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TPC Distortions in the Transverse Plane: An Update Jim Thomas

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

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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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