In this blog post, we’ll explore everything from the initial processes of plasma generation via gas discharge to the principles behind the formation and maintenance of low-temperature plasma, as well as real-world applications such as fluorescent lights.
Plasma is commonly referred to as the fourth state of matter, following solids, liquids, and gases. Plasma is a state in which a gas absorbs energy, causing some or a significant number of its atoms and molecules to become ionized; free electrons and cations coexist, exhibiting collective electrical properties. Therefore, it is not necessary to heat the entire gas to a very high temperature to create plasma. In fact, by using discharges that selectively transfer energy to electrons, plasma can be formed even when the temperature of the gas as a whole is relatively low. Representative examples of places where plasma forms naturally include the Sun’s corona and the Earth’s ionosphere. It is widely accepted that a significant portion of the matter in the universe exists in a plasma state, which is why plasma is commonly found in celestial bodies and outer space.
However, if you look around, you’ll find that plasma is applied in various aspects of daily life, such as in fluorescent lights and dust collectors. This is because plasma can be artificially generated even without extremely high temperatures. Such plasma is referred to as low-temperature plasma or non-thermal plasma. Low-temperature plasma, which is widely used industrially, can be generated in laboratories and industrial settings through various types of discharges. Let’s now examine the initial processes leading up to the formation of plasma through gas discharge.
When a circuit like the one shown above is configured, the gas between the cathode and anode initially consists mainly of neutral particles and can be regarded as an insulating medium that does not conduct electricity well. Therefore, when a voltage is applied between the cathode and anode, an electric field is formed; assuming the electric field between the electrodes is uniform, the potential varies almost linearly with distance from the cathode. As the voltage applied between the cathode and anode increases, the electric field between the electrodes also increases. This potential distribution is a critical condition in the gaseous state prior to the onset of discharge.
However, as the voltage between the cathode and anode is gradually increased, a phenomenon occurs at a certain point where the current suddenly surges and light is emitted. This is called a discharge or gas breakdown, and the voltage at this point is referred to as the breakdown voltage or spark voltage. The breakdown voltage varies depending on conditions such as the type and pressure of the gas, the distance between the electrodes, the shape and surface condition of the electrodes, and the initial electrons supplied from an external source. Therefore, rather than assuming that there is always a single fixed voltage at a specific pressure and electrode gap, it is more appropriate to understand it as a value that varies depending on the discharge conditions. In glow discharge occurring in low-pressure gases, various regions—such as dark discharge, glow discharge, and abnormal glow discharge—can appear depending on the voltage-current characteristics, and the cathode drop voltage in normal glow discharge can reach several hundred volts, depending on the gas and conditions.
The plasma generated under such conditions is called direct-current glow discharge plasma. As mentioned earlier, plasma is an ionized gas; because it contains freely moving electrons and ions, it exhibits electrical properties similar to those of a conductor. While the electric potential inside the plasma varies relatively gradually compared to the total electrode voltage, a thin region where the potential changes abruptly forms near the cathode. This region is called the sheath. In particular, the sheath formed near the cathode in direct-current glow discharge is also referred to as the cathode shadow or cathode shadow region. A strong electric field forms within the sheath, accelerating electrons toward the plasma and accelerating cations toward the cathode. The cathode shadow appears dark because the electron density in this region is relatively low, limiting excitation and luminescence. Therefore, it is not accurate to explain that the sheath appears dark simply because it emits wavelengths with higher energy than visible light. In actual glow discharge, the cathode shadow, along with the potential drop, electric field, and charge distribution around the cathode, plays a crucial role in sustaining the discharge.
Let us now examine the process by which plasma is generated inside the discharge tube. This process can be broadly explained in terms of three actions, which we will refer to here as α, β, and γ actions.
