Cryocooler - Temperature Controlled Test Chamber
Speed of response, thermal gradients, vibration, and vacuum stability are all important factors to consider in the cryocooler chamber design:.
Temperature controlled chambers are designed and tested to quickly obtain stable operation. Many of the measurements require stabilization at 1-5 Kelvin steps over several hundred Kelvin. Optimizing the speed of response is a high priority to obtain timely results. Stabilization is done using a PID controller. The user is able to modify the PID values as necessary.
Semetrol's stage design places the heater and temperature sensor close together, and also close to the sample. High thermal conductivity grade copper is used for the stage. Temperature testing not only optimizes the response time to a step increase or decrease in temperature, but also tests the time it takes for the sample to get to a stable temperature near the set point.
Samples with low thermal conductivity, or with a thermal impedance between the sample active region and the stage, such as with solar cells on glass substrates, may take more than a minute to stabalize for a 5K step. Other samples such as SiC stabilize more quickly. The temperature lag can be characterized using your samples. Thermal radiation shielding is also used to minimize the significant effect of the temperature difference between the sample and chamber walls.
Each chamber is tested for pump down rate and leak-up rate. The data is supplied to the end user to compare the performance after years of use. A slow pump down rate may mean contamination, e.g. from handling the sample with fingers or from high humidity. A leak-up rate exceeding the initial, new condition, may indicate an o-ring that is contaminated with lint, for example. Having the data available allows you to quickly answer questions such as "Is the pump working as it should?", "Am I pumping on the chamber long enough before making measurements?".
Each chamber has four ports: temperature control feedthrough, 4 BNC connectors, vent valve, and vacuum port. Other configurations are possible. Components are selected for hermeticity and reliability.
Each cryocooler chamber is tested for vacuum leak rates, thermal response over the full temperature range, and vibration levels. All test results are available to the end user to compare behavior after years of use.
Representative pump down curve. Black curve is initial pump down after the chamber was open for 12 hours. Red curve is a subsequent pump down after venting momentarily. The difference is due to water vapor on the walls of the chamber.
Leak up test after pumping at room temperature for 12 hours. The average leak rate is less than 2 microns/hour. The information is useful for comparison to determine the state of the chamber after years of use.
Frequency spectrum of the vibrations. Black trace is the spectrum measured directly on the cold finger. Vibration isolation reduces the acceleration by about 100X (red).
Low vibrations allow lower probe pressure, and lower noise from piezoelectric material.