcb

G10 isolator

glue

+HV

glue

G10 isolator

glue

glue

carbon frame

pcb



He / C2H6

Malte Hildebrandt


PCB Cross Section

GND +HV

pcb

G10 isolator

glue

+HV

glue

G10 isolator

glue

glue

carbon frame

He / C2H6

pcb


→ 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:




1780 V1850 V

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

Malte Hildebrandt


dc09, dc11, dc14


observations:




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

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