Non-thermal Plasma Generation Research Articles

Innovative Curved-Tip Reactor for Non-Thermal Plasma and Plasma-Treated Water Generation: Synergistic Impact Comparison with Sodium Hypochlorite in Dental Root Canal Disinfection.

Non-thermal plasmas (NTPs), known as cold atmospheric plasmas (CAPs), hold great potential for diverse medical applications, including dentistry. However, traditional linear and rigid dielectric barrier discharge reactors used for NTP generation encounter limitations in accessing oral cavities and root canals. To address this issue, we have developed an innovative NTP reactor featuring an angled end for improved accessibility. The central copper electrode, with a 0.59 mm diameter and adjustable length for desired angulation, is coated with zircon powder (ZrSiO4) to ensure stable NTP generation. This central electrode is housed within a stainless steel tube (3 mm internal diameter, 8 mm external diameter, and 100 mm length) with a 27° angle at one end, making it ergonomically suitable for oral applications. NTP generation involves polarizing the reactor electrodes with 13.56 MHz radio frequency signals, using helium gas as a working medium. We introduce plasma-treated water (PTW) as an adjunctive therapy to enhance biofilm eradication within root canals. A synergistic approach combining NTP and PTW is employed and compared to the gold standard (sodium hypochlorite, NaOCl), effectively neutralizing Enterococcus faecalis bacteria, even in scenarios involving biofilms. Moreover, applying NTP in both gaseous and liquid environments successfully achieves bacterial inactivation at varying treatment durations, demonstrating the device's suitability for medical use in treating root canal biofilms. The proposed NTP reactor, characterized by its innovative design, offers a practical and specific approach to plasma treatment in dental applications. It holds promise in combatting bacterial infections in root canals and oral cavities.

A review on dielectric barrier discharge nonthermal plasma generation, factors affecting reactive species, and microbial inactivation

Nonthermal plasma treatment has been proven effective in decontaminating microorganisms in various foods on a laboratory scale. Various techniques for creating plasma have been investigated extensively due to their potential application in multiple settings; one of these methods is dielectric barrier discharge. This article summarizes our current understanding of plasma physics, different methods for producing plasma, the resulting reactive species, and their role in microbial degradation. The effect of plasma treatment on the inactivation of microorganisms is discussed, along with the intrinsic and extrinsic factors involved. In addition, the use of dielectric barrier discharge plasma in microbial destruction in various foods, as well as the effect of the treatment on other relevant properties, is also addressed.

Selected Materials and Technologies for Electrical Energy Sector

Ensuring the energy transition in order to decrease CO2 and volatile organic compounds emissions and improve the efficiency of energy processes requires the development of advanced materials and technologies for the electrical energy sector. The article reviews superconducting materials, functional nanomaterials used in the power industry mainly due to their magnetic, electrical, optical, and dielectric properties and the thin layers of amorphous carbon nitride, which properties make them an important material from the point of view of environmental protection, optoelectronic, photovoltaic and energy storage. The superconductivity-based technologies, material processing, and thermal and nonthermal plasma generation have been reviewed as technologies that can be a solution to chosen problems in the electrical energy sector and environment. The study explains directly both—the basics and application potential of low and high-temperature superconductors as well as peculiarities of the related manufacturing technologies for Roebel cables, 1G and 2G HTS tapes, and superconductor coil systems. Among the superconducting materials, particular attention was paid to the magnesium di-boride MgB2 and its potential applications in the power industry. The benefits of the use of carbon films with amorphous structures in electronics, sensing technologies, solar cells, FETs, and memory devices were discussed. The article provides the information about most interesting, from the R&D point of view, groups of materials for PV applications. It summarises the advantages and disadvantages of their use regarding commercial requirements such as efficiency, lifetime, light absorption, impact on the environment, costs of production, and weather dependency. Silicon processing, inkjet printing, vacuum deposition, and evaporation technologies that allow obtaining improved and strengthened materials for solar cell manufacturing are also described. In the case of the widely developed plasma generation field, waste-to-hydrogen technology including both thermal and non-thermal plasma techniques has been discussed. The review aims to draw attention to the problems faced by the modern power industry and to encourage research in this area because many of these problems can only be solved within the framework of interdisciplinary and international cooperation.

