Magnets

In the process of editing and updating this page. Please contact Juliette Mammei crowder@jlab.org with questions or suggestions.

PREX Main<< HOW TOs

Nominal Settings

PLEASE READ HOW-TO BEFORE SETTING THE MAGNETS!!!!!

• When starting a shift, it is prudent to verify that the magnets are set correctly. If it's not right, consult the RC. Currents should be within about 0.5 A of the values. Also, note that "readback" values are slightly different from "set point" values.

• LHRS dipole NMR doesn't lock. Need to set manually to match the field value (iterative procedure, see the instruction below)

• Below are set points. Readbacks will be a bit lower.

• CREX LHRS

Dipole	Q1	Q2	Q3
Hall Probe: -0.7775	225.387 A	934.273 A	981.301 A

• CREX RHRS

Dipole	Q1	Q2	Q3
p0 = 2.18350	230.916 A	925.955 A	981.301 A

PREX Tune P
LHRS Dipole	LQ1	LQ2	LQ3
p0 = 0.95285	118.50 A	407.70 A	450.76 A
RHRS Dipole	RQ1	RQ2	RQ3
p0 = 0.95284	118.55 A	404.07 A	446.90 A

Setting the Magnets

If one can not reset the magnet remotely: Do NOT go into the hall to reset by yourself. Contact RC and then tech on-call/magnet expert.

Regulators for Q1, Q2, Q3 are always OFF. Q1, Q2, Q3 magnets will be set by current.

Set HRS Magnets

0. Clear all faults (if there are still problems with flows or liquid levels there is no point ramping back up to crash again).

RHRS Dipole

1. Regulator ON
2. Set the target momentum and hit enter. The dipole should start moving towards that setting.

LHRS Dipole (New instruction for CREX)

1. Regulator OFF (why not using it? https://logbooks.jlab.org/entry/3747901), how to deal with it (https://logbooks.jlab.org/entry/3761815)
   
2. Set the current to 744.0 followed by the procedure https://logbooks.jlab.org/entry/3802168.
3. Wait until the field is stable (strip chart!). Check the Hall Probe if it matches the target value (nominal CREX: -0.7775 +/- few 1.e-4) when it is stable.
4. Adjust the current if they don't match. Wait until it gets stable and repeat #3 if needed.

If the dipole turns off, the first way to try to turn it back on is to do the following

1. Zero
2. Reset
3. On
4. Type in the desired current and hit Enter

If that doesn't work then notify the RC

Quads

1. Check regulator OFF for Q1, Q2 and Q3
2. Turn on the Q1 power supply if not ON already
3. If turning a quad magnet down, we do not need to cycle the magnet. Just set the set point.
4. If turning a quad magnet up, it must be cycled to always follow the same hysteresis curve
   1. Right:
      1. Q1: The RQ! should be taken to 900A. Let it stay at 900A for about 2 minutes before loading the desired set point. (NB in the past the RQ1 has had problems reaching 900A so if you cannot do that make a log entry and go to 700A)
      2. Q2: The RQ2 should be run to 1400A. Let it stay at 1400A for about 2 minutes before loading the desired set point.
      3. Q3: The RQ3 should be run to 1400A. Let it stay at 1400A for 1 minute before loading the desired set point. Do NOT leave RQ3 at 1400 A more than 5 min (but leave it there for 1 min)!!
   2. Left:
      1. Q1: The LQ1 should be run to 900A. Let it stay at 900A for about 2 minutes before loading the desired set point.
      2. Q2: The LQ2 should be run to 1600A. Let it stay at 1600A for about 2 minutes before loading the desired set point. First ramp it to 100 Amp. If it doesn't trip, you can ramp to 1600A. (Last updated: 08/26/2020)
      3. Q3: The LQ3 should be run to 1600A. Let it stay at 1600A for about 2 minutes before loading the desired set point.

Set Septum Magnet

• The polarity of the septum is positive
• Instructions for setting the current Jack's elog entry.
• CREX set points determined here.

