Fixed Discharge Power Research Articles

Experimental demonstration of a water electrolysis Hall Effect Thruster (WET-HET) operating with a hydrogen cathode

This study presents the first experimental demonstration of a Hall Effect Thruster operating with water electrolysis-derived constituents: oxygen for the anode (thruster) and hydrogen for the cathode, both from pressurised bottles. In contrast to prior research where the thruster was operated with oxygen to the anode and xenon/krypton to the cathode, this investigation uses an advanced set-up that safely allowed the combined use of pressurised oxygen and hydrogen. Firstly, a comparative assessment between the established plasma filament cathode (utilised in previous research) and a novel LaB6 cathode (referred to as Hydrocat), both krypton-fuelled, is conducted. Results show a different IV curve for these cathodes, but negligible disparities in thruster performance for fixed discharge power and anode mass flow. Thrust, anode specific impulse, thrust-to-power ratio, and both, anode and total efficiency of the system using oxygen (anode) and hydrogen (cathode) are then presented and compared against oxygen (anode) and krypton (cathode). The cathode successfully operated on hydrogen, with a reduction in thrust performance of under 10% compared to when krypton is used as the cathode propellant. This performance difference can be attributed to different factors, such as the reduced efficiency of the cathode running with hydrogen due to a higher ionisation potential and the cathode propellant’s contribution to thrust, more significant when using heavier propellants such as xenon/krypton compared to hydrogen. The impact of changing the hydrogen-to-oxygen ratio is also assessed and shown to have no impact on the thruster’s performance. The most optimal performance was obtained at the highest discharge power tested (Pd=1953±1 W), using a hydrogen:oxygen ratio of Φ = 1:25. This operating point corresponded to a thrust of 20.9±0.2 mN, anode specific impulse of 2130±20 s, thrust-to-power ratio of 10.7±0.5 mN/kW, anode thrust efficiency of 11.2±0.3 %, and total efficiency of 10.1±0.3 %.

Stability of two-dimensional complex plasma monolayers in asymmetric capacitively coupled radio-frequency discharges.

In this article, the stability of a complex plasma monolayer levitating in the sheath of the powered electrode of an asymmetric capacitively coupled radio-frequency argon discharge is studied. Compared to earlier studies, a better integration of the experimental results and theory is achieved by operating with actual experimental control parameters such as the gas pressure and the discharge power. It is shown that for a given microparticle monolayer at a fixed discharge power there exist two threshold pressures: (i) above a specific pressure p_{cryst}, the monolayer always crystallizes; (ii) below a specific pressure p_{MCI}, the crystalline monolayer undergoes the mode-coupling instability and the two-dimensional complex plasma crystal melts. In between p_{MCI} and p_{cryst}, the microparticle monolayer can be either in the fluid phase or the crystal phase: when increasing the pressure from below p_{MCI}, the monolayer remains in the fluid phase until it reaches p_{cryst} at which it recrystallizes; when decreasing the pressure from above p_{cryst}, the monolayer remains in the crystalline phase until it reaches p_{MCI} at which the mode-coupling instability is triggered and the crystal melts. A simple self-consistent sheath model is used to calculate the rf sheath profile, the microparticle charges, and the microparticle resonance frequency as a function of power and background argon pressure. Combined with calculation of the lattice modes the main trends of p_{MCI} as a function of power and background argon pressure are recovered. The threshold of the mode-coupling instability in the crystalline phase is dominated by the crossing of the longitudinal in-plane lattice mode and the out-of plane lattice mode induced by the change of the sheath profile. Ion wakes are shown to have a significant effect too.

Decolorization of indigo carmine solution by argon dielectric barrier discharge produced on liquid surface

Decolorization of indigo carmine solution was performed by excitation of argon dielectric barrier discharge (DBD) on a target solution and the effect of the gap distance on the water treatment characteristics was investigated for a gