Plasma (Part-VII)- Converting Plasma Into Electricity

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Technology That Converts High Velocity Plasma into Electricity: Magneto-Hydrodynamic Converter

Here comes the seventh and last part of the blog on plasma. Those who have missed the sixth part can read it from Here. It will help to connect with this last part of the blog which provides a detail discussion on technology which enables conversion of high velocity plasma into electricity. How does High Velocity Plasma converts into electricity through the help of Magneto-Hydrodynamic converter? To know all these explore this blog. In words of Michael Faraday:

 

"It is right that we should stand by and act on our principles; but not right to hold them in obstinate blindness, or retain them when proved to be erroneous".

Introducing Magneto-Hydrodynamic Converter

In the year 1960, the concept of  magneto-hydrodynamic (MHD) converters was used in order to bring MHD power conversion to market with commercial power plants of a new kind, converting the kinetic energy of a high velocity plasma into electricity with no moving parts at a high efficiency.  Research was also conducted in the field of supersonic and hypersonic aerodynamics to study plasma interaction with magnetic fields to eventually achieve passive and even active flow control around vehicles or projectiles, in order to soften and mitigate shock waves, lower thermal transfer and reduce drag. In the year 1832, Michael Faraday  attempted the first time to test an MHD converter. MHD converters involving plasma were highly studied in the 1960s and 1970s, with many government funding and dedicated international conferences. The research almost stopped after it was considered the electro-thermal instability would severely limit the efficiency of such converters when intense magnetic fields are used, although solutions may exist. Such ionized gases used in "plasma technology" ("technological" or "engineered" plasma) are usually weakly ionized gases in the sense that only a tiny fraction of the gas molecules are ionized. These kinds of weakly ionized gases are also non-thermal "cold" plasmas. In the presence of magnetic fields, the study of such magnetized non-thermal weakly ionized gases involves resistive magneto-hydrodynamics with low magnetic Reynolds number, a challenging field of plasma physics where calculations require dyadic tensors in a 7-dimensional phase space. 

How does the Generation of MHD Power Occurs?

A magneto-hydrodynamic generator is an MHD converter that transforms the kinetic energy of an electrically conductive fluid, in motion with respect to a steady magnetic field, into electricity. MHD power generation has been tested extensively in the 1960s with liquid metals and plasma as working fluids. Basically, a plasma is hurtling down within a channel whose walls are fitted with electrodes. Electromagnets create a uniform transverse magnetic field within the cavity of the channel. The Lorentz force then acts upon the trajectory of the incoming electrons and positive ions, separating the opposite charge carriers according to their sign. As negative and positive charges are spatially separated within the chamber, an electric potential difference can be retrieved across the electrodes. While work is extracted from the kinetic energy of the incoming high-velocity plasma, the fluid slows down during the process.

MHD Propulsion

A magneto-hydrodynamic accelerator is an MHD converter that imparts motion to an electrically conductive fluid initially at rest, using cross electric current and magnetic field both applied within the fluid. MHD propulsion has been mostly tested with models of ships and submarines in seawater. Studies are also ongoing since the early 1960s about aerospace applications of MHD to aircraft propulsion and flow control to enable hypersonic flight: action on the boundary layer to prevent laminar flow to become turbulent, shock wave mitigation or cancellation for thermal control and reduction of the wave drag and form drag, inlet flow control and airflow velocity reduction with an MHD generator section ahead of a scramjet or turbojet to extend their regimes at higher Mach numbers, combined to an MHD accelerator in the exhaust nozzle fed by the MHD generator through a bypass system. Research on various designs are also conducted on electromagnetic plasma propulsion for space exploration. In an MHD accelerator, the Lorentz force accelerates all charge carriers in the same direction whatever their sign, as well as neutral atoms and molecules of the fluid through collisions. The fluid is ejected toward the rear and as a reaction, the vehicle accelerates forward.

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-V)- Exploring Complex Plasma

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Diving into the Amazing World of Complex Plasma

Now we continue with the fifth part of our blog on plasma. Those who have missed our fourth blog can read it from Here. It will help to connect with this fifth part of the blog which provides a deep insight into the world of complex plasma. Let us explore the blog to find out more. In words of Albert Einstein-

"A human being is part of the whole, called by us 'Universe'; a part limited in time and space. He experiences himself, his thoughts and feelings 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 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. Nobody is able to achieve this completely but the striving for such achievement is, in itself, a part of the liberation and a foundation for inner security".

