Introduction
The detailed simulation of particle detectors that use gas and semi-conductors as sensitive medium can be done by Garfield++. This is toolkit developed by RD51 collaboration.
In order to perform a simulation with Garfield++ information about fields are required. For that we use a software called CST Studio Suite© which makes use of the finite element method. At the beginning an interface between Garfield++ and CST was needed and developed in our group. Another possible software for simulating fields is called ANSYS®. It can be used as reference and also makes use of the finite element method.
CST simulation
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Figure 1:
a) Photo of a GEM (click to see larger version)
b) Sketch of the GEM with the bsic cell used in simulation depicted by a orange box (click to see larger version)
Figure 1a shows a microscopic view of a GEM (pitch p = 140 μm, outer radius R = 35 μm, inner radius r = 25 μm) and Figure 1b shows a sketch of it. The orange box is the basic cell used to reconstruct the GEM structure build in CST. This basic cell is sufficient to reconstruct the whole GEM structure using a x and y mirror symmetry afterwards in Garfield++.
After the implementation of the interface class in Garfield++ the result of a field simulation obtained in CST can be compared with the field calculated in Garfield++ (using the interface). The result of such a comparison is shown in Figure 2. Here a scan of the different components of the electric field along a vertical axis with respect to the GEM surface is is shown.
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Figure 2:
Coparison of the electric field and the potential between the original results from CST (black dots) and the calculated results in Garfield++ (red curve).
a) potential
b) z-component of the electric field
c) x-component of the electric field
d) y-component of the electric field
(click to see larger version)
Garfield++ simulation
Finally Figure 3 shows a result of a Garfield++ simulation of a single GEM. Above the GEM in the drift volume where a field of 250 V/cm is applied one electron is released (organge line). It travels towards the GEM where a applied voltage across the GEM (350 V) causes an amplification process. Ions produced during this process are represented by brown lines. An intrinsic supression of ions drifting towards the drift volume is visible. This feature will be further studied in order to supress ions in the ILD TPC.
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Figure 3:
Garfield++ simulation of a single GEM with one electron (orange track) entering the GEM from the dirft volume (top)
(click to see larger version)
