Plasma (Part-VI)- Facts About Generating Artificial Plasma

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What is Artificial Plasma? How is it Generated? What is Piezoelectric Direct Discharge Plasma?

Now here comes the sixth part of the blog on plasma. Those who have missed the fifth part can read it from Here. It will help to connect with this sixth part of the blog which provides a detail discussion on artificial plasma, its generation and piezoelectric direct discharge plasma. Let us explore the blog to find out these in more details. In words of Albert Einstein:

"A human being is a part of the whole called by us universe, a part limited in time and space. He experiences himself, his thoughts and feeling as something separated from the rest, a kind of optical delusion of his consciousness. This delusion is a kind of prison for us, restricting us to our personal desires and to affection for a few persons nearest to us. Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty".

How are Artificial Plasma Generated?

Just like the many uses of plasma, there are several means for its generation, however, one principle is common to all of them: there must be energy input to produce and sustain it. For this case, plasma is generated when an electric current is applied across a dielectric gas or fluid  as can be seen in the adjacent image, which shows a discharge tube as a simple example. The potential difference and subsequent electric field pull the bound electrons (negative) toward the anode (positive electrode) while the cathode (negative electrode) pulls the nucleus.  As the voltage increases, the current stresses the material beyond its dielectric limit into a stage of electrical breakdown, marked by an electric spark, where the material transforms from being an insulator into a conductor .The underlying process is the Townsend avalanche, where collisions between electrons and neutral gas atoms create more ions and electrons. The first impact of an electron on an atom results in one ion and two electrons. Therefore, the number of charged particles increases rapidly only "after about 20 successive sets of collisions", mainly due to a small mean free path. Some of the major techniques for generating artificial plasma are-

1. Electric Arc

With ample current density and ionization, this forms a luminous electric arc between the electrodes. Electrical resistance along the continuous electric arc creates heat, which dissociates more gas molecules and ionizes the resulting atoms and as per the sequence: solid-liquid-gas-plasma, the gas is gradually turned into a thermal plasma. A thermal plasma is in thermal equilibrium, which is to say that the temperature is relatively homogeneous throughout the heavy particles (i.e. atoms, molecules and ions) and electrons. This is so because when thermal plasma are generated, electrical energy is given to electrons, which, due to their great mobility and large numbers, are able to disperse it rapidly and by elastic collision to the heavy particles.

2. Low-Pressure Discharges

i) Glow Discharge Plasma: Non-thermal plasma generated by the application of DC or low frequency RF (<100 kHz) electric field to the gap between two metal electrodes. Probably the most common plasma; this is the type of plasma generated within fluorescent light tubes. 

ii) Capacitively Coupled Plasma (CCP): Similar to glow discharge plasma, but generated with high frequency RF electric fields, typically 13.56 MHz. These differ from glow discharges in that the sheaths are much less intense. These are widely used in the microfabrication and integrated circuit manufacturing industries for plasma etching and plasma enhanced chemical vapour deposition.

iii) Cascaded Arc Plasma Source: A device to produce low temperature (≈1eV) high density plasma (HDP).

iv) Inductively Coupled Plasma (ICP): Similar to a CCP and with similar applications but the electrode consists of a coil wrapped around the chamber where plasma is formed. 

v) Wave Heated Plasma: Similar to CCP and ICP in that it is typically RF (or microwave). Examples include helicon discharge and electron cyclotron resonance (ECR)

3. Atmospheric Pressure 

i) Arc Discharge: This is a high power thermal discharge of very high temperature (≈10,000 K). It can be generated using various power supplies. It is commonly used in metallurgical processes. For example, it is used to smelt minerals containing aluminium oxide to produce aluminium.

ii) Corona Discharge: This is a non-thermal discharge generated by the application of high voltage to sharp electrode tips. It is commonly used in ozone generators and particle precipitators.

