sPHENIX discussion of the superconducting solenoid

["Yip, Kin" <kinyip AT bnl.gov>]

Hi Carl,

Below is what Martin Berndt sent to me when I asked him why they claimed that BaBar only needed half an hour for a full ramp down.  I'll come to talk with you.  But I'd just like to send it to you while relaxing in restaurant ...

Diode accelerates decay ...and they did fast discharge at ~ 1500 A !

I still don't understand why half an hour only but you may ... and you may find the document that Martin mentions that Bob has passed to you ?? ... 😀

Kin

PS: He doesn't charge me anything for his answers 😀

------ Original message------

From: Martin Berndt

Date: Thu, Apr 21, 2016 1:20 PM

To: Yip, Kin;

Cc: Lambiase, Robert;

Subject:Re: half an hour a full ramp-down or longer in BaBar ?

Dr Kin Yip:
    Good to hear from you.  The diode through which the magnet discharges
has a relatively constant forward voltage drop of about 0.75 Vdc.  Thus the
current decays towards a new value of
-577 Amperes (-0.75 V /1.3 mOhm), which of course it can't reach abecause
the diode is not a battery.  It does mean however that the diode accelerates
the decay, and becomes predominatly more important at reduced currents.  Our
practice at SLAC was to initiate a slow discharge rate through the diodes
until the current came down to about 1500 Adc, at which point we opened the
DC breaker for a fast discharge through the dump resistor.
    Somewhere in the documents I sent to Robert L I have the precise numbers
for the current decay with the diodes.  I don't have that here with me.
    Feel free to contact me if you have further questions.  It makes me feel
still useful!
            Martin Berndt


----- Original Message -----
From: "Kin Yip" <kinyip AT bnl.gov>
To: <martinberndt AT sbcglobal.net>
Cc: "Lambiase, Robert" <lambiase AT bnl.gov>
Sent: Thursday, April 21, 2016 7:25 AM
Subject: half an hour a full ramp-down or longer in BaBar ?


> Dear Martin,
>
> My name is Kin Yip at Brookhaven National Lab. and I'm the physicist now
> in charge of the sPHENIX Magnet,
> formerly BaBar Magnet.   I found your email address in one of the emails
> that Bob Lambiase CC-ed his email to you.
>
> I've come across a memo which has been also shown in this webpage :
>
> http://www.slac.stanford.edu/grp/ad/op/solnramp.html
>
> At the end of the 3rd sentence (near the top), it said "a full rampdown
> takes about 30 minutes".
>
> I understand that this is ramp-down through the freewheeling diode with
> low resistance (cable).   From the document
> you wrote (2nd line on p. 4 of the attached) and other places, the
> input/cable resistance is ~ 1.3 mOhm.  With the inductance
> of 2.57 H at BaBar, the time constant is close to >30 minutes or so.  In
> the time of one time constant, the current would only
> get down to about 63% and so you need a couple of 30 minutes before you
> can ramp down to 0 (from ~4600 A), right ?
>
> So, in BaBar, when you ramped down to 0 A (from 4596 A or so), it'd take
> more than an hour or really just half an hour ???
>
> Kin Yip
>
>