Controlling Energy Transfer in Plasma-Driven Ammonia Synthesis by Adding Helium Gas

Plasma-driven ammonia (NH3) synthesis is regarded as a green complementary to the conventional Haber–Bosch process, especially for decentralized and on-demand production. However, a major challenge remains in reducing its energy cost because a huge amount of energy is consumed but not specifically for exciting, ionizing, and dissociating the feedstock. Therefore, controlling the energy transfer to predominantly generate more reactive species instead of gas heating is crucial to enhancing energy yields in the plasma synthesis of ammonia. Helium (He) is commonly used in atmospheric-pressure gas discharges to assist the generation of stable nonthermal plasmas with reduced breakdown voltages and gas temperatures while enhancing thermal conductivity and diffusion rate. In this work, a simple process based on adding He, which can be recycled and returned to the reactor along with unreacted N2 and H2, has been developed for enhancing NH3 synthesis. Optical emission spectra (OES) show an increased electron density with a significantly enhanced N2+ emission centered at 391.5 nm. With the addition of He, the energy transfer in the plasma is modified, with electron impact leading to the formation of metastable He and He2+ species, which collide with N2 molecules to generate N2+ through Penning ionization and charge transfer, contributing to more efficient N2 activation and NH3 production.

The Effects of Reactive Oxygen and Nitrogen Species (RONS) Produced by Surface Dielectric Barrier Discharge (SDBD) Non-Thermal Plasma with Treatment Time and Distance Variations to Kill Escherichia coli

Research on the inactivation of Escherichia coli causing diarrheal disease using non-thermal plasma SDBD has been carried out. SDBD is a new technique for non-thermal plasma generation with several advantages: low power generation, comprehensive treatment area coverage, and reducing the potential effects of burning and drying tissue. This study aimed to analyze the effect of treatment time variations, namely 0 as control, 60, 75, 90, 105, and 120 seconds and treatment distance variations of 3, 6, 9, 12, and 15 mm of non-thermal plasma treatment of SDBD on E. coli. The results of the non-thermal plasma SDBD treatment with variations in time and distance showed that the longer the treatment time, the more bacterial cells died. Colony counts decreased to 4.33 x 107 CFU/mL compared to the control, 409 x 107 CFU/mL, with a treatment time variation of 120 seconds, yielding the best treatment results. At the same time, the results of the treatment for variations in the non-thermal plasma distance of SDBD showed that the smaller the treatment distance, the greater the bacterial death rate, with the best treatment results at a 3 mm treatment interval, with colony counts of 8 x 107 CFU/mL, compared to 409 x 107 CFU/mL in control. Based on these results, SDBD non-thermal plasma treatment can be used to inactivate or kill bacteria with effectiveness in killing bacteria depending on the length of treatment time and the distance of treatment.

Identification of Reactive Species Produced by Surfaces Dielectric Barrier Discharge Nonthermal Plasma with Gas Sources Variation (Air, N2, O2) to Kill Bacteria

Non-thermal plasma is one of the new techniques that is being focused on in the medical world, one of which is used for sterilization because non-thermal plasma is known to have a bactericidal effect. Reactive species produced by non-thermal plasma are antimicrobial. One factor affecting plasma species' reactive composition is the gas source used. Therefore, this study aimed to determine the effect of reactive species produced from non-thermal plasma generation of SDBD using different gas sources, namely free air, oxygen, and nitrogen, in treating Escherichia Coli bacteria. The gas flow used is 1 liter/minute. Reactive species produced by plasma were characterized using Optical Emission Spectrometry (OES). SDBD Non-thermal plasma treatment for Escherichia Coli bacteria samples was repeated three times for each treatment, and then the Anova test was performed. The results of this study indicate that bacterial death at the decontamination level depends on the composition of the gas used during treatment. Plasma using an O2 gas source has a more effective inactivation ability, namely 6 x 107 CFU/ml, compared to the control, which is 409 x 107 CFU/ml. At the same time, the treatment results with free air and nitrogen gas sources were 6.33 x 107 CFU/ml and 41.67 x 107 CFU/ml. These results indicate that the composition of ROS and RNS influences bacterial inactivation, where ROS is more effective in inactivating bacteria than RNS

Self-powered air purifier with coupling of non-thermal plasma and photocatalytic oxidation for formaldehyde degradation based on triboelectric nanogenerator

