FDC Weekly Meeting
Date: March 21, 2007
Participants: Daniel, Tim, Kim, Simon, Brian, Chuck, Roger, Fernando, Joe
Next Meeting: Wednesday, March 28, 2007 @ 1:30 p.m.
Magnetic Field Issues
- We have heard back from Andrey Korytov from CMS who is one of their cathode chamber experts. He invited us to set up a phone conference with him to discuss the magnetic field issues. Simon will arrange to set this up in the next few weeks. Daniel will prepare a list of talking points and circulate it for comment. - Simon is working on preparing a list of gas mixtures used by various collaborations in their cathode chambers. He will then perform GARFIELD studies to plot the Lorentz angle as a function of cell position (equalizing the gas gains) so that we can come to some understanding of what are some candidate gas mixtures that we can consider. He will also study how the cell size affects the Lorentz angle. We need to also understand the impact of the time to distance relation on the gas choice (linearity, effective drift velocity) and we also need to consider chamber life time and performance issues with the different mixtures. Daniel has some articles on this that can be used for reference. - Brian has sent out several inquiries to folks at BNL and SLAC to find out about availability of suitable magnets with which to perform our B-field studies. He will also provide the information to Elke as their may be contacts in Germany that she can follow up on. - Simon and Daniel will work to develop a test plan for these studies and circulate for comments.
Material in the Active Area
- We are currently investigating ways in which to reduce the material in our active area. One plan is to remove the Rohacell backing from the cathodes. This would reduce the thickness in the active area by roughly 1/3. Simon will perform deflection calculations on the surface of the cathode (as a function of tension) so that we can understand how the electrostatic forces affect the surface. Once these are completed, Simon will perform calculations of the variation with gain across the FDC package from the edge (where the deflections are minimal) to the center (where the deflections are maximal). - We are also pursuing ideas to reduce the cathode trace thickness below 1/7 oz of Cu. Brian is contacting companies to find out what they can provide. 1/7 oz of Cu amounts to roughly 5 microns of Cu. Brian is looking into manufacture of cathode planes with thicknesses down to 1 micron of Cu. Fernando will think about a bench tests that can be performed to test the electrical properties of the cathodes to understand if there are any performance issues. He stated that he is more concerned about mechanical limitations. Namely that with very thin copper layers we will not be able to reliably solder to the surface (although gold-plating may be an option around this) and he also stated that with very thin Cu layers, vias are typically not reliable. - Brian is also contacting manufacturers to find out about aluminum conductors on these circuit boards. This cannot be handled within the PCB industry, and he is working to contact folks who can sputter on thin aluminum layers through a mask. - Roger has been doing some digging trying to find companies that can provide stock for our cathode boards that will allow us to manufacture our full-scale cathode as a single board. Note that the issue is not with the PCB manufacturer, but with coming up with the stock. Roger has had no luck in finding any companies, but he is still pursing some leads. - We will set a design goal of material in the active area of 1 to 1.5% of a radiation length for all 4 FDC packages. Starting from this, Simon will do some calculations to find what the associated copper thickness is. This will be our upper limit on Cu thickness that we can use as a basis for the design.
