M9B is a backward-muon beamline with a superconducting solenoid for the pion-decay leg. The solenoid functions well when driven by 300 to 500 A; typically 400 A.

Set polarity

Set polarities while supplies are off.

Most of the polarity switches are big copper manual throw switches, with plastic red "+" and black "-" labels affixed nearby to indicate beam polarity. Ignore any labels you see on the cables or their attachments — they indicate electrical polarity, not beam polarity! Ignore any stickers indicating polarity for experiment 930 ‐ they are for some strange set-up. Follow the guidance of this table

MagnetRemotefor µ+for µ-Notes
B1   yes normal reverse Front panel. Set polarity control switch to "local" and press the desired polarity button (the little LEDs are push-buttons)
B2   yes normal reverse Like B1
B3   no +/normal -/reverse Rotating switch on front panel. Beware that resetting interlocks remotely sometimes resets - polarity to +.
Q1   yes normal reverse Like B1
Q2   yes normal reverse Like B1
Q3W   no red + (none) Ignore white "930" sticker
Q3E   no red + black - P/S at far east of mezzanine
Q4   no sticker + sticker - Single white sticker
Q5   no sticker + sticker - Single white sticker
Q6   no red + black - Ignore white "930" sticker
Q7   no red + black - Ignore white "930" sticker
Q8   no red + black -  
Q9   no red + black - The colored labels are up high. There are also "+[-] particles" labels.
Q10   no red + black -  
Q11   no red + (none) Small internal switch. Read instructions on front door of P/S

Turn on supplies

Most of the power supplies can be turned on remotely, if they are set to remote control, but may also be switched on at the front panel. Exceptions are BB3 and BQ11:

B3 should only ever be switched on remotely, to ensure that it obeys remote current settings. Also, beware that resetting the interlocks may switch from - polarity to + polarity, and the little "on" button on the main epics window performs an interlock reset. Therefore you should click on the black "B3" button to bring up a device-settings window, and click on the Turn On button there.

Q11 can only be turned on (sucessfully) locally because it requires special procedures. When switched on ("main power") it will immediately trip with a false ground fault. Just reset the interlock and switch on again. This time it should turn on successfully, and you should press the round red button (upper-right) to initiate operation.

Choose momentum

The nominal settings for B1 and B2 for a few beam momenta are:

p(mu) in MeV/c
B2DAC = 630.1*p(mu)
B2 Amps = B2DAC*7.534e-3
p(pi) = 1.511*p(mu) + 36.15 MeV/c (linear approximation)
B1DAC = p(pi)*254.914
B1 Amps = B1DAC*5.4054e-3/

p(mu)  B2DAC  B2 Amps       p(pi)    B1DAC  B1 Amps
  60. 37806. 284.830 126.8   32326. 174.733
  65. 40957. 308.566 134.4   34251. 185.143
  70. 44107. 332.302 141.9   25324. 195.554
  75. 47258. 356.038 149.5   36177. 205.964
  80. 50408. 379.774 157.0   40029. 216.374

Tune for desired polarization

The Q3 quadrupole magnet, placed immediately after the solenoid, has two power supplies to drive it asymmetrically, giving some control over the beam's polarization. Asymmetric settings are used in conjunction with an offset position for the horizontal slit (BHSL) to produce transversely-polarized muon beams.

There are several tunes saved in the "ref" section, named such as "L70", optimized for good transverse polarization (T), good longitudinal polarization (L), or for absolute maximum rate (M), at various momenta. Currently, a freely-optimized tune in the momentum range 60-80 MeV/c gives a muon spin polarization rotated almost 30 degrees to the right (and a transverse asymmetry of typically 0.10; the "M" tunes)

To produce a more highly spin-rotated tune, one should start with an existing tune for the desired momentum, then increase the difference between Q3E and Q3W so Q3E is about double Q3W, and with the same average setting. Set the slit BHSL to width 70 and position -50. Tune the beam, especially Q4, but not Q3 (or B2). Then scale the injection (B1 Q1 Q2) momentum to lower values, say 10% lower. Autotune again, excluding B1, B2, and Q3E/W. This should give a transverse asymmetry in excess of 0.15. Both the magnet current and the slit width/position are flexible ‐ choose settings that strike the right balance between the desired polarization and the desired beam flux.

To produce a normal longitudinally-polarized beam, set Q3E and Q3W equal, set the horizontal slit narrow (width 30 if the rate is high enough) and centred (position 0), and auto-tune. The slit may be opened further when using the tune.

When autotuning, watch out for magnets that respond slowly; make sure the "settle time" is long enough.