Plasma is like any other gas like oxygen, nitrogen etc. in many respects. However a plasma has many more interesting and complex properties. In an ordinary gas the molecules or the atoms are electrically neutral i.e. they carry no net electric charge either positive (+) or negative (-). A neutral atom has equal number of negatively charged electrons and positively charged protons. Electrons are very light and they move around the centre of the atom in different orbits some what similar to the planets in our solar system. Protons are about 2000 times heavier than electrons and they are concentrated at the centre of the nucleus. In a plasma there are always some atoms where one of the electrons is removed. Such atoms thus carry one electron less and hence have one net positive charge. These atoms are said to be ionized. If such ions and electrons are present in a gas in substantial numbers the gas is said to be ionised and is called a plasma. It is believed that more than 90% of the universe is in plasma state. The sun, stars, inter stellar space are all made up of plasma. Only cold places like the earth and other planets are not in plasma state. On earth one can find plasma in the ionosphere a region beyond the earth's atmosphere. Many of the modern gadgets like the fluorescent lamps ( tube lights), sodium lamps, mercury vapour lamps, electric arcs used for welding and cutting metals, plasma displays etc all have plasmas in them.

Some plasma properties:
A plasma has many interesting and useful properties. These arise due to the presence of ions, electrons, neutral atoms in higher energy states as well as neutral atoms in the ground state which form a majority in most of the man made plasma. Plasmas can be manipulated by electric and magnetic fields. As in a gas, the particles in a plasma are always colliding with each other. In the process electrons and ions may recombine to form a neutral atom in an excited state or a ground state, a neutral atom may get ionised. Excited atoms loose energy and go to ground state by emitting light. This is how light is generated in fluorescent lamps ( tube lights ), neon tubes, sodium lamps, mercury vapour lamps, CFL's, electric arcs etc. This property makes plasma attractive to look at with various colours. Sun and stars also generate light by the same process.


+ - + - + -
- + - + - + - +
+ - + - + - + - + Plasma
- + - + - + -
+ - + - + -




Plasma temperature:
Like an ordinary gas, plasma would also have some temperature. But in a plasma there can be more than one temperature! For example the electrons generally have an higher temperature than the ions. The ions themselves could be at a higher temperature than the neutral atoms. In other words, each species i.e. electrons, ions and neutral behave like three different gases with different temperatures. Colliding plasma particles may exchange energy. The energy lost by one would be gained by the other. Such collisions help in equalising the temperatures of the different species. This happens when plasma densities are very high and collisions are quite frequent as in the sun or in welding and cutting electric arcs. Such plasmas are said to be in thermal equilibrium. The plasma in tube lights, CFL's, sodium lamps, mercury vapour lamps etc are all non equilibrium plasmas. Some times even a given species, say electrons, may not have a well defined unique temperature. Some times they are characterised by two temperatures. These properties play an important role in some plasma applications.

Shielding and quasi neutrality:
An important property of a plasma is the so called quasi neutrality. In any small region (or volume ) of a plasma there are equal number of positive and negative electric charges.

n = n_e = n_i

Here n is the plasma density, ne is electron density and ni is ion density. The densities are expressed in number of particle per unit volume.

This ensures that there can be no appreciable electric field in a plasma. The quasi neutrality comes about because a positive ion surrounds itself with a number of negatively charged electrons so that the effective charge in a sphere of radius l_d, called the Debye (shielding) length, is zero. Beyond this Debye sphere of radius l_d the electric field is zero. This is seen in following sketch.



Electric field due to a free charge q is E = q / r²


Electric field due to a shielded charge q in a plasma is
E = (q / r² ) E^-r / l_d
What happens when we immerse two electrodes in a plasma and apply a voltage? When this is done in air or vacuum a uniform electric field is established between the two electrodes. This is given by
E = V/d where d is the distance between the electrodes and V is the applied potential.

But in a plasma most of the potential falls very close to the cathode within a few Debye lengths and the bulk plasma would not have any electric field. This region where the potential falls is called the plasma sheath. Close to a cathode the sheath would have excess positive charge. Quasi neutrality condition does not hold good in this region.

In fact, some kind of sheath develops around any solid object placed in a plasma. Sheaths play an important role in all laboratory and industrial plasmas. Positive ions from the plasma moving towards the cathode are accelerated by large negative potential and bombard the cathode with large energy. This would lead to many consequences as we would see later.



The Debye length depends on plasma parameters like plasma density and temperature. It is given by

l_d = (kT/4pne²)1/2^½

where k is the Boltzmann constant, T is electron temperature of the plasma, n is the plasma density and e is the unit electric charge.


Collisions:
As mentioned earlier, plasma particles would be moving randomly in all directions. In the process they collide with each other. During such collisions several things can happen. Some of them are

1. Elastic collissions:
   The kinetic energy colliding particles can alter during such collisions. This can lead to sharing and equalisation of energy ( temperature) of different species in the plasma. This is also called thermalisation. However, interna energy or the internal structure of the colliding partners does not change.

2. Inelastic collisions: Here the internal energy and structure of the colliding partners may be altered. Some such processes are:
   
   Excitation: e + A ® A* ® A + radiation
   Ionisation: e + A ® A+ + 2e
   Dissociation: e + B2 ® B + B + e
   Recombination: e + A+ ® A + radiation
   
   The rate at which a particular reaction occurs depends on the type of reaction, the energy of the colliding partners, and the frequency of collisions. While the energy depends on plasma temperature, collision frequency depends on temperature as well as density.
   Such processes play crucial roles in all plasmas.

Collision frequency:
Collision frequency tell us how many times a particular type of collision ( elastic, ionization etc) takes place per second in given plasma. This, of course, depends on plasma density, temperature and the type of collision.

Mean free path:
This is the average distance a particle travels in the plasma between to successive collisions.

(to be continued)

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