MadSci Network: Physics |
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....
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