HV BOARDS and MUMETAL Integration. Claudio Arnaboldi Tito Bellunato Emanuele Panzeri Gianluigi Pessina (*) INFN Milano Bicocca

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1 HV BOARDS and MUMETAL Integration Claudio Arnaboldi Tito Bellunato Emanuele Panzeri Gianluigi Pessina (*) INFN Milano Bicocca 1

2 HV BOARDS and Magnetic field SHIELDING 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 -20 kv The acceleration and focusing of the photoelectrons need 3 high voltages kv kv mumetal 2

3 HV BOARDS: half column organization -20 kv 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 kv kv A B C A B C 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 semicolumn. Therefore, the boards are not fully populated. Monitoring simplified diagram 5GΩ A B C A B C 3

4 Column view: summer test 2005 Board with/without splitter Board with splitter Half-column HPDs Board with splitter Board with/without splitter Picture from Didier Piedigrossi 4

5 HV boards and LV boards LV boards Picture from Didier Piedigrossi 5

6 HV connections to the HPDs HV cables from HPDs Picture from Didier Piedigrossi 6

7 HV Boards layout: the PCB 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. 2 mm PCB cross-section Copper FR4 0.8 mm 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) 7

8 HV Boards layout: silicon rubber protection A silicon rubber having 20 kv/mm of break-down voltage covers the boards on each side. 8

9 HV Boards layout: silicon rubber protection and GND New version 9

10 HV boards characterization To test the boards after production a sort of analog boundary scan is provided. GND BS Main GND 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. = current monitor = voltage monitor Every board is checked in such a way for days under HV bias before passing the test. 10

11 Test and monitoring of the HV boards in Milano ISEG UPS Watch dog Programmable power supply HV Power Supply ISEG GPIB PC with MATLAB GPIB HV boards Multichannel multimeter Protection network = analog values = Digital data 11

12 Results from irradiation This board was covered with the so called DC The leakage current is very small, at the limit of the instrumentation set-up. Neutron irradiation gave no effects on the working condition of this board. Average Ambient conditions: 20±2 C and 50±8 % relative humidity. Leakage current (na) Board covered by DC (after neutron irradiation) Days High Voltage (KV) 12

13 Monitoring We tested the monitoring system with the ELMB boards: 1.2 MΩ 10 GΩ 2.5 V Pt MΩ 1.8 MΩ Pt1000 Elmb HV board Keithely ½ digit multimeter 13

14 Elmb results 0,04 0,0395 0,039 0,0385 0,038 0,0375 0,037 0,0365 0, day Board 1 Board ,9 0,8 0,7 0,6 mv 0,5 0,4 0,3 0,2 0,1 0 Board 1 Board 2 mv day ZOOM The system works fine.

15 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. 15

16 Which insulator? We tested two different insulator types: Kapton 8x25.4 µm: excellent dielectric properties, good mechanical characteristics excellent radiation tolerance hygroscopic extremely long (6 months!) delivery time Polyester 3x125 µm: very good dielectric properties (only 10% less than Kapton) excellent mechanical characteristics good radiation tolerance (tested up to 50 times the total estimated dose in LHCb) delivery time 4-5 weeks non hygroscopic slightly flammable thicker 16

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. 17

18 Mumetal Integration: Kapton The measured leakage was consistent with the specs. Leakage current from capton (na) Leakage current from capton HV (kv) Days 18

19 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 minisystem test. 19

20 Measurement conditions (Slightly) dummy HPD: complete and functional with high 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) 20

21 Leakage from polyester The leakage from the system is adequately small. Current from mumetal (na) Leakage current from mumetal (20-Jul :33:58) Shut Down during week-end HV (kv) Days

22 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) 22

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 23

24 Spare slides 24

25 Actual monitoring circuit Since the ELMB it is able to read only positive voltage levels we propose the following scheme where an offset is added > 0 V V H A A 5GΩ R A B R B V H R A C B C Monitoring R B R A R B V REF >0 The voltage measured is: Vmeas = R R B B RB + R RB + R A A ( V V ) V H H REF 25