What is Complex Plasma Phenomena? 

Although the underlying equations governing plasma are relatively simple, plasma behavior is extraordinarily varied and subtle: the emergence of unexpected behavior from a simple model is a typical feature of a complex system. Such systems lie in some sense on the boundary between ordered and disordered behavior and cannot typically be described either by simple, smooth, mathematical functions, or by pure randomness. The spontaneous formation of interesting spatial features on a wide range of length scales is one manifestation of plasma complexity. The features are interesting, for example, because they are very sharp, spatially intermittent or have a fractal form. Many of these features were first studied in the laboratory, and have subsequently been recognized throughout the universe. 

Examples of Complexity and Complex Structures in Plasma

Some of the major examples of complexity and complex structures in plasma includes-

Filamentation

Striations or string-like structures, also known as Birkeland currents, are seen in many plasmas, like the plasma ball, the aurora, lightning, electric

arcs, solar flares, and supernova remnants. They are sometimes associated with larger current densities, and the interaction with the magnetic field can form a magnetic rope structure. High power microwave breakdown at atmospheric pressure also leads to the formation of filamentary structures.

Filamentation also refers to the self-focusing of a high power laser pulse. At high powers, the nonlinear part of the index of refraction becomes important and causes a higher index of refraction in the centre of the laser beam, where the laser is brighter than at the edges, causing a feedback that focuses the laser even more. The tighter focused laser has a higher peak brightness that forms a plasma. The plasma has an index of refraction lower than one, and causes a defocusing of the laser beam. The interplay of the focusing index of refraction, and the defocusing plasma makes the formation of a long filament of plasma that can be micrometers to kilometers in length. One interesting aspect of the filamentation generated plasma is the relatively low ion density due to defocusing effects of the ionized electrons.

Non-Neutral Plasma

The strength and range of the electric force and the good conductivity of plasma usually ensure that the densities of positive and negative charges in any sizable region are equal. A plasma with a significant excess of charge density, or, in the extreme case, is composed of a single species, is called a non-neutral plasma. In such a plasma, electric fields play a dominant role. Examples are charged particle beams, an electron cloud in a Penning trap and positron plasma. 

Dusty Plasma/Grain Plasma

A dusty plasma contains tiny charged particles of dust (typically found in space). The dust particles acquire high charges and interact with each other. A plasma that contains larger particles is called grain plasma. Under laboratory conditions, dusty plasma are also called complex plasma. 

Impermeable Plasma

Impermeable plasma is a type of thermal plasma which acts like an impermeable solid with respect to gas or cold plasma and can be physically pushed. Interaction of cold gas and thermal plasma was briefly studied by a group led by Hannes Alfvén in 1960s and 1970s for its possible applications in insulation of fusion plasma from the reactor walls. However, later it was found that the external magnetic fields in this configuration could induce kink instabilities in the plasma and subsequently lead to an unexpectedly high heat loss to the walls. In 2013, a group of materials scientists reported that they have successfully generated stable impermeable plasma with no magnetic confinement using only an ultrahigh-pressure blanket of cold gas. While spectroscopic data on the characteristics of plasma were claimed to be difficult to obtain due to the high pressure, the passive effect of plasma on synthesis of different nanostructures clearly suggested the effective confinement. They also showed that upon maintaining the impermeability for a few tens of seconds, screening of ions at the plasma-gas interface could give rise to a strong secondary mode of heating (known as viscous heating) leading to different kinetics of reactions and formation of complex nanomaterials.

Continued in the next 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...

Plasma (Part-II)- Lamenting its Traditional Neglect and Potential

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What is the potential of Plasma? How is the temperature of plasma measured? Comparison between Thermal and Non-Thermal Plasma? What is Plasma Magnetization?

