Layout of the system maintaining dry cooling on a rotating table. A cryostat (i.e., a vacuum chamber that holds a cryocooler) and other instruments are set on the table. Wireless LAN technology allows real-time communication with the instruments from the ground. A custom-made rotary joint was developed for the helium gas transfer; there are two lines, a supply from and a return to a compressor located on the ground. We also use a commercial rotary joint for the electricity. Thus far we use seven electrical paths in parallel: three paths for the cryocooler control (three-phase AC 200 V), two for the other instruments (single-phase AC 100 V), and two for each ground. Both the electrical wires and gas hoses are routed to the top of the table through a series of access holes at the center of the base and attachment tables.
Photograph of the system shown in Fig. 1 (left). The compressor for the cryocooler is not shown. A close-up of the interface between the rotating table and the ground; a series of two rotary joints connects to the bottom of the base table (middle). Photographs of the rotary joints removed from the system (right).
Setup of a leak test of the gas rotary joint. The closed system was filled with high-pressure helium gas at 2.0 MPa; a 1-l buffer tank and the input port of the joint are connected with a copper tube (diameter of 1/4 in. and length of 1 m), and the output port of the joint is sealed with a ball valve. A geared motor rotated the inner part of the joint. We monitored the pressure while maintaining the rotation of the joint at 17 rpm. We observed a zero-consistent leak rate in the test over eight days.
Time trends of temperatures at each cold stage of the GM cryocooler on the rotating system. The specifications of the cryocooler, namely, the obtained temperatures and the cool-down time from room temperature, were confirmed. The motion of the displacer of the cryocooler produces a periodic temperature vibration with frequency of 1 Hz. No temperature instability was found except for this effect.
Spectra of each of the cold-head's temperature in Fourier space: first stage (top panel) and second stage (bottom panel). The line style (solid or dashed) indicates the condition of table rotation. The motion of the displacer in the GM cryocooler creates a cyclic temperature oscillation at 1 Hz. This creates the lines in the spectrum at 1 Hz, 2 Hz, and so on. No difference was found between the spectra with and without rotation of the table. In particular, there is no characteristic feature around the rotation cycle at 20 rpm, i.e., 0.33 Hz. We thus found no instability associated with rotation.
Long-term trends of temperatures in each cold stage. We continuously maintained the operation of the cryocooler and table rotation. We cooled down from room temperature; Fig. 5 presents the 1 h trends at the beginning of this plot.
Time trends of temperatures in each cold stage when changing the rotating condition. We repeated the start and stop sequence of rotation. Shaded areas correspond to durations of the table rotation at 20 rpm. Neither the starting nor stopping of rotation produced any instability.
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