Minutes 4-11-2007

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FDC Weekly Meeting

Date: April 11, 2007

Participants: Daniel, Simon, Brian, Tim, Kim, Roger, Chuck

Next Meeting: Wednesday, April 18, 2007 @ 1:30 p.m.

Cathode Prototyping

 - Brian has been working on preparing the tensioning of one of our
   new dummy cathode planes with the copper strips.  His plan is to
   begin the tensioning process tomorrow.  An important aspect of this
   work is to determine the tension in the cathode.  Tim has produced
   a write-up describing the procedure.  The plan is as follows:
    1. Determine the force on the cathode from the electric field.
    2. Determine the allowable deflection of the cathode at its center.
    3. Convert the field force to a point force at the center of the
       cathode plane.
    4. Calibrate a dial indicator in the deflection range with twice
       the approximate expected deflection.
    5. Setup the cathode plane in the tensioning device.
    6. Set up the dial indicator over the cathode plane until it just
       touches the cathode surface.
    7. Tension the cathode plane until the indicator moves the desired
 - In this process we do not want the dial indicator to damage the
   surface.  Brian will work to include a piece on the end of the
   meter to spread out the force over a larger area.
 - Tim will participate in the measurements to be sure that we can
   get a meaningful result.  Simon will work to transform this tension
   measurement into a measure of the deflection at the center of the
   cathode.  Assuming the surface shape is parabolic, we can then
   map out the shape of the surface over its full extent.  Simon
   will also work to compute some measure of the gain variation
   across the chamber.
 - On the dummy cathode Brian has drawn a circle.  His plan after the
   tension is to measure this circle to check for lateral distortions
   of the cathode surface.
 - Brian has given a piece of the cathode to the JLab Survey Group.
   They have passed this on to a company called Brown and Sharp in
   North Carolina who will attempt to perform an optical measurement
   of the surface flatness.  Once they develop the technique, Brian
   will bring the completed cathode to them for measurement.  The
   plan is to perform these measurements with the cathode in a
   vertical orientation to avoid issues with sag due to gravity.
 - We discussed an alternative plan for the cathode backing.  Given
   the worries associated with electrostatic distortions, it may be
   that we can employ a modified design for the backing.  Instead of
   a solid piece of Rohacell, we could consider using two thin Rohacell
   skins sandwiching an inner core of a low density foam or of a
   material like Nomex honeycomb.  Brian will make some contacts to
   get us some material samples.

Cathode Frames

 - Simon has passed the design numbers for the composite cathode frame
   on to Dave for MC work.  He will also pass this information on to
   Richard and Eugene so that they can complete some studies.


 - DSC has been in contact with David regarding MC work on two fronts.
   1). Determine the size of the beam hole that we can deaden to give
       us acceptance down to 1 deg.  The present nominal design is to
       deaden a fixed circle of 7-cm diameter independent of z.  However
       the 1 deg point increases as a function of z.  Given that the
       rates increase dramatically as we go downstream, it makes sense
       to increase the size of the deadened region for each package.
       Once we have an answer on the deadened region, we can then complete
       the design of the cathode planes and address whether we want to
       split the central cathodes in two.  This has an issue with
       calibrations due to the change in capacitance near the end of
       the half strips as well as in reconstruction of the cathode
       coordinate near the ends of the strips.
   2). To determine the HV granularity, we need to consider minimizing
       the number of wires per HV channel (in case of wire breakage and
       the need for HV disconnects) and we also need to consider equalizing
       the current per channel.  These two issues will force us to
       compromise.  David will provide information on the rate per wire.
       This is essentially equivalent to the current per wire.

Circuit Boards

 - Roger has not yet found a company to produce the cathode material
   in a single piece.  He still has two companies left to contact.  However
   he is confident that we can probably reduce the number of cathode
   pieces from 3 to 2.  More to come.
 - Brian had some exchanges with a PCB company (CCT Marketing).  They
   can make our STB and HVTB boards as a single piece provided that we
   go to a two-layer design.  There may be other options out there so
   that we do not have to move away from our current 4-layer design.
 - With the new gas mixtures that we are considering for the FDCs, it
   seems that our current design with 3-kV rated capacitors will be
   on the edge.  We will most likely have to move to 5-kV rated components.

Wire Frames

 - We discussed again the idea of leaving cut-outs in the G10 wire
   frame behind the capacitors.  This is something that we will move
   to design in detail.  This will give us access to replace the
   capacitors should we ever have to and they will therefore not be
   buried under the circuit boards.

Attachment Feet

 - Last week Chuck presented a design for the feet that will attach the
   FDC packages to the rails.  His initial design was quite beefy and
   we discussed ways to reduce the material.  Chuck will revisit this
   and consider thinner materials and plastics.  More to come on this.

Small-Scale Prototype Cathodes

 - Roger has completed a first layout of the new +/-75 deg cathode
   boards for the small-scale prototype.  His design was a single
   layer design.  We asked him to modify this to be a two-layer design
   with a ground plane layout similar to the existing boards.  We
   do not want to change too many things at once.
 - We want to order boards on a 1-mil Kapton backing with 5 microns
   of copper (1/7 oz) and 2 microns of copper.  The 2-micron choice
   was made because this seems to be the thinnest copper layer that
   we can find.

Work List

 - The FDC short-term work list has been posted on the FDC web site
   (see http://www.jlab.org/Hall-D/detector/fdc/).  DSC is still
   waiting for feedback and comments from the group.

Magnetic Field Tests

 - We are still waiting to hear back from Brian's contact at BNL
   regarding a magnet for our tests.  It may be that the magnet
   does not have its orientation in the proper direction.  DSC
   reminded Elke about her contacts in DESY and she said that she
   will get back on this.
 - After the meeting DSC talked with Elton.  He suggested that we
   consider using the old pair spectrometer magnet in the downstream
   alcove of Hall B.  DSC will find out more about the field strength
   and the gap of this magnet, as well as its availability this
   summer for tests.  The field in this magnet is vertical.  Elton
   seemed to remember that the gap was 6-8 inches high.

Minutes prepared by Daniel. Send any comments or corrections along.