Re: Skin Effect & Primary Current?

>Message-ID: <199607300425.WAA18801-at-poodle.pupman-dot-com>
>Date: Mon, 29 Jul 1996 22:25:01 -0600
>To: Tesla-list-subscribers-at-poodle.pupman-dot-com
>Subject: Re: Skin Effect & Primary Current?
>From jim.fosse-at-bdt-dot-comMon Jul 29 21:26:44 1996
>Date: Mon, 29 Jul 1996 06:48:49 GMT
>To: tesla-at-pupman-dot-com
>Subject: Re: Skin Effect & Primary Current?

   [ snip ]

>> but the actual time it takes to
>>   strike an arc in mercury vapour is incredibly small, less than a
>>   nanosecond.  Mercury wetted relays are used in line-discharge
>>   pulser circuits for nanosecond pulses.  The "make" part of the
>>   cycle is extremely fast.
>I always guessed that mercury wetted relays were used to reduce the
>bounce time. As the contacts bounced apart,upon closure, the liquid
>mercury formed a thread between them, keeping them 'closed'.

   That is true, but they do close very fast.  We had to use this to
   fire off our short charged cables into the cathode to produce 10
   nanosecond pulses (and shorter).  The turn-on time or the rise time
   of the current in the line-charge pulser fired by triggering the
   merecury-wetted relay was faster than a nanosecond.  We're talking
   several hundred volts on a 50-ohm coax line fired into a microwave
   vacuum-tube (a triode) cathode.  The tube was run in grounded-grid
   configuration, and the plate circuit was the mouth of the
   accelerator tube.  So the electrons shot down the tube and got
   accelerated to a few million electron-volts of energy.  They made
   really hard x-rays.

>>  We used these on our old Van de Graaff to
>>   pulse electrons into the cathode at the top of the accelerator
>>   tube. 
>I'd would have liked to see the throw on that relay! How many leagues
>did you say it was?;)

   You could easily hold it in your hand.  I think it fit in a
   standard round relay socket.  The physical mounting orientation was
   important.  The whole VdG generator (column, belt, motor, controls
   and th HV top end) was inside a pressure vessel of 20 atmospheres
   of a mixture of nitrogen and carbon-dioxide gas to suppress tank
   sparks since the top-end ran at 2 million or more volts DC and the
   diameter of the tank was under 6 feet.  The top-end HV VdG terminal
   cathode-pulsing circuit was controlled optically.  The mechanical
   switching was done by driving motors at the bottom end which drove
   long lucite rods up alongside the belt up to the top end.  Some
   other Van de Graaff accelerators used nylon (or similar) cables and
   pulleys to turn switches and pots at their top ends.  I remember
   the day several pulley systems were melted by the addition of SF6

   The time jitter on the closing of the mercury-wetted relay was plus
   or minus 25 milliseconds from the time we asked it to fire.  Since
   we could not do nanosecond timing of the capture of the actual
   experimental data with that huge kind of slop (!), we had an early
   beam-pickoff trigger on at the bottom of the accelerator tube which
   fed a fast beam signal into a special microwave cable (a carrier
   telephone-system rigid trunk cable) which had a velocity factor of
   90% the speed of light instead of ordinary RG58 cables which are
   about 66% the speed of light and down which slower cables the rest
   of our signals passed through a longer route.  This fast pickoff
   was fed straight through a hole bored in the concrete shield wall
   and straight into the control room to the trigger electronics
   there.  Thus we could fire the storage-scope traces and the data
   acuisition systems to synchronize with the beam pulse itself, and
   the trace would actually start before the beam hit the target.

>>    Somebody ought to investigate the possibly advantageous properties
>>    of mercury for fast switching.  We know it turns on fast.  I wonder
>>    if it turns off as fast.
>I'm having visions of an open-air, rotary paddle-wheel, mercury gap

   Why not put the whole thing in a sealed cavity, motor, contacts,
   mercury and all?  If physical orientation were a problem for our
   relay, it ought to be a bigger problem for a motor-driven rotary
   switch.  A job for a mechanical engineer maybe.

>On the other hand, a cold-cathode fluorescent light is just what we
>are talking about. Anyone out there care to steal the backlight out of
>their LCD portable computer for some gap testing?
>	Regards,
>	jim

    Maybe we're on the right track with gas arcs.

 Fred W. Bach ,    Operations Group        | Internet: music-at-triumf.ca
 TRIUMF (TRI-University Meson Facility)    | Voice:  604-222-1047 loc 6327/7333
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