Here we continue with the second part of our blog on plasma. Those who have missed our first blog can read it from Here. It will help to connect with this second part of the blog discussing details about the potential of plasma, measuring its temperature, comparison between thermal and non-thermal plasma and plasma magnetization. Let us explore about all the minute details of plasma, while reading this blog. In words of Hannes Alfvén-

"Students using astrophysical textbooks remain essentially ignorant of even the existence of plasma concepts, despite the fact that some of them have been known for half a century. The conclusion is that astrophysics is too important to be left in the hands of astrophysicists who have gotten their main knowledge from these textbooks. Earthbound and space telescope data must be treated by scientists who are familiar with laboratory and magneto-spheric physics and circuit theory, and of course with modern plasma theory."

What is the Potential of Plasma?

Plasma are very good electrical conductors, electric potentials play an important role. The average potential in the space between charged particles, independent of how it can be measured, is called the "plasma potential", or the "space potential". If an electrode is inserted into a plasma, its potential will generally lie considerably below the plasma potential due to what is termed a Debye sheath. The good electrical conductivity of plasma makes their electric fields very small. This results in the important concept of "quasi neutrality", which says the density of negative charges is approximately equal to the density of positive charges over large volumes of the plasma on the scale of the Debye length there can be charge imbalance. In the special case that double layers are formed, the charge separation can extend some tens of Debye lengths. The magnitude of the potentials and electric fields must be determined by means other than simply finding the net charge density. In astrophysical plasma, Debye screening prevents electric fields from directly affecting the plasma over large distances, i.e., greater than the Debye length. However, the existence of charged particles causes the plasma to generate, and be affected by, magnetic fields. This can and does cause extremely complex behavior, such as the generation of plasma double layers, an object that separates charge over a few tens of Debye lengths. The dynamics of plasma interacting with external and self-generated magnetic fields are studied in the academic discipline of magneto hydrodynamics.

How is the Temperature of Plasma Measured?

Plasma temperature is commonly measured in kelvins or electron volts and is, informally, a measure of the thermal kinetic energy per particle. High temperature are usually required to bolster ionization, which is a defining feature of a plasma. The degree or the magnitude of plasma ionization is determined by the electron temperature relative to the ionization energy (and more weakly by the density), in a relationship called the Saha equation. At low temperatures, ions and electrons tend to reintegrate into bound states atoms and the plasma will eventually become a gas.

In most cases the electrons are close enough to thermal equilibrium that their temperature is relatively well-defined, even when there is a significant deviation from a Maxwellian energy distribution function, for example, due to UV radiation, energetic particles, or strong electric fields. Because of the large difference in mass, the electrons come to thermodynamic equilibrium among themselves much faster than they come into equilibrium with the ions or neutral atoms. For this reason, the ion temperature may be very different from (usually lower than) the electron temperature. This is especially common in weakly ionized technological plasma, where the ions are often near the ambient temperature.

Comparison between Thermal and Non-Thermal Plasma

Based on the relative temperatures of the electrons, ions and neutrals, plasma are classified as "thermal" or "non-thermal" (also referred to as "cold plasma").

• Thermal plasmas have electrons and the heavy particles at the same temperature, i.e. they are in thermal equilibrium with each other.
• Non-thermal plasma on the other hand are non-equilibrium ionized gases, with two temperatures: ions and neutrals stay at a low temperature (sometimes room temperature), whereas electrons are such hotter. A kind of common non-thermal plasma is the mercury-vapor gas within a fluorescent lamp, where the "electrons gas" reaches a temperature of 10,000 kelvins while the rest of the gas stays barely above room temperature, so the bulb can even be touched with hands while operating. A particular and unusual case of "inverse" non-thermal plasma is the very high temperature plasma produced by the Z-machine, where ions are much hotter than electrons.

What is Plasma Magnetization?

Plasma with a magnetic field strong enough to influence the motion of the charged particles is said to be magnetized. A common quantitative criterion is that a particle on average completes at least one gyration around the magnetic field before making a collision, is the "electron gyro frequency" and  is the "electron collision rate". It is often the case that the electrons are magnetized while the ions are not. Magnetized plasmas are anisotropic, meaning that their properties in the direction parallel to the magnetic field are different from those perpendicular to it. While electric fields in plasmas are usually small due to the high conductivity, the electric field associated with a plasma moving in a magnetic field and is not affected by Debye shielding.

Continued in the next blog...

