Increasing Gas Pressure Research Articles

Analysis of energy relaxation kinetics for control of the electron energy distributions in capacitively coupled RF discharges

The transition of electron energy probability functions (EEPFs) through the change of heating mode is an important issue in plasma science. A well-known example is that the increase of gas pressure, which was analyzed in terms of the ratio of the energy relaxation mean free path to the electrode gap distance, changes the EEPF from bi-Maxwellian to Maxwellian or Druyvesteyn. In this study, a new aspect of the temporal decay of kinetic energy during the energy relaxation time is theoretically analyzed and compared with a particle-in-cell Monte Carlo collision simulation of capacitively coupled plasmas. A fully kinetic description of electron transport and collisions shows drastic changes of EEPFs with the variation of the driving frequency due to the heating mode transition.

Ion beams in multi-species plasmas

Argon and xenon ion velocity distribution functions are measured in Ar-He, Ar-Xe, and Xe-He expanding helicon plasmas to determine if ion beam velocity is enhanced by the presence of lighter ions. Contrary to observations in mixed gas sheath experiments, we find that adding a lighter ion does not increase the ion beam speed. The predominant effect is a reduction of ion beam velocity consistent with increased drag arising from increased gas pressure under all conditions: constant total gas pressure, equal plasma densities of different ions, and very different plasma densities of different ions. These results suggest that the physics responsible for the acceleration of multiple ion species in simple sheaths is not responsible for the ion acceleration observed in expanding helicon plasmas.

PIC Simulation to Analyze Peak Electron Current Generation in a Triggered Pseudospark Discharge-Based Plasma Cathode Electron Source

In this paper, an investigation has been carried out for the analysis of peak electron current at the anode surface in pseudospark discharge-based plasma cathode electron (PCE) source using 2-D kinetic plasma simulation code OOPIC-PRO. It has been observed that peak electron current is a function of hollow cathode (HC) dimension, gas pressure, and seed electron energy. The investigation has also been carried out for temporal evolution of the plasma formation inside the HC and electron-beam generation processes. Trends in size of the generated electron beams have been presented as a function of the potential distribution at the cathode aperture and applied gas pressure. As a result, on increasing gas pressure for the constant aperture size of HC, electron beam of higher current is generated while on other hand, by increasing cathode aperture for constant gas pressure, electron beam of lower current is achieved. The spatial characteristics of cathode fall region have also been presented for the different gas pressures and cathode aperture sizes. The simulation has been carried out for the variation of cathode apertures 1–10 mm as well as argon gas pressures 1–80 Pa. The gas pressure, cathode aperture and seed electron energy strongly influence the potential distribution in PCE source, and therefore, the peak electron currents as well as electron beams of different amplitudes and sizes can be realized.

Numerical simulation and analysis of electromagnetic-wave absorption of a plasma slab created by a direct-current discharge with gridded anode

In this paper, we present investigation of a direct-current discharge with a gridded anode from the point of view of using it as a means of creating plasma coating that could efficiently absorb incident electromagnetic (EM) waves. A single discharge cell consists of two parallel plates, one of which (anode) is gridded. Electrons emitted from the cathode surface are accelerated in the short interelectrode gap and are injected into the post-anode space, where they lose acquired energy on ionization and create plasma. Numerical simulations were used to investigate the discharge structure and obtain spatial distributions of plasma density in the post-anode space. The numerical model of the discharge was based on a simple hybrid approach which takes into account non-local ionization by fast electrons streaming from the cathode sheath. Specially formulated transparency boundary conditions allowed performing simulations in 1D. Simulations were carried out in air at pressures of 10 Torr and higher. Analysis of the discharge structure and discharge formation is presented. It is shown that using cathode materials with lower secondary emission coefficients can allow increasing the thickness of plasma slabs for the same discharge current, which can potentially enhance EM wave absorption. Spatial distributions of electron density obtained during simulations were used to calculate attenuation of an incident EM wave propagating perpendicularly to the plasma slab boundary. It is shown that plasma created by means of a DC discharge with a gridded anode can efficiently absorb EM waves in the low frequency range (6–40 GHz). Increasing gas pressure results in a broader range of wave frequencies (up to 500 GHz) where a considerable attenuation is observed.

Experimental investigation on the development characteristics of initial electrons in a gas pressurized closing switch under DC voltage

As one of the most important elements in linear transformer driver (LTD) based systems, the gas pressurized closing switches are required to operate with a very low prefire probability during the DC-charging process to ensure reliable operation and stable output of the whole pulsed power system. The most direct and effective way to control the prefire probability is to select a suitable working coefficient. The study of the development characteristics of the initially generated electrons is useful for optimizing the working coefficient and improving the prefire characteristic of the switches. In this paper an ultraviolet pulsed laser is used to generate initial electrons inside the gap volume. A current measuring system is used to measure the time-dependent current generated by the growth of the initial electrons so as to study the development characteristics of the electrons under different working coefficients. Experimental results show that the development characteristics of the initial electrons are influenced obviously by the working coefficient. With the increase of the working coefficient, the development degree of the electrons increases consequently. At the same times, there is a threshold of working coefficient which produces the effect of ionization on electrons. The range of the threshold has a slow growth but remains close to 65% with the gas pressure increase. When the working coefficient increases further, γ processes are starting to be generated inside the gap volume. In addition, an optimal working coefficient beneficial for improving the prefire characteristic is indicated and further tested.

Effect of deposition conditions on physical properties of sputtered silicon oxynitride thin films on float glass

AbstractSilicon oxynitride layers have been widely used to tailor the optical performance and mechanical resistance of multilayer optical coatings. The key issue for their reliable large‐scale application on a glass substrate is the effect of technological parameters on their performance. Here we report on the interrelationship between the conditions of the reactive magnetron deposition process and composition, structure and in turn optical and mechanical properties. The process gas composition and pressure level exhibit a strong influence on the film properties, whereas only moderate effect of magnetron power was observed. The O/(O+N) ratio of the films changes from 0.33 to 0.90 and the film density varies between 2.66 g.cm−3 and 1.85 g.cm−3 along with the increase in the process gas pressure from 0.58 Pa to 1.02 Pa. The refractive index shifts between 1.81 and 1.48 and the residual stress of the film varies in the range from compressive (−385 MPa) to tensile (26 MPa). Hardness and reduced modulus follow the same trend and decline with the increase in process pressure from 12.1 GPa to 2.1 GPa and from 120.3 to 31.6 GPa, respectively. The abrasive wear resistance decreases with the nitrogen content in the films.

Characteristics of CO2 adsorption on biochar derived from biomass pyrolysis in molten salt

AbstractThis work focused on the preparation and characterization of a promising biochar as a novel solid adsorbent towards CO2. The biochar was prepared by catalytic pyrolysis of waste roasted peanut shell in molten salt; it was characterized by means of SEM‐EDS, BET, FTIR, and TGA, followed by determining the adsorption characteristics, such as adsorption capacity, isosteric heat of adsorption, uptake rate, and selectivity via adsorption temperature and gas pressure. The results indicated that the as‐prepared biochar had a rich microporous structure with a peak pore size in the range of 0.69–1.3 nm, and exhibited a good performance of CO2 adsorption with a capacity of 3.8 mmol/g at 273 K and 100 kPa. Moreover, the adsorption selectivity of CO2 over N2, O2, CO, and CH4 was found to be above 12, 11, 8, and 7, respectively. In addition, an interesting phenomenon of an initial increase and then a decrease in the selectivity of CO2/N2 adsorption with increasing gas pressure was experimentally revealed.

Advanced PIC-MCC simulation for the investigation of step-ionization effect in intermediate-pressure capacitively coupled plasmas

A two-dimensional parallelized particle-in-cell simulation has been developed to simulate a capacitively coupled plasma reactor. The parallelization using graphics processing units is applied to resolve the heavy computational load. It is found that the step-ionization plays an important role in the intermediate gas pressure of a few Torr. Without the step-ionization, the average electron density decreases while the effective electron temperature increases with the increase of gas pressure at a fixed power. With the step-ionization, however, the average electron density increases while the effective electron temperature decreases with the increase of gas pressure. The cases with the step-ionization agree well with the tendency of experimental measurement. The electron energy distribution functions show that the population of electrons having intermediate energy from 4.2 to 12 eV is relaxed by the step-ionization. Also, it was observed that the power consumption by the electrons is increasing with the increase of gas pressure by the step-ionization process, while the power consumption by the ions decreases with the increase of gas pressure.