The emission of various volatile organic compounds (VOCs) caused by indoor decoration and furniture placement are the causes of health hazards in people's lives. Here, we demonstrate a self-powered air purifier for indoor organic pollutant removal based on triboelectric nanogenerator (TENG), coupling non-thermal plasma (NTP) with photocatalytic oxidation (PCO). An adjustable external high voltage field is generated by our homemade TENG, in which the output open-circuit voltage (Voc) ranges from 10 to 30 kV. After connecting the TENG with a non-thermal plasma generator (NTP-G), a maximum ozone concentration of 14.7 mg/m³ and NTP can be generated. The prepared three-dimensional porous framework is used as a photocatalytic layer, which can support more titanium oxide and increase the contact area between the photocatalyst and formaldehyde. This designed self-powered air purifier can reduce the formaldehyde concentration from 2.06 ppm to 0 ppm in 21 min in a 13 L closed air chamber. The design and concept proposed in this work has multiple ways for formaldehyde degradation and are not limited by sunlight, and can remove indoor formaldehyde around the clock, providing an effective way to create a cleaner and healthier indoor environment.

Evaluation of the Degradation Process of Diethyl (3H-1-Ethoxy-3-phenoxazinylidene) Ammonium Chloride in Water, after Exposure to Nonthermal Plasma at Atmospheric Pressure

The present study characterizes the degradation of diethyl dye (3H-1-ethoxy-3-phenoxazinylidene) ammonium chloride in water (concentration of 20.00 mg/L), after its exposure to nonthermal plasma of air at atmospheric pressure. The exposure time of the solution of interest to the plasma was 250 minutes per treatment. Four different nonthermal plasma generation conditions were considered (15, 20, 25, and 30 mA) at a voltage of 2000 V. The change in dilution was determined by UV/VIS spectrophotometry (absorbance and concentration), chemical oxygen demand (COD), pH, and bioassays with lettuce seeds (Lactuca sativa). Given the increase in temperature of the sample under study due to exposure to plasma, the change in each of the measured parameters was also explored as an exclusive function of the increase in temperature (27°C to 44°C). The measurements did not show the influence of temperature on the changes observed in the parameters of interest in this experiment. In general, the maximum degradation obtained from the samples analyzed at the end of the treatments corresponds to the 30 mA treatment. For this, reductions of 70.4% in absorbance, 71.5% of dye concentration, 81.6% of COD, and 51.85% of pH were achieved. The germination percentage of the lettuce seeds obtained was 74%. The degradation of the dye in the solution of interest depends on the plasma generating parameters high power and the high degradation rate.

Confined Water for Passive Generation and Accumulation of Non-Thermal Plasma

Introduction: In a recent publication, we described the production method for Non-Thermal Plasma (NTP) without added external energy. Bulk water contains 12-19% free water molecules. Once filtered through glass into a sealed confined jar, free water molecules reacted to produce an new form of NTP, a gas/water combination, Hybrid-Plasma (HP). This report describes a second production method for HP. Methods: We added 800mL distilled water to a 4000mL acrylic cylinder. Inside the cylinder, an uncovered 500mL collection jar equipped with an ion counter and hygrometer was placed on a platform above the water line. When the acrylic cylinder was sealed with a cover, it formed the confined chamber similar to the water through glass method. Results: After 24 hours, maximum readings were registered on the ion counter and hygrometer inside the open jar confined in the chamber. The uncovered collection jar was sealed with a plastic cover and placed on a shelf for observation (n=6). Initially, the ion counters (n=6) registered a mean ion count of 1443 and a mean hygrometer reading of 93%; both readings gradually declined until day 6. Discussion: We produced HP by a second method with the same properties as HP produced by the original water-throughglass method. Analysis of the HP indicated the presence of Hydroxide/OH- and H2 O2 /Hydrogen Peroxide. Conclusions: We discovered how to separate highly kinetic free water molecules in a confined chamber to produce stable NTP. Hybrid plasma was accumulated, analyzed and used in experiments to determine the effects it has on living systems.

