VTI

This note records the manual of performing cryogenic transport measurements in the Variable Temperature Insert (VTI) system.

The VTI system discussed here is a traditional He4 liquid cooled superconducting magnet system invented by CRYOMAGNETICS. It can cool down to a base temperature of 1.5K, with a superconducting magnet that can apply 11T magnetic field (dual axis).

Sample probe and cooling system

In order to load a sample and cool down to 1.5K, VTI uses a delicate sample probe integrated with (from bottom to top): a 16-pin sample holder, a heater, a thermometer, twisted pair bronze cables and RF cables. The sample holder with the heater and the thermometer are installed together at the very end of the probe. The sample holder is sitting at a rotatable orthogonal-dual-gear mechanical station that can be rotated to face downwards or 90-degree to the side, in order to achieve a switchable dual axis magnetic field. All of the cables connected with the above elements go along the probe upwards and go throughout the plastic probe shell that sustains cryogenic temperature to the top end. The top end of the probe has two ears of electrical plugs for the usual twisted pairs and the RF cables, respectively, that are further connected with the outlines. The middle probe ends at a knob at the top end that allows for rotating. Below is a picture of the sample probe.

The sample should be installed on a standard 8-pin or 16-pin chip carrier, then attached to the sample holder on the probe. The probe will be further insert vertically into the VTI main chamber, which is thermally separated from the 40K chamber that stores the 4K superconducting magnet. In order to cool down the system to the base temperature, He4 will be circulated by an external compressor inside the circulation loop. The He4 circulation loop of VTI is controlled by a needle valve system: He4 is stored in a helium tank, pumped into the circulation line by a scroll pump, with its flow rate controlled by the needle valve, finally compressed into liquid by the compressor, evaporated and circulated back to the tank. This part of the system is often inert and does not need much adjustments if the system is in a good status. Usually, one just needs to gently tune the needle valve so as to make sure the flow rate of He4 is maintained between 3-4(sccm?)

Sample loading

To load in the sample, one needs to first purge the exchange chamber and the main chamber with helium. Starting with the status where the sample probe is inserted inside the main chamber and the temperature has been warmed up to room temperature, the first thing one needs to do is to purge the exchange chamber before opening the main chamber. The main chamber and the exchange chamber is separated by a gate valve.

Below is the two-valve setup of the exchange chamber. The left valve (HV) controls the flow of the external helium, while the front valve (PV) controls the pump line that is connected with a scroll pump. First, only open HV, and let a fraction of the helium (kept at 20psi flow rate) to feed into the exchange chamber; then close HV, open PV to purge out the fed helium, finally close PV. Repeat the above procedure for three times in order to fully purge the exchange chamber. Make sure the last time stay a long pump for about one minute.

After the exchange chamber is fully purged, fully open the external helium valve and HV, then slowly open the gate valve between exchange chamber and main chamber. You will notice the pressure of the main chamber is slowly increasing until capped at 80(?)psi, with the sound of helium gas keeps popping out. Then, loosen the vacuum cap between the main chamber and the sample probe, and slowly pull out the probe, and lay it down on the table in a horizontal position. Finally, seal the main chamber with the vacuum cap again and close HV.