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    User name J. Singh

    Log entry time 14:34:25 on August 1,2003

    Entry number 108498

    keyword=Neutron Spin Crisis

    Short Version:
    Target spin angle was 90 deg instead of 180 deg. This was caused by a software glitch. We lost about 1 hr, 1 elastic data run, & an estimated 3% absolute polarization (~40% to 37.6%). Moral of the story: Make sure that the software is actually doing what you tell it to do.

    Long Version:
    @ 1120 The target was rotated to 180 deg. see 108460
    @ around 1220, JP called the counting house b/c he was concerned that the target halfwave plate was still in the OUT position. (for a target field orientation of 180 deg, the target halfwave plate must be IN.) He was worried about it b/c one of the variables/comments in a recent start/end summary looked incorrect.

    Vince and I checked the the position of the target halfwaveplate on the monitor viewing the laser table. The target halfwaveplate was in the correct orientation. In addition the GUI on the target operator terminal was reading the correct value. (For future reference 0=IN & 1200000=OUT.)

    Everything appeared okay. However, upon closer inspection Vince noticed that the currents in the coils displayed on the GUI were:
    Small (Long) = +0.64A
    Large (Trans) = +7.0A
    This corresponds to 90 deg! We checked the target field rotation software. The datafile stored on the computer recorded the present target angle as 180 deg. In addition, both Ron and Jing saw the needle in the field rotation vi point to 180 deg at the end of the rotation. Since there was a discrepenancy, we checked the function generators (FG) that control the power supplies to the coils. The FGs were set to values which would correspond to a field angle of 90 deg. The following steps were followed to remedy the situation:

    (1) Beam OFF.
    (2) Long Lasers OFF (4,6,7).
    (3) Moved target halfwaveplate from IN->OUT.
    (3) Field reinitialized to 0 deg.
    (4) Verified that field was actually 0 deg by checking the coil currents GUI and the settings of the FG themselves.
    (5) Rotated back to 180 deg.
    (6) Verified that field was actually 180 deg by checking the coil currents GUI and the settings of the FG themselves.
    (7) Moving target halfwaveplate from OUT->IN.
    (8) Verified on monitor that target waveplate was in correct position.
    (9) Turned on LONG lasers (4,6,7)
    (10) Took NMR = 37.6%.

    (some of these steps were unnecessary and in retrospect our response could have been more efficient. here, i am simply reporting what we did step by step.)

    Everything looks okay now. We did not take a big hit in polarization b/c we were pumping transverse to the holding field. This has no effect. The polarization was lower b/c the Rb was unpolarized for about 1 hr, the beam was on at 5 microA, and we abruptly changed the holding field angle (step 3, 90 deg to 0 deg). All in all, it could have been worse. In the future, please check that the software is actually doing what you think that it is doing. If anything looks fishy, then please don't hesitate to page Vince.

    P.S. I checked all the start/end of run summaries. Everything was correct except for the user-entered header comment thing that pops up at the beginning of every run. for right arm runs 3110,3111,3112,3113 (left arm runs 23110,23111,23112,23113), the target halfwaveplate is mislabeled as OUT. It is actually IN. see also 108497