To check/set the septum current:

• In JMenu:
  • Operations < Magnets < Special Hall Magnets < Septum Magnet (Hall A)
  • The Hall A Septum screen will come up (see Figure 1)
• From the Hall A Tools screen:
  • Find the HES power supply link in the top right of the GUI
• For CREX we are running high, with negative polarity in both the SBS and HES power supplies
  • Cycle the magnet to 400 A (PREX), -900 A (CREX)
  • Set it to 333 A -- nominal Tune (PREX), -801.248 A (CREX)

To reset the Septum magnet power supplies (CREX) after some problem (from Jack's log entry):

• Auxillary switches on the main contactors are wired in series with a relay
  • Relay contacts are wired to fault inputs on the supplies
  • Turning one supply OFF, or the supply faulting OFF, will fault both supplies to OFF
• To turn the supplies ON you need to open the GUIs for the HES supply and the Super BigBite supply (above)
  • On the HES GUI you need to click on the Handshake Bypass button
    • This will short out the contactor switches for two minutes, 120 seconds
    • State will change to On and Counter will change to 120 and count down to 0
  • You now have 120 seconds to do the following:
    • Reset a supply and the fault should clear
    • Turn the supply ON
    • Reset the other supply and its fault should clear
    • Turn the supply ON. After this point, if the Counter reaches 0, the supplies will not trip off unless there is a fault or one is turned OFF
  • Please note that the Super BigBite supply is not stable at 0A
    • If it needs to sit for some time while ON, it is best to have it set to 150A or higher
    • Brief periods at 0A are not too bad, though

If Power Supply is on, and amplitude set, but magnet staying at 0A log entry

• A presumed software glitch can prevent the SBS (and possibly HES) from registering the amplitude set point if this happens
  • step the set point more gradually from 0A (ie. 50A, 100A, 500A, 700A, 800A, 900A, waiting a few seconds after each step)
  • then cycle down to desired set value (ie. 801.248A during CREX)
  • then cycle the septum properly (down to 50A. wait 1 min. cycle to 900A. wait 1 min. then to setpoint)

Monitoring the Magnets

HRS

GUI can be found at:

OPS Menu -> EDM (HLA) -> Hall A -> Tools Display

How to check polarity

LHRS dipole, RHRS dipole polarity changes require an access. The rest can be changed with the power supply gui.

One should check the polarity with Gaussmeter. The Hall probes should read positive for positive particles and negative for negative particles (WITH CURRENT IN THE MAGNETS)

• PREX polarity and CREX polarity (Signs of field should be the same!)

• 

  PREX polarity

• 

  CREX polarity

Septum

GUI can be found at:

OPS Menu -> EDM (HLA) -> Hall A -> Magnets -> Super Big Bite
OPS Menu -> EDM (HLA) -> Hall A -> Tools Display -> Top Right of GUI has HES Dipole button

Checking and Controlling Drifts

While on shift, it's important to check the magnetic fields of the HRS magnets and the Septum. This is part of the shift checklist.

For the Quads, you should check the setpoints against the setpoints written on the paper near the controls computer. Or check the readbacks versus the tune E readbacks. Note, the readbacks are not necessarily the same as the setpoints, so don't mix these !

For the Dipole, the R-HRS is regulated by NMR and should be accurate for the given momentum. The L-HRS NMR is not currently working, so one should go by the Hall probe field, which however is not as accurate as NMR.

When an IOC is rebooted, the set points are put to the observed read-back values, and this might not be what you want ! (probably not, in fact)

More, from Javier:

The dipole is an iron dominated magnet so its actual field will change with current history (the reason to define hysteresis cycles) so, neither set or read-back currents are reliable. After a large current change, you will see both Hall probe and NMR (if it was working) changing (dp ~ several 10-3) over many hours.So, you want to use the Hall probe if the NMR is not available and adjust by hand as needed. Unfortunately, the Hall probe was not calibrated vs NMR measurements - you have to find a previous setting, preferably near to desired setting. Also, keep in mind that the Hall probe has lower precision than the NMR probe.

Rebooting an IOC. Note that there are differences between the set and read-back currents of the power supplies. There are many reasons for these differences. We use the read-back currents to determine the momentum of the quads based on cross checks done long time ago. The dipole currents are nor really relevant for the reasons described above. When an IOC is rebooted, it needs to initialize all its variables (these IOCs do not have such thing as an entire image of the system from x -time ago). One thing that it needs to do is to initialize the set current variables. It uses the read back currents to do so which will cause some changes in the magnets currents

(end of Javier's statement)

Physics concern:

The location of the elastic peak is very important for CREX. If the peak partly falls off the detector, our sensitivity to energy and subsequent noise in the asymmetry become significant. If the peak is too far in the other direction, we start getting contamination from inelastic states, which are dangerous at this relatively high Qsq -- ratio of inelastic to elastic is a few percent, which introduces a systematic error. Of course we should try to notice these things and make alignment adjustments. The alignment procedure and a checklist will be written elsewhere.