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Item added to your basket View basket. Proceed to Basket. View basket. Continue shopping. Results 1 - 6 of 6. This region is filled with a positive space charge of ions moving in the other direction. When the electrons acquire sufficient energy to ionize, they begin to do so as they enter the negative glow, the brightest part of the discharge. In the width of the negative glow, all the positive ions that the discharge needs are continually being created, and then they are accelerated towards the cathode.
Even the ions may be excited by the fast electrons, and both the neutral and ionized molecules contribute to the light. The electrons are exhausted by this effort, and drift out into the Faraday dark space at low velocity. There are now much fewer ions around, so the electrons can be accelerated in the gentle field. As they speed up, their number density goes down. Except close to the cathode, the electrons carry practically all of the discharge current.
When they have sufficient energy to excite the molecules of the gas, the emitted light becomes evident. This light is produced with minimal excitation, and consists only of light emitted by neutral atoms which have been excited above their resonance energy, which explains its different character from the light emitted by the negative glow. From here to the vicinity of the anode, the potential gradient must only be sufficient to allow the most energetic electrons to form ions to replace those that diffuse to the walls.
The electrons carry most of the discharge current in this region, the ions mainly just ensuring electrical neutrality. As the anode is approached, adjustments must be made to suit the anode current. The random electron current may be larger than required, so the electrons must be slowed down by an extra electron density near the anode, or, alternatively, a positive ion sheath may be required to draw extra current out of the plasma.
This is the case shown in the figure. The extra ions required are made in the anode glow, while little excitation occurs near the anode, making a dark space. What happens at the anode is variable, and depends on the details of the individual discharge.
The diagram is slightly different than the one that appears in many references, which seems to be copied from an original by Loeb.
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The common diagram has, I believe, some inaccuracies, the most important of which is the positive ion density in front of the cathode, which cannot fall to zero, as Loeb seems to indicate, but must remain finite. We should now discuss the important property of similitude in gas discharges. The diameter of an equivalent sphere is d. We have already seen this in Paschen's Law.
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The diagram at the right shows two discharge tubes of similar shape, in which the linear dimensions are in the ratio a. The potential difference across the tubes is the same. If the pressures scale in the inverse ratio a, then the number of mean free paths in any similar distances is the same, and pd is a constant for similar behavior. From the expression for the currents in the Townsend region, we find that the current densities are constant if the initial current densities at the cathodes are the same.
Note that the ratios of charged particle densities are not the same as the ratio of neutral molecule densities. Rates of change of particle densities are also in a different ratio. Not everything scales properly, but in most cases similitude is a very good approximation.
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This shows, incidentally, that tests at reduced voltage cannot be made on models, since the discharges will not be comparable if the voltages are different. Most of the voltage between anode and cathode is represented by the cathode fall V c near the cathode.
Aston found experimentally that the electric field is greatest at the cathode, and falls linearly to zero at the end of the cathode fall in the negative glow. If K is the mobility of the positive ions, then the positive ion current can be found. The electrons are accelerated by the strong field near the cathode to a rather high energy. They are then moving much faster than their mobility in the electric field, but are not slowed down much by elastic collisions with neutral atoms, and continue to ionize with increasing efficiency as they slow, doing the most work in the negative glow.
The appearance of the spectrum of ionized atoms as well as neutral shows that the electrons still have high speeds in this area, though they are moving in random directions. The width of the negative glow is approximately equal to the "range" of the electrons at the prevailing pressure.
This range is not exactly the same as the concept of the same name in the stopping of radioactive beta rays. It may be naively believed that the electrons are accelerated in the cathode drop as if in a vacuum, and then start ionizing when they reach sufficient energy, but this is not a valid picture, and leads to confusion when trying to understand the discharge.
The mean free path is still short even at 1 mmHg, and neutral molecules far outnumber electrons and ions. While the neutral density is about 4 x 10 15 cm -3 at 1 mmHg, the electron and ion densities do not exceed 10 10 , and are usually no greater than 10 8. Only one in ten million molecules is ionized, and the motion of electrons and ions is governed by mobility rather than by free acceleration.
The electron mean free path in neon at 1 mmHg is about 0. It is no wonder that the cathode phenomena hug the cathode tightly at higher pressures, and expand as the electron range increases. A very high electric field at the surface of the cathode is a feature of all discharges, and here is where the electrons get their initial energy.
Only a few of the necessary positive ions are created in the cathode dark space, where most of the potential rise takes place. Most appear in the negative glow where ionization is most effective, then drift towards the cathode. The range of a fast charged particle is familiar from radioactivity. It is the projected distance in the general direction of motion, not the actual path length, especially for electrons, and is the distance to the point where the particles have insufficient energy for further ionization.
The negative glow grades into the Faraday dark space as the electrons become relatively slow and unable to ionize. Now they simply have to make their way to the anode somehow without using up much voltage. The distribution of light intensity in the cathode dark space and the negative glow is a good subject for contemplation. The light does not come from the positive ions unless they have been further excited, which does happen , but from ions that have recombined.
The entire region between the cathode and the Faraday dark space is full of positive ions, but recombination is more likely where the electrons are moving slowly, and this means electrons that have already ionized and are now drifting about, and slower ions that have not yet been accelerated by the field. Electrons in the dark space are still moving too rapidly for this, so there is little light there. These equations can be used to find empirical values for the constants A and B. Note that V is independent of the pressure, d varies inversely with the pressure, and j is proportional to the square of the pressure.
This applies, of course, in the range of pressures where the approximate analysis is valid. The current through a glow discharge is controlled by the external circuit. If this cathode area is not available, then the cathode fall must increase to provide a greater current density. This region is called the abnormal glow.
The characteristics of Electrical Discharges in Magnetic Fields
The increased cathode fall generates faster electrons and faster ions, so the negative glow expands and brightens, and the fast ions bombard the cathode. In a glow lamp, it is easy to see the negative glow spread as the current through the lamp is increased. Fast ion bombardment causes sputtering of the cathode material.
The rate of sputtering is proportional to V c - V , where V is the normal cathode fall, and V c the established cathode fall. The exact mechanism of sputtering is in doubt, but it seems that cathode material is ejected to the cathode surface, and then evaporated by the local heating. The rate of deposition obeys the inverse-square law.
The films produced are quite coherent and uniform. Silver sputters relatively easily, aluminum with difficulty.
The Characteristics of Electrical Discharges in Magnetic Fields
Where cathode surfaces are specially prepared, perhaps with low-work function substances as in thermionic cathodes, sputtering can destroy them quickly, even at relatively low overvoltages. There seems to be little sputtering at the normal cathode fall. The electrodes of glow lamps are treated with low-work-function materials, so they are injured by overcurrent, and their striking voltages may rise.
In the Faraday dark space, the electric field is again controlled by space charge and the mean free path, this time of the electrons, which carry nearly all of the current.
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