Plasma (Part-I)- Explore the 4th State of Matter

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What is the 4th state of matter? Is it Plasma? Who are the scientist involved to discover the fourth state of matter and how is the nature of 4th state of matter? What are factors that define plasma? Let us explore these topics by reading the following blog.

In the words of Carl Sagan-

"There is a place with four suns in the sky—red, white, blue, and yellow; two of them are so close together that they touch, and star-stuff flows between them. I know of a world with a million moons. I know of a sun the size of the Earth—and made of diamond. There are atomic nuclei a few miles across which rotate thirty times a second. There are tiny grains between the stars, with the size and atomic composition of bacteria. There are stars leaving the Milky Way, and immense gas clouds falling into it. There are turbulent plasmas writhing with X- and gamma-rays and mighty stellar explosions. There are, perhaps, places which are outside our universe. The universe is vast and awesome, and for the first time we are becoming a part of it".

Introducing the 4th State of Matter: Plasma

Plasma is one of the four basic and fundamental states of matter, and was first stated by chemist Irving Langmuir in the 1920s. It is not the other three states, solid, liquid, and gas, plasma does not exist freely on the Earth's surface under normal conditions. Plasma can be artificially produced by heating or subjecting a neutral gas to a strong electromagnetic field to the point an ionised gaseous substance becomes increasingly electrically conductive, and long-range electromagnetic fields dominate the behaviour of the matter. Plasma and ionised gases have properties and display behaviours unlike those of the other states, and the transition between them is mostly a matter of nomenclature and subject to interpretation. Based on the surrounding environmental temperature and density, partially ionised or fully ionised forms of plasma may be produced. Neon signs and lightning are examples of partially ionised plasma, while the interior of the Sun is an example of fully ionised plasma, along with the solar corona and stars. Positive charges in ions are achieved by stripping away electrons orbiting the atomic nuclei, where the total number of electrons removed is related to either increasing temperature or the local density of other ionised matter. This also can be accompanied by the dissociation of molecular bonds, though this process is distinctly different from chemical processes of ion interactions in liquids or the behaviour of shared ions in metals. The response of plasma to electromagnetic fields is used in many modern technological devices, such as plasma televisions or plasma etching. Plasma may be the most abundant form of ordinary matter in the universe, although this hypothesis is currently tentative based on the existence and unknown properties of dark matter. Plasma is mostly associated with stars, extending to the rarefied intra-cluster medium and possibly the intergalactic regions.

Phrase History of Plasma

The word plasma comes from Ancient Greek πλάσμα, meaning 'moldable substance' or 'jelly', and describes the behaviour of the ionised atomic nuclei and the electrons within the surrounding region of the plasma. Very simply, each of these nuclei are suspended in a movable sea of electrons. Plasma was first identified in a Crookes tube, and so described by Sir William Crookes in 1879 (he called it "radiant matter"). The nature of this "cathode ray" matter was subsequently identified by British physicist Sir J.J. Thomson in 1897. The term "plasma" was coined by Irving Langmuir in 1928. Lewi Tonks and Harold Mott-Smith, both of whom worked with Irving Langmuir in the 1920s, recall that Langmuir first used the word "plasma" in analogy with blood. Mott-Smith recalls, in particular, that the transport of electrons from thermionic filaments reminded Langmuir of “the way blood plasma carries red and white corpuscles and germs.” 

The factors that define Plasma:

i) The Plasma Approximation: The plasma approximation applies when the plasma parameter, Λ, representing the number of charge carriers within a sphere (called the Debye sphere whose radius is the Debye screening length) surrounding a given charged particle, is sufficiently high as to shield the electrostatic influence of the particle outside of the sphere. 

ii) Bulk Interactions: The Debye screening length is short compared to the physical size of the plasma. This criterion means that interactions in the bulk of the plasma are more important than those at its edges, where boundary effects may take place. When this criterion is satisfied, the plasma is quasi neutral.

iii) Plasma Frequency: The electron plasma frequency (measuring plasma oscillations of the electrons) is large compared to the electron-neutral collision frequency (measuring frequency of collisions between electrons and neutral particles). When this condition is valid, electrostatic interactions dominate over the processes of ordinary gas kinetics.

Continued in the next blog...