Content
- Introduction
- Magnet Setup
- Chamber Setup
- Results
- Participants
Introduction
In a collaborative effort of RWTH Aachen and DESY the charge transfer in multiple GEM structures is being measured in high magnetic fields up to 5 T. These are important measurements to evaluate the applicability of a TPC with a GEM based gas amplification system for the TESLA detector.
The drift velocity of electrons in a TPC is described by the Langevin formula:
with 
Because 
for the TESLA TPC, the drift direction of the electrons close to a GEM hole (where large angles between E and B can occur) will be dominated by the E × B and B directions. As shown in Figure 1, most of the electric field lines close to a GEM hole bend into the hole whereas the magnetic field lines hit the GEM surface perpendicular to its copper layer.
Therefore one could naively expect that the anode current drops in high magnetic fields. But this has been proven to be false by the Aachen group some time ago:
As can be seen in Figure 2 the collection efficiency drops down as expected but the effect is more than compensated by a large increase in the extraction efficiency so that, in total, an increase of the anode current is observed. Unfortunately the magnet used for the shown measurement was only capable to go up to 2 T field that will be applied in the TESLA detector) could not be accomplished.
This result shows that a thorough study of the charge transfer in GEM structures in high magnetic fields is badly needed.
