How to operate and monitor the Spectrometer Magnets

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NOTICES IN EFFECT

The HMS Dipole NMR field setpoint is no longer automatically updated by the go_magnets_HMS_current script.  It is not clear why.  For now, you will need to manually update the 'Field Input (Tesla)' field in the HMS Dipole NMR magnet panel once you have reached the correct field value.  Use the value from the go_magnets_HMS_current script.
[Aug 20, 2020] 
When ramping up the SHMS or HMS magnets, do it in steps to minimize the chance of a trip.  So, ramp to the same MOL current in the end, but get there in 3--4 steps.  There is no need to pause between steps, just watch the magnet screens and adjust the set current in 3--4 stages.  You can do multiple magnets at a time -- just watch them.
(This makes it easier for the lead-flow controllers to reach the needed flow and remain stable.)
HMS Q1 has issues ramping down from a current. Once at MOL, ramp down in increments of no more than 50A. You can change the current you are ramping to (decrease it) by no more than 50A at the same time it is still ramping down.
Do not ramp HMS Q2 to 0A. If you need to ramp it down, ramp it to 10A and then turn it off. HMS Q2 takes about 5 minutes to stabilize at currents. If HMS Q2 quenches off for any reason, call RC before turning it back on.
Before re-setting interlocks, check the magnet power supply on the camera for smoking and look below it for water on the floor.

Magnet Controls Screens

  • Magnet screen is brought up on any hcdesk machine by the command go_magnets
    • Note: only ONE copy of the magnet control GUI screen can be running. Creating a new one will cause the old one to disappear.

SHMS Magnets and HMS Quads

  • The SHMS magnets are set by current from the desired momentum. From any hcdesk computer, run:
go_magnets_SHMS_current P
  • The HMS quad magnets are set by current from the desired momentum. From any hcdesk computer, run:
go_magnets_HMS_current P

where P is the desired momentum in [GeV/c]. This will return the current (in Amps) needed to set each of the magnets.

  • This script will ask if alarm limits should be set. If you answer yes (you should), then the alarm handler will show an alarm for any magnet with a set current that does not agree with the desired momentum. Also, the MOL (Minimum On Loop) and Target (desired) currents will load in the individual magnet GUIs for each magnet. MOL currents always have a greater magnitude than the Target currents and refer to putting the magnet response on the hysteresis loop.
  • For precision physics data-taking, the magnets should be put "on loop". Turn off the beam prior to ramping the magnets to put them on loop.
  • From the main magnet GUI, click on the box for the current to bring up the screen to set each magnet by current. The procedure to put a magnet on loop is described here:
Ensure beam is off.
1. Select polarity for electrons or positrons in the magnet GUI. If you need to change polarity, first ramp the magnet to 0 [A], and then you can change the polarity.
2. Run the go_magnets_XXX_current P command as described above.
3. If you are increasing momentum, you should first go to the MOL current. You can click the Go command next to the MOL current, or if needed, you can type the desired current in the box next to Set Current, click enter, and then click Set Current. 
4. If you are decreasing momentum or have already gone to the MOL current, you can click the Go command next to the Target current, or if needed, you can type the desired current in the box next to Set Current, click enter, and then click Set Current. 
  • An example of the power supply (PSU) page for a magnet where you will set the current from is shown below. This is the SHMS dipole: (And you should do the same for SHMS HB, Q1, Q2 and Q3 as well. Similar is for HMS case. If you are changing polarity or momentum in HMS first start with HMS Dipole as it takes longer time to complete the change we want. Also, for safety if you are changing momentum and/or polarity in SHMS and HMS, you can do so for one arm first and once it is done then do for the other arm. When you change HMS momentum we have to adjust the HMS diploe NMR value correctly. This takes longer time and patience is important.)
A large clock tower and other buildings line a great river.
Example SHMS dipole PSU
  • Explanation of the MOL and the Target current:
A large clock tower and other buildings line a great river.


  • The program for the magnet settings is in sync with the repository.

HMS Dipole

  • We set the HMS dipole first by current, and then adjust the current to get the desired magnetic field as given from the command above ("go_magnets_HMS_current P"). To do this without field regulation, ensure the Start command in the HMS dipole NMR screen is not selected/illuminated.
  1. Run the go_magnets_HMS_current P command to see the MOL and Target currents as well as the B-field we will set the magnet to.
  2. If obtaining these currents for the first time, type 'y' when asked if they should be loaded into the alarm handler.
  3. If changing polarity, ramp to 0 A first, then change the polarity.
  4. In the HMS PSU screen, go to the MOL current if increasing momentum or after having changed polarity (this is at least 1500 A, with 300 A buffer above the set current).
  5. In the HMS PSU screen, after reaching the MOL current (or if descending in momentum), decrease the current to the Target current.
  6. Open the "HMS Dipole NMR" screen and Make sure you have an NMR Lock (NMR Lock will be illuminated bright green). Wait 5 minutes at the Target current, and begin to slowly adjust the current in small steps to reach the desired B-field setting. The field read back is in the HMS dipole NMR screen. This process will take approximately 10-15 minutes for the dipole to settle. Be sure to agree with the desired B-field from the program to within 5E-5 T.
  7. In the same screen, manually type the "Recommended NMR B" value into "Field Input (Tesla)" (if it was not loaded automatically), then hit 'enter'.
  8. After the dipole settles and the field read back agrees with the desired field, proceed to take data (approximately 5 more minutes).

Troubleshooting the dipole NMR lock

  • If the dipole NMR Lock does not illuminate, ensure that Search Mode is turned off (if the box to the left of the words is illuminated, then it is on. It can be toggled off by clicking this box), and select Manual .
  • Click on the probe to the left in the NMR GUI that corresponds to the desired field in units of Tesla. Wait 1-2 minutes.
  • If NMR Lock illuminates, then all is well.
  • If the NMR Lock does not illuminate, call an expert.
Issue with engaging HMS dipole NMR locks : https://logbooks.jlab.org/entry/3818975

Communication Error

  • If you see a Communication Error, the NMR may need to be power cycled. This is accomplished by clicking the green button, Power is On to the right of the field value box in the HMS NMR tab. Wait for it to say Power is Off.
  • Next, click it back on and proceed to set the current to obtain the desired field. You want to see NMR LOCK illuminated green.
Old Instructions for Setting the HMS dipole by NMR
These are archived.  Do not use these instructions unless directed by an expert

  1. On the NMR screen, click the "Stop" button and wait for it to be stopped.
  2. Enter the desired field in the input box and Enter.
  3. Click Start and wait. The system will calculate the initial current to set the dipole, and you should see the current setting change. This ought to be close to what you see from "go_magnets_HMS_current".
  4. The system is now going to wait about 1 second/amp for the power supply to reach the initial current, then it will start trying to get NMR lock.
    1. Do not do anything with the dipole controls for at least 20 minutes. Patience is important at this stage.
    2. You can watch the current read back in the lower-left of the Dipole NMR window. It needs to get close to the current reported by the 'go_magnets_HMS_current' command before it will start searching for an NMR lock.
    3. Eventually it should get an NMR lock and begin making adjustments in the current. You can watch the dipole current stripchart to see when it begins regulating.
  5. Once the current and NMR values stabilize you should be good to go.

Magnet Cryogenics