1

HV BOARDS and MUMETAL Integration

Claudio Arnaboldi

Tito Bellunato

Emanuele Panzeri

Gianluigi Pessina (*)

INFN Milano Bicocca

2

HV BOARDS and Magnetic field SHIELDING

-20 kV

-19.7 kV

-16.4 kV

The acceleration and focusing of the photoelectrons need 3 high voltages

The magnetic field must be shielded. A mumetal is foreseen around each HPD with a small gap, less than 1 mm: good insulation must be provided.

mumetal

mumetal

3

HV BOARDS: half column organization

-20 kV

-19.7 kV

-16.4 kV

A

B

C

A

B

C

A

B

C

A

B

C

5GΩ

Monitoring simplified diagram

The single -20 kV line enters each semi-column. It is split by a voltage divider in the final 3 HV voltages.

The 3 voltages feed the rest of the semi-column.

Two kind of boards are used: one with the splitter, the other with the voltage monitoring. Both of them bias a pair of HPDs.

A monitoring system is provided. Only one monitoring system is needed per semi-column. Therefore, the boards are not fully populated.

4

Column view: summer test 2005

HPDsHalf-column

Board with splitter

Board with splitter

Board with/without splitter

Board with/without splitter

Picture from Didier Piedigrossi

5

HV boards and LV boards

Picture from Didier Piedigrossi

LV boards

6

HV connections to the HPDs

Picture from Didier Piedigrossi

HV cables from HPDs

7

HV Boards layout: the PCB

The PCB measures 165 mm x 90 mm x 2 mm.The PCB has four layers.The HV tracks are buried under 0,8 mm of FR4 fiber glass to obtain very large insulation (FR4 has a dielectric strength of 80 KV/mm)

FR4

CopperPCB cross-section

2 m

m

0.8

mm

Many precautions has been taken to suppress any form of electrostatic breakdown.

The first step was a 4 layer PCB with buried tracks. Cuts has been implemented to be filled with rubber to shield the electric field parallel to the board itself.

8

HV Boards layout: silicon rubber protection

A silicon rubber having 20 kV/mm of break-down voltage covers the boards on each side.

9

HV Boards layout: silicon rubber protection and GND

New version

10

HV boards characterization

GND BS

= current monitor

= voltage monitor

To test the boards after production a sort of “analog boundary scan” is provided.

The line GND BS collects only the leakage current (nA range), while in the Main GND the whole biasing current from the splitter and the monitoring resistors is measured (tens of µA).

After the test the GND BS is connected to the main GND to provide low a impedance path.

Every board is checked in such a way for days under HV bias before passing the test.

Main GND

11

Test and monitoring of the HV boards in Milano

PC with MATLAB

Programmable power supply

HV Power SupplyISEG

HV HV boardsboards

Multichannel multimeter

Protection network

= analog values

= Digital data

Watch dog

ISEG

GPIB

GPIB

UPS

12

Results from irradiation

This board was covered with the so called DC 3-4241.

The leakage current is very small, at the limit of the instrumentation set-up.

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5Board covered by DC 3-4241 (after neutron irradiation)

Days

Lea

kage

cur

rent

(nA

)

-22

-21.5

-21

-20.5

-20

-19.5

-19

Hig

h V

olta

ge (K

V)

Average Ambient conditions: 20±2 °C and 50±8 % relative humidity.

Neutron irradiation gave no effects on the working condition of this board.

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Monitoring

2.5 V

1.2 MΩ

1.4 MΩ Pt1000Pt1000

Keithely 2700 6½ digit

multimeter

Elmb

10 GΩ

1.8 MΩ

HV board

We tested the monitoring system with the ELMB boards:

14

ElmbElmb resultsresults

0,036

0,0365

0,037

0,0375

0,038

0,0385

0,039

0,0395

0,04

1

260

519

778

1037

1296

1555

1814

2073

2332

2591

2850

3109

Board 1Board 2

day0 1 2 3

mV

00,10,20,30,40,50,60,70,80,9

1

1

241

481

721

961

1201

1441

1681

1921

2161

2401

2641

2881

3121

Board 1Board 2

day0 1 2 3

mV

ZOOM

The system works fine.

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Mumetal Integration

mumetal

mumetal The gap between the mumetal and the HPD is very small, less than 1 mm. The mumetal is to be connected to GND for safety reasons.

It needs an electric shielding material with very good insulating properties between the HPD and the mumetal to avoid discharges.

16

Which insulator?

Kapton 8x25.4 µm: excellent dielectric properties,good mechanical characteristicsexcellent radiation tolerancehygroscopic

extremely long (6 months!) delivery time

Polyester 3x125 µm: very good dielectric properties (only 10% less than Kapton)excellent mechanical characteristicsgood radiation tolerance

(tested up to 50 times the total estimated dose in LHCb)delivery time 4-5 weeksnon hygroscopicslightly flammablethicker

We tested two different insulator types:

17

Experimental results

The testing concerned the electrostatic discharge properties when the insulator is put in the final set-up.

The magnetic shielding investigation is in progress.

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Mumetal Integration: Kapton

0 0.5 1 1.5 2 2.5 3 3.5 4-5

-4

-3

-2

-1

0

1

2

Leak

age

curre

nt fr

om c

apto

n (n

A)

Leakage current from capton

Days0 1 2 3 4

-25

-21

-17

-13

-9

-5

-10

HV

(kV

)

The measured leakage was consistent with the specs.

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Mumetal Integration: Polyester

A dummy HPD has been biased with the 3 HV provided from an HV boards biased with a ISEG supply voltage.

The measurement is a sort of mini-system test.

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Measurement conditions

(Slightly) dummy HPD: complete and functional withhigh leakage current from the silicon sensor

Polyester film 3x125 micron wrapped around

Mumetal shield 760 micron thick, dimensions in specs

HV board prototype and ISEG power supply at nominal value 20 kV

Monitoring HV, current from the HPD pins (grounded)and from the mumetal (grounded)

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Leakage from polyester

0 2 4 6 8-5

-4

-3

-2

-1

0

1

2

3

4

5C

urre

nt fr

om m

umet

al (n

A)

Leakage current from mumetal (20-Jul-2005 11:33:58)

Days0 2 4 6 8

-20

-15

-10

-5

0

HV

(kV

)

Shut Downduringweek-end

The leakage from the system is adequately small.

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Conclusion: Polyester vs Kapton

• Dielectric strength: ~10% less for same thickness

• Non hygroscopic• Cheaper (~factor 20)• Promptly available (4 weeks vs 6

months)• Easier to handle• Slightly less radiation resistant, but

still factor 50 safe (data from NASA)

23

Future actions

Continue test in dry environment

Stop cycling and leave maximum HV for one month

Independently test breakdown properties(current profiles, sparks) without HPD

24

Spare slides

25

Actual monitoring circuit

A

B

C

A

B

C

5GΩ

VREF>0

Monitoring

Since the ELMB it is able to read only positive voltage levels we propose the following scheme where an offset is added > 0 V

RA

RA

RA

RB

RB

RB

The voltage measured is:( )

HAB

B

REFHAB

B

VRR

R

VVRR

RVmeas

+≈

−+

=

VH

VH