1. Micromegas TPC and wire TPC First measurements in a magnetic field
P. Colas1, Y. Giomataris1, J. Jeanjean2, V. Lepeltier2, J. Martin1, A. Olivier1
1) DAPNIA Saclay 2) LAL Orsay
A small-gap wire chamber TPC and a Micromegas TPC (both 1cm drift) have been put in 1-2 Tesla magnetic field
Feedback has been measured and progress on simulation has been made.
Hamburg, July5, 2002
P. Colas - Micromegas and wire TPCs

2. P. Colas - Micromegas and wire TPCs
Experiment
We ran end of June with a small-gap TPC and a micromegas TPC
The primary ionization was provided by
an 55Fe source (25 MBq, gain typically 100,000) for the wire chamber
a 90Sr source (1 GBq, gain a few 100) for the micromegas
The currents from the supplies are monitored, allowing to measure the feedback.
Mainly the results from micromegas are shown here.
The results from the wire chamber are: no significant effect of the magnetic field, though simulation predicts an increase of 50% on the feedback from 0 to 2 T. To be understood.
Hamburg, July5, 2002
P. Colas - Micromegas and wire TPCs

3. P. Colas - Micromegas and wire TPCs
SETUP
Hamburg, July5, 2002
P. Colas - Micromegas and wire TPCs

4. P. Colas - Micromegas and wire TPCs
Ar:CH4 90:10
detector
2T Magnet
High voltage and monitoring
Hamburg, July5, 2002
P. Colas - Micromegas and wire TPCs

5. P. Colas - Micromegas and wire TPCs
Small-gap wire TPC
Micromegas TPC
KV V
V V
55Fe
90Sr
wires
grid
Cathode
anode
mesh
Cathode
2mm 2mm cm
50 mm 1cm
Hamburg, July5, 2002
P. Colas - Micromegas and wire TPCs

6. Feedback : theory and simulation
We (Jerôme Martin) carried out simulations with 2D geometry to:
Understand the feedback
look at the effect of the magnetic field : predicts 50% increase in the wire chamber, and no effect in micromegas
Ideally, the feedback is given by the inverse field ratio E(drift)/E(amplif)
It is less good (i.e. larger) if the dispersion in the avalanche is small with respect to the hole pitch
The two important quantities are s : the dispersion after 50 microns and
l, the pitch. The hole size does not matter when the field ratio is high
Hamburg, July5, 2002
P. Colas - Micromegas and wire TPCs

7. Theoretical ion feedback
Hypothesis on the avalanche
Periodical structure
Gaussian diffusion
Avalanche
Resolution l
2*sigma
Hamburg, July5, 2002
P. Colas - Micromegas and wire TPCs

8. Theoretical ion feedback
Calculations
Sum of gaussian diffusions 2D 3D
Hamburg, July5, 2002
P. Colas - Micromegas and wire TPCs

9. P. Colas - Micromegas and wire TPCs
Theoretical ion feedback
Results
1500 lpi (sigma/l=0.75)
1000 lpi (sigma/l=0.5)
500 lpi (sigma/l=0.25)
Hamburg, July5, 2002
P. Colas - Micromegas and wire TPCs

10. Theoretical ion feedback
Hamburg, July5, 2002
P. Colas - Micromegas and wire TPCs

11. P. Colas - Micromegas and wire TPCs
Hamburg, July5, 2002
P. Colas - Micromegas and wire TPCs

12. P. Colas - Micromegas and wire TPCs
Hamburg, July5, 2002
P. Colas - Micromegas and wire TPCs