We now get into the meat of the empirical aspects of plasmas. Our theoretical tools are wonderful but provide models that fall apart easily with a slight disturbance. Yet we do find repetitive processes at work and causes that can be tracked as well. All good but straining on our computer power to model.
In the meantime empirical knowledge is intuitive and surprisingly stable in its own right. Thus we have advanced steadily.
There just too many active centers in a plasma to allow predictive methods to ever be either exact or sufficiently inexact as well. It is a little like building large stone structures where you over engineer it all.
However, an electron carries the same magnitude of (negative) charge as a positively charged proton, the lightest form of ion. Therefore, the higher velocity of the electrons means that they are more effective than the ions at carrying current in a plasma.
- Within the tube, there are visible bands along the axis wherein the plasma is seen to glow, interspersed with ‘dark’ bands where there is no such glow. The different bands represent two of the three possible modes of operation of plasma when carrying a current.
- The dark bands represent, unsurprisingly, the Dark Current Mode. In these regions the electron velocity is below that necessary to cause visible excitation of the atoms of neutral gas, although ionization will start to occur at higher currents. However, radiation will be given off at wavelengths outside the visible even in the Dark Current Mode and so may be detected by non-optical means.
- The glowing bands represent the Normal Glow Mode. Here, the velocity of the electrons causes ionization to occur. The glow is caused by radiation from the electrons of neutral atoms after they have been excited by collisions with fast free electrons.
- The third possible mode of plasma operation is the Arc Mode, familiar in painfully bright welding applications or lightning, for example.
- Returning to the glow discharge tube, one might expect that the potential difference between the electrodes would cause a uniform electric field along the length of the tube. However, the plasma behaves differently.
- It is found that a Double Layer (DL) develops in the tube which modifies the externally applied electric field between the anode and cathode. The DL forms in such a way that the majority of the potential drop occurs across the DL. Away from the DL region, much of the remainder of the plasma is a glow discharge region known as the positive column. This can extend for a significant part of the length of the discharge tube.
- Within the positive column there are approximately equal numbers of electrons and ions. The plasma here is therefore quasi-neutral. Because most of the potential drop occurs across the DL, only a small but constant voltage gradient, or electric field, exists within the positive column.
- There appear to be analogies between the positive column in a discharge tube and the plasma within the Sun’s heliosphere.
- Another result of the discharge tube
experiments is also relevant to our discussion of plasma behavior and
will be discussed in the next section.
The confinement of field-aligned filamentary currents to definite cylinders of current by electromagnetic forces is also consistent with the falling characteristic of the J-V curve seen in laboratory experiments in discharge tubes. If the plasma is in Glow Mode, which in space plasmas may mean a glow in wavelengths outside the visible range, then the radius of the current cylinder will be determined by a combination of the effects of the electric and magnetic fields and the shape of the current density-Voltage curve. Read more about the filamentation process in dense cosmic z-pinches in this_paper by Russian physicists A.B Kukushkin and V.A. Rantsev-Kartinov of the Kurchatov Institute, Moscow.
Of course, once the matter has been sufficiently compressed and if it is neutralized by recombination of ions and electrons, then the electromagnetic forces may be reduced to the point that gravity becomes significant and continues the compression started by the electromagnetic forces.
- The first is that radiative cooling from the regions of increased density can result in a temperature decrease nearer the center, contrary to the increase one might intuitively expect from increasing the density.
- The second is that recombination of ions and electrons starts to occur.
End of Chapter 6