# Presentation: 484120

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

## A Mnemonic for Listing the Potential Distortions

## 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

## We think of our events like this ...

- Data Taken June 25, 2000.

## Most of our events look more like this ...

## SpaceCharge from (all) events cause distortion

**Z  / ****5 cm**

**Radius / ****5 cm**

**Z**

**Radius**

**Distortion**

## 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/R****2 ****distribution of charge in the TPC**
**The charge from the collisions is not significant in the 2001 data, but will be in the future ****(?)**

## Uniform  .vs.  1/R2  Space Charge Distribution

**Radial********Distortions**

**Z**

**Z**

**Radius**

**Radius**

**Distortion**

**Distortion**

## Two sources of SpaceCharge

**Beam gas and other up stream events**
**not synchronous with our trigger**
**Scales with beam intensity (*****not Luminosity*****)**
**1/R****2**** distribution of charge ****(?)**
**The collisions at STAR**
**synchronous with our trigger**
**Scales with Luminosity**
**A/R + B/R****2**** 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**

## A Wide Range of Charge Distributions

**1/R****3**

**1/R****2**

**Wieman’s HiJet**

**1/R**

**Linear 2:1**

**Linear**

**These (and other) Distributions are Available in StMagUtilities**

**Radius**

**Voltage**

## 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/R****2****, HiJet, etc.**
**Choose the RICH scaler normalization constant**

**with the DCA held constant**

## 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

## 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 ...**

## DCA, Steering at the RICH, and p

**The RICH Scaler has an arbitrary normilization**
**The DCA’s have been tuned to be the same in both cases**
** ********p is different by a factor of 2**
**Steering at the Rich changes sign under these conditions**