Plasma: Basics, Applications, and Diagnostics

The three states of matter include solids, liquids, and gases. However, plasma, as we all know, is the fourth state of matter, in which matter exists as electrons and ions. So it is an electrified gas with both positive ions and negative electrons moving freely. This usually happens when a gas is given more energy and the negatively charged electrons, which are held by the pull of the nucleus, break free.

In a more focused way, plasma can be defined as a partially ionized gas, a mixture of electrons, atomic ions, molecular ions, neutral atoms, and molecules in the ground and excited states. The negative and positive charges compensate each other, and thus, most of them are electrically neutral. This is known as the property of quasi-neutrality. The presence of the charged particles in the plasma causes it to have a high electrical conductivity.

The motion of the particles in the plasma can cause local concentrations of positive and negative electric charges. These charge concentrations create long-ranged Coulombic field that affect the motion of charged particles far away from the charge concentrations. Thus, elements affect each other, even at large separations, giving plasma its characteristic collective behavior. In a more rigorous way, plasma can be defined as a quasi-neutral gas of charged and neutral particles characterized by a collective behavior.

In a plasma, both ions and electrons are free to move about, while in a gas they are bonded to the atom. This happens when we heat and energize the gas to much higher temperatures; and when the temperature is brought down the electrons and ions bond back to form individual atoms.

(Energy in) (Energy out)
Gas—–> Plasma —-> Gas

Plasma state has more energy content than the solid, liquid, and gaseous states, hence, it is known as the fourth state of matter. The dynamics of motion of plasma are unique compared to other states of matter. In the case of a neutral particle collision between two particles, each particle moves undisturbed until it makes a collision with the other. While in the case of plasma it is much different. As they consist charged particles, their movement causes separation between the charges giving rise to electrical fields and the charged particle motion giving rise to a magnetic field. These forces affect the motion of other particle and gives rise to long range forces in a plasma which add to the complexity of its behavior.

It was Irving Langmuir, Nobel Prize laureate who gave plasma its name. While working on rarefied gas discharges, he observed an electric oscillation in them and referred to them as ‘plasma oscillations’. Since then, the word ‘plasma’ (which means ‘moldable substance’) has been used to represent conducting gas, and because it has properties that are quite different from those of ordinary neutral gases, plasma has acquired the nature called the fourth state of matter.

Plasma Sources

‘99% of matter in the universe exists in the plasma state’.

► On a clear sunny day, just look up towards the sun. Our Sun, powered by nuclear fusion is a giant phenomenon of seemingly endless plasma activity.

► A neon sign or a fluorescent lamp is also a plasma phenomena, in which the gas becomes a conducting plasma when a voltage is applied across it.

► Lightning occurs when there is a heavy potential difference in the clouds and they finally discharge causing a heavy current to flow.

► Welding is a phenomenon similar to lightning, and the bright arc we see is nothing but the plasma produced when a current flows through the gap between the electrode and the substrate.

► The gaseous nebulae and the glow of the aurora borealis are amongst the other several places we encounter plasma.

Plasma Generation

Plasma is usually obtained when sufficient energy, higher than the ionization energy, is added to atoms of a gas, causing ionization and production of ions and electrons. Parallel and concomitant to the ionization occur the opposite process of recombination of electrons with ions to form neutral atoms or molecules. Though we could also make plasma by heating a gas to very high temperatures, it would not be a good way to do so as the container itself would heat up and vaporize and ionize. Due to this reason we usually make plasma by heating a gas to moderate temperatures and driving a current through it or using radio frequency waves to energize it. Although they are commonly produced by electric discharges in gases, plasma can also be obtained when sufficient energy is provided to a liquid or a solid to cause its vaporization and ionization.

The most commonly used method of generating and sustaining a low-temperature plasma for technological and technical application is by applying an electric field to a neutral gas. Any volume of a neutral gas always contains a few electrons and ions that are formed, for example, as the result of the interaction of cosmic rays or radioactive radiation with the gas. These free charge carriers are accelerated by the electric field and new charged particles may be created when these charge carriers collide with atoms and molecules in the gas or with the surfaces of the electrodes. This leads to an avalanche of charged particles that is eventually balanced by charge carrier losses, so that a steady-state plasma develops.

There are various other ways to supply the necessary energy for plasma generation to a neutral gas. One possibility is to supply thermal energy, for example in flames, where exothermic chemical reactions of the molecules are used as the prime energy source. Adiabatic compression of the gas is also capable of gas heating up to the point of plasma generation. Yet another way to supply energy to a gas reservoir is via energetic beams that moderate in a gas volume. Beams of neutral particles have the added advantage of being unperturbed by electric and magnetic fields. Neutral beams are primarily used for sustaining plasma or for plasma heating in fusion devices.