- Given the goal to reduce the material thickness in the active layer to 1 - 1.5% of a R.L., we are working on several aspects of the FDC package design. We are still assuming a nominal design with the preamp daughter boards connected to the frames. For the design we should assume that the connector thickness is 4.2 mm, so we need to design the system for clearances of 4.3 mm. The current thinking is that the wire plane connectors on the STB will be mounted to the face of the STB. For the cathodes, we want a design with all of the connectors on the back side of the cathode boards. So the daughter board connectors for the cathodes will reside in the middle of the cathode sandwich. Note that this will increase the layer to layer gap within a package by a few mm to accommodate this. This is not viewed as an issue. - Tim will work to produce a sketch of this layout so that we can understand the clearance issues for both the cathodes and the anodes. We will discuss this at our next meeting. - Fernando is continuing to contact connector manufacturers to find out what options are available for even thinner connectors. - Note that we have decided that our nominal half-gap should be 5 mm as the smaller the cell size the smaller the magnetic field affects. It may even be that we will have to plan for half-gaps below 5 mm, so we need to be prepared. - The issue with getting the cathode signals out from the back side of the cathodes is a bit of a tricky issue. Fernando will think about this and provide us with some options at our next meeting. - Our new nominal design for the cathodes is that the stereo angle will be +/- 75 deg with respect to the wires. Chuck will provide some CAD drawings so that we can understand the placement and clearance issues that we will be faced with. - The strip pitch will be a constant across the cathode surface. This pitch will be the same as the half-gap. However to minimize the rates on the hottest strips near the beam hole, we will most likely remove the conductor from the beam hole area and readout half strips on opposite sides of the circuit board. Note that the highest rate cathodes are presently in the last package. To combat this we have nominally decided to employ an increasing deadened area in the FDCs as we move from the upstream to the downstream package. David Lawrence will provide this size. This would amount to a 7-cm diameter deadened area for the most upstream package increasing to something like 10-cm for the most downstream package. This will likely reduce the occupancies to an acceptable level without having to cut the central strips in half.
Cathode Frame Design
- Tim has produced a drawing of a new sandwich design to replace the 5-mm thick (10-cm radial thick) G10 cathode frames. This sandwich would be constructed from thin graphite skins, with fiberglass and Rohacell on the inside. This would reduce the effective radial thickness of these pieces by a factor of 3 to 4. This is important to reduce the photon conversions in the FDC frames and the preshowering thickness. We are still nominally planning to employ G10 for the wire frames (necessary for rigidity) and CH2 for the spacers. - Simon will do some calculations of the effect of the new design from Tim on the thickness. - We need to contact Eugene to study the effect on his simulation results with the new design.
Low Voltage System
- Daniel had estimated that we should employ a single 30-A supply for each FDC package. Fernando has indicated that this may not be sufficient. His calculations indicated that we should employ 50 or 70A supplies for each package. We need to converge on this and make a decision so that we can update the budget appropriately. - Joe needs information on the LV granularity and fusing specs so that he can make some decisions on cable choices.
Small-Scale Prototype Plans
- Roger will complete the design of the new cathodes with the +/-75 deg stereo angle as well as the associated frames. These should be ready so that we can write the PR in about 3 weeks. Roger indicated that this should not be a problem. We will need to review the drawings to check for conflicts before they go out the door. - The 90 deg. stereo angle cathodes are available, however we cannot use both in the chamber as we cannot quantify the resolution without defining an external track (which we cannot yet do). However Daniel suggested to use one 90 deg plane and one 45 deg plane to do some intermediate studies. Simon will do some calculations to check what we can expect in terms of resolution. - Summer student: Simon and Daniel have completed the project definition for a summer student and have passed this on to Elke. The project would be to calibrate the scintillator hodoscope and the cosmic ray chambers in our test setup.
- Daniel circulated a request for budget items for the FDC to Tim, Brian, Fernando, and Joe. These are needed ASAP as Elke needs the budgets by early next week. Things are starting to trickle in, and folks should contact Daniel if they have any questions on what to provide.
- Daniel has prepared a draft of the FNAL MOU for stringing the FDC wire planes based on an example provided by Mike Syptak. Brian and Simon need to provide feedback. We will then distribute the draft to Elke, Elton, and Karen Kephart for feedback. The next step is to turn this over to Mike Syptak and the procurement folks here at JLab. - Note that our plans are to wind several test planes this summer so that we can finalize the specs and the winding technique.
Dummy Cathode Planes
- The dummy cathode planes are set to arrive at JLab next week. Simon will perform strip-to-strip capacitance measurements on them. He will also contact Gerard to find out if there is anything else useful that we could measure. Joe will provide a unit to perform the measurements. - Brian will then begin construction of a second Rohacell-backed sandwich and a full sandwich without the backing so that we can understand the distortion issues and the cathode construction issues in this alternative design. - Brian will follow up the JLab Survey Group about the flatness measurements (non-contact) of the new cathode planes. We need this ready in about 3 weeks or so.
Minutes prepared by Daniel. Send any comments or corrections along.