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TPC Distortions in the Transverse Plane: An Update Jim Thomas

This update covers potential distortions in the transverse plane, their correction methods, and models to understand space charge effects in detectors. Different sources of distortion and ways to address them are discussed, providing insights into data analysis and preparation for future observations.

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TPC Distortions in the Transverse Plane: An Update Jim Thomas

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  1. TPC Distortions in the Transverse Plane: An Update Jim Thomas

  2. Time Projection Chamber Magnet Coils Silicon Vertex Tracker TPC Endcap & MWPC FTPCs ZCal ZCal Endcap Calorimeter Vertex Position Detectors Barrel EM Calorimeter Central Trigger Barrel or TOF Central Membrane Outer Field Cage Endwheel and Padplane RICH Inner Field Cage A Mnemonic for Listing the Potential Distortions

  3. The List of Distortions in the Transverse Plane • The list can be enumerated by surfaces: • Outer field cage corrections • Inner field cage corrections • Central membrane corrections • End-wheel and pad-plane corrections • Pad Row 13 corrections and other local electrostatic defects • Rotation and miss-alignment of sectors with respect to their ideal locations • Rotation of either TPC end-wheel with respect to its ideal location • and by volume: • Space Charge corrections due to charge in the volume of the TPC • Magnetic field corrections due to B fields in the volume of the TPC • Twist of the TPC with respect to the magnetic field axis and/or the measured map • General coordinate transformations • A few additional items are listed for completeness. (These items affect the drift of the electrons in the Z direction but do not strongly affect the distortions in the transverse plane.) • Gas composition and variations in the drift velocity • Barometric pressure changes and variations in the drift velocity • Pressure variations as a function of height in the TPC • Temperature gradients in the TPC

  4. We think of our events like this … Data Taken June 25, 2000.

  5. Most of our events look more like this …

  6. SpaceCharge from (all) events cause distortion Radius / 5 cm Z / 5 cm Distortion Radius Z

  7. Model for the distortions • Old Model • Beam gas events leave a uniform deposition of charge in the TPC • The charge from the collisions is not significant • New Model • Beam gas events leave a 1/R2 distribution of charge in the TPC • The charge from the collisions is not significant in the 2001 data, but will be in the future (?)

  8. Uniform .vs. 1/R2 Space Charge Distribution Distortion Distortion Radius Radius Z Z RadialDistortions

  9. Two sources of SpaceCharge • Beam gas and other up stream events • not synchronous with our trigger • Scales with beam intensity (not Luminosity) • 1/R2 distribution of charge (?) • The collisions at STAR • synchronous with our trigger • Scales with Luminosity • A/R + B/R2 distribution of charge (?) • In the future, the average Luminosity will go up a factor of 40 but the beam intensity will only go up a factor of 2 to 4 • We have to prepare for a significant increase in space charge due to the collisions in the detector. We have to be able to distinguish the two sources of distortion

  10. A Wide Range of Charge Distributions Voltage Radius 1/R3 1/R2 Wieman’s HiJet 1/R Linear 2:1 Linear These (and other) Distributions are Available in StMagUtilities

  11. 2 Equations, 2 Unknowns • We can simultaneously fit the DCAs and match the steering at the RICH due to the beam gas induced space charge • Choose the right charge distribution 1/R, 1/R2, HiJet, etc. • Choose the RICH scaler normalization constant with the DCA held constant

  12. These Items on the List Are Ready to Go • The list can be enumerated by surfaces: • Outer field cage corrections • Inner field cage corrections • Central membrane corrections • End-wheel and pad-plane corrections • Pad Row 13 corrections and other local electrostatic defects • Rotation and miss-alignment of sectors with respect to their ideal locations • Rotation of either TPC end-wheel with respect to its ideal location • and by volume: • Space Charge corrections due to charge in the volume of the TPC • Magnetic field corrections due to B fields in the volume of the TPC • Twist of the TPC with respect to the magnetic field axis and/or the measured map • General coordinate transformations • A few additional items are listed for completeness. (And these I don’t know about.) • Gas composition and variations in the drift velocity • Barometric pressure changes and variations in the drift velocity • Pressure variations as a function of height in the TPC • Temperature gradients in the TPC

  13. Conclusions • Tools are available to calculate all known distortions • in the transverse plane • gain, t0, and drift velocity corrections have not been discussed • SpaceCharge corrections are significant • especially at 40x <L> • We need to track the beam current and/or the distribution of charge in the TPC for untriggered events. We will need scalers and diagnostics for each source of charge • Monitor L • Monitor beam current • The RICH Mult Scaler is gone and we need a replacement • Recent progress with the laser cluster finder means we might be able to use this data …

  14. DCA, Steering at the RICH, and Dp • The RICH Scaler has an arbitrary normilization • The DCA’s have been tuned to be the same in both cases • Dp is different by a factor of 2 • Steering at the Rich changes sign under these conditions

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