Vibrations & Vibration Spectrum
The Cryostation was designed for both vibration and temperature stability of the sample. This stability of the sample space is a unique performance differentiator among closed cycle cryostats, and enables many experiments which otherwise would not be possible. The Cryostation controls vibrations by coupling the sample in the sample space very rigidly to the optical table (just like any other optic on the table would be), while isolating the cryocooler vibrations through a unique mounting design and thermal connection. This design is effective in controlling external vibrations both from the optics on the table and from the sample area.
VIBRATION RESULTS AT 3.2K
The data below shows the typical vibrational displacement of a Cryostation s50. The y-axis corresponds to the displacement (nm) of a standard sample mount relative to the optical table. The x-axis corresponds to time (sec). Note the peak-to-peak displacement shown is <5nm peak to peak. Displacement results will vary as sample mount options are added.
The data below shows the acceleration of the optical table (floating) in the direction of the X-axis (blue trace), Y-axis (red trace), and Z-axis (green trace). The y-axis in the data below corresponds to the acceleration magnitude (µg), while the x-axis corresponds to time (sec). The orientation of the axes is shown in the picture below. Note: The z-axis is oriented perpendicular to the optical table (into the optical table).
MAGNETO-OPTIC OPTION VIBRATIONS
The peak to peak vibrations are <12nm with the magnet option installed, primarily due to the increased sample height off the table, whether or not the chiller pump or magnet is on. The graph below shows the measured vibrations during startup and during field ramp up.
VIBRATION MEASUREMENT PROCESS
Vibration measurements were taken on a standard Cryostation s50 (C2) optical cryostat using a Model CPL190 capacitive displacement sensor made by Lion Precision. The sensor provides an analog voltage output proportional to the displacement and is valid over a relatively large range.
The raw sensor voltage contained some high frequency variations due to local EMI. These were attenuated with a front end analog single pole low pass filter using an RC circuit as shown in the diagram. The values for R and C were chosen to provide a cutoff frequency of 1kHz.
The Lion sensor was mounted through an optical access hole with a hermetic seal and bellows and positioned approximately 150 microns away from the sample mount. The sensor output an analog voltage proportional to the distance from the sensor to the sample mount. This analog signal was captured using a PicoScope 4424 PC-based digital oscilloscope. Data sets of about 250k – 1M points per file with up to 32 of these files depending on the setup were saved. The digital filter feature of the scope was also set to attenuate spurious high frequency signals above 1KHz. With the 1kHz low pass filtering used as described, the peak to peak noise level of the sensor was about 250-300 picometers. All measurements were made using DC coupling to the scope.
The image and line drawing below show the measurement setup. The large sensor mounting block is very rigidly coupled to the table and has an interface which extends into the sample space with the capacitive sensor on it. A user thermometer was taped to the side of the sensor mounting block to monitor temperature effects on the hardware due to room temperature variations. The cable that exits the right of the image goes to the Lion control unit. The output of the Lion unit is an analog signal that is connected via coax to the Picoscope. All measurements used for this paper correspond to the X or Y direction of movement which are identical due to symmetry. Z axis vibrations were measured to be similar or better than X and Y. All the sample stage measurements taken for this paper were below 4 Kelvin.
Measurements were taken once the Cryostation had converged and stabilized at its target temperature and the compressor was operating in the stabilized idle run mode.