Small-scale cryogenic cooler applications include medical imaging for MRI systems and infrared sensor cooling.

A system was designed to automate cryogenically cooled low-noise amplifier systems used in the NASA Deep Space Network. It automates the entire operation of the system including cool-down, warm-up, and performance monitoring. The system is based on a single-board computer with custom software and hardware to monitor and control the cryogenic operation of the system. The system provides local display and control, and can be operated remotely via a Web interface.

The system controller is based on a commercial single-board computer with onboard data acquisition capability. The commercial hardware includes a micro-processor, an LCD (liquid crystal display), seven LED (light emitting diode) displays, a seven-key keypad, an Ethernet interface, 40 digital I/O (input/output) ports, 11 A/D (analog to digital) inputs, four D/A (digital to analog) outputs, and an external relay board to control the high-current devices.

The temperature sensors used are commercial silicon diode devices that provide a non-linear voltage output proportional to temperature. The devices are excited with a 10-microamp bias current. The system is capable of monitoring and displaying three temperatures.

The vacuum sensors are commercial thermistor devices. The output of the sensors is a non-linear voltage proportional to vacuum pressure in the 1-Torr to 1-millitorr range. Two sensors are used. One measures the vacuum pressure in the cryocooler and the other the pressure at the input to the vacuum pump. The helium pressure sensor is a commercial device that provides a linear voltage output from 1 to 5 volts, corresponding to a gas pressure from 0 to 3.5 MPa (≈500 psig).

Control of the vacuum process is accomplished with a commercial electrically operated solenoid valve. A commercial motor starter is used to control the input power of the compressor. The warm-up heaters are commercial power resistors sized to provide the appropriate power for the thermal mass of the particular system, and typically provide 50 watts of heat.

There are four basic operating modes. “Cool” mode commands the system to cool to normal operating temperature. “Heat” mode is used to warm the device to a set temperature near room temperature. “Pump” mode is a maintenance function that allows the vacuum system to be operated alone to remove accumulated contaminants from the vacuum area. In “Off” mode, no power is applied to the system.

This work was done by Michael J. Britcliffe, Theodore R. Hanson, and Larry E. Fowler of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47246

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