Wiring & Thermal Lagging
When routing wires and other connections into your sample space, a number of best practices and wiring techniques should be followed to ensure optimal performance.
- Choose the proper wiring material. Typically in cryogenics work, the researcher is accustomed to wrapping long lengths of wire around bobbins to properly lag the wire. By considering the material of the wire or cable, optimal base temperatures can be achieved without excessive lengths of wiring or cable. See our Wiring Guide: A Complete Introduction for details.
- Build wiring harnesses if needed.
- Follow thermal lagging instructions to properly route wires through thermal clamps, if necessary.
- Follow wire management guidelines to properly organize wires within the sample space and radiation shield for best performance.
- When making wiring harnesses, GE varnish should be applied between the wires at the connector as this will protect the wires at the solder joint, and more importantly keep the wires from shorting to each other.
- The wire can be coiled around a smooth round object, as shown below, to achieve more length inside the sample space while managing the wire, and not inducing a heat load with a touch to the 30K radiation shield.
The Cryostation 50-class and older Nanoscale Workstations require the user to thermally lag wires that are coming from room temperature into the 4K sample space. The 100-class (Fusion) and 200-class have a cold circuit board, so in most cases the thermal lagging is conveniently already handled for the user.
Legacy Style Thermal Lagging
In the Cryostation 50-class and older models of the Nanoscale Workstation, all wires and cables must be thermally lagged under thermal clamps which are found on the 30K ring, as shown in Figure 1 below in the blue and green rectangles.
- The clamp does not need to be very tight. Excessively tightening down the clamp can short or break the wires.
- The wires under the clamp should not overlap each other, as this can cause the wires to break.
- In some cases, kapton tape is added to the wire when it passes under the clamp to avoid damage to the wire.
- Ensure there are a few inches of the wire before the thermal clamp, and at least a few inches after the clamp for proper thermalization.
The thermal clamp does not offer a large thermalization area; however, Montana Instruments has found that by using proper wiring materials, such as phosphor bronze and manganin, and proper wiring techniques, such as not touching the wire to the radiation shield near the sample or not touching the sample space right after the thermal clamp, the minimum base temperature can be achieved. The area inside the sample space is cooled through thermal conduction from the cold finger. If the user is using the wiring to cool a component and the component is isolated from the thermal conduction of the cold finger, more length of wire may be needed to appropriately thermalize the wires.
The blue rectangle shows that two RF coax are lagged on the lower level of the thermal clamp. This part of the clamp needs to be tight to thermally lag the coax cables. The upper level of the clamp has the wiring necessary for the positioners installed inside the sample space.
The green box shows heater and thermometer wires going through the bottom part of the clamp. Through the upper level, a gas tube is routed into the sample chamber. The gas tube is wrapped in Teflon and intentionally not thermally lagged to allow the gas to stay warm and avoid freezing inside the tube. As long as the tube does not touch any components in the 4K sample space, the temperature should not increase.
Pre-Lagged Style Routing
The 100 & 200-class models have all user wiring pre-lagged into the cold circuit board. The 100-class (Fusion) is shown below.
The blue rectangle shows two RF coax. The user simply has to plug in RF coax to the adapters inside the sample space. This option is only available on the 100-class model.
The orange rectangle shows the MDR26 side panel which connects to wiring that is already routed into the 4K space (shown in the green rectangle). The user only has to hook up the internal wires, allowing for the use of any material and length of wire. The user must still adhere to the principal of avoiding touching the wire to the inside of the radiation shield.
Proper wire management can be the most time consuming step:
- Wires should be routed neatly. The wires can be tucked under the PCB and routed through the hooks before going to the connectors.
- The wires should not touch the radiation shield, as this will induce a heat load. If the wires touch the radiation shield from the 4K stage this will raise the entire base temperature at the 4K stage.
- BEFORE attaching the radiation shield with screws, remove the radiation shield lid to check for touches. If the wires do NOT touch the radiation shield at any point, you can proceed to screw down and secure the radiation shield. If wires do touch, then remove the radiation shield and re-organize the wires.
Problem: Temperature from the thermometer reading seems too high to be real!
Solution: Most likely due to poor lagging, refer to lagging section. Also, check the calibration file, as well as a radiation shield around the thermometer to keep background radiation from heating the unit.
Problem: Heater does not work! The software shows 0 power going to the heater!
Solution: The heater is most likely just broken and will show an open circuit when tested by a Fluke. Another option is that the heater is simply shorted to ground and the thermal clamp is too tight.
Problem: Heater is showing a power that is different than what was set
Solution: Check for shorts on the heater. Also, if the heater was recently swapped out, then make sure the calibration file was adjusted, if not, this could be the issue.
Base temperature issues
Problem: Base temp is too high when I connected all my new wires!
Solution: Check that the wires are all properly lagged under the thermal clamps.
Problem: I can see some strange effects in my signal that somehow depend on the source!
Solution: There is a an unshielded line in the detection measurement lines which are not twisted pair, or possibly there is a ground loop so check the grounding for the instruments used in the experiment.
Excessive electronic noise
Try to separate the source and sensitive measurement line from each other. Do not share the thermal lag points with the voltage and current leads. Make sure all DC lines are twisted pairs. Make sure the extra loops of wire are securely fastened to the system to minimize mechanical vibration. Use RF filters on the measurement circuit.