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Abstract

Lsym is a Penning trap which is currently being developed at the MPIK Heidelberg. Its goal is to test for possible CPT violations in the lepton sector by determining the difference in Larmor frequencies of a positron and a bound electron up to a magnitude of 10⁻¹⁴. To reach this precision the cyclotron motion of the positron must be cooled to the ground state, which is done by coupling it to the black body field of the trap. This trap will be cooled with a helium-3 evaporation fridge to a temperature of less than 500mK. This refrigerator reduces the pressure above a liquid helium bath with charcoal pumps to reduce the boiling point of the helium. The fridge contains a total of four of these Pump systems with two helium-3 and two helium-4 parts. First pumping the helium-4 reduces the temperature to approximately 0.8K then pumping the helium-3 reduces it further down to approximately 300mK. Having two of these modules allows the trap to stay below 500mK continuously. Before the fridge is implemented into the Lsym experiment, it is tested to ensure that it works as expected and that it can handle the expected heat load of the Lsym experiment.