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Potential Hardware Improvements, Related R&D

Potential Hardware Improvements, Related R&D. Wieman RNC LBNL TPC Review Friday 6 October 2006. Issues, prioritized? list. Inner field cage, single gap fluctuating resistance Outer field cage, single gap fluctuating resistance Grid Leak, space charge distortion

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Potential Hardware Improvements, Related R&D

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  1. Potential Hardware Improvements, Related R&D 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

  3. 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

  4. 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 IFC East variable resistance 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.

  5. 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 Outer field cage West bi-stable problem 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

  6. 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

  7. Fix grid leak to reduce positive ion distortions? Ground stripes A. Lebedev

  8. Inner sector A. Lebedev

  9. Outer sector A. Lebedev

  10. 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 Fix grid leak to reduce space charge distortion

  11. 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

  12. 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

  13. 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

  14. MAGBOLTZ result for Ar+CF4 gas mixture ( 2.5, 5., 7.5, 10. %) in 0.5 T magnetic field, and data for P10. N. Smirnov

  15. END • No easy or free lunch

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