1. Numerical simulations on single mask conical GEMs
Fri, 13th March 2009 – CERN
Marco Villa

2. The standard GEMs

3. Photolithographic technique used to
prepare standard GEMs

4. Why simulations on conical GEMs?
Why conical GEMs?
Why simulations on conical GEMs?
Large Area GEMs ↓
very difficult to align the two masks on the top and bottom side
What are the properties of the GEM foils obtained with single mask lithographic technique? How do these properties depend on the geometry?
spatial uniformity
time stability
electron transparency
discharge probability
maximum achievable gain
field shape
avalanche shape
charging up properties

5. Simulations: basics Simulation ANSYS PACKAGE GARFIELD PACKAGE
Ansys is used to define:
the geometry;
the material properties;
the electrodes voltage;
the e.m. boundary conditions;
and to solve the e.m. equations with a finite elements analysis method
GARFIELD PACKAGE
Garfield is used to:
read the Ansys fieldmaps;
define the gas properties;
simulate the behavior of electrons in the gas

6. ANSYS(1): definition of the geometry
Geometrical properties:
kapton thickness = 50 µm
copper thickness = 5 µm
drift gap thickness = 800 µm
induction gap thickness = 800 µm
holes pitch = 140 µm
hole smaller diameter = 55 µm
hole larger diameter = 55 µm  95 µm
In order to speed up the simulation, only the elementary cell has been considered
the mesh density has been doubled on all the gas boundary surfaces

7. ANSYS(2): definition of the voltages
cathode
1MΩ
GEM 1 top
0.5MΩ
In the chamber:
conversion gap = 3 mm
transfer gap = 2 mm
induction gap = 2 mm
Therefore, the field values in the chamber are:
drift field = kV/cm
induction field = 4 kV/cm
GEM voltage ≈ V
I ≈ 800 µA
GEM 1 bottom
1MΩ
GEM 2 top
0.5MΩ
5MΩ
GEM 2 bottom
1MΩ
GEM 3 top
0.5MΩ
2.5MΩ
GEM 3 bottom
1MΩ
anode to GND

8. GARFIELD(1): field and electrons drift

9. GARFIELD(2): field 95-55 85-55 75-55 65-55 55-55 55-65 55-75 55-85
55-95

10. GARFIELD(3): electron drift
95-55
85-55
75-55
65-55
55-55
55-65
55-75
55-85
55-95

11. GARFIELD(4): electron drift & diffusion
Finally, the electron diffusion is implemented in the Garfield code
a Monte-Carlo technique is used to simulate the longitudinal and transversal diffusion processes
the Monte-Carlo integration step is set much smaller than the typical detector dimensions (good compromise = 250 nm)

12. GARFIELD(5): electron drift & diffusion
55-55
95-55
55-95

13. Preliminary conclusions
Moving from the open-top geometry (95-55) to the cylindrical geometry (55-55) and then to the open-bottom geometry (55-95):
the field decreases, while the field increases;
if the thermal electron diffusion is switched off, the electron transparency decreases from to 55-55, then it increases slightly;
if the diffusion is off, all the stopped electrons end up on the upper copper surface  not a meshing problem!
if the diffusion is off, the diameter of the outcoming electron beam decreases from to 55-55, then is remains constant;
the Monte-Carlo diffusion technique seems to work fine  is there a real need for a microscopic diffusion procedure?

14. implementation of the avalanche process (Monte-Carlo or microscopic?)
Outlooks
check the behavior of a microscopic diffusion process and compare the results with the Monte-Carlo method;
electron transparency study using electrons from random coordinates on a “far” plane (70 µm above the upper copper foil);
implementation of the avalanche process (Monte-Carlo or microscopic?)
charging up studies