TPC Distortions in the Transverse Plane: - PowerPoint PPT Presentation

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Title: TPC Distortions in the Transverse Plane:


1
  • TPC Distortions in the Transverse Plane
  • An Update
  • Jim Thomas

2
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.)

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
Radial Distortions
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
Wiemans HiJet
1/R
Linear 21
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 dont know about.)

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 ltLgt
  • 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 DCAs 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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