The enduring contributions of low temperature plasmas to both technology and science are largely a result of the atomic, molecular, and electromagnetic (EM) products they generate efficiently such as electrons, ions, excited species, and photons. Among these, the production of light has arguably had the greatest commercial impact for more than a century, and plasma sources emitting photons over the portion of the EM spectrum extending from the microwave to soft x-ray regions are currently the workhorses of general lighting (outdoor and indoor), photolithography for micro- and nano-fabrication of electronic devices, disinfection, frequency standards (atomic clocks), lasers, and a host of other photonic applications. In several regions of the EM spectrum, plasma sources have no peer, and this article is devoted to an overview of the physics of several selected plasma light sources, with emphasis on thermal arc and fluorescent lamps and the more recently-developed microcavity plasma lamps in the visible and ultraviolet/vacuum ultraviolet regions. We also briefly review the physics of plasma-based metamaterials and plasma photonic crystals in which low temperature plasma tunes the EM properties of filters, resonators, mirrors, and other components in the microwave, mm, and sub-mm wavelength regions.

Mercury trapped ion clocks have demonstrated great long-term frequency stability and robustness. In this paper, we report a demonstration of an integrated 100-cc physics package in an effort to develop a micromercury trapped ion clock with high frequency stability. The physics package consists of a sealed 30-cc vacuum tube with one layer of magnetic shielding, light source, and detector assembly. A field emitter array and a 194-nm microplasma lamp were employed together with a microtrap tube to reduce the size and power consumption for a mercury trapped ion clock. We show that the 100-cc physics package is capable of providing a fractional frequency stability of $1\times {10}^{-11}{\tau }^{-1/2}$ down to $5\times {10}^{-14}$ after a few hours of integration. We also show a set of environmental sensitivity evaluations as well as the clock frequency retrace.

A greater than four log ₁₀ reduction in the concentration of Legionella pneumophila in municipal wastewater has been achieved in 1 min with ozone produced by a microchannel plasma reactor. Requiring less than 22 W of electrical power, and ambient air as the feedstock gas, the microplasma ozone generator is robust and a promising alternative to conventional corona and dielectric barrier discharge (DBD) technologies. Contrary to previous studies, the Ct model for pathogen deactivation (i.e. rate proportional to the product of the available disinfectant concentration and the exposure duration) is found to be valid for L. pneumophila . Accordingly, wastewater-specific Ct equations have been developed to predict the deactivation of L. pneumophila in the secondary wastewater environment. Inactivation of this pathogen was found to be dependent on temperature only in the absence of wastewater organic matter (WOM). In the presence of WOM, pathogen deactivation is controlled by the disinfection contact time, initial ozone concentration (varied between 15 and 281 µ g l ⁻¹ ), and initial WOM loading. The data reported here will assist in the implementation of plasma ozone generators for L. pneumophila deactivation in cooling towers, point-of-use systems, and wastewater reclamation facilities.

Energy efficiencies above η = 12% have been observed for the nonthermal dissociation of pure carbon dioxide (i.e. in the absence of carrier gas) at atmospheric pressure in arrays of microchannel plasmas. Microplasma ‘chips’ similar to those developed for the plasmachemical production of ozone from O ₂ or air (Kim et al 2017 Eur. Phys. J. Spec. Top . 226 2923) generate spatially uniform, abnormal glow plasmas in ⩾ 1 atm. of CO ₂ and consume 1–8 W of average power when driven by sinusoidal or bipolar pulsed voltage waveforms (pulse repetition frequency of 15–20 kHz). Fabricated in nanoporous alumina, the microchannels are 250 µ m in width, 150 µ m in depth, and 3 cm in length, and the array on each chip typically comprises 12 channels. Intense emission from microplasma arrays, recorded over the ~335–700 nm spectral interval when E / N ≈ 100 Td, is dominated by fluorescence which indicates that both and electronically-excited CO (e.g. CO ^* ) are generated primarily by multistep electron impact processes. Dissociative recombination of the ground state species appears to also be a contributing factor in CO ^* production. Owing to the modest power consumption and active volume of a single chip, the CO generation efficiency for a single array is 45 g kWh ⁻¹ for a CO ₂ flow rate of 70 sccm and the dissociation efficiency is 4.3%. The connection of four chips in tandem in a commercial module increases the CO ₂ dissociation efficiency to 20% and ~11% for flow rates of 70 sccm and 200 sccm, respectively. Data acquired with multiple chips confirms the scalability of the CO ₂ dissociation process, and suggests the economic viability of the commercial production of CO for the synthesis of formic acid, methanol, and syngas.

A series of miniature, microcavity plasma lamps emitting predominantly at 194 nm has been successfully developed and tested as the optical pump for the microwave ¹⁹⁹ Hg-ion atomic clock (40.507 GHz), replacing low pressure, RF-powered Ar/Hg discharges. Intense fluorescence on the 6 p ² P _1/2 → 6 s ² S _1/2 transition of the singly-charged ²⁰² Hg ion at 194.23 nm has been generated in arrays of cylindrical microplasmas through electron-impact excitation of He, followed by three-body formation of He ₂ ( a ³ Σ u + ) and Penning ionization of Hg. Emission spectroscopy and kinetic modeling of He/Hg vapor plasmas demonstrate that the population of the Hg ⁺ (6 ² P _1/2 ) radiating state (16.82 eV), produced by direct or two-step electron impact processes, is < 1% of that generated by excitation transfer to Hg by H e 2 * . Flat, fused silica lamps having emitting areas as small as 4 mm ² and containing several mg of ²⁰² Hg and 50–800 Torr of He have been fabricated and serve as optical drivers for the Hg ⁺ atomic clock cycle. Based on small arrays of 500 μ m–1 mm diameter microcavities, these lamps produce peak and average intensities at 194 nm greater than those associated with the Hg resonance transition at ∼ 254 nm, despite the factor of > 3 difference between the energies of the 6 p ³ P ₁ and 6 p ² P _1/2 states of neutral Hg and Hg ⁺ , respectively. These lamps are unique in the sense that the desired radiating species is an excited ion and the background He gas pressure can reach 1 atm, both of which contribute to a dense glow plasma placing severe demands on E / N , power conditioning, and materials selection. Nevertheless, with proper attention given to design, vacuum processing, and preparation of the lamps, lifetimes above 1500 h have been realized to date. When these lamps drive Jet Propulsion Laboratory Hg ⁺ clocks, the stability floor has been measured to be ∼ 10 ⁻¹⁴ . The implications of this lamp for gas-phase and solid-state photochemistry are also discussed.