Historically many of the faults which force a rampdown of the BaBar solenoid come from the cryogenic cooling system (the magnet is superconducting...you knew that).
The first thing to do is to dispatch the cryogenics group and/or the power techs as appropriate.
It is often the case that the fault can be cleared before the solenoid has completely ramped to zero (a full rampdown takes about 30 minutes).
Let's say you hear back from the cryo folks right away and they say the fault has been cleared. Presume this happens 5 minutes into the rampdown. The magnet will have about 3850 amperes circulating and will be dropping. The power supply will be "tripped".
Addendum 9/20/2004 HVS:
I strongly recommend launching the striptool config called "babarramp.cnf" before doing anything.
Much of what follows can be ignored if you are in a hurry. William and I have a button macro which does it all for you. On the SCP BaBar Power Supply Touchpanel do a keyboard "F8" and run the button macro called "BABARRMP". It should do everything listed below automatically as long as the power supply comes on OK.
Use the SCP to reset the LGPS then turn on the LGPS. This will leave the power supply reference voltage at zero.
Here is where you need to understand a little fact (Martin Berndt explained this to me...it is key) about the way BITBUS ramping works. When you setup ramp, BITBUS starts with the reference voltage it presently has (in our case zero), then determines a final reference voltage which corresponds to the present BDES. When you start ramp, it ramps the reference from the initial value to the final value in the time interval specified.
So now let's say the magnet is down to 3500 amperes. If you setup a ramp to 3300 amperes in 1 minute (enter BDES 3300, ramp time 1 minute, setup ramp, start ramp) in one minute the power supply reference will be that which corresponds to 3300 amperes. Soon thereafter the magnet will ramp down to the 3300 ampere level. At this point the power supply will begin to regulate (at 3300 amperes) and the LGPS will be "green".
Typical rampdown rates are about 120 amperes per minute
A point of caution: There is historical evidence that if you quickly ramp the reference voltage to a level which corresponds to a current greater than that which is still circulating in the magnet, then the quench detection circuits will detect a voltage spike and cause a fast discharge to happen. For this reason don't overshoot the BDES when trying to turn around a rampdown.
Now you can ramp it back to 4600 amperes by typing in the nominal BDES of 4600 amperes, an appropriate ramp time, setup and start a new ramp back to normal.
Presently (Nov 2002) E.F.D. is requesting that we ramp the solenoid at a rate of 130 amperes per minute. This corresponds to a ramp time of 35 minutes for full ramp from 0 to 4600 amperes.
Don't forget to deal with the issue of what this excursion has done to the magnetization of BaBar's DIRC shield. There are button macros which are made to use the bucking coil to degauss the DIRC shield. Details of that process are beyond the scope of what I want to cover in this document.
Happy Ramping!