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Ion Effects [message #939] Thu, 28 June 2007 03:24
Messages: 23
Registered: June 2006
Location: University of Bonn
Below are Ron's minutes of the meeting on ion effects at DESY on June 2, 2007:

WP#30 Special discussion on ion effects
2 June 2007, 14:00-ca.17:00
Sem Room 1

Chair: Takeshi

Ron - Boundary conditions, Beijing report
Adrian - TPC occupancies
Astrid (presented by Martin) - Space charge
Akira - GEM gating
Vincent - Gas possibilites
Dean - Ideas on how to measure/correct for ions using photoelectrons
Everybody - Concluding discussion

All talks are avialable at the LCWS07 site: onId=144&materialId=1&confId=1296


1. Ron reminded of the Snowmass 2005 and other work which is summarized in our Beijing TPC report (now published as LC Note LC-DET-2007-005 (see

-Space charge effects can be minimized by choosing a gas with large \omega\tau.

-The Star TPC review Oct.2006 estimates mean for us (large \omgea\tau) up to 200fC/cm3 in the volume would give rise to about 10cm drift-electron dispacement over the full drift, and this is the magnitude of the B-field effects we have to correct.

-Back-of-the-envolope and the Tesla TDR estimates give 0.5% occupancy for nominal backgrounds and about one fC/cm3 in the volume assuming 100e per occupied voxel. (Adrian at this meeting give more solid numbers based on simulation, and his numbers are lower as seen below).

-In the sheet, the density might be as large as 100 fC/cm3, but the sheet is thin next to the Gem/Micromegas plane so its effect should be small (must be simulated). In the volume, the sheets can be eliminated by gating between trains.

-The correction for space-charge and B-field (antiDID) of about 1 cm means measuring the effects to 2x10-5, the tools for doing this are known (see the Beijing report); this order of correction was achieved by the Aleph TPC.

-The distortion effects and thier correction can and must be studied further by simulation.

-When thinking of the solution to the space-charge issue, we should not compromise the point resolution or the dE/dx resolution.

2. Adrian's study of the backgrounds (based on Guinea Pig) and their effect on TPC occupancy have now advanced to a rather sophisticated level. He described in his talk the various background sources. The main result on occupancy is seen in slide 12, where it is seen that we would expect on average about 0.04% occupancy (the radial dependence is on slide 13) for the pad sizes we are now considering, which is an order of magnitude lower than the TDR days. That plot also demonstrates what we have said since the beginning (2001) that we should make the granularity as fine as possible to reduce the background occupancy as much as possible. Adrian's
slide 14 on the electron space charge for 100 BX must be multiplied by 150 since the ions take about one second to drift out (worst case) and itegrate over 15000 BX (near the anode); nevertheless, Adrian's values are an order of mignitude lower than above, point 1. Note that backgrounds from minijets and muons are not yet included in Adrian's simulations.

3. Astrid's thesis work was reported by Martin. She has developed a tool which can be used to study the ion-sheet effect. It is based on Heeds, Magboltz and Gem charge-transfer parameterization. It gives the charge density in the back-drifting ion sheet (slides 5, 6, 7). This can be used
for optimizing Gem settings and simulation distortion effects due to the sheets.

4. Akira, and CDC colleagues have studies many properties of gating using wires or Gems (the micromesh-gating was not reported on). The ExB effect for wire gating seems to deteriorate about 10% of the gap between the wires. For Gem, many ideas were looked into; no attempt will be made here to summarize all of the facits of the parameters simulated and/or measured; Akira's slides are the best summary of the understanding. The conclusion for the moment is that a Gem gate should have larger holes, thinner Gem, a low drift field and high (but not too high) transfer field: in that case the electron transmission up to 70% might be possible (is this enough?). The basic idea was to decide on a scheme using Gem gating and then to test it at using the MPTPC at Kek or, see Akria's final slide no. 21, at the 5T magnet at Desy.

5. Vincent gave a nice review of issues and numerical values for the electron and ion drift properties. Possible solutions involve exotic ideas (e.g. choosing a gas with fast ions or making a very short TPC) and less exotic possibilites (e.g. gating to avoid the sheets in the drift volume). He points out that the first thing to do is more simulation work in order to understand the magnitude of the effects. (One comment to his
"extra slide" No.16: the Star TPC has such a sheet, but a cylindrical one due to a mismatch between the wire grids on the inner and outer sectors. The correction for this distortion is rather well understood.)

6. Dean explained his proposal to study ion distortions at the LCTPC using the displacement of photoelectrons emitted by a pattern on the cathode illuminated by UV light. This interesting technique will be tested at LP1.

7. Discussion (Takeshi et al): no show-stoppers were identified
and simulations indicate that the effects may be smaller than originally estimated. We agreed that these studies must be continued so that we have a robust strategy for handling such corrections by the time the ILCTPC roles around.
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