Magnets

Mass control window

Each magnet operates independently. The mass factor for MAG90 is approx 797 and for MAG60 it is approx 801, (where the High Voltage property is set to the effective acceleration voltage = Accelerating voltage + Anode voltage). The masses and mass factors may be set directly in the mass control application (select Specialist Mode in the menu).

Modes

There are 2 modes of operation, set by the value of rgmode (read/written via the Equipment Info program).

Mode 0 is BH15 (hardware field controller) field regulation only, using feedback from a hall probe
Mode 3 is software field regulation, using feedback from the NMR probes

In all cases, AQN is the readout from the NMR probes.

To access the Equipment Info program, select the equipment name in the working set and then click on "Eqp Info" in the working set menu.

Normal operation

Property rgmode=3 (software regulation via NMR probes):

The mass control window is used in the same manner as for the GPS. After each mass change, the field may take 5-10 minutes to stabilise. After each field change the magnet field is cycled (message cycling magnet appears in the mass control window). During cycling the power supply is operating under current regulation: the field is changing too rapidly for the NMR to lock, so there is no field readout. The cycling takes 2-3 minutes, depending on the field precision required and the setup of the software. When field regulation resumes, AQN returns the NMR readout. After a few minutes more, the desired field is reached.

As always, close the beam-gate during mass changes.

Note that the magnets take 5-10 minutes to stabilise after each field change. It is possible to change the field in both magnets simultaneously (ie. there is no need to wait for MAG90 to stabilise changing MAG60). It is also possible to change the requested field (CCV) before the magnet has stabilised -- the magnet cycling routine will be automatically interrupted and restarted.

Re-sending a field request without changing the CCV will be ignored. To provoke a magnet cycle without changing the field, open the field control knob (HRS.MAG90 and HRS.MAG60 in the HRS working set), then request a field change of +0.00001 T, immediately followed by -0.00001 T.

The field range of the NMR probes is 0.09 - 0.52 T, corresponding to a mass range of A = 7 - 217 at 60kV (A = 13 - 432(!) at 30kV).

The mass-control program always shows the field error, which is just the difference between CCV and AQN. Once stable, the field-error should be less than 10 uT and the beamgate may be opened.

Whenever the User Beamgate is open, slit SL470 should be IN (access via the WG470 control window).

Magnet calibration

Calibrate MAG90 first. Ensure that the Mass and High-voltage properties are set correctly. With the field stable and WG470 out, note the distance between the beam and the axis (centre of SC482/3). Set this distance in the calibration calculator (+ve values move the beam to the right). Click calculate, check that the new mass-factor is 800+/-5, and click send. The field and calibration are updated in one go.

Once MAG90 has stabilised, calibrate MAG60 on the centre of SC680. Ensure that the Mass and High-voltage properties are the same as those used for MAG90.

Note the High-Voltage and Mass-Factor values for each magnet in the logbook!

Trouble shooting

Lost calibration values

In case the magnet calibration has been totally lost, click "Set Default Values" in the mass control menu. Then fine-tune the calibration using the procedure above.

Field AQN reads zero

NMR probes are working, but not locked. Send a new field request, and check that the message cycling magnet appears in the mass control window. Check that the magnet is really being cycled by checking YHRS.SEPMAG90DEG and YHRS.SEPMAG60DEG in the working set. It should be changing by 20A roughly every 2 seconds.

Field AQN reads -1

Communication has been lost with the NMR probes (hardware bug in NMR controllers). Find rack RA50 in the hall, between GLM and the merging switchyard. In the back of this rack, find the orange 220V plug labelled "NMR". Pull it out, wait 10 seconds, and plug it back in again.

Using the HRS working set, check the CCV of the power supply of the affected magnet, YHRS.SEPMAG90DEG or YHRS.SEPMAG60DEG. If it reads 470 or above, reduce it by 100A.

Send a new field request and check that the magnet is cycling (see above).

Magnet is not cycling after a new field request

The control software in the DSC has jammed. Either reload the DSC, or get a controls expert to restart the hrsmagrt process. Once restarted, the field CCV will be set to 0.00010 T. Send a new field request and check that the magnet is cycling (see above).

Operation using the BH15 field controller

~~~ Not recommended!!! ~~~

To switch to hardware field regulation:

Switch off both magnet power supplies via the control system
In the power supply room, switch over the signal used to regulate the power supplies: on the panel press local, M, remote. Check that the display reads "BH15".
Switch on both magnet power supplies via the control system
Set the property rgmode to 0 via the Eqp. Inf. program

To switch back to software field regulation:

Switch off both magnet power supplies via the control system
In the power supply room, switch over the signal used to regulate the power supplies: on the panel press local, M, remote. Check that the display reads "IG64".
Switch on both magnet power supplies via the control system
Set the property rgmode to 3 via the Eqp. Inf. program

This mode switching occasionally fails, in which case you will need a controls expert to untangle the system. The troubleshooting section above will not help you. In other words, 6pm on Friday is a bad time to try switching modes!

In BH-15 mode the Mass Control window is used in the same manner as for software regulation. The field stabilises more rapidly, but the calibration is extremely poor. Note that when calculating mass factors, the field AQN from the NMR probes is used (whereas the CCV is used for the GPS). In hardware field regulation the NMR probes are taken out of the feedback loop, and therefore the AQN does not exactly match the CCV. Outside the range 0.09 - 0.52 T the NMR probes will not read at all.