[Sneap] sparking problem

Mon Feb 2 18:33:34 EST 2009

Re: [Sneap] sparking problemTom et. al.

It is really great how the harvest of SNEAP replies about spark protection
and chain motors allows a richer understanding and choice of options that
just one of us could devise on his own.  Our instinct is to locally shield
the motor but another tack is to reduce the spark insult to the motors by
providing alternative RF paths to ground.  I think that the motors and their
wiring are the default path for spark energy.  So a strategy to consider is
defence at a distance rather than last ditch defence.

Ken Purser had real insight into this.  At Rochester, he recognised that the
column was like a coax transporting RF energy and that its connection to the
tank wall was like a short circuit which would double the voltage spike when
the RF wave was reflected.  He installed an array of resistors between the
end of the column and the end of the tank, to better impedance match the
column.  An inspired solution.  Unfortunately, the resistors were not up to
the challenge.  But in their failure, they highlighted that the model was
valid.  Spark energy at the ground end of the column is powerful and must be
confronted.

The SNEAP postings from FN labs with Pelletron conversions highlight the
difference in topography in the region between the end of the column and the
tank wall.  In the 14UD, the chain motors are partially hidden under the
grounded end of the column.  Chris' photo clearly shows that the motors in
the FN are out in space and so need extra protection.

Our early experience on the 14UD ,with sparks at the low energy end of the
machine, provides some strategies that could be applied to the FN chain
motors.   In the 14UD there was consistent spark damage to electronic
devices outside on the top of the tank.  Indeed, there was more spark damage
here than outside the high energy end. The problem was solved by installing
8 flat conduction strips between the grounded end casting and the top flange
of the tank.  These strips are about 20 mm wide and 0.5 mm thick made of
phosphor-bronze.  Silver plated copper would be better but not necessary.

I believe, but can not prove, that most of the spark energy travels as
ring-to-ring sparks and so the RF conductive paths, from the ground plane
adjacent to the last ring, to the tank wall take much more spark power than
is travelling in the space between the column and the wall. So giving the
spark energy good RF paths in parallel with that accidentally provided by
the motor wiring is probably the most effective first step.

In spite of saying that the space between the column and tank wall is a less
serious spark energy location, nevertheless, I believe it would be useful to
shield the motors by radial conducting sheets from the end of the column to
the tank wall.  These sheets would also be a safe path for spark energy.
They should be mounted as far away from the motors as practical and,
ideally, form a complete bulkhead from the end of the column to the wall.
There is an issue of how to electrically connect it to the wall.  Drilling
and tapping into the pressure vessel wall would have to be sanctioned by a
responsible engineer.  However, the sparks will jump a gap to wall and the
bulkhead will still do the job without metal-to-metal contact.  The bulkhead
would need to be perforated for cooling and easy to open for service.

Finally, Art Haberl reminded me that we also used MOVs on the motor leads
and that, at least once, one of them failed.

Forgive me for going on so long - SNEAP enthuses me.

David
********************************************
D.C. Weisser
Nuclear Physics Department
Research School of Physical Sciences and Engineering
Australian National University
Canberra A.C.T. 0200
AUSTRALIA

Tel:    + 61 (0)2 6125 2080
Fax:    + 61 (0)2 6125 0748
Mobile: + 61 (0)414 249 209

david.weisser at anu.edu.au

  -----Original Message-----
  From: sneap-bounces at tunl.duke.edu [mailto:sneap-bounces at tunl.duke.edu]On
Behalf Of Chris Westerfeldt
  Sent: Tuesday, 3 February 2009 1:17 AM
  To: SNEAP
  Subject: Re: [Sneap] sparking problem

  Hi Tom,
      We don't have any surge suppression in our motor circuits and have
never had any problems
  such as you describe.  Our motor leads are run in that flexible conduit
that NEC provided with the kit.
  The only preventative measure that we employ is to "hide" the motor behind
a well grounded,
  perforated metal "screen".  The screen in bolted to the motor mount right
at the ground plane.
  Our thinking was that any spark coming down the equipotential rings would
likely hit the motor or
  it's mount as they were 'hanging out in the open".  I've attached a photo
to illustrate.
  Regards,
  - Chris
  --

  Chris R. Westerfeldt
  Research Scientist / T.U.N.L. Radiation Safety Manager
  Duke University Physics Department &
  Triangle Universities Nuclear Laboratory
  Science Drive, Box 90308
  Durham, NC  27708-0308
  Tel: (919) 660-2600
  Fax: (919) 660-2634
  Email: Cwest at Tunl.Duke.Edu

----------------------------------------------------------------------------
--
  From: "Miller, Thomas Edward" <millerte at purdue.edu>
  Reply-To: Symposium of Northeastern Accelerator Personnel
<sneap at tunl.duke.edu>
  Date: Sun, 1 Feb 2009 08:34:45 -0500
  To: <sneap at tunl.duke.edu>
  Conversation: sparking problem
  Subject: [Sneap] sparking problem

----------------------------------------------------------------------------
--
  SNEAPERs:

  I need your advice. For our 2 drive motors inside the tank we have PI
filters inside a metal box in series with the 2 motors. Connections from the
tank feedthrus are made with copper conduit to the box and then to the
motor. It was found that spark damage inside the copper conduit between the
wires and the copper conduit had damaged the wires that are between the
motor and PI filter box. What was done was the PI filter box was mounted
outside the tank and the conduit connecting both the PI filter box to the
tank feedthrus and the tank feedthrus to the motors was replaced with
plastic conduit. What this did was bring the spark energy outside the tank
where it can now damage equipment outside the tank.

  What is your recommended solution, i.e. how have you guys done it?

  I would like to have a better robust filtering for the motors on the
inside to prevent the spark energy from getting out of the tank and a better
conduit system. Seems like spark energy is traveling thru the motor 208 VAC
wires and needs to be removed right at the motor. Any help would be great.

  Thank you. Cheers!!!!

    Tom Miller
   Purdue University
    millerte at purdue.edu

----------------------------------------------------------------------------
--
  _______________________________________________
  Sneap mailing list
  Sneap at daytona.tunl.duke.edu
  http://daytona.tunl.duke.edu/mailman/listinfo/sneap
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://daytona.tunl.duke.edu/pipermail/sneap/attachments/20090203/9cd549ad/attachment.html 