Re: RF banding effects in vacuum tubes (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Sun, 05 Oct 2003 19:08:11 -0300
From: Antonio Carlos M. de Queiroz <acmq@xxxxxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: RF banding effects in vacuum tubes (fwd)

High Voltage list wrote:

> From: Mike Novak <solschoice@xxxxxxxxxxx>

> Anywho, I was wondering if anyone knew of a source of info on the "string of
> pearls", "dotting", or "banding" effect caused by high voltage RF in vacuums
> at moderate pressures.

Take a look at old books that mention Geissler tubes. I have one here
that discussed the banding effects. They are not high-frequency
phenomena,
but appear when the tubes are powered by DC.
Starting from the cathode, what is seen is:
A bright area around the cathode (first cathodic layer). Formed by
positive ions hitting the cathode.
A dark space (Hittorf's dark space). Charges there move slowly and don't
cause ionization.
A bright space (second cathodic layer), where electrons have enough
speed to cause ionization on atoms hit by them.
A second dark space (Faraday's dark space). The electrons slow down
after
the first ionization region, and proceed to the anode, gaining speed
again.
A bright region (positive light), extending to the anode, where
ionization occurs constantly, because the electric field is high
enough to accelerate the electrons quickly, and there are enough
atoms to be hit. At lower pressures, this region is reduced, eventually
disappearing completely at high vacuum.
At certain pressures and applied voltages, the sequence of the
second cathodic layer and Faraday's dark space repeats many times,
forming the observed banding.

(I took this description from a book from 1928, but I don't think that
something substantially new was discovered about this behavior since
then.)

I have observed this in a neon sign tube that I use for experiments
with electrostatics, but just once, and could not reproduce the effect.

Antonio Carlos M. de Queiroz