Potential Hardware Improvements, Related R

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Potential Hardware Improvements, Related RD

• Wieman
• RNC
• LBNL
• TPC Review
• Friday 6 October 2006

2
Issues, prioritized? list

• Inner field cage, single gap fluctuating
  resistance
• Outer field cage, single gap fluctuating
  resistance
• Grid Leak, space charge distortion
• Reduction of space charge distortion, increase
  drift field
• Use corrected pad response function to improve
  tracking precision

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Inner field cage, single gap fluctuating
resistance

• The problem A short developed in the inner field
  cage that unlike others could not be found and
  removed. Solution was to lock down the short and
  increase neighbor resistors to restore
  undistorted drift field. Later developed reduced
  resistance with humidity dependence. It was
  assumed that epoxy used was bad and bridged a gap.

Resistor chain cover
Inner field cage outside of drift volume
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IFC East variable resistance

• Distortion now being minimized by adjusting
  current to hold most of the cage to the proper
  boundary condition
• Possible because flaw is very close to the sector
• Proposed solution clean up epoxy
• Risk low
• Effort relatively low remove SVT with cones to
  access
• Will be done when TPC rolls out and SVT is
  removed

For scale one resistor error 420 nA
B. Stringfellow
Field cage currents in IFCW (blue) and IFCE
(brown) showing excess current and noise in IFCE.
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Outer field cage West bi-stable problem

• Distortion small, but can be detected
• Cause unknown, concern if becomes worse
• Action being considered
• Develop small contact probe to insert into
  insulating gap to locate z position
• Develop clean techniques with outward air flow
  when opening the gap
• Knowing z position allows precise correction
• Risk could introduce dust or relocate existing
  dust and cause a corona leak

For scale one resistor error 420 nA
B. Stringfellow
Field cage currents in OFC west (blue) and OFC
east (brown) showing the bi-stable excess current
in OFC west
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Outer field cage West bi-stable problem

• To replace resistor in the outer field cage
• Requires putting people inside TPC
• Opening of resistor chain cover
• Difficult and very risky

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Fix grid leak to reduce positive ion distortions?
Ground stripes
A. Lebedev
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Inner sector
A. Lebedev
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Outer sector
A. Lebedev
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Fix grid leak to reduce space charge distortion

• Tasks (if work done outside TPC)
• Rework sector tool
• Construct clean rooms
• Develop and fab barrier electrodes
• Strip sectors
• Remove inner sectors and add electrode
• Reinstall sectors
• Reinstall cooling manifolds
• Reinstall electronics and test
• Effort 44 man months
• TPC down time 6 months
• Cost (if sector and electronics handling labor
  free) 424 k
• Risk significant, need to redevelop lost skills
  and know-how
• Alternative approach work inside very risky,
  but could save effort

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Increased drift field to reduce space charge
distortion

• Double field and reduce distortion by 4
• Double positive ion velocity reduces space charge
  by two
• Increase drift E field component relative to
  space charge E field
• Requires C2F6 gas in the 5 cm outer field gas
  insulator gap
• Present gas nitrogen
• Present voltage 27 kV
• One problem a leak between drift volume and the
  insulator volume which prevents use of C2F6
• We observer rapid onset of CH4 in the insulator
  gap when TPC pressurized to 2 mBar operating
  pressure

Distorted path
E space charge
E drift
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Increased drift field to reduce space charge
distortion

• Alexei Lebedev would like to try to find the leak
  using gas transit times an interesting
  approach, but one would have to be lucky to find
  the leak. There is a lot of surface. Only if it
  is at the end joint could one ever hope to solve
  the problem.
• Other risks
• Inner field cage hold off
• Corona leaks
• More shorts between equipotential rings

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Additional improvements that have been considered

• Correction for non-Gaussian pad response function
  possible tracking improvements on 200 micron
  scale (A. Lebedev )
• Mostly a software issue
• Low frequency laser shots during particle runs to
  dynamically monitor space charge ( A. Lebedev )
• Could be done using a laser shutter
• CF4-Ar TPC gas for modest improvement in
  transverse diffusion ( N. Smirnov )
• Requires higher drift voltage

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MAGBOLTZ result for ArCF4 gas mixture ( 2.5,
5., 7.5, 10. ) in 0.5 T magnetic field, and data
for P10.
N. Smirnov
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END

• No easy or free lunch