Characterizing gas bubble size distribution of laboratory foamed cement using X-ray micro-CT

The objectives of this study are to use micro-computed tomography (micro-CT) to elucidate the influencing factors of the microstructure of laboratory foamed cement and develop statistical models to quantify the gas bubble size distributions. During this study, foamed cement slurries were prepared using sealed foam cement mixers at various operating conditions. The influences of shear rate, mixing energy, base cement slurry composition, blender geometry, and applied pressure on the gas bubble size distribution of set foamed cement were investigated. Test results indicate foam quality and gas pressure are the primary determining factors of gas bubble size in foamed cement. The gas bubble size in foamed cement produced by a sealed foam mixer approximately follows the normal distribution. For samples generated with a standard multiblade blender at atmospheric condition, the mean and standard deviation of gas bubble size distribution increase exponentially with increasing foam quality. A dramatic decrease in gas bubble size is observed with increasing gas pressure. On the other hand, variations in mixing energy, shear rate, base slurry composition, and blender geometry have relatively little effect on the microstructure of the foamed cement within the range investigated.

PIC/MCC simulation of breakdown dynamics inside high power microwave output window

High power microwave (HPM) has important applications in controlled thermonuclear fusion heating, microwave high-gradient accelerator, high-power radar, directed-energy weapon, super jammer, impact radar, etc. The window breakdown of HPM has been extensively studied, and some research progress in this respect has been made. However, the researches on the transition of window breakdown from multipactor discharge to rf plasma are still not enough in-depth. Especially, the influences of microwave frequency and microwave amplitude during breakdown need further studying. This paper focuses on the process of dielectric multipactor and background argon ionization during the discharge breakdown near the HPM dielectric window/vacuum interface. A one-dimensional-spatial-distribution-and-three-dimensional-velocity-distribution (1D3V) electrostatic model with using particle-in-cell simulation is adopted in present work. The model includes secondary electron emission, electrostatic field induced by the remaining positive charge on the dielectric window, the motion of charged particles under electrostatic and microwave field, and the collision process between electron and background gas, and the corresponding PIC/MCC code is also developed. We examine the effects of gas pressure, microwave frequency and microwave amplitude on discharge breakdown. It is found that there exists only electron multipactor process during the discharge breakdown on dielectric window in vacuum. At low pressures (10 mTorr, 500 mTorr) and slightly high pressure (10 Torr), electron multipactor and gas ionization are coexistent. However, at an extremely high pressure (760 Torr), the gas ionization dominates the breakdown process. At the same time, the position of plasma density peak moves away from the dielectric window as the gas pressure increases, which is the consequence of the competition between secondary electron multiplication on the dielectric window and gas ionization in the body region. It can be seen that the advantage of gas ionization gradually increases as the gas pressure increases. In addition, it is also observed that at 500 mTorr, the moment of gas ionization moves forward first and then backward with the increase of the microwave amplitudes or the microwave frequency, especially when the increment of frequency is numerically twice that of the amplitude, gas ionization occurs earliest. This phenomenon is explained by the secondary electron emission model. Meanwhile, the results show that the position of plasma density peak from gas ionization gradually approaches to the dielectric window as the microwave amplitude increases. However, with continually increasing the microwave frequency, the plasma density peak moves away from the dielectric window first and then approaches to the dielectric window.

Study on the Dielectric Properties of C4F7N/N2 Mixture Under Highly Non-Uniform Electric Field

As an environment-friendly gas insulating medium, C4F7N has attracted great attention in recent years due to its excellent environmental protection and insulation performance. However, studies on the insulation performance of C4F7N/N2 gas mixture are not comprehensive at present. In this paper, the breakdown and partial discharge characteristics of the C4F7N/N2 gas mixture under highly non-uniform field were tested using the gas insulation performance test platform. The effects of gas pressure and mixing ratio on the negative partial discharge inception voltage (PDIV±), positive PDIV (PDIV−), and breakdown voltage of the gas mixture were analyzed. The engineering application potential of the C4F7N/N2 gas mixture was also discussed considering the limitation of liquefaction temperature. It is found that an increasing gas pressure or a mixing ratio can effectively improve the insulation performance of the C4F7N/N2 gas mixture. The breakdown properties of a gas mixture for a minimum operating temperature of x00B1;%at 0.3, 0.4, 0.5, and 0.6MPa can reach 63.4%, 54.6%, 49%, and 56.4% of pure SF6, respectively, and the PDIVx00B1; can reach 80.4%, 66.9%, 62.8%, and 68.8% of pure SF6, respectively. Relevant results not only reveal the influence of the mixing ratio and pressure on insulation performance of the C4F7N/N2 gas mixture, but also provide significant guidance for its engineering application.

