Cryogenics

SHARC II incorporates a ³He sorption fridge to cool the detectors to about 350 mK. This fridge must be cycled every day prior to the start of observing. The cycle takes a bit more than 90 min. Before cycling the fridge, please consult with the staff and any other observers about the weather forecast and the observing plan.

The ³He fridge is controlled electronically. Normally the staff will set up the controller and its software. To cycle the fridge:

1. Connect to view the fridge monitor.
   comp% csovnc.pl -l sharc sharc.fridge
   • Consult staff for passwords.
   • There should be three windows: the fridge program (top), an xterm (lower left), and a clock (lower right).
   • The fridge server and client processes should already be running. If they are not, see setup below.
2. For an immediate cycle:
   1. Verify the Start Immediately box is checked.
   2. Verify the ³He cycle only and ³He cycle only boxes are unchecked.
   3. Press START.
3. For a delayed cycle:
   1. Uncheck the Start Immediately box.
   2. Verify the ³He cycle only and ³He cycle only boxes are unchecked.
   3. Enter the delay in the Delay til start (hrs) box.
   4. Press START.
4. Warning: Do not logout (exit) from the xterm. This will abort the server and client processes, stopping the cycle.
5. Close the vnc viewer window if desired.

Examples of the fridge monitor at the start, middle, and completion of the cycle.

The Neocera LTC-21 controller must be powered up, connected to the sharcii computer with a serial cable, and connected to the cryostat. Two processes must be running on sharcii for properly fridge cycling: a background server that controls the Neocera and logs temperatures and a graphical client. Only one server should be running but multiple clients can run at the same time. For simplicity, these are run in a detached (vnc) desktop.

Normally the staff staff will set up the controller and its software. To start or restart the fridge processes:

1. Connect to view the fridge monitor.
   comp% csovnc.pl -l sharc sharc.fridge
   • Consult staff for passwords.
2. There should be three windows: the fridge program (top), an xterm (lower left), and a clock (lower right).
   • If the xterm or the clock is missing, (left) click on a blank area in the and select from the pop up menu.
3. Login to sharcii if necessary. In the (lower left) xterm,
   kilauea% ssh sharc@sharcii
4. If necessary, stop the existing processes.
   1. Check what processes are running.
      sharcii% ps -ef | grep java
   2. The server and client each show up as java 1.4 processes. For example,

      sharc   598     1  0 14:18:00 pts/0    0:03 /home/sharcii/sharc/j2sdk1.4.2_30/bin/java -Djava2d.font.usePlatformFont=true F
      sharc   600     1  0 14:18:14 pts/0    0:04 /home/sharcii/sharc/j2sdk1.4.2_30/bin/java -Djava2d.font.usePlatformFont=true G
      sharc   621   606  0 14:22:05 pts/1    0:00 grep java

5. Determine the server and client PIDs (second column) and kill them.
   sharcii% kill -9 PID
   • Other than the sequence of the PIDs, it is not possible to distinguish the server and client processes in the ps listing.
   • If in doubt about what processes are running, kill all of the java 1.4 processes.
6. Start the server program.
   sharcii% cd ~/recycle
sharcii% cycle
   • After a few seconds, a message should appear similar to
     10 Minutes;Both; 500;One Shot;0;
7. Start the graphical client.
   sharcii% gclient
   • There will be several warning messages, none crucial.
8. Warning: Do not logout (exit) from the xterm. This will abort the server and client processes, stopping the cycle.
9. Close the vnc viewer window if desired.

Cycle logs are on sharcii in ~sharc/recycle/logs/. Temperatures can be read directly from the Neocera front panel near the instrument. The left temperature is a “charcoal pump” temperature (³He or ⁴He), which is normally 6.7 K except during a cycle. (For the pump temperature, there is a scale factor difference between the front panel and the cycle logs.) The right temperature is the detector temperature, which is normally 0.36 K in operation.

If a cycle hangs in a state where continuous heat is applied to the refrigerator, the heating can be stopped by pressing the Monitor button on the Neocera front panel.

Normally the initial cooldown is done many days in advanced to allow the detectors to passively cool to 4 K. During the cooldown, the detector temperature will be between 10 K and 100 K. If necessary, the cooldown may be accelerated by actively cooling the detectors. Active cooling uses more LHe than passive cooling. Run a “³He only” cycle then wait for the detector temperature to drop below 8 K. If one “³He only” cycle was not sufficient to cool the detectors below 8 K, try another one. Once the detectors are below 8 K, follow with a standard cycle.

In 2003 August and 2004 January, the first standard cycles ran succesfully, but the detector took several hours to cool below 0.4 K and the liquid ³He ran out prematurely. This problem disappeared by the next day. The best explanation is a poorly heat sunk component that causes a transient heat load on the ³He system. Performing the first ³He cycle as much as a day in advance is a suitable way to address this problem.

During the work week, the CSO staff handle cryogen service. Please consult the staff about the schedule and the plan for weekend and holiday service. Please service cryogens yourself only if you are trained by the staff and wear the proper safety equipment.

The LN₂ reservoir must be filled at least every 48 h, i. e., at least every other day. The LN₂ fill port is in the middle of the cryostat. To allow access for the funnel, position the telescope at a zenith angle of 45° (uip> za 45). The reservoir capacity is 7 liters and the usage is about one white pail per day.

SHARC II needs LHe every other day during active observing, i. e., when the ³He fridge is cycled. Otherwise, the LHe reservoir must be refilled every six days. The LHe fill port is the one closest to the edge of the cryostat. To allow access for the fill tube, position the telescope at a zenith angle of 45° (uip> za 45). The reservoir capacity is 11 liters. The distance from the bottom of the reservoir to the exit hole is 20.5 in and the reservoir is 7 in deep. Before the ³He fridge is cycled, the LHe depth should be at least 3 in. Please replace the downward-pointed black rubber hose assembly after filling the LHe. This helps prevent ice plugs.

In case of a suspected ice plug in the SHARC II LHe reservoir, CALL STAFF IMMEDIATELY FOR ASSISTANCE. Ice plugs can eventually result in a cryostat explosion.