An Electrohydraulic Direct Current Discharge for Inactivation of <i>Escherichia coli</i> in High-Bacterial Density Wastewaters

The United Nations, through its Sustainable Development Goals, have identified access to clean water as one of the challenges facing society. With reported global deaths exceeding 1 million annually linked to untreated water consumption, which is usually contaminated by pathogenic micro-organisms, further research continues in water disinfection. The direct generation of non-thermal plasma in water is a promising method for the inactivation of disease-causing bacteria present in the wastewater. This study explored the efficacy of plasma in the inactivation of different bacterial densities (4.0×104, 1.5×105, and 2.5×107 CFU/mL) using a 500 mL plasma batch reactor operating at atmospheric pressure. The plasma discharge was generated in water by a Technix-SR-10R-5000 high voltage direct current power supply in negative polarity with a set current of 0.45 A and a maximum pre-set ignition voltage of 9 kV. The electrodes used in the discharge was a copper material. A bacterial culture of Escherichia coli ATCC® 25922TM (E.coli) was used as a model for the direct plasma discharge. The study further investigated the contribution of copper ions (0.4 and 0.7 mg/L) released into the water during treatment by having two control reactors that were not exposed to plasma. The results show a complete inactivation at 180 seconds for the bacterial densities from 4.0×104 to 2.5×107 CFU/mL. The results from this study indicated the potential of a direct electric discharge in handling water source with high-bacteria densities.

Synthesis and Spectroscopic Characterization of Platinum Nanoparticles by Plasma Jet Method

In this research, Argon gas was used to generate atmospheric plasma in the manufacture of platinum nanomaterials, to study the resultant plasma spectrum and to calculate the cellular toxicity of those manufactured nanomaterials. This research is keen on the generation of nonthermal atmospheric pressure plasma using aqueous platinum salts (H2PtCl6 6H2O) with different concentrations and exposure of cold plasma with a different time period used to produce platinum nanoparticles, to ensure typical preparation of nanoparticles. Visible UV and X-rays were performed for this purpose, and the diameter of the system probe was (1[Formula: see text]mm) with the Argon gas flow of 2.5[Formula: see text]min/L to prepare the platinum nanoparticles, and spectroscopic study of plasma parameter including, electron temperature, electron density, Debye length and plasma frequency, were computed using spectral analysis techniques. The effect of nanoparticles on natural lymphocytes was studied to calculate cytotoxicity and the greatest proportion was at the concentration of 100% nanoparticle platinum is 37.4%. The study results revealed that cold in the atmosphere is a promising technology when used in the production of nanoparticle materials which can be used for many industrial and medical applications.

A New Driving Circuit With Delay Trigger for the Solid-State Marx Modulators

The rising edge of the output pulses affects the discharged mechanism and efficiency of the nonthermal plasma generation. In the traditional Marx-type pulse generators, the magnetic isolation driving circuit could not adjust the rising edge of the output pulses and limits the application on a capacitive load. In this article, the factors affecting the rising edge of output pulses are analyzed. And a driving circuit based on magnetic isolation is proposed, which can control the rising edge of the output pulses. A synchronous driving signal is used to control multiple groups of switches using different sequences, and the magnetic core does not limit the driving signals. The experiments were carried out using a seven-stage solid-state Marx modulator. From the results, the rectangular pulses and stepped pulses are obtained at a capacitive load, and the 0.1– $2.3~\mu \text{s}$ adjustment rising edge can be obtained under 0–5 kV output voltage.

PFAS Degradation in Ultrapure and Groundwater Using Non-Thermal Plasma.

Perfluoroalkyl substances (PFAS) represent one of the most recalcitrant class of compounds of emerging concern and their removal from water is a challenging goal. In this study, we investigated the removal efficiency of three selected PFAS from water, namely, perfluorooctanoic acid (PFOA), perfluorohexanoic acid (PFHxA) and pefluorooctanesulfonic acid (PFOS) using a custom-built non-thermal plasma generator. A modified full factorial design (with 2 levels, 3 variables and the central point in which both quadratic terms and interactions between couple of variables were considered) was used to investigate the effect of plasma discharge frequency, distance between the electrodes and water conductivity on treatment efficiency. Then, the plasma treatment running on optimized conditions was used to degrade PFAS at ppb level both individually and in mixture, in ultrapure and groundwater matrices. PFOS 1 ppb exhibited the best degradation reaching complete removal after 30 min of treatment in both water matrices (first order rate constant 0.107 min−1 in ultrapure water and 0.0633 min−1 in groundwater), while the degradation rate of PFOA and PFHxA was slower of around 65% and 83%, respectively. During plasma treatment, the production of reactive species in the liquid phase (hydroxyl radical, hydrogen peroxide) and in the gas phase (ozone, NOx) was investigated. Particular attention was dedicated to the nitrogen balance in solution where, following to NOx hydrolysis, total nitrogen (TN) was accumulated at the rate of up to 40 mgN L−1 h−1.

