Re: What's the cause of Crookes Dark Space & striations in discharge tubes

Dear Professor Matthew:

You are inquiring about the phenomena which occur in glow-discharge 
plasmas.  Typically these plasmas are used at pressures from ~0.05 torr to 
~10 torr.   Since you are using visual observation for your detection, let 
us keep our discussion on the qualitative level to enable us to present a 
picture of what is going on without getting into  the "theoretically 
complicated" realm which you refer to.

Your reference to the Crooke's dark space speaks towards experiments with 
DC discharges where the vacuum envelope is optically transparent, usually 
made of quartz or pyrex glass.  Two electrodes are suspended within, on 
metal rods vacuum sealed to go through the glass and allow application of 
voltage across the electrodes within the vacuum environment.

Now the glow discharge plasma is known as a non-equilibrium plasma, which 
means that the ions and neutral atoms are generally at a temperature close 
to room temperature, while the electrons are at extremely high 
temperatures.  This means their kinetic energy is very high and they move, 
or drift or diffuse, at velocities many orders of magnitude more rapidly 
than do the heavy ions or neutral molecules.

The net effect of this is that when the plasma is initially ignited, and a 
large burst of free electrons and positive ions are created from the 
ionization in the initial breakdown discharge, the hot electrons diffuse 
very rapidly away from the discharge to the walls of the chamber and to the 
positive electrode which leaves a cloud of positive ions within the bulk 
region of the plasma. The plasma stabilizes as more ionization takes place 
and diffusion out of the glow occurs until a steady state is 
established (all in microseconds!).Thus the negative electrode assumes a 
very negative charge from these electrons, and the insulating quartz walls 
of the chamber are also covered with a surface charge of negative electrons 
which adhere. 

The region of the plasma which is glowing glows because there are electrons 
there recombining with the ions and this makes the neutralized ion give off 
a photon of light; so wherever you see a glowing region in the plasma you 
know there are a lot of ions and free electrons present in the same 
location. Remembering now that the plasma is a continuous phenomenon, i.e. 
we are continuously ionizing atoms, making ions and free electrons, the 
electrons are continuously diffusing rapidly to the walls, and the ions are 
diffusing away slowly, and so a steady  state situation is established.

Now the electrons have a very wide distribution of energies.   The negative 
electrode is at a negative potential in relation to the somewhat positive 
glowing plasma region because of the excess electron coverage.   Therefore 
although electrons are continuously diffusing away from the glowing region, 
only the higher energy ones can reach the negative electrode because the 
lower energy electrons are repelled by the negative charge on the negative 
electrode.  Thus the density of electrons in the space near to the negative 
electrode drops considerably in a manner dependent on the plasma parameters 
(pressure, gas mixture, power level, electrode shape and spacing,etc.,) 
because these factors determine the energy distribution of the electrons.  
And as we approach nearer and nearer to the negative electrode the density 
of moving electrons drops down to a low level.  But since the electrons 
colliding and recombining with the ions are the reason for the glow in the 
gas, if the electron density is falling the glow intensity will also fall. 
And so the dark space develops near the negative electrode (the Crooke's 
dark space).

Now the glass walls of the chamber are also covered with negative electrons 
(remember the quartz is not conducting and so the electrons normally are 
trapped on the walls) and for the same reason there will be a dark space 
all over the inner walls of the chamber because these electrons repel the 
electrons from the plasma just as the negative electrode does.  

This is a qualitative explanation of the dark-space phenomenon.  This can 
all be shown quantitatively with a  very complex mathematical analysis 
which I think is unnecessary for your purposes.

Now what happens when you bring your hand up to the quartz tube?  You hand 
makes an electrical capacity between the quartz tube and the ground and 
that changes the coupling of the electrical plasma to the walls at the 
point of your hand location. This causes a redistribution of the electrons 
on the wall of the chamber and therefore changes the potentials at the wall 
which then changes the number of electrons in the gas which can reach the 
wall and so the glow around the location of your hand changes.

Also because the inner walls of the chamber are generally contaminated with 
water vapor and other surface contaminants the electrons which pile up on 
the walls sometime develop unstable concentrations and build up high 
charges which rise to a point where they suddenly  rearrange themselves and 
cause the striations which you see in the glow as the electrons pile up, 
develop a big charge, then become unstable and discharge off to some other 
region of the walls, and this can repeat itself over an over depending upon 
the surface of the glass and other contaminants in the plasma.

This is a simple phenomenological explanation of the behaviour of the 
discharge tube which I hope will be adequate for your purposes.  If you 
wish to ask me some questions about this please submit them and I will 
explain further if I can. However it can get very complicated-because a 
plasma is such a complex environment.  

Best wishes,

Dick Bersin....