sinqwiki: HelioxVT

HelioxVT

Oxford Intruments 3He Sorption Cryostat

HEOX is a 3He cryostat with a sorption 3He pump. All gas handling is inside the cryostat, there are therefore no tubes to a pump outside.

System operation

Loading the HelioxVT insert

The sample mounting

procedure is equivalent to a dilution refrigerator. After closing and pumping the IVC. 2-5mbar 4He exchange gas is added. The HelioxVT insert can be treated like any other sample rod for a Variable Temperature Insert (VTI).

System control

All Heliox sorption pumped 3He refrigerators have only a finite quantity of 3He which can be condensed into the system. Experimental heat loads are taken up by the latent heat of vaporisation of the liquid as it is transformed into vapour. Once all the liquid 3He is consumed in this way, the system will start to warm.

To cool the system again requires the "regeneration" of the 3He sorption pump such that the adsorbed gas is released from the sorb and re-condensed into the 3He pot. The process for cooling and temperature controlling Heliox 3He refrigerators is described in the following section.

3He regeneration

In order to run the refrigerator, the 3He charge must be condensed. This is achieved by warming the 3He sorption pump, using its in-built heater, to a temperature of ~30 K such that it cannot pump Helium gas (any gas previously adsorbed in the pump will be released). This raises the pressure of the 3He gas in the insert.

Simultaneously, the VTI should be run to ensure there is a cold surface for the gas to condense onto. The condensed liquid 3He then runs down into the 3He pot, filling it with liquid at a similar temperature to that of the VTI.

Low-temperature operation

To control the temperature of the 3He pot between the lowest attainable temperature and a temperature approximately equal to that of the VTI pot it is most efficient to control the temperature, and therefore the pumping efficiency, of the 3He sorption pump.

Varying the pressure above the liquid surface (see Figure 3-1) can control the temperature of the 3He without supplying an additional heat load to the 3He pot, maximising the low-temperature hold time. Closed loop control of the 3He pot temperature can be achieved by servo-controlling the 3He sorption pump heater to control the 3He pot at a set point.

High-temperature operation

To control the temperature of the 3He pot at temperatures above that of the VTI it is most efficient to apply electrical power directly to the 3He pot heater.

To provide cooling the 3He sorb is warmed to ~15 K to partially release the 3He charge from the sorption pump, thus providing a thermal link (through the gas) between the VTI and the 3He pot.

Closed-loop control of the 3He pot temperature can be achieved through PID control of the 3He pot heater.

Further Information can be found here: LdmWiki:HelioxVT

Sea/SICS Commands

condense

disable the sample heater

ts set 0

set the sorb heater loop to regulate the sorb (channel C)

ts setsorb/channel C

set the sorb heater to 30 K

ts setsorb 30

wait 15 min, then disable sorb heating

ts setsorb 0

regulate at temperatures below ~2 K

disable the sample heater

ts set 0

set the sorb heater loop to regulate the samlow sensor (channel B)

ts setsorb/channel B

regulate at 0.5 K

ts setsorb 0.5

when oszillating, you might need to reduce the prop parameter from the default value 500 to e.g. 100

ts setsorb/prop 100

regulate in T range 2 K < T < 20 K

set the sorb heater loop to regulate the sorb (channel C) ts setsorb/channel C

regulate the sorb at an intermediate T (15 K)

ts setsorb 15

regulate at 4.5 K

ts set 4.5

higher temperatures and warmup (e.g. 80 K)

tt set 80

For a more precise control tt set might be set to value slightly lower the the desired T, and ts set to the desired target.