Generation of non-thermal plasmas over large and complex surfaces

Atmospheric non-thermal plasma is gaining increasing attention for various applications including food, medical and healthcare technologies because of its unique capability in producing reactive species. In spite of its promising potential, generating non-thermal plasma over large and complex geometries such as the human body or a narrow channel is still challenging and is limiting the use of atmospheric non-thermal plasma. In this study, we propose two new electrode systems, printed and knitted electrodes, to enhance scalability and flexibility of a conventional atmospheric non-thermal plasma source. The flexibilities of both electrode systems are quantified for varying curvatures to generate non-thermal plasma over complex geometries. Moreover, both electrode systems are assessed for varying system size to assess the ability of large scale plasma geometries. Electrical and optical diagnostics including Optical Emission Spectroscopy (OES), are used to monitor the property of plasma generated by these systems. The present study shows that both printed and knitted electrodes can produce non-thermal plasma, however both have certain limitations. Concluding from these findings, a schematic of new hybrid electrode system for the treatment of large surfaces or narrow long channels is proposed to eradicate these limitations.

Healing of wounds in lower extremities employing a non-thermal plasma

Purpose: Venous, neuropathic, and mixed (venous and arterial) ulcers are complex chronic wounds that have a tremendous long-term impact on the mortality, morbidity, and quality of life of patients. In this work it was evaluated the efficacy of the non-thermal helium plasma at atmospheric pressure applied in patients with this suffering.Methods: Using three elements: a RF generator, a needle-type reactor, and helium gas flow, for non-thermal plasma generation were used. The power density on the tissue of patients was of 0.6–0.67 W/cm2. The application time was 30 s/cm2 daily, at 5 to 10 mm of distance from the wound.Results: The thirty-two patients with venous and mixed ulcers indicated that they had a decrease progressively of the pain with the therapy, without the need to continue with analgesics. Besides, the ten patients with chronic wounds showed 100% healing in neuropathic ulcers. For eight patients with venous leg ulcers, the percentage was reduced to 75% healing. While for fourteen patients with mixed leg ulcers, the rate of healing was 59%.Conclusion: The non-thermal plasma generated in a needle reactor with helium gas is a promising candidate for clinical and therapeutic use in the treatment of neuropathic leg ulcers, with time reduction healing, and it can be applied everyday. In the study conducted, it was found that the non-thermal plasma applied to patients with autoimmune diseases clinically was not beneficial.

Elemental analysis and mixture ratio determination in fine powder metals using microwave-sustained plasma ball spectroscopy

Elemental analysis of fine powder metals and their mixtures using plasma ball spectroscopy has been developed. This method is suitable for in situ and on-site industrial/ non-industrial monitoring applications; pre-treatment or powder fixation methods which are necessary for laser-induced breakdown spectroscopy were not required. The metals analysed were Cu, Mo and Pb with average grain size of ≤75 μm. A plasma seed (generated by laser or spark discharge) was spatially sustained in air at atmospheric conditions in a spherical form and controlled temporally using a semiconductor microwave (MW) oscillator; hence, the name plasma ball. The frequency and output power range of continuously oscillated MW were 2.45 GHz and 20–150 W respectively. Longer lifetime, controllability, non-thermal and stable plasma generation, as well as comparatively large volume of MW-sustained plasma made PBS of powder metals practically possible. Calibration curves were obtained for Cu-Mo and Cu-Pb mixtures, from which the concentration limit of detection (LOD) for both Mo and Pb powders were obtained as a function of the MW oscillation duration. The optimum concentration LOD for Mo and Pb powders in their constituent mixtures were respectively 0.004% and 0.002% by weight.

High-Voltage, Multiphasic, Nanosecond Pulses to Modulate Cellular Responses.

