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PREX Main << HOW TOs

This page is intended for PREX-II/CREX shift crew and instructs the correct usage of the compton DAQ, as well as some troubleshooting procedures.

Things to Check Before Running Compton

The power supply sits just behind the glass next to the shift leader station and has a big sign pointing to it on the lower shelf of the rack.
  1. Check that the black BNC cable is plugged into the module labeled "Compton PMT Power Supply." This module is located in the room with the DAQ crates, on the rack furthest from the door, facing the front room.
  2. Check that the power supply is set to exactly 5.13 V.
  3. Check that the Compton laser table rates (look for ComptonUSbg and ComptonDSbg on the EPICS computer strip charts) are acceptable.
  4. After verifying all of the above, then follow the instructions to start compton CODA.

Compton DAQ CODA

All compton runs and analysis are taken through a VNC session on the compton machine. The VNC acts as a remote desktop for the compton machine in the counting house. Having the VNC allows compton experts to centralize control over the compton DAQ, offer that control to shifters, and still allow off-site monitoring.

Annotated window of what the compton VNC should look like when open to workspace 1. Note the fact that both the compton CODA and analysis links exists entirely in one window, and shifters should be careful not to enter commands intended for adaq1 in the VNC.

Opening the VNC & CODA

  • Log in to the compton machine with ssh compton@compton (PW can be found from the paper next to the CH white board)
  • Once logged in run vncviewer :9 -shared
    • This will open a window in which a second desktop will appear.
  • If CODA is not open, then open a terminal window, and run kcoda (to be safe) and then startcoda
    • The Compton CODA looks almost exactly like the parity DAQ CODA with one extra window: the blue terminal window titled "ROC6." When you start up the Compton CODA if ROC6 keeps spitting out an error called udpSend: write failure then you've done it correctly.
  • With the compton CODA opened in the VNC window, first click Platform -> Connect.
  • Afterwards click "Configure."
    • The final line in the ROC6 window should be an error message saying callbackControl: Do not understand the command: session/control/setSession. If you see this error, then you've done it correctly.
  • Then click "Download."
    • If you've done it correctly, then ER1, EB6, and ROC6 will all read "downloaded" as a state.
    • If ROC6 begins showing errors other than the ones mentioned above, then contact the RC immediately.

How the VNC Works: There are a few important features about the VNC, the most important of which is: the same VNC can be open in multiple windows at one time! If someone else has the same VNC window open then you will both have simultaneous control over the mouse pointer, keyboards, etc. If you open the VNC window and you see it moving on its own then contact either the compton expert on-site or the RC to determine if the compton experts need the VNC. Generally it is the experts' responsibility to maintain control over the VNC. If an expert asks you to close your VNC window then it is safe to close without interrupting the current compton run.

Also important: if you close the VNC window it will not kill CODA or stop the compton run! However you last closed the VNC window will be the state in which it will be opened.

Also also important: Make sure you are only using the VNC for compton CODA and analysis! Any other work should be done through terminals on the adaq machines!

Starting and Stopping a Compton Run

  • Once CODA is open, configured, and the ROCs downloaded to, simply click "Start" to start a compton run.
  • Comment with the run configuration, and the current status of the beam.
  • If possible have all compton runs last approximately 90 minutes. At 240 Hz that means once every three parity runs that you start, you should also start a new compton run.
  • To stop a compton run, simply click "Stop."
  • After stopping each run, follow the steps to analyze it below:

Analyzing Compton Data

The compton has a much simpler and faster analysis procedure than the parity DAQ. To analyze a compton run:

  1. Open a new terminal window.
  2. Enter command gocompton
  3. Enter command source online.csh -r <run_num> --webupload
    • The --webupload flag will automatically place plot PDFs in the hallaweb comptonPlots directory, but they won't appear on the webpage until the compton shifter later decides which group they're in.

That's it! If you see any errors that indicate replaying did not end successfully, then inform the compton expert on-call.

ROC6 Hard Reboot

The following instructions detail how to restart ROC6 if/when it dies during compton operation:

  1. In either the vnc or on the compton machine open a Firefox window. Go to the page or [follow this link].
  2. Login with the correct hlauser credentials.
  3. Go to outlet 8 and do "Immediate off."
  4. Wait at least 30 seconds
  5. Select "Immediate On" for outlet 8.
  6. Check CODA to see if ROC6 is responding.