iii) Dielectric Barrier Discharge (DBD): This is a non-thermal discharge generated by the application of high voltages across small gaps wherein a non-conducting coating prevents the transition of the plasma discharge into an arc. It is often mislabelled 'Corona' discharge in industry and has similar application to corona discharges. It is also widely used in the web treatment of fabrics. The application of the discharge to synthetic fabrics and plastics functionalizes the surface and allows for paints, glues and similar materials to adhere. The dielectric barrier discharge was used in the mid-1990s to show that low temperature atmospheric pressure plasma is effective in inactivating bacterial cells. This work and later experiments using mammalian cells led to the establishment of a new field of research known as plasma medicine. The dielectric barrier discharge configuration was also used in the design of low temperature plasma jets. These plasma jets are produced by fast propagating guided ionisation waves known as plasma bullets. 

iv) Capacitive Discharge: This is a non-thermal plasma generated by the application of RF power (e.g., 13.56 MHz) to one powered electrode, with a grounded electrode held at a small separation distance on the order of 1 cm. Such discharges are commonly stabilized using a noble gas such as helium or argon. 

Piezoelectric Direct Discharge Plasma 

It is a non-thermal plasma generated at the high-side of a piezoelectric transformer (PT). This generation variant is particularly suited for high efficient and compact devices where a separate high voltage power supply is not desired.

Continued in the last blog...

Plasma (Part-IV)- Mathematical Model and Artificial Formation

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How is Plasma described mathematically? What are Artificial Plasma and how are they categorized? 

Here we continue with the fourth part of our blog on plasma. Those who have missed our third blog can read it from Here. It will help to connect with this fourth part of the blog discussing details about the mathematical description of plasma and also artificial plasma along with their category. To know in details let us explore this blog. In words of Hannes Alfven:

"I have never thought that you could obtain the extremely clumpy, heterogeneous universe we have today, strongly affected by plasma processes, from the smooth, homogeneous one of the Big Bang, dominated by gravitation".

Mathematical Description of Plasma

To completely describe the state of a plasma, all of the particle locations and velocities that describe the electromagnetic field in the plasma region would need to be written down. However, it is generally not practical or necessary to keep track of all the particles in a plasma. Therefore, plasma physicists commonly use less detailed descriptions, of which there are two main types- the first one is Fluid Model and the second one is Kinetic Model.

Fluid Model

Fluid models describe Plasma in terms of smoothed quantities, like density and averaged velocity around each position (see Plasma parameters). One simple fluid model, magneto hydrodynamics, treats the plasma as a single fluid governed by a combination of Maxwell's equations and the Navier–Stokes equations. A more general description is the two-fluid plasma picture, where the ions and electrons are described separately. Fluid models are often accurate when collisionality is sufficiently high to keep the plasma velocity distribution close to a Maxwell–Boltzmann distribution. Because fluid models usually describe the plasma in terms of a single flow at a certain temperature at each spatial location, they can neither capture velocity space structures like beams or double layers, nor resolve wave-particle effects.

Kinetic Model

Kinetic models describe the particle velocity distribution function at each point in the plasma and therefore do not need to assume a Maxwell–Boltzmann distribution. A kinetic description is often necessary for collision less Plasma. There are two common approaches to kinetic description of a plasma. One is based on representing the smoothed distribution function on a grid in velocity and position. The other, known as the particle-in-cell (PIC) technique, includes kinetic information by following the trajectories of a large number of individual particles. Kinetic models are generally more computationally intensive than fluid models. The Vlasov equation may be used to describe the dynamics of a system of charged particles interacting with an electromagnetic field. In magnetized Plasma, a gyro-kinetic approach can substantially reduce the computational expense of a fully kinetic simulation.

What are Artificial Plasma? How they are Categorised?

Most artificial Plasma are generated by the application of electric and/or magnetic fields through a gas. Plasma generated in a laboratory setting and for industrial use can be generally categorized by:

The type of power source used to generate the plasma—DC, RF and microwave. The pressure they operate at—vacuum pressure (< 10 mTorr or 1 Pa), moderate pressure (≈1 Torr or 100 Pa), atmospheric pressure (760 Torr or 100 kPa)The degree of ionisation within the plasma—fully, partially, or weakly ionised. The temperature relationships within the plasma thermal plasma , non-thermal or "cold" plasma. The electrode configuration used to generate the plasma. The magnetization of the particles within the plasma magnetized partially magnetized (the electrons but not the ions are trapped by the magnetic field), non-magnetized.

Continued in the next blog...