Ion Backflow in the Micromegas Tpc for the Future Linear Collider

We present ion backflow measurements in a Micromegas (MICRO-MEsh GASeous detector) TPC device developed for the next high energy electron-positron linear collider under study and a simple explanation for this backflow. A Micromegas micro-mesh has the intrinsic property to naturally stop a large fraction of the secondary positive ions created in the avalanche. It is shown that under some workable conditions on the pitch of the mesh and on the gas mixture, the ion feedback is equal to the field ratio (ratio of the drift electric field to the amplification field). Measurements with an intense X-ray source are in good agreement with calculations and simulations. The conclusion is that in the electric field conditions foreseen for the Micromegas TPC (drift and amplification fields respectively equal to 150-200 V/cm and 50-80 kV/cm) the expected ion backflow will be of the order of 2 − 3 × 10. In addition, measurements have been done in a 2T magnetic field: as expected the ion backflow is not altered by the magnetic field. 1 The Micromegas TPC In a TPC, especially in high background conditions, it is very important to have a very limited ion backflow from the secondary ions produced in the amplification region to the drift volume, in order to avoid distortions of the drift electric field. The MWPC TPC’s (as ALEPH, DELPHI or STAR) are equipped with a gating grid, where two consecutive wires are polarised at opposite voltages, so creating a transverse field stopping most of the secondary ions before they reach the drift space. For the physics to be studied on the next linear collider [1], it is proposed to build a high performance large TPC, using instead of MWPC, new MPGD (Micro-Pattern Gaseous Detector) readout. The Micromegas [2] (MICROMEsh GASeous detector), under development for the future TPC [3][4] by the Saclay-Orsay collaboration is a parallel plate device, simply composed of a very thin (5 μm) metallic micro-mesh, with a pitch of 25 to 50 μm, set at a small distance from the anode plane (50-100 μm). Primary electrons coming from the drift space cross the micromesh, which is fully transparent, and avalanche in the small gap, where a voltage of ∼300-500 V is applied between the two electrodes (see figure 1).