Re: Van de Graaff /Pelletron (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Sun, 18 Jan 2004 17:44:20 -0800
From: Ed Phillips <evp@xxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Van de Graaff /Pelletron (fwd)

">I guess all of those who've looked at those pictures again have come
up
>with our own way of making a "belt".  In thinking about that I came up
>with a fundamental question which might inhibit further efforts.  If the
>individual pellets are conducting, which seems necessary, don't their
>edges form a chain of little spark gaps and isn't the maximum output
>voltage for a given length limited by breakdown between the pellets?  If
>so, it would seem that to get interesting voltages the machine would
>have to be run in an insulating gas under pressure.
>
>       Comments?
>
>Ed


Ed,

Makes sense, but the links in the chain must also form a series of
equipotentials dividing the voltage gradient from top to base by the
number of links.  Some of these machines appear to be using hundreds
of links, which would be necessary for multi megavolt differentials.
Here is a very interesting page with a bunch more info on the use of
large equipotential rings along the length of these machines to even
out the electrostatic field.  At the bottom Antonio Carlos M.
deQueiroz is quoted as stating that a pelletron does require a longer
chain than a a standard Van de Graaff to avoid belt sparking:

http://members.aol.com/lyonelb/vdg2.html

I think there are open air pelletrons.  As long as your belt is long
enough...

Lastly according to the page above, a "string of beads" approach was
in fact one of the steps that led to the modern pelletron.   Whoever
proposed that idea on this list was on the mark!


Zap!

Charles Brush"

	The thought of making hundreds of beads sort of discourages me.  I have
been wondering if a jewelry or art supply store might have metallic
beads with holes for stringing already there.  Might not be too much
work to make up a "necklace" and give it a try.

	In the discussions about separating the plates of a charged capacitor I
can't remember anyone mentioning the good old electrophorus (sp?).  With
that you can feel the extra force as you remove the plate.  Likewise,
even in my small VDG I can hear the motor slow down as the top terminal
charges, then speed up after a nice spark.

	Here's another question whose answer should be obvious, but has puzzled
lots of guys.  Suppose we take a capacitor charged to voltage V and
connect it to an identical one with no charge.  The original energy was
1/2 CV^2.  Now we have two capacitors with voltage V/2, and each has an
energy of 1/2 C(V/2)^2; the sum is CV^2/4, or half the starting energy.
Question is "Where did the LOST energy go?".

Ed

P.S. Antonio does such beautiful work that I wish he'd make a pelletron
in his spare time, post the pictures, and describe the results!!!!!