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    User name Calarco

    Log entry time 17:46:15 on February25,2001

    Entry number 56953

    keyword=How to set the right dipole

    The problem with the right dipole has been diagnosed to be due to the characteristics
    of the short changing from before. It now appears that the ramp rate is too high and
    causes a reactance in the superconducting coils that exceeds the resistance of the
    short. This produces voltage spikes across the coils that is interpreted as a quench
    by the power supply and shuts it down (thereby causing a quench). The procedure
    to avoid this is either (1) reduce the ramp rate by about a factor of 2, or (2) increase
    the current manually in small increments, pausing in between the intervals.

    In detail: a strip chart has been set up on the magnet control screen which displays
    the voltage across the coils as well as the current. Neither coil voltage should exceed
    +/- 1 V. One of the voltages gets close to this value if the ramp proceeds without
    pause, and occasional spikes can exceed the limit. The procedure is to watch this
    voltage as the current ramps up and pause for a few minutes if the voltage exceeds
    0.6 - 0.7 V. The voltage will slowly decrease. After a "rest", ramping can proceed
    again until the voltage builds up again.

    This must be done with the NMR regulator "OFF".

    This procedure will have to be followed when ramping down as well.