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Potential Hardware Improvements, Related R&D

Wieman RNCLBNL

TPC ReviewFriday 6 October 2006

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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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Ground stripes

Fix grid leak to reduce positive ion distortions?

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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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

.474

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 charge

Distorted 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

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END

No easy or free lunch