dear meg committee,psi, april 14 th 2009

Malte Hildebrandt

DC Status Report, 08.04.2009

Dear MEG Committee, PSI, April 14th 2009

this is the first report concerning the drift chamber status as requested inthe „Presentation of the Committee recommendations to the spokespersons“ at this year‘s PSI Open User Meeting held in February, as well as, in yourwritten report „Review of the MEG Experiment“.

The most important message of this report is the first and direct observation of a discharge connected to the „HV via hypothesis“ !

This is a breakthrough as up to now these HV vias, which face the GND layer, were seen as the most plausible weak point, but these observations give clear evidence for such a hypothesis !

With the new pcb design, which was already presented at the review meeting, this weak point will be eliminated.

Best regards, the DC Team

Malte Hildebrandt


dc activities in lab

• helium cabin

• dc01 skeleton in „aquarium“

• „via-sample“ in HV test box

• next steps

Malte Hildebrandt


DC System in Helium Cabin

The next transparency summarises the activities with the dc system in thehelium cabin.

The important observations after ~3 more months with helium operation since the end of last years run are:

• dc planes which needed to be operated with reduced HV settings at the end of run 2008 continue to deteriorate.

• even planes which were still fine at the end of last years run start to deteriorate.

Malte Hildebrandt


DC System in Helium Cabin

• dc system in helium cabin

• dc system inside helium environmentsince Jan 16th

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

• 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:

• „weak“ planes (run 2008) get worse

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

• characteristics of weak anode channels: similar to dc01 in „aquarium“ (Jan / Feb)

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

Malte Hildebrandt


dc01 Skeleton in „Aquarium“

The next transparency summarises the activities with the dc01 skeletonin the „aquarium“.

The dc skeleton was operated ~2 months („equivalent“ running conditions) inside the „aquarium“.

The most important observation is the discharge between HV via and GND layer on the bottom side of the HV pcb!

There are some more transparencies to remind you about the „via hypothesis“.

Malte Hildebrandt


dc01 Skeleton in „Aquarium“

• dc01 skeleton: anodemiddle cathodeanode(no hood cathode)

• since Feb 19th mounted inside „aquarium“

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

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

Mar 9th: HV tests („rather“ stable)

Mar 9th – Mar 20th:helium (22 days)

since Mar 20th: helium / ethane (until Mar 30th: 10 days)

since Mar 23rd: HV tests

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

• trip during night at 1850 V • to match with exposure time of digital camera increase HV to 2000 V

→ 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

MEG Review Meeting, 18.02.2009

HV Via

top layer

bottom layer

+HVGND

7 mm

Malte Hildebrandt


dc01A

no glue glueno glue

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 anode 3

Malte Hildebrandt


dc01A anode 2

Malte Hildebrandt


dc01A anode 3

Malte Hildebrandt


HV Print 2009

The next transparency explains some of the modifications which are already implemented in the design of the new HV anode pcb in order to avoid this weak point on the bottom layer.

These changes were already presented in the review meeting in February,this is just a reminder.

Malte Hildebrandt


HV Print 2009

HV print 2009

• traces for HV on middle layer

→ no HV traces on bottom layeronly GND 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


„Via Discharge“

The direct observation of discharges between HV vias and GND is the first„positive“ observation during all of these investigations.

Up to now we had a long list of suspicious and possible weak points concerning the construction and operation of the drift chambers.

Numerous tests during the last months have excluded step by step the „most common suspects“ from this list, with one main point remaining on the list:the bottom layer of anode HV pcb where the HV-via faces the GND-layer.

The observation of these discharge is the first positive evidence during these investigations!

Malte Hildebrandt


„Via Sample“ in HV Test Box

Certainly, there are still some open questions.

For example: why do the „via samples“ in the HV test box, which have no glue on the via copper ring not discharge immediately? Helium should penetrate rather fast, as there is an „open“ channel.

The situation of the HV test box is summarised in the next transparency.

Malte Hildebrandt



• 2 pcb samples installed in HV test box

1st pcb: 3 vias without glue on via copper ring

4 vias with glue only on via copper ring

2nd pcb: 4 vias with glue on via copper ring and with glue in via hole

• since Mar 3rd mounted inside HV test box

• flushed with pure helium interruptions: Mar 9th (sealing of HV feedthroughs)Mar 14th (sealing of GND on pcb)

• HV applied

• T = 40-45° C (partially off from Mar 16th to 30th due to broken heating blanket)

→ not yet a clear result → test will continue

→ 50 days „run conditions“(up to Mar 30th)

Malte Hildebrandt





→ why are there no discharges ?

GND +HV

pcb

G10 isolator

glue

+HV

glue

He

pcb

expected situation:

Malte Hildebrandt



There is one possible explanation for this situation: The edge of the GNDlayer may be somehow passivated with glue (as shown in the next transparency).

This situation may be caused by the special way of applying the glue on the GND layer whereas the HV via copper ring is protected.

Malte Hildebrandt



GND +HV

pcb

G10 isolator

glue

+HV

glue

He

pcb

possible situation:



→ why are there no discharges ?

Malte Hildebrandt


HV Via

To answer this question we prepared another „via sample“.It was ensured that the glue was not applied on the edge of the GND layer and the via copper ring.

Malte Hildebrandt


HV Via

protection with tapebefore application of glue

free edges of GND layerafter glueing

Malte Hildebrandt


New „Via Sample“ in HV Test Box

GND +HV

pcb

G10 isolator

glue

+HV

glue

He

pcb

new situation:

• new pcb samples installed in HV test box

3rd pcb: 7 vias without glue on via copper ring and edge of GND layer

Malte Hildebrandt


New „Via Sample“ in HV Test Box

→ observation: after a few hours of flushing the HV test box with helium we observe the discharges between HV via and (not passivated) GND edge.

→ conclusion:in case of the previous „via samples“ in the HV test box, most likely the edge of the GND layer is passivated with glue and consequently no discharges will occur!

top layer of pcb is covered with (red) capton tapein order to isolate / shield HV and GND on top layer

rectangular cut out of the capton tape to allow helium to penetrate through the via holeinto the gap between HV via and GND layer

discharge in via samplemounted in HV test box

Malte Hildebrandt


Further Tests in HV Test Box

→ Even the new pcb design, which will be used for chamber construction, will avoid this weak point of HV via and GND layer, we will continue with tests in the HV test box to understand better thereason for the accumulation of helium in this gap between HV via and GND layer.This will also include tests concerning the relation of breakdown voltageand gas mixture / pressure inside this gap.

Malte Hildebrandt


Next Steps

The pcb with the modified layout is ordered and is expected to arrive in the weekafter Easter.There is a small delay compared to the schedule presented at the review meeting. This is caused by intensive discussions (e.g. with the experts from the Pisa group) about further potential modifications of the design in order to avoid any other possible weak point.

A new test is planned which represents a cross section through the wing of a dc plane and the transition of anode frame - isolators - pcb - G10 isolator (small) - G10 isolator (wide) - hood framein order to exclude a „hidden“ discharge problem within this sandwich structure due to unexpected polarisation effects.

Immediately after the delivery of the new pcb two „new“ anode wire frames will be constructed (wires are already prepared on take-over frames). Together with recycled hood cathodes and middle cathodes two new dc modules will be assembled and they will be placed inside the „aquarium“ for a long term test.

In parallel we will then continue the repair work of the remaining dc modules as presented at the review meeting.

dear meg committee,psi, april 14 th 2009

Documents

hv status

hv print

hv problem

hv testsmar

hv vias

meg dc hv systemgoal

hv pcb trip

helium ethane