Tuning

In contrast to μSR experiments at TRIUMF, tuning the radioactive beam for a β-NMR experiment is done primarily by the ISAC operators, not the β-NMR experimenter(s).

The role of the experimenter then is largely to provide guidance to the operators so that they may deliver a beam with a “good” tune that satisfies the (rather stringent) criteria for the experiment. This mainly comes in towards the end of the startup - once the RIB has been delivered up to the front of either spectrometer.

Table of contents

1. General Instructions
2. The β-NQR “extension”
   1. Objectives
   2. Procedure

General Instructions

Once the ISAC operators have established transport of ⁸Li⁺ up the last Farady cups before the NMR and NQR spectrometers, they will notify us (usually by phone) and prompt us for some input. This is, pragmatically, the beginning of the “fine” tuning of the beam (but we’ll just call it tuning for brevity).

The tuning “algorithm” is:

1. Perform a 1n scan (i.e., Na or Rb cell). The is to find the optimal bias needed to Doppler-shift the beam onto resonance with the counter propagating polarizing laser light. Use the NBM for this. For ⁸Li, a typical optimum is ~100 V.
2. Tune to the NMR spectrometer at 0 T with BNMR:EL3 on (at its theory value). Get a centred beamspot. Does varying ILE2A3:EL1 or ILE2A3:EL2 by ±300 V move the beamspot or just vary its shape/intensity? If the former (i.e., it moves), adjust the upstream elements. The goal is to get the beam going into the centre of the magnet (i.e., on axis with the field).
3. Move the cryostat so the beamspot is centred (if necessary).
4. Turn BNMR:EL3 off and degauss the magnetic field to the desired setpoint (e.g., 6.55 T or whatever field is planned for the run).
5. Look at the beamspots at NQR (but do not place too much emphasis on it).
6. Tune on the forward detector rate (ILE2:SCALER:CH4 in EPICS) at high platform bias (e.g., 19.5 kV for a ~20 keV beam).
7. Check the NMR beamspots at all biases. Should have a single tune when the beamspot does not move position, but focuses/de-focuses at different implantation energies.
8. Fine tune the beamspot at NQR (usually easy - just vertical/horizontal steering to compensate for field/bias). Except for at low magnetic fields (e.g. < 50 G), a separate tune is usually necessary for each field/bias combination (i.e., to have beamspots the exact same size in the exact same positions). For some experiments, this may be overkill.

The optimum settings for the steering elements on the beamline can be calculated and viewed using the tuneX application.

Note that when using a ³¹Mg⁺ beam, it is perhaps best to skip the “adjustments” given in step 2 and go directly to step 4 (i.e., ramp up the magnet). This will focus the betas into the detectors and give a better metric for the rates/beamspot. Without the focusing effect from the field, the rates may be lower enough (on the order of ~1k s^-1) that it is hard to judge if all of the beam is delivered to the spectrometer. See the bnmr logbooks from, for example, October 2019 and July 2021 for further notes.

The β-NQR “extension”

In 2021, the high parallel field extension of the “β-NQR” leg of the ISAC low-energy beamline was completed. This new end station offers the ability to apply ~2 kG magnetic fields perpendicular to a sample surface - a desirable feature for certain scientific applications.

There is less experience using the new beamline optics and these notes (originally by Edward Thoeng) serve as an aid for tuning to the extension.

Objectives

• Establish a common tune to both the NMR and NQR spectrometers with the NMR magnetic on.
• Make sure that the beam is always centered on-axis going into the NQR leg. This detail is crucial for the ability to focus the beam using electrostatic quadrupoles without moving the beamspot (i.e., the beam position).

Procedure

1. Turn all magnets off at both the NMR and NQR spectrometers.
   • Set the NMR magnet (in CAMP) to 0 T.
   • Set BNQR:HH3 to 0 A.
   • Set BNQR:HH6 to 0 A.
2. Tune using stable ⁷Li⁺ to NQR.
   • Make sure the beam is centred on ILE2A:RPM2. Should see peaks at 0.5 mm (horizontal) and 1.5 mm (vertical). If not, only adjust ILE2:B21 for X-steering and ILE2:YCB19 for Y-steering. The beam profile should also be circular and roughly 2 rms in both X- and Y-directions. If not, adjust ILE2:Q16, ILE2:Q17, ILE2:Q18, and/or ILE2:Q19. Check that the ILE2A:RPM2 peak positions do not move when ILE2A:Q2 is changed. If they do, the beam is not centred and ILE2:B21/ILE2:YCB19 need to be adjusted.
   • Make sure the beam in centred on ILE2A:LPM0.
3. To be completed in the future…