Nanosecond electric pulses are an effective power source in plasma medicine and biological stimulation, in which biophysical responses are governed by peak power and not energy. While uniphasic nanosecond pulse generators are widely available, the recent discovery that biological effects can be uniquely modulated by reversing the polarity of nanosecond duration pulses calls for the development of a multimodal pulse generator. This paper describes a method to generate nanosecond multiphasic pulses for biomedical use, and specifically demonstrates its ability to cancel or enhance cell swelling and blebbing. The generator consists of a series of the fundamental module, which includes a capacitor and a MOSFET switch. A positive or a negative phase pulse module can be produced based on how the switch is connected. Stacking the modules in series can increase the voltage up to 5kV. Multiple stacks in parallel can create multiphase outputs. As each stack is independently controlled and charged, multiphasic pulses can be created to produce flexible and versatile pulse waveforms. The circuit topology can be used for high-frequency uniphasic or biphasic nanosecond burst pulse production, creating numerous opportunities for the generator in electroporation applications, tissue ablation, wound healing, and nonthermal plasma generation.

Suitability of thermal plasmas for large-area bacteria inactivation on temperature-sensitive surfaces – first results with Geobacillus stearothermophilus spores

The application of thermal plasma for large-area bacteria inactivation on temperature-sensitive surfaces is not a common one. Nonetheless, there are thermal plasma generators which offer a high sheath homogeneity and have proven to be suitable for treatment of thermally sensitive materials in the past. To investigate the suitability of such plasmas, agar dishes plated with endospores of Geobacillus stearothermophilus have been treated with a long arc plasma generator called LARGE. The achieved results have been compared with a commercially available non-thermal plasma generator. A significant inactivation of the endospores could be observed only after 60 s of treatment with the thermal plasma source. This was not possible with the non-thermal generator. Moreover, no temperature damage or increase of the specimen could be detected. An attempt to determine the main agents responsible for the microbicidal effects have been made – the influence of plasma gas composition, discharge current and treatment time has been investigated. Significant improvements in the disinfection rates after adding small amounts of nitrogen to the plasma gas could be observed. A first discussion regarding the suitability of thermal plasmas for bacteria inactivation has been given.

Nonthermal Argon Plasma Generator and Some Potential Applications

Abstract A laboratory - made nonthermal plasma generator is presented. It has a diameter of 0.020 m and length of 0.155 m and contains two electrodes. The first electrode is a 2% Th-W alloy, 0.002 m in diameter bar, centred inside the generator’s body by means of a four channel teflon piece; the other three channels, 0.003 m in diameter, are used for Ar supply. The second electrode is a nozzle of 0.002 m - 0.008 m diameter and 0.005m length. A ~500 kV/m electric field is generated between the two electrodes by a high frequency source (13.56 MHz ±5%), equipped with a OT-1000 (Tungsram) power triode. For Ar flows ranging from 0.00008 m3/s to 0.00056 m3/s, a plasma jet of length not exceeding 0.015 m and temperature below 315 K is obtained. Anthurium andraeanumis sample , blood matrix, human hair and textile fibers may be introduced in the plasma jet. For time periods of 30 s and 60 s, various effects like, cell detexturization, fast blood coagulation or textile fiber or hair cleaning and smoothing are obtained. These effects are presented and discussed in the paper.

Reduction of NOx and PM in marine diesel engine exhaust gas using microwave plasma

Abatement of NOx and particulate matters (PM) of marine diesel exhaust gas using microwave (MW) non-thermal plasma is presented in this paper. NOx mainly consist of NO and less concentration of NO2 in a typical two stoke marine diesel engine and microwave plasma generation can completely remove NO. MW was generated using two 2kW microwave sources and a saw tooth passive electrode. Passive electrode was used to generate high electric field region within microwave environment where high energetic electrons (1-3eV) are produced for the generation of non-thermal plasma (NTP). 2kW gen-set diesel exhaust gas was used to test our pilot-scale MW plasma reactor. The experimental results show that almost 100% removal of NO is possible for the exhaust gas flow rate of 60l/s. It was also shown that MW can significantly remove soot particles (PM, 10nm to 365nm) entrained in the exhaust gas of 200kW marine diesel engine with 40% engine load and gas flow rate of 130l/s. MW without generating plasma showed reduction up to 50% reduction of PM and with the plasma up to 90% reduction. The major challenge in these experiments was that igniting the desired plasma and sustaining it with passive electrodes for longer period (10s of minutes) as it required fine tuning of electrode position, which was influenced by many factors such as gas flow rate, geometry of reactor and MW power.