Potential Hardware Improvements, Related R&D
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Transcript of Potential Hardware Improvements, Related R&D
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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.
Inner field cage outside of drift volume
Resistor chain cover
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Field cage currents in IFCW (blue) and IFCE (brown) showing excess current
and noise in IFCE.
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 error420 nA
B. Stringfellow
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Field cage currents in OFC west (blue) and OFC east (brown) showing the bi-stable excess current in OFC west
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 error420 nA
B. Stringfellow
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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 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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SECTOR
TOF RAIL
OUTER FIELD CAGE
OUTER FIELD CAGE GAS INSULATOR
157.425 WEST
157.415 MIDDLE
157.430 EAST
157.489 WEST
157.499 EAST
214.72
84.5345
9.14
3.641 ACTUAL
1.2
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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
E drift
E space chargeDistorted path
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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 resultfor Ar+CF4 gas mixture( 2.5, 5., 7.5, 10. %)in 0.5 T magnetic field,and data for P10.
N. Smirnov