Minutes of the brainstorming meeting concerning the backframe and the HV connectors
- Stefano Caiazza
- Doerte David
- Ralf Diener
- Felix Mueller
- Volker Prahl
- Christoph Rosemann
- Ron Settles
- Robert Volkenborn
The focus of this meeting was to present and provide ideas for the optimization of the backframe production and the external HV distribution of the new LP-TPC module (Saphir). By previous consensus it was agreed to keep the design modifications to a minimum to be able to field 3 new modules by the beginning of the summer 2012. The deadline for the production of the new backframes and the procurement of cables and connectors for the HV distribution of such new module was fixed at the end of April 2012.
The general design seems to be adequate for the requirements of the new module. The new mounting tool should be compatible with the old module so no major design changes are foreseen. The drawings of the latest iteration of the module design are provided in the following attachment BackFrame5.3.pdf. A few minor design changes were discussed:
- To simplify the module insertion Volker proposed a small chamfer to the module insertion edge (where the screw holes for the mounting bracket are). This design modification was judge to produce no negative effects on the back-frame function
- One important new function of the new back-frame is to provide a support for the mounting structure of the new HV connectors. Two options were evaluated:
- To mount the connector bulkhead over the mounting bracket. This type of structure will only make use of existing features of the existing back-frame. On the other hand, during the module insertion procedure the bulkhead and the attached connector would be completely decoupled from the board or the back-frame leaving it prone to damage.
- The second option is to fix the connector bulkhead to the size of the back-frame. This solution needs a modification of the back-frame design with the introduction of several screw holes on the side of the device. Volker and Robert confirmed that such a modification of the back-frame could even be done in a post-production process without damaging the devices.
To align the pad board to the back-frame, the old system foreseen already for the Korund module was kept. This system consist on 1 mm wide pins to be fit through the board and the backframe during the gluing procedure and to be removed afterwards
Robert will take care of the design modifications, Volker is taking care of the procurement of the best material.
For the previous modules the back-frames were produced at the University Workshop. Something in the procedures at that workshop changed thus Volker and Robert will check whether we can still use that workshop for the production and at which conditions. Alternative solution are the Central workshop (very busy ATM) and the ZEUS CNC machine. Robert and Volker also expressed the will to try to perform the liquid nitrogen cold shock procedures suggested by Dan Peterson for the production of the original backframes. Considering that we have still 1 available backframe of the old production cycle the consensus agreed that an optimal solution could be the production of 2 back-frame with the same procedure used for the old back-frames (simple CNC machining), and to try and produce three more back-frames with the liquid nitrogen cold shock procedure so that we would have 3 back-frames even if some unforeseen event would delay the production of the back-frames with the new procedure. For the liquid nitrogen procedure we will need to build some additional tooling. After the production we will arrange for a survey of the module that will measure the position of the alignment holes and the flatness of the mounting surface of the pad plane
HV Multipolar Connectors
To reduce the space occupied by the HV distribution on the connector side of the board where, in the new design, there will be about 150 connectors compared to about 20 in the Korund design, the pad board has been designed with the following improvements over the simplified design of the Korund module:
- The protection resistors will be mounted on board
- The 20 independent HV lines of the simplified module are reduced to 8 independent HV level to be provided from outside. The decoupling of the 4 sectors will be done on board via the protection resistors
- The 8 voltage levels will be collected in 2 multipolar HV connector, each one providing 4 independent voltage levels
After some research done last year by Stefano and Doerte we decided to look for products from Fischer Connectors. This company provides safe and compact HV connectors which we already used in the LP-TPC. The chosen connector is the Model 104 of the Core Series that you can find on page 5-4-1 and 5-5 of the Fischer Catalogue In particular we plan to use:
- The receptacle (to be mounted on board) D 104 Z062-130 at 25.23€ per piece for an order between 5 and 20 pieces
- The plug (to be mounted on cable) S 104 Z062-130 at 43.60€ per piece for an order between 5 and 20 pieces
These quotes come from an offer we received last year and need to be update but it's unlikely there will be a substantial price difference. To produce 3 modules we need at least 6 modules. Foreseeing at least a couple of spare pieces we need at least 8 connectors. Considering that the production of the multi-cable assembly was never tested it could be safer to order up to 10 connectors
Mounting the connector on-board
To install the connector receptacle on board we need to build a small bulkhead and fix this element to the board or to the backframe. Due to the limited available space on board and the impossibility to drive holes through the board that must gas-seal the TPC this bulkhead must be fixed on the back-frame. This connection can be done, as previously stated:
- Mounting the connector bulkhead over the mounting bracket or
- Modifying the back-frame to fix the bulk-head on its side.
A suitable amount of space was already reserved for this structure in the pad board design. Assuming the problem of the mounting of the bulkhead is solved there it remains to address the issue of properly grounding this device and the related connector with it. Two solution where analyzed:
- A wire could be connected from the ground pad of the PCB to the back-frame. The bulkhead will be screwed to the back-frame as well thus closing the ground loop. To improve the connection a thick copper wire could be also routed around the backframe to directly connect the point where the pad board ground is fixed to the grounding point for the bulkhead. This solution requires some bricolage but it's easy to implement and safe
- A small modification to the pad board will introduce a small grounding area in the area reserved for the connector. The risks involved in this solution is that, introducing a live soldering area closer to the HV pads could induce some discharging. This problem could be solved if the live connection could be insulated after the soldering.
Because the first solution can always be implemented we thought it is wise to pursue the second one and try to solve the related problems
HV Cable Assembly
The main problem in the HV distribution with a multipolar connector is the cable assembly. Ordering of special multi conductor cable is a tricky business because these type of order usually involve a minimum amount of the order of a few Km of cable. Our need is for about 25m of cable. The alternative solution is to use standard issue HV cables, as thin as possible and bundle them in the connector. The feasibility of this solution was confirmed by prior experience (there is one cable like this in the lab with more than 20 cables bundled together) and by the Fischer company. We have two choices of cable available. CERN provides 5KV cable with 3 mm diameter while in the DESY store the smallest cable available which stands up to 5KV has a diameter of 3.3 mm. Doerte prepared and measured a test bundle of the first type and, after the meeting, a second bundle of the second type. The first type will fit the biggest available collet of the connectors we want to order. The 3.3 mm cable will teoretically fit the same collet but the range in the measured values of this bundle is quite wide and the fit will be just fine. To make sure that the solution will work we should order first a set of collets, test them and, only afterwards, order the rest of the material. This will increase the delays in the module construction.
To perform the cable assembly itself we thought about asking ZE. They should assemble the cable boundle with the multipolar connectors we will provide and install the SHV connectors on the other end. An optimal length for the whole cable could be about 2.5 m, enough to clear the electronics support rings of the TPC and optimal for the assembly considering that each single cable strand could be obtained splitting a standard 5 m cable. With this solution each connector would need about 10 m of cable