ACES will bring a new generation of atomic clocks to the ISS enabling accurate time measurements for research in the field of fundamental physics.

One of the key components of the ACES payload is the Space Hydrogen Maser (SHM) developed by Safran (Spectratime), Switzerland. The H-maser operates at the frequency of the hyperfine transition between the two mF=0 energy levels of the hydrogen ground state. Hydrogen atoms are contained in a glass bulb kept under vacuum conditions (at the 10-7 mbar level) and surrounded by a microwave cavity. Here, the spontaneous emission signal on the clock transition is amplified by the cavity until the maser action starts, resulting in a stable output at 1.420 GHz. This signal is used to stabilize an oscillator which is then providing the 100 MHz output frequency of the atomic clock.

The glass storage bulb is connected to the vacuum system with two O-rings. Vacuum conditions are ensured by a getter pump. During the storage of the clock on the ground, air can permeate through the O-rings inside the SHM vacuum system and passivate the getter material. This exposure to air during the ground storage can significantly reduce the pumping speed and the getters capacity, thus degrading the clock performance and limiting its lifetime.

Therefore, a series of tests aiming to analyse the performance of the ACES SHM vacuum system before the launch are conducted. The results of the tests will allow to assess and minimise risks related to a prolonged storage of the atomic clock on the ground during test and integration activities.