# Re: Cockroft-Walton & Marx Generators

```Dick:
Looking at your multiplier setup, indeed I would have only been
surprised if there were no problems!

A calculation of the setup gives the following:

Cockroft-Walton Multiplier Work Sheet
-------------------------------------
November 6,1998 K.Ottalini
-------------------------------------
Enter the number of stages:? 15
Enter the input voltage (volts):? 15000
Enter the input AC Frequency (Hz):? 60
Enter the capacitor values (Uf):? .0035
Enter the load current (amps) or 'R':? .004    <--- assumes a 15kv,60ma

-------------------------------------------------------
The estimated ripple at  4 ma is:  2285.715 KVolts     <------ Ouch!
The output voltage is:  225 KVolts          <------ Whew!
The estimated ripple is:  507.9366 % maximum at this
current.
The estimated Vdrop is:  44952.39 KVolts     <------ oh my!
The real output voltage is: -44727.39 KVolts     <------ oh my!
The percent regulation is:  9989.419 %
The power requirement for  225 KVolts at  4  ma is: 900.0001 Watts
This is equivalent to a load resistance of  5.625E+07  Ohms

====================
I would say that this setup, even if it had been immersed in highgrade oil
would have been marginal at best.  Above about 30KV, the coronal effects
become so significant that it is impossible to build a multiplier that
isn't encapsulated or immersed or in SF6 gas (or something).

For TC work, you rarely need more than 15KV to drive the coil, and the
total instantaneous power delivery is even more important.

Imagine that you have a hammer and you are using it to ring a bell. If
the bell is too big, the hammer just bounces.  If the hammer is too big,
the bell tone is heavily distorted (or the bell cracks! this is secondary
arc-over).

If the hammer is too soft, the tone will be muted, and if the hammer is
too hard, again the tone will be distorted.

TCs are resonant transformers and the driving impulse characteristics are
critical to achieve good efficiency.  The rise time and magnitude of the
pulse need to closely match (harmonically) the resonance of the secondary
and terminal capacitance.

A "large" or "small" hammer is like a small primary capacitor.  A "hard"
or "soft" hammer is equivalent to the discharge time of the primary spark
gap.

If the primary capacitance is too small, there isn't enough energy there
to create a field that will completely envelope the secondary coil.

If the primary capacitance is too large, most of the energy goes places
other than where it shouldn't, like into heat and damaging the spark gap.

If the rate of discharge is too slow (soft hammer) then the rise time of
the primary field won't resonate with the secondary so most of the
energy gets lost.

If the rate of discharge is too fast (hard hammer), then again, the
coupling to the secondary will be compromised and energy wasted.

The Marx generator is very interesting in that it can accumulate a
tremendous amount of energy and then deliver it in one mighty blast.
Unfortunately, the delivery losses are extreme because the impulse is
so large, and chances are very good that the rise time is too fast to
efficiently couple to the secondary (HUGE hammer, small bell problem).

Since TCs are AC devices, there isn't any "real" storage of energy at
the terminal capacitor (toroid or whatever).  Since terminal capacitors
are rarely perfect and the atmosphere is rarely dry and dust free, the
overall effect is that a good portion of the high voltage energy will
leak off without consolidating into that massive 60" discharge you
really want.

The only way to overcome these losses is to push more energy through the
system so the terminal losses represent a small portion of the total
energy available to create streamers.  This means that the rep-rate
must be increased to that critical point where it matches the total
available power delivery from the NST or whatever.

Marx generators all seem to run at sub-second rep-rates, and the
energy impulse is both too large and too fast for typical TC use.

That doesn't mean it won't work, just that you could have bigger and
better fireworks with less BANG!

I would recommend a simpler setup, using just the NST with the
rectifier, coupling to the primary capacitor through a large wattage
limiting resistor (20K-40K, 200-400 watts).  DC rotary gaps are
very easy to build and deliver excellent pulses.  See pictures of my
first gen RSG here:

ftp://ftp.mindspring-dot-com/users/ottalini/highvoltage/rotgap

One of the pics shows my current 8-pole rotor made of 1/2"
polycarbonate.  The motor is a 24VDC surplus motor and the box is
all polycarbonate (acrylic would have been fine).  An open box is
actually more desirable (I just got a little carried away).  I've
added a fan also.  You can easily get 200bps (or more) with a
similiar arrangement.

There are lots of other pics and info there, so feel free to look
around (I'll put it on a web page some day ...).

You can easily take all those 30kv caps and put them in parallel for
a nice primary capacitor.  30 caps (from the 15 stages of the cockroft)
at .0035 each = 0.100 uFd at 30KV, quite excellent (maybe even too
much!).

Best,
Kevin

*---------------------------*
* Kevin Ottalini            *
* WhoSys / Who Systems      *
* High Voltage with Style!  *
* ottalini-at-mindspring-dot-com   *
* Often in Another Reality  *
*---------------------------*

----------
> From: Hamly Richard D CONT NSSC <HAMLYRD-at-NAVSEA.NAVY.MIL>
> To: 'ottalini-at-mindspring-dot-com'
> Cc: tesla-at-pupman-dot-com; 'David Speck [dspeck-at-relex-dot-com]'
> Subject: Cockroft-Walton & Marx Generators
> Date: Friday, November 12, 1999 12:10 PM
>
> Kevin,
>
> I saw you very informative email on the Tesla List.
> I am interested in building a spark chamber.
>
> For the power supply, I built a Cockroft Walton with fifteen stages of
caps
> and diodes.
> The caps are 3500 pF door knobs and were rated at 30KV.
> The input power was a 15KV neon transformer.
> It worked for a while, throwing fat foot long sparks.
> However, I kept blowing diodes.   I do not now remember their rating.
> But either it was too low or I had very high ripple.
> Also, as your email points out, my capacitance should have been much
higher
> for the 60 Hz input to avoid bad ripple.    But they were all I had
laying
> around.
>
> So, using the same caps,  series spark gaps and 10 meg parallel
resistors,
> I built a Marx generator.   I powered it with the same transformer with a
> rectifier bridge.
> The Marx was much more reliable and produced about the same spark.
> However, it takes longer to charge up the caps.   The value of the
resistors
> may be too high or I may need smaller caps (in this case).
> The resistance of the old diode stacks may also have been to high and I
have
> some new ones, now, to try out.  I am now about to test several new
> rectifier bridges on it.
> I have not yet tackled replacing all the resistors or the caps, due to
the
> expense.
> The Marx is exciting to watch because it humms and then hisses and then
> crackles and finally, "BANG!", like a rifle shot.   It scares the crap
out
> of me every time.
> The Cockroft Walton was faster and smoother but more costly because of
the
> diodes.
>