Effect of Channel Geometry and Properties of a Vapor–Gas Mixture on Volume Condensation in a Flow through a Nozzle

A method of direct numerical solution of the kinetic equation for the droplet size distribution function was used for the numerical investigation of volume condensation in a supersonic vapor–gas flow. Distributions of temperature for the gas phase and droplets, degree of supersaturation, pressure, fraction of droplets by weight, the number of droplets per unit mass, and of the nucleation rate along the channel were determined. The influence of nozzle geometry, mixture composition, and temperature dependence of the mixture properties on the investigated process was evaluated. It has been found that the nozzle divergence angle determines the vapor–gas mixture expansion rate: an increase in the divergence angle enhances the temperature decrease rate and the supersaturation degree raise rate. With an increase or decrease in the partial pressure of incondensable gas, the droplet temperature approaches the gas phase temperature or the saturation temperature at the partial gas pressure, respectively. A considerable effect of the temperature dependence of the liquid surface tension and properties on gas phase parameters and the integral characteristics of condensation aerosol was revealed. However, the difference in results obtained with or without considering the temperature dependence of evaporation heat is negligible. The predictions are compared with experimental data of other investigations for two mixtures: a mixture of heavy water vapor with nitrogen (incondensable gas) or n-nonane vapor with nitrogen. The predictions agree quite well qualitatively and quantitatively with the experiment. The comparison of the predictions with numerical results from other publications obtained using the method of moments demonstrates the usefulness of the direct numerical solution method and the method of moments in a wide range of input data.

Influence of laser linewidth on light-induced drift velocity of atom

Light-induced drift has many applications in astrophysics, semiconductor physics, and isotope separation. Light-induced drift velocity is a key parameter to characterize the effect of light-induced drift. Laser linewidth exerts a great influence on light-induced drift velocity through influencing the velocity selectivity of atomic excitation, so it is an important factor that cannot be ignored in the study of light-induced drift. However, in existing theoretical studies, the influence of laser linewidth is seldom considered and the exciting light is always treated as monochromatic light. Furthermore, in a few theoretical studies about laser linewidth, the numerical model adopted does not include all the factors of light-induced drift, such as energy level degeneracy, hyperfine structure, and collision model, which will cause the error of calculation. In order to study the influence of laser linewidth on light-induced drift velocity, a four-level rate equation model is established to describe the atomic energy level transition in the process of light-induced drift. In the theoretical model, we introduce strong collision model to describe collisions between atoms and buffer gas. The influences of energy level degeneracy and hyperfine structure are also taken into account. Numerical method is used to calculate the four-level rate equation. According to the calculation results, the influence of laser linewidth on drift velocity of alkali metal atoms is analyzed. The results show that as the linewidth increases, the value of drift velocity first increases and then decreases. There is an optimal linewidth that maximizes the drift velocity. For the best light-induced drift effect, the laser should work under the optimal linewidth condition. When the laser linewidth fluctuates near the optimum linewidth, the laser linewidth should be set to be slightly wider than the optimal linewidth. This can reduce the influence of fluctuation and obtain a better drift effect. In addition, as the laser linewidth increases, the optimum power density corresponding to the maximum drift velocity decreases. When the laser linewidth is narrow, small fluctuations near the optimal laser power density will not have great influence on drift velocity. When the laser linewidth is wide, the power density should be set strictly. The optimum linewidth is related to laser power density, temperature and buffer gas pressure. As the laser power density increases, the value of optimum linewidth first increases rapidly and then decreases slowly. The value of optimal linewidth also increases linearly with the increase of temperature, and it decreases with the increase of buffer gas pressure. In conclusion, the laser linewidth does play a key role in the process of light-induced drift. The results of this study can provide a theoretical basis for future experiments, and be a good reference to the selection of exciting light.

