Microplasma

A microplasma is a plasma of small dimensions, ranging from tens to thousands of micrometers. Microplasmas can be generated at a variety of temperatures and pressures, existing as either thermal or non-thermal plasmas. Non-thermal microplasmas that can maintain their state at standard temperatures and pressures are readily available and accessible to scientists as they can be easily sustained and manipulated under standard conditions. Therefore, they can be employed for commercial, industrial, and medical applications, giving rise to the evolving field of microplasmas.

There are 4 states of matter: solid, liquid, gas, and plasma. Plasmas make up more than 99% of the visible universe. In general, when energy is applied to a gas, internal electrons of gas molecules (atoms) are excited and move up to higher energy levels. If the energy applied is high enough, outermost electron(s) can even be stripped off the molecules (atoms), forming ions. Electrons, molecules (atoms), excited species and ions form a soup of species which involves many interactions between species and demonstrate collective behavior under the influence of external electric and magnetic fields. Light always accompanies plasmas: as the excited species relax and move to lower energy levels, energy is released in the form of light. Microplasma is a subdivision of plasma in which the dimensions of the plasma can range between tens, hundreds, or even thousands of micrometers in size. The majority of microplasmas that are employed in commercial applications are cold plasmas. In a cold plasma, electrons have much higher energy than the accompanying ions and neutrals. Microplasmas are typically generated at elevated pressure to atmospheric pressure or higher.

Successful ignition of microplasmas is governed by Paschen's Law, which describes the breakdown voltage (the voltage at which the plasma begins to arc) as a function of the product of electrode distance and pressure,

where pd is the product of pressure and distance, and ${\displaystyle A}$ and ${\displaystyle B}$ are the gas constants for calculating Townsend's first ionization coefficient and ${\displaystyle \gamma }$ is the secondary emission coefficient of the material. As the pressure increases, the distance between the electrodes must decrease to achieve the same breakdown voltage. This law is proven to be valid at inter-electrode distances as small as tens of micrometers and pressures higher than atmospheric. However, its validity at even smaller scales (approaching debye length) is still currently under investigation.

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