Plasma Applications

Plasma, apart from their use in achieving thermonuclear fusion, have several industrial and commercial application:

1. Processes like plasma etching and deposition find their use in semiconductor industry. The utility and flexibility of plasma technology stems from the fact that it can be used to modify a variety of surfaces under precisely controlled conditions, without the safety hazards and liquid waste associated with wet processes.

2. They could be used for surface treating, thin film deposition, and other plasma processing techniques (Plasma Nitriding, Plasma Diffusion, Plasma Assisted Chemical Vapor Deposition, Plasma Ion Implantation, etc.)

3. Plasma Propulsion and Thrusters

4. Plasma Arcs used for cutting, drilling and welding processes.

5. Modification of surfaces by plasma

6. In plasma chemistry―transforming specific compounds, production of precursors, production of excimers, clean-up of gases, flue gases, diesel exhaust.

7. Plasma techniques could be used to treat fibers and textile more quantitatively and selectively.

8. Plasma finds application in sterilizing water and air purification.

9. Plasma displays, high intensity discharge, neon and fluorescent lamps.

10. Hardening processes for industrial tools.

Plasma Diagnostics

To understand better the behavior of plasma, every experiment must incorporate different means of sampling and monitoring its properties. Plasma diagnostics refers to the techniques used to gather information about laboratory plasma and other ones. We could say that progress in plasma research can be measured by the stage of development of its measurement techniques and the adequacy of accompanying theoretical interpretation.

Diagnostics help us measure both the large-scale properties (the macroscopic properties) as well the properties at much smaller scale (the microscopic properties) which arise due to atomic processes. Thus, total current through the plasma, the voltage across it, its conductivity, all refer to its macroscopic properties; microscopic measurements include spectral line measurement, microwave, X-ray, and other techniques. Of the hundreds of diagnostics used, given below are some of them.

1. Magnetic Probe
A magnetic probe is used to measure the magnetic field of a plasma. This simplest arrangement involves a sensor which is usually a light-gage wire, placed according to the direction of the field to be measured. It’s possible to measure the field in all directions by appropriate positioning of the probe. The basic principle underlying the probe is the variation of the magnetic field (dB/dt) that induces a voltage across the loop. The signal is then integrated to get the value of B(t) characteristics of position of the coil.

2. Rogowski Coil
A Rogowski coil gives a direct measurement of the total current flowing through is center. As shown in the figure, it is a solenoidal coil whose ends are brought around together to form a torus. The voltage output of the coil can be integrated to give a signal proportional to I.

3. Langmuir Probe
A Langmuir probe is a small conductive electrode used to measure the density, temperature, and electric potential (voltage) of a plasma. Langmuir could be used to diagnose the particle distribution functions within a plasma by insertion of a probe that could directly sense the particle fluxes.

4. Far Infrared and Interferometry
Real-time density profile measurements have been identified as essential for advanced fusion tokamak operation. Multichannel Far Infrared (FIR) Interferometry is a proven method for measuring density profiles.

5. Reflectometry
Plasmas are dispersive media whose refractive index is a function of plasma density; higher frequency radiation reflects from higher density plasma layers. Reflectometry, as its name suggests, exploits the reflection of electromagnetic waves from plasma cut-offs to either measure density profiles or spatially resolve density fluctuations.

6. Electron Cyclotron Emission Imaging
Electrons that gyrate around magnetic field lines give rise to emission at harmonics of the electron cyclotron frequency. If the electrons are sufficiently hot and sufficiently dense, then the plasma is considered optically thick. Under these conditions, the electron cyclotron emission (ECE) is directly proportional to the electron temperature and independent of all other plasma parameters. As the electron cyclotron frequency is proportional to the total magnetic field, the emissions at a given frequency are emitted from a very specific layer of the plasma corresponding to a given magnetic field. Measuring the emission power as a function of frequency allows the electron temperature to be computed as a function of plasma radius. Spatially imaging the emission onto an array of detectors expands the capabilities of ECE radiometry to include ECE imaging.

7. Bolometry
It is a technique which involves the direct measurement of radiation loss. Most bolometers consist of an absorbing element designed to absorb all the incident energy, whose temperature rise, measured by some appropriately sensitive method, is then equal to the total energy flux divided by the bolometer’s thermal capacity.

8. Microwave Techniques
These techniques involve measuring plasma properties through the interaction of electromagnetic fields with the free charges of the plasma.