Vibrations in OptiCool – A Closer Look
OptiCool is an optical cryostat by Quantum Design, with superconducting magnets and closed helium circuit. The superconducting magnet requires a large cold head which, as a result of its design, emits significant vibrations. Thanks to a novel vibration isolation, the sample chamber is almost completely mechanically decoupled from the cold head. There are different ways to obtain a low vibration amplitude, as this article will show, using the X130 positioning system as an example. The amplitude alone is not sufficient to predict the performance of certain test setups.
At the sample holder, called sample pod, the amplitude is just 10 nm peak-to-peak at 1.7 K base temperature. For most applications, a piezo positioning system is required for the sample pod. These positioners have a low resonant frequency and consequently vibrate strongly when excited.
The X130 positioning system consists of two ANPx311/RES/LT/HV piezos (X and Y) and an ANPz102/RES/LT/HV (Z) by Attocube. For measuring, a SmarAct PicoScale laser interferometer, set to a bandwidth of 153 Hz, was used.
Fig. 1
Figure 1 shows the time-dependent amplitude in (a) horizontal and (b) vertical directions. The peak-to-peak amplitude is just 8-9 nm horizontally and 1.5 nm vertically. These are the same values as those that are measured directly on the surface of the sample pod. This shows that, despite the positioning system, the vibrations are not stronger. This is due to the low forces acting on the sample pod. A low amplitude can be achieved either by minimizing the acting forces and/or by a design with high stiffness, which means high resonant frequency. The fact that the low stiffness of the piezos does not negatively affect performance shows clearly that hardly any kinetic energy is transferred from the cold head to the sample pod. It is thus not necessary to use a design with high stiffness. Accordingly, this also applies to all hardware that is installed on the sample pod in connection with the experiment (e.g. sample holder or positioning system).