Anti Surge Control of Centrifugal Compressor at PT. Pertamina EP Asset 2 Field Pendopo

Gas Compression Package (GCP) is a vital system in the distribution of gas. Pertamina EP, in this regard as a state-owned company managing oil and gas on the upstream, is in the project phase of purchasing and installing compressor facilities. GCPs are used to increase gas pressure hence it can be transferred from gas wells to industrial areas. Dynamic analysis becomes important to observe the performance of the compressor and to ensure safety during operation, in order to avoid surge phenomenon. Surge is an undesirable phenomenon in the form of a backward flow of gas. The surge may damage the internal components of a centrifugal compressor, hence needs to be prevented from happening. The anti-surge controller manipulates anti-surge valve to give actions in the recycle lines when a surge occurred. PID controller is used for controlling and stabilizing the system. PID tuning is determined using Auto Tuning Variation method. This method is based on the process frequency response from relay oscillations. After the dynamic model of the plant had been developed, some simulations were carried out with various scenarios. The scenarios were based on a variety of issues that may occur in a gas compression system. The applied scenarios were a decrease in flow rate on the input side, a pressure drop in gas wells, and changes in gas composition. The control configuration of gas compression system was designed and modified in order to compensate disturbances and avoid surge phenomenon.

Effects of Ethanol Blends on Performance of Spark Ignition Engine-A Review

Bio-fuels have grabbed attention of engine researchers ever since the oil crisis and increasing cost of crude oil in the 70’s. Ethanol has been the most widely researched alcoholic fuel in IC engine. The present paper discusses the effect of ethanol-gasoline blends as a bio-fuel in the conventional spark- ignition engine. The paper reviews and brings into light the properties of ethanol, production procedure and the effect of various ethanol blends on the performance of engine. The parameters focused for this study are the engine brake torque, engine volumetric efficiency and thermal efficiency with the effect on the exhaust emissions, which is one of the present prime concerns. The merits as well as the disadvantages of using ethanol as a fuel in Spark Ignition Engines also discussed. It has been seen that with the increasing proportion of ethanol in the blend, the overall thermal efficiency, heat release rate, engine volumetric efficiency and cylinder gas pressure increases. The requirement of compression ratio for occurrence of ignition also rises with the proportion of ethanol in the blend.

Ozone Production Influenced by Increasing Gas Pressure in Multichannel Dielectric Barrier Discharge for Positive and Negative Pulse Modes

ABSTRACTThis paper describes the influence of gas pressure on the conversion of O2 to O3 and the ozone production efficiency in a multichannel dielectric barrier discharge (DBD) reactor utilizing positive and negative pulses. Results show that conversion of O2 to O3 is continuously enhanced by the increase of gas pressure (0.1–0.24 MPa) while the rising speed of oxygen conversion with the increasing gas pressure at fixed specific input energy is reduced above 0.15 MPa. The maximum ozone generation efficiency is increased with increasing gas pressure (0–0.2 MPa) while positive pulse exhibits higher energy efficiency. The maximum ozone generation efficiency is suppressed with further increase of gas pressure (0.2–0.24 MPa) while no significant difference in ozone generation efficiency is observed for two unipolar pulse modes. Results also show that 0.2 MPa is the optimal working gas pressure to obtain the maximum ozone generation efficiency and increasing gas pressure would lead to remarkable increase of ozone generation efficiency for ozone production at high energy densities in multichannel DBD.

Application of Argon Plasma Jet for Methane Hydrate Decomposition by Radio Frequency Irradiation

In this study, decomposition of methane hydrate using argon plasma jet was investigated in the pressure range of 0.1MPa to 2.0MPa. The plasma was successfully generated under high-pressure condition, which is difficult to achieve when using radio frequency (RF) plasma in liquid method. Using emission spectrometer analysis, the excitation temperature is found to increase as the gas pressure increases, whereas, it decreases as the argon flow rate increases. During the process of plasma irradiation, the required basic reactions for methane hydrate decomposition i.e. methane hydrate dissociation (MHD), steam methane reforming (SMR), and methane cracking reaction (MCR) were not completely satisfied due to an insignificant amount of methane. The gas chromatograph analysis confirmed that the methane cracking reaction (MCR) was only occurred to generate hydrogen and the C (s) , due to the absence of C 2 H 2 and C 2 H 4 as the byproducts. In comparison with the other basic reactions of methane hydrate decomposition, steam methane reforming reaction became dominant in converting methane into hydrogen. Although the hydrogen production efficiency is less than that of radio frequency plasma in liquid, the reduction of CO 2 by the thermal decomposition of Teflon from CO making it possible to be considered as an advanced promising technique in the future .

