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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.
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