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

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

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

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

P. Colas - Micromegas and wire TPCs Small-gap wire TPC Micromegas TPC 0 +2KV 0 - 300 V 0 -340 V - 640 V 55Fe 90Sr wires grid Cathode anode mesh Cathode 2mm 2mm 1cm 50 mm 1cm Hamburg, July5, 2002 P. Colas - Micromegas and wire TPCs

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

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

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

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

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

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

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