RF magnetized ring-shaped plasma for target utilization obtained with circular magnet monopole arrangement

We proposed a new setup for generating outer ring-shaped radio frequency (RF) magnetized plasma near the chamber wall using monopole magnet setups. Three monopole magnet setups with (a) R = 5 mm, (b) R = 20 mm, and (c) R = 35 mm were investigated, where R is the gap between the magnets in consecutive circles. The distributions of the two dimensional magnetic flux lines, the absolute value of the horizontal magnetic flux density, and the discharge voltage were investigated for the proposed setups to produce outer ring-shaped plasma. A highly luminous ring-shaped plasma was observed for the setup (a), whereas multi-ring discharges were observed for the setups (b) and (c). It was found that the electron temperature decreases with increasing gas pressure for all cases. The electron temperatures were 2.42, 1.71, and 1.15 eV at an Ar gas pressure of 4 Pa for setups (a), (b), and (c), respectively. The plasma density was approximately the same for setups (b) and (c) at all gas pressures. The highest plasma densities were 6.26 × 1015, 1.06 × 1016, and 1.11 × 1016 m−3 at 5 Pa for setups (a), (b), and (c), respectively. It was found that the electron mean free path was 41.4, 63.17, and 84.66 mm at an Ar gas pressure of 5 Pa for setups (a), (b), and (c), respectively. The electron neutral collision frequency for setup (a) was higher than those for setups (b) and (c) at a constant RF power of 40 W and an axial distance of z = 13 mm from the target surface. The radial profile of the ion saturation current for setup (b) was more uniform than those for setups (a) and (c).

Effects of direct current discharge on the spatial distribution of cylindrical inductively-coupled plasma at different gas pressures

Stable operations of single direct current (DC) discharge, single radio frequency (RF) discharge and DC + RF hybrid discharge are achieved in a specially-designed DC enhanced inductively-coupled plasma (DCE-ICP) source. Their plasma characteristics, such as electron density, electron temperature and the electron density spatial distribution profiles are investigated and compared experimentally at different gas pressures. It is found that under the condition of single RF discharge, the electron density distribution profiles show a ‘convex’ shape and ‘saddle’ shape at gas pressures of 3 mTorr and 150 mTorr respectively. This result can be attributed to the transition of electron kinetics from nonlocal to local kinetics with an increase in gas pressure. Moreover, in the operation of DC + RF hybrid discharge at different gas pressures, the DC discharge has different effects on plasma uniformity. The plasma uniformity can be improved by modulating DC power at a high pressure of 150 mTorr where local electron kinetics is dominant, whereas plasma uniformity deteriorates at a low pressure of 3 mTorr where nonlocal electron kinetics prevails. This phenomenon, as analyzed, is due to the obvious nonlinear enhancement effect of electron density at the chamber center, and the inherent radial distribution difference in the electron density with single RF discharge at different gas pressures.

Two-dimensional particle-in-cell simulations of standing waves and wave-induced hysteresis in asymmetric capacitive discharges

Asymmetrically excited, high frequency cylindrical capacitive discharges are widely used for materials etching and thin film deposition. Two-dimensional (2D) electrostatic particle-in-cell (PIC) simulations show the existence of standing waves and wave-induced hysteresis of the plasma density, i.e. two different steady states for the same driving rf voltage amplitude, when the voltage is increased from a low value or decreased from a high value. The phenomenon is explored over a range of pressures (10–30 mTorr) and frequencies (60–80 MHz). Examined at 73 MHz, with increasing gas pressure, the hysteresis loop gradually shrinks and vanishes. To understand the hysteresis induced by z-symmetric and z-antisymmetric radial wave propagation modes, the PIC results are compared with a nonlinear transmission line model assuming uniform bulk plasma density, to determine the symmetric and antisymmetric voltage amplitudes. The model results are in good agreement with the PIC observations, showing central-low and central-high profiles of the antisymmetric mode voltage at low density and high density, respectively. The results are then used to determine the parameters of a lumped circuit model of the two modes, from which the hysteresis is induced by the density dependence of the symmetric and anti-symmetric wave mode absorbed electron powers. For the low density state, the discharge is sustained mainly by the symmetric mode excitation. At high density, the discharge is sustained by both symmetric and anti-symmetric modes, with the latter partly showing a spatial resonance. The results are also shown to be frequency dependent, with an onset of the hysteresis at about 66 MHz.