sPHENIX discussion of the superconducting solenoid

[Kin Yip <kinyip AT bnl.gov>]

Hi Carl,

Below is further detailed answer from Martin.  I feel I understand more ......

Kin

-------- Forwarded Message -------- Subject: Re: 2nd try: done many fast discharges after ~30 minutes in BaBar ? Date: Mon, 25 Apr 2016 17:56:37 -0700 From: Martin Berndt <martinberndt AT sbcglobal.net> To: Kin Yip <kinyip AT bnl.gov>

Hi Kin:
   Sorry I didn't reply to you earlier.  I got side tracked.  I am retired, 
but keep very busy.
   I will reply to the various points you raised
1)  The reason for doing a fast discharge after discharging the magnet 
slowly for about 35' was simply that the experimenters didn't want to wait 
more than about 45' for completely discharging the magnet when they needed 
access to the detector for one reason or another.  Remember that most of the 
remaining energy goes into the dump resistor, and only some small fraction 
is dissipated as eddy currents losses in the coil cylinder and in the matrix 
of aluminum that encloses the superconductor.  Yes, some helium would boil 
off, but within an hour or so the coil would again have cooled off 
sufficiently to allow restarting of the magnet.  Remember that this is an 
extremely rugged magnet.  It survived a couple crashes at full current (4600 
Amps) into the dump resistor, at which time it must have generated over 300 
V across the coil terminals.  The insulation was never damaged.  Such 
crashes were always generated by a malfunction of the Quench Detector, never 
on purpose.  I suspect that during such a crash quenching of the super 
conductor occurred fairly uniformly throughout the coil, not localized in 
one spot, thus not damaging the insulation.  A magnet crash at full current 
into the dump resistor always caused a lot of helium to boil off, and it 
would typically take more than 4 hours to cool down the magnet to again make 
it superconductive and allow a restart.
2)  I don't recall whether the magnet was charged at 2 A/s or 2.5 A/s during 
routine operation.  The problem we always had was with the Quench Detector 
malfunctioning during the voltage transient whenever a current ramp (L 
di/dt) is initiated, even to start a slow discharge.  Turning off the power 
supply for whatever reason would cause the current to decay through the 
freewheeling diodes, generate about 6.5 V across the teminals at 4600 Amps 
and an initial  di/t = 2.3 A/s.  This set the maximum di/dt we could allow.
3)  From my comments perhaps you can see why I have stated before that the 
best thing you can do is to completely rebuild the Quench Dector, particular 
the divider network that senses the voltage across themagnet.  You would 
then be able to run the magnet at higher ramp rates (di/dt), and not worry 
about crashes when nothing is wrong with the magnet.
   Keep me posted!
               Martin





----- Original Message ----- 
From: "Kin Yip" <kinyip AT bnl.gov>
To: <martinberndt AT sbcglobal.net>
Sent: Monday, April 25, 2016 11:44 AM
Subject: 2nd try: done many fast discharges after ~30 minutes in BaBar ?


>
>
> Dear Martin,
>
> I've just found a page in a scanned PDF file (from the BaBar documents) 
> which seems to be
> exactly what you talked about in the last email.   I've attached that 
> page.
>
> I've talked with engineers such as Bob Lambiase ... People seem to be a 
> bit surprised that you guys are
> "brave" enough to do "fast discharge" at 1338 A.
>
> Even though power ~ (1338/4600)^2, so the remaining energy left is ~8.5% 
> of the total in this example.
> But 1338A/38 seconds is like >35 A/s.   Did this (fast discharge at 1338 
> A) not generate heat and quench
> in the Magnet ??   You guys have done this (fast discharge after ~one time 
> constant) many many times ??
>
> In the years of mature BaBar operation, did you guys constantly ramp the 
> magnet up at 2.5A/s ?
> ( I asked this because you seemed to be using 2A/s in your Word document 
> reply to Bob Lambiase. )
>
> Kin
>
> -----Original Message-----
> From: Martin Berndt [mailto:martinberndt AT sbcglobal.net]
> Sent: Thursday, April 21, 2016 1:20 PM
> To: Yip, Kin <kinyip AT bnl.gov>
> Cc: Lambiase, Robert <lambiase AT bnl.gov>
> 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
>>
>>
>
>
>
>
>