It is well recognized at present that the unique, high energy density plasma environment provides suitable conditions to dissociate/atomize molecules in remediation systems, to convert waste and biomass into sustainable energy sources, to purify water, to assemble nanostructures, etc. The remarkable plasma potential is based on its ability to supply simultaneously high fluxes of charged particles, chemically active molecules, radicals (e.g. O, H, OH), heat, highly energetic photons (UV and extreme UV radiation), and strong electric fields in intrinsic sheath domains. Due to this complexity, low-temperature plasma science and engineering is a huge, highly interdisciplinary field that spans many research disciplines and applications across many areas of our daily life and industrial activities. For this reason, this review deals only with some selected aspects of low-temperature plasma applications for a clean and sustainable environment. It is not intended to be a comprehensive survey, but just to highlight some important works and achievements in specific areas. The selected issues demonstrate the diversity of plasma-based applications associated with clean and sustainable ambiance and also show the unity of the underlying science. Fundamental plasma phenomena/processes/features are the common fibers that pass across all these areas and unify all these applications. Browsing through different topics, we try to emphasize these phenomena/processes/features and their uniqueness in an attempt to build a general overview. The presented survey of recently published works demonstrates that plasma processes show a significant potential as a solution for waste/biomass-to-energy recovery problems. The reforming technologies based on non-thermal plasma treatment of hydrocarbons show promising prospects for the production of hydrogen as a future clean energy carrier. It is also shown that plasmas can provide numerous agents that influence biological activity. The simultaneous generation in water discharges of intense UV radiation, shock waves and active radicals (OH, O, H2O2, etc), which are all effective agents against many biological pathogens and harmful chemicals, make these discharges suitable for decontamination, sterilization and purification processes. Moreover, plasmas appear as invaluable tools for the synthesis and engineering of new nanomaterials and in particular 2D materials. A brief overview on plasma-synthesized carbon nanostructures shows the high potential of such materials for energy conversion and storage applications.

https://iopscience.iop.org/article/10.1088/0963-0252/23/6/063002/pdf

Plasmas for environmental issues: from hydrogen production to 2D materials assembly

Natural gas is catalytically converted into several bulk chemicals such as ammonia, methanol, dimethyl ether, and synthetic liquid fuels by Fischer-Tropsch synthesis and similar processes. The main step in the conversion of natural gas to these products is the production of synthesis gas with the desired composition ranging from H2/CO = 3:1 used for the production of ammonia to the 1:1 mixture preferred for production of dimethyl ether. Catalysts and catalytic processes are important in the production of synthesis gas from natural gas. In this work, relevant catalytic systems employed recently in the production of syngas by the catalytic partial oxidation of methane, as well as experimental evidences on the reaction mechanisms are examined. Differences in methane dissociation, binding site preferences, stability of OH surface species, surface residence times of active species and contributions from lattice oxygen atoms and support species are considered. The methane dissociation requires reduced metal sites, but at elevated temperatures oxides of active species may be reduced by direct interaction with methane or from the reaction with H2 and CO (or C). The comparison of elementary reaction steps on Pt and Rh illustrates the fact that a key factor to produce hydrogen as primary product is a high activation energy barrier to the formation of OH. Another essential property for the formation of H2 and CO as primary products is a low surface coverage of intermediates, such that the probability of O-H, OH-H and CO-O interactions is reduced.

/pdf/recent-developments-in-the-partial-oxidation-of-methane-to-44zkutctc0.pdf

Recent Developments in the Partial Oxidation of Methane to Syngas

Purpose:
To measure the acoustic signal generated by a pulsed proton spill from a hospital-based clinical cyclotron.
Methods:
An electronic function generator modulated the IBA C230 isochronous cyclotron to create a pulsed proton beam. The acoustic emissionsgenerated by the proton beam were measured in water using a hydrophone. The acoustic measurements were repeated with increasing proton current and increasing distance between detector and beam.
Results:
The cyclotron generatedproton spills with rise times of 18 μs and a maximum measured instantaneous proton current of 790 nA. Acoustic emissionsgenerated by the proton energy deposition were measured to be on the order of mPa. The origin of the acoustic wave was identified as the proton beam based on the correlation between acoustic emission arrival time and distance between the hydrophone and proton beam. The acoustic frequency spectrum peaked at 10 kHz, and the acoustic pressure amplitude increased monotonically with increasing proton current.
Conclusions:
The authors report the first observation of acoustic emissionsgenerated by a proton beam from a hospital-based clinical cyclotron. When modulated by an electronic function generator, the cyclotron is capable of creating proton spills with fast rise times (18 μs) and high instantaneous currents (790 nA). Measurements of the proton-generated acoustic emissions in a clinical setting may provide a method for in vivoproton range verification and patient monitoring.

Experimental observation of acoustic emissions generated by a pulsed proton beam from a hospital-based clinical cyclotron.