  1. Enter the command telnet hatsv5 2005 after selecting "Immediate On" to monitor ROC6 startup.
  2. If you see the VXWorks logo in ascii-art pop up, startup is proceeding regularly.
  3. Exit the environment with "Ctrl+]" and quit.

IOC hacp Reboot

Have the RC contact the compton laser expert on call. This reboot should be entirely expert-driven.

Strip Tool Monitoring

What the compton strip tools should look like when opened. NOTE: this screenshot was taken without beam so the BPM and rate numbers with beam should be different. Use the numbers in the table, and not the ones in this screenshot.

Shifters are tasked with - at the start of every new compton run - briefly checking the strip charts to ensure that there are no technical issues.

Starting StripTool

If the StripTool isn't running follow these instructions:

  1. Click into the VNC window. Open a new terminal.
  2. Enter cd
  3. Then enter the command StripTool ~/compton/shift_crew.stp. This will open up a window with plots of eight different EPICS variables.

Nominal Values

At the start of each compton run, briefly check that the Strip tool values reported are approximately what they should be in the following table:

Variable Name Explanation Correct Behavior If Incorrect...
COMPTON_PW1PCAV_ca Cavity laser power When laser is on, should be >2000 W. When laser is off should be <5 W.

Should be flipping semi-periodically between off and on.

Contact RC, tell them to contact on-call laser expert
ComptonCentralRate Trigger rate in main detector When both beam and detector are on, should be ~200 kHz. Otherwise 0.* If rate is high immediately unplug photon detector,

and contact compton DAQ expert on-call.

IPM1P02A.XPOS BPM 2A X position Should be approx. 0.35 Call MCC tell them to enable compton lock
IPM1P02A.YPOS BPM 2A Y position Should be approx. -0.41 Call MCC tell them to enable compton lock
IPM1P02B.XPOS BPM 2B X position Should be approx. 0.27 Call MCC tell them to enable compton lock
IPM1P02B.YPOS BPM 2B Y position Should be approx. -0.22 Call MCC tell them to enable compton lock
COM_DETPH_XCPOSai Compton photon det table X position Should be approx. 70 Contact RC, have them call compton DAQ expert
COM_DETPH_YCPOSai Compton photon det table Y position Should be approx. 32 Contact RC, have them call compton DAQ expert
ComptonDSbg1 Rate in laser scintillator DS 1 Should be approx. 2000 Hz Contact RC, have them call compton laser expert
ComptonDSbg2 Rate in laser scintillator DS 2 Should be approx. 2000 Hz Contact RC, have them call compton laser expert

* While both beam and laser is running the ComptonCentralRate should be noticeably higher than when either is off. The numbers in this table are assuming beam is on, lead target is in, and the laser is off.

Troubleshooting Common Problems

A few common problems have occurred when running the compton and should have easy fixes. A few of them are:

CODA is saying something disconnected and is asking me if I want to reset.

  • This is a common occurrence. Proceed with the reset. Re-configure and re-download when you're done. If CODA still doesn't behave after that, then run kcoda and startcoda (up to five times if necessary) to attempt to recover.

CODA hangs at the end of a run in a "Waiting for..." state indefinitely.

  • Reset CODA if the option is available to you. If the reset works, try configuring and downloading again. If that doesn't work, or you cannot reset CODA, then simply run kcoda (multiple times if necessary) and then startcoda.

CODA says it can't communicate with ROC6.

  • First attempt a reset. If that fails, click on the blue "ROC6" window. Press enter a few times. If a new line pops up, simply kcoda and startcoda again. If you cannot get a new line on that window then call the RC and instruct them to call Adam Zec and tell them that ROC6 needs a manual power cycle.

The VNC crashed on me!

  • First, make sure you can still ssh into the compton computer. If yes, then it's likely just a network hiccup and you can start running again. If you can't, briefly walk over to the compton terminal (the one farthest from the door in the counting house back room) to make sure it's still running. If not, then contact the RC who should then contact a compton expert.