Re: Skin Effect & Primary Current?
>Date: Mon, 29 Jul 1996 22:25:01 -0600
>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
>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
>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?
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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