This paper is a review of studies on the generation of low-energy high-current electron beams in electron guns with a plasma anode and an explosive-emission cathode. The problems related to the initiation of explosive electron emission under plasma and the formation and transport of high-current electron beams in plasma-filled systems are discussed consecutively. Considerable attention is given to the nonstationary effects that occur in the space charge layers of plasma. Emphasis is also placed on the problem of providing a uniform energy density distribution over the beam cross section, which is of critical importance in using electron beams of this type for surface treatment of materials. Examples of facilities based on low-energy high-current electron beam sources are presented and their applications in materials science and practice are discussed.

Generation of Low-Energy High-Current Electron Beams in Plasma-Anode Electron Guns

The results of a comprehensive study of a gigawatt level power pulsed ion beam used for material modification are presented. The ion beam is formed using a diode with self-magnetic insulation. The study was performed using the TEMP-4M accelerator in double pulse formation mode. The ion current density is 30–300 A/cm 2 (for different designs of diodes), and the beam is composed of protons (15%) and carbon ions (85%). It was shown that a diode with planar configuration generates an ion beam with a fairly homogeneous energy density over a cross section of 80 cm 2 ; the energy density is 0.3–0.4 J/cm 2 and the fluence is 2 × 10 13  cm − 2 per pulse. A diode with focusing geometry forms an ion beam with an energy density of 2–2.5 J/cm 2 at the focus and a pulse energy of 80–100 J.

A gigawatt power pulsed ion beam generator for industrial applications

An externally biased matching transformer was used to increase the efficiency of transfer of the energy stored in the double forming line (DFL) of a high-current electron accelerator to the electron beam. The transformer was placed between the DFL and the diode module. Saturation of the transformer core prior to line charging allowed us to significantly reduce the amplitude of the prepulse and its destructive effect on the anode foil without using a pulse-sharpening gap. An electron beam with a pulse energy of 200 J was produced; the kinetic energy of the electrons was 500 keV and the pulse duration was 60 ns.

A High-Current Pulsed Accelerator with a Matching Transformer

The conversion of methane in electric discharges of different types and under electron beam irradiation are considered. The influence of nonequilibrium conditions of conversion in low-temperature plasma on the energy consumption, product composition, and selectivity is analyzed. The results of works on plasma pyrolysis, partial plasma oxidation, and steam and carbon dioxide reforming of methane in a low-temperature plasma are discussed. It is shown that the use of chain processes makes it possible to substantially reduce the power consumption for methane conversion by an electrophysical device.

Methane conversion in low-temperature plasma

The research results of the speed dynamics of explosive emission plasma expansion in the planar diode with cathodes made of graphite, carbon fiber, and copper (solid and multiedged) and with the multiedged tungsten cathode are presented. The plasma speed was determined by the volt-ampere characteristics of the diode with a time resolution of 0.2 ns. The experiments have been performed at the electron accelerator TEU-500 (350-450 kV, 100 ns, and 10 kA). It was found out when the plasma formation at the cathode is completed, and until the pulse ends, the plasma speed is constant and equal to 2 plusmn 0.5 cm / mus for graphite and carbon fiber cathode, 3 plusmn 0.5 cm /mus for tungsten cathode, and 4 plusmn 0.5 cm / mus for cathode made of copper. The analysis of various mechanisms of anode plasma action on the deviation of electron current from the value described by the correlation of Child-Langmuir-reduction of anode-cathode gap, compensation of volumetric electron charge, and additional input of electrons and ions of anode plasma to the total diode current-has been performed.

Research of Cathode Plasma Speed in Planar Diode With Explosive Emission Cathode

Thermal imaging diagnostics of the total energy of a pulsed ion beam and energy-density distribution over the cross section is described. The diagnostics was tested on the TEMП-4M accelerator in the conditions of formation of two pulses: (i) the first plasma-forming pulse is negative (300–500 ns, 100–150 kV) and (ii) the second generated one is positive (150 ns, 250–300 kV). The beam composition includes carbon ions (85%) and protons, and the power density is 0.2–3.0 J/cm2 (for various diodes). The diagnostics was applied in studies of the powerful ion beam, formed by an ion diode with self insulation (two-pulse mode) and external magnetic insulation in the single-pulse mode. The diagnostics was intended to measure the beam energy density in a range of 0.05–5.00 J/cm2 in the absence of erosion and ablation processes on the target. When an infrared camera with a 140 × 160-pixel matrix is used, the spatial resolution is 0.9 mm. The measurement time does not exceed 0.1 s.

Alexander I. Pushkarev

Papers

A High-Current Pulsed Accelerator with a Matching Transformer

A gigawatt power pulsed ion beam generator for industrial applications

Methane conversion in low-temperature plasma

Research of Cathode Plasma Speed in Planar Diode With Explosive Emission Cathode

Thermal imaging diagnostics of powerful ion beams