Janis cryo
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Contents |
Important notes:
Never pump the helium chamber before evacuating the vacuum chamber!
Never leave the helium transfer unattended!
Always make sure that the system is safe and steady before leaving it alone!
Drawing
Cooling down:
Checklist:
- check if the helium level meter is operational (the battery is not drained);
- have the cables necessary for thermometry and measurements ready: commonly used cables are labeled „Janis”;
- check if the pump is operational: try to pump the tube when its closed, pressure should go down to zero in a few minutes.
- Pump the vacuum chamber until the pressure goes below 1E-6 mbar. It is usually enough to pump overnight.
- Clean the helium chamber and the VTI with periodically pumping them with the rotary pump and filling them with helium gas from the recovery. It is enough to do this procedure three or four times. Check if the needle valve is not blocked.
- Fill the nitrogen chamber with liquid nitrogen twice every twelve hours and let the device to cool down. The temperature of the VTI reaches about 200K in less than one day.
- Fill the helium chamber with liquid helium: start the helium transfer slowly so that both the device and the transfer tube cools down with the cold helium gas. High evaporation rate can be noticed after liquid helium starts to flow through the transfer tube which reduces dramatically after liquid helium appears in the bottom of the cryostat (which is also visible on the level meter). Helium level reaches 30 cm after which the evaporation rate rises rapidly again. Cooling down should not take more than 12l of liquid helium and apart from the top the exterior of the cryo should not freeze!
Note: if the sample is not sensitive for rapid cooling it is also appropriate to cool down the sample holder together with the first filling: put the sample holder in the VTI before the cooldown procedure and open the needle valve as soon as liquid helium is present in the helium chamber. For larger inserts (e. g. point contact sampleholder) closing the recovery and pumping the VTI (500…800mbar) during filling also aids cooldown. If you do this, do not leave the transfer unattended as evaporation rate might rise rapidly in the VTI so pressure buildup has to be avoided by opening the pumping valve further.
Operation:
- Fill the cryo with liquid nitrogen at least two times every day.
- Periodically check the helium level and fill when it is necessary but do not leave the cryo overnight with low helium level (less than 15 cm), because it can warm up quite fast once there is no liquid helium left in the system.
- Filling usually takes less than 8 liters of liquid helium. Filling up to 20 cm takes virtually no time while the rest goes slower up to 30 cm.
VTI operation:
Continuous mode:
Cool down the insert as above and then set a stable inner pressure and flow rate (visible on the pressure sensors) using the needle valve and the pumping valve. In order to reach low temperatures only open the needle valve slightly and continuously open the pumping valve so that the flow rate stays around 10…20 mbar. In the end both the inner pressure and the flow rate shall drop. If you intend to warm up the insert above 4K, it is preferable to operate with a slightly open needle valve, but to set a higher (more than 100 mbar) inner pressure. In this case the VTI temperature can be regulated with the VTI heater and usually no liquid helium is present.
Single shot mode:
Start with filling the VTI: pump it so that the inner pressure drops, close the pumping valve and the carefully open the needle valve. You will notice that the VTI is full when the pressure start to rise rapidly in which case close the needle valve and start open the pumping line. Try to regulate the flow rate at around 10…20 mbar so that you will not evaporate too much helium from the inside. This means that as the system cools down, the pumping valve needs to be opened further and further. The base temperature with this setting is just below 1.3K.
Temperature sensors
VTI temperature sensor is a DT-670 silicon diode
Sample holder temperature sensor is a DT-670 silicon diode with the calibration curve D6004512