First, the α process refers to a chain reaction in which initial electrons are accelerated by an electric field, collide with gas particles, and ionize gas molecules into electrons and cations; the electrons emitted in this process then collide with other gas molecules, causing further ionization. Since the gas is initially composed mostly of neutral particles, photoelectrons emitted from the metal surface when the cathode is exposed to ultraviolet light can be used as the initial electrons. The electrons generated in this way are accelerated by an electric field and collide with gas particles; those that acquire sufficient energy cause further ionization. As this process repeats, the number of electrons increases, and electron multiplication generally follows an exponential pattern depending on the distance traveled.
Beta action refers to the process in which a cation collides with a gas molecule and causes further ionization. However, when an electron collides with a neutral particle to cause ionization, the electron’s mass is very small compared to that of the neutral particle; consequently, there is a significant difference in the energy and kinetic state acquired by the electron and the cation during the collision. In particular, while an electron accelerated by an electric field can acquire the energy necessary for ionization even in a short time, a relatively heavy cation finds it difficult to gain sufficient kinetic energy under the same conditions. Consequently, the likelihood of a cation colliding with a gas molecule to cause further ionization is lower than that of an electron. Therefore, in the initial stages of a typical low-pressure glow discharge, this process can be considered less significant than ionization caused by electrons. However, in actual discharges, various processes—such as ion-molecule collisions—can influence discharge behavior depending on conditions such as the type of gas, pressure, and electric field.
Finally, the γ-process is associated with the process in which, following a collision between an electron and a gas particle, the resulting electron moves toward the anode while the cation moves toward the cathode. The electron moving toward the anode can continue to multiply through the α-process, while the cation that has moved toward the cathode collides with the cathode. At this point, new electrons may be emitted from the cathode surface due to the collision energy of the cations; this is called secondary electron emission. The secondary electrons emitted in this way are accelerated again by the electric field, collide with gas particles, and cause ionization, thereby playing a crucial role in sustaining the discharge. This secondary electron emission and the resulting electron multiplication process are at the heart of γ-action.
In other words, ionization begins through α-processes, and the discharge can sustain itself as the next generation of electrons is supplied through γ-processes. As the creation and annihilation of electrons and cations repeat in this manner, a portion of the gas is continuously ionized, and when sufficient conditions are met, a plasma state is formed. An important point here is that it is not necessary for the majority of gas molecules to be ionized for a plasma to form. Even if only a fraction of the electrons and ions are present, collective charge behavior and electrical properties can emerge; in typical low-temperature plasmas, neutral particles still constitute the majority.
During the maintenance of the discharge, ionized electrons and cations move in opposite directions. Within the plasma region, a quasi-neutral state forms where the number densities of electrons and cations are generally similar; however, near the electrodes, this balance is disrupted, leading to the formation of a sheath. In particular, near the cathode, electrons move relatively quickly, while cations are accelerated toward the cathode by the electric field, creating a region of low electron density. Consequently, a large potential drop occurs between the cathode and the plasma, and the strong electric field in this region plays a crucial role in sustaining the discharge. As time passes and the discharge reaches a stable state, a sheath forms at a constant distance from the cathode, while the plasma region beyond it maintains a state where the number densities of cations and electrons are nearly in equilibrium. When discharge conditions change, the thickness, charge density, and potential distribution of both the plasma region and the sheath also change accordingly.
A prime example of the application of this low-temperature plasma is the fluorescent lamp. A fluorescent lamp consists of a discharge tube filled with low-pressure mercury vapor and a small amount of inert gas to aid the discharge, with electrodes positioned at both ends. The electrodes are designed to easily emit electrons; when power is applied, electrons are emitted from the electrodes and travel through the discharge tube. As these electrons collide with mercury atoms, they excite or ionize them, and the excited mercury atoms emit ultraviolet (UV) light. This UV light interacts with the fluorescent material—known as the phosphor—coated on the inner surface of the discharge tube, producing visible light. Fluorescent lamps are a representative example of discharge light sources that combine this low-pressure mercury discharge with the light emission of the phosphor. The process by which electrons and ions are generated and move due to gas discharge, the acceleration of electrons by an electric field, and the processes of atomic excitation and light emission form an important foundation for understanding the principles behind the creation of low-temperature plasma.
