Special efforts for high precision
For a target uncertainty of 10-11, efforts and precautions must be taken to ensure that such a measurement can be done. These include:
- Precision electrodes:
The hyperbolic electrodes used in the traps are manufactured with a precision of about 2.5 μm.
- Correction electrodes:
Even with precised machined electrodes, tolerances occur and have an impact on the electric field in the trap. Therefore, in total two azimuthally segmented correction electrodes are placed between the endcaps and the ring. With them, the electric field can be fine-adjustable and radio frequency excitation can be shone in the trap.
- Strong magnetic field:
A strong magnetic field is required to keep the cyclotron orbit small and the frequency high. The radius of the orbit must be small so that the particle remains within the most harmonic region. Also in a larger magnetic field with the same mass difference a larger frequency difference occurs, which leads to better mass resolution.
- Stable magnetic field:
Because the frequencies of the ion species are measured sequentially, a temporal change of the magnetic field interferes with the measurement significantly. Therefore, it is attempted to keep the magnetic field as stable as possible. As a first step in the development phase of the laboratories, magnetic material in the vicinity of the experiment were avoided wherever possible. As a second step, changes in the Earth’s magnetic field and changes generated by magnetic objects are actively compensated for by a system based on a fluxgate magnetometer and a pair of Helmholtz coils. In addition the pressure in the magnet is stabilized. This leads to a constant boiling temperature of the helium in the magnet. Also the helium level is stabilized, which results in a constant spatial temperature distribution and therefore the magnetic susceptibilities of the surrounding materials do not change. Overall, these efforts lead to a stability of the magnetic field of a few 10ppt/h.
- Decoupling from the surrounding:
Another important aspect of the design of the laboratory is the need for a stable environment. In order to avoid the influence of external vibrations on the measurement, the whole structure is placed on a second foundation separated from the rest of the building. The result is that the oscillation amplitude is reduced to below 1 μm. By heating the room, it is also actively temperature stabilized at 60 mK/day.
- Good vacuum and low temperatures:
For the storage of single ions for extend periods of time, an ultra-high vacuum is essential, as interactions with other particles in the trap make the experiment difficult or even impossible. Starting from a pressure of 10-9 mbar at room temperature, the traps are immersed in liquid helium. This reduces the thermal noise in the detection electronics and most of the residual gas freezes out, whereby the vacuum is improved by up to 6 orders of magnitude. This also has a positive effect on the storage time of the ion, which can be up to a few weeks.