Microplasmas ignited and sustained by microwaves

Abstract

The challenges and benefits of microwave-induced microdischarges are reviewed. Transmission lines, resonators and surface wave launchers may be used for coupling microwave power to very small plasmas. Fortunately, microplasmas are typically much smaller than the wavelength of microwaves, and the electromagnetic problem may be treated electrostatically within the plasma. It is possible to trap electrons within small discharge gaps if the amplitude of electron oscillation is smaller than the plasma size. Typically occurring above 0.3 GHz, this condition results in lower breakdown fields than are required by direct current or radio frequency systems. Trapping of electrons also decreases the electrode potential to only tens of volts and makes the plasma density invariant in time. The steady-state microplasma produces electron densities of up to 10 15 cm −3 in argon but the electrons are not in equilibrium with the low gas temperatures (500-1000 K). Microwave discharges are compared with other forms of microplasma and guidelines for device selection are recommended. Scale-up of microplasmas using array concepts are presented followed by some exciting new applications.

Electron density measurement in atmospheric pressure plasma jets: Stark broadening of hydrogenated and non-hydrogenated lines

“…Recently the method was applied to a MW plasma jet working in Ar and it was found that in the core of the jet the electron density is about 10 16 cm −3 [35]. …”

“…The plasma density was found to be in the order of 10 13 cm −3 which is in good agreement with other methods. MW scattering can be applied to plasmas of low ionization degree with a minimum n e of approximately 10 9 cm −3 and shows possibilities of obtaining the decaying plasma parameters and information about loss rates of electrons [41,44]. It has to be noted here that a drawback of the method is that only volume averaged value of n e can be measured.…”

Electron confinement and heating in microwave-sustained argon microplasmas

“…The plasma was generated by applying microwave power to a transmission line terminated in a small discharge gap. This type of source was compared to other microwave source designs in a review paper; 13 the characteristics of the present design are outlined in Sec. II A.…”

A novel, all-dielectric, microwave plasma generator towards development of plasma metamaterials

“…Thus, the present design adds a new class of generators to the existing plasma generator flora, which are discussed in several reviews. [14][15][16][17][18][19] Unlike all previous plasma generators, the all-dielectric design of our generator makes it ideal for interactions with high-frequency electromagnetic waves.…”