> For the complete documentation index, see [llms.txt](https://arpes-1-2.gitbook.io/arpes-1-2/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://arpes-1-2.gitbook.io/arpes-1-2/manuals-arpes-1-2/starting-the-experiment/cooling-down-with-lhe.md).
# Cooling down with LHe
{% hint style="warning" %}
Preliminary setup and change of the dewar should be done together with a beamline scientist. This instruction is only for the case when the needle valve is already inside the dewar.
{% endhint %}
1. Check that helium line is deep inside the dewar, ca. 5 cm above the bottom.
2. Check that Helium pump is running (see panel inside the frame from the side of the load lock):
3\. Check that black valve below the frame is open:
In this configuration gas is passing through a black valve reducing the flow. To fully open rotate 180 deg.
4\. Make sure that helium recovery line is connected to a He return port on the wall-panel.
5\. Use ladder and open the needle valve 1 full turn. Rotate counter clockwise if you look from below.
6\. Wait until temperature reading starts to steadily go down by 0.01 K. If this doesn't happen in 10 minutes - it is likely that there is a problem (\*).
7\. When sample temperature reaches ca. 40K start closing needle valve in small steps and wait for the temperature to stabilize. Try to close as much as possible to reduce helium consumption, but don't let temperature to go up. **Optimal values are achieved when coldfinger temperature is around 10K (gaseous helium return rate should be around 900 L/h, which is ca. 1.2 L of LHe per hour).**
8\. The lowest sample temperature reading is currently \~26 K (condition on Jan 2025).
{% hint style="info" %}
To reach the lowest possible temperature on the sample one can improve thermal contact by tightening the screws on the sample pocket of manipulator head. See [here ](/arpes-1-2/manuals-arpes-1-2/starting-the-experiment/cooling-down-with-lhe/improving-thermal-contact.md)for more details.
{% endhint %}
{% hint style="danger" %}
Check that helium line is **not at the very bottom** of the dewar. If it slides down, retract at least **5 cm** back. This way there is less chance that it will be blocked.
Make sure that the needle valve part is straight, if not - adjust the support.
{% endhint %}
{% hint style="danger" %}
Underpressure in the LHe dewar should be avoided at all times! If this happens, air can reach inside the dewar and moisture will turn into ice that freezes on cold parts inside. This ice can block helium line from moving, then it cannot be neither closed normally nor taken out to change the dewar. Too much ice may lead to a dangerous condition!
#### Monitor the manometer of the dewar periodically.
Since 2025 we are required to leave at least **30L** of LHe in the 100L dewars (45L in 250L ones) to make sure that they do not get warm by the time they arrive at the filling station.
#### Change dewars timely, before helium level goes below this limit!
LHe level should be measured with no excess pressure in the dewar, if manometer doesn't show approximately 0, the measurement will probably be wrong.
{% endhint %}
## Dynamics of manipulator cooling
{% file src="/files/KvB7qwF15u1ZyFjNi0Kv" %}
T vs time for the first cooling down and for [setting temperatures using the manipulator heater](/arpes-1-2/manuals-arpes-1-2/starting-the-experiment/cooling-down-with-lhe/t-dependent-measurements.md).
{% endfile %}
## Troubleshooting:
(\*) As a first step try to disconnect helium recovery line from the wall (see Fig. 1) and check if there is a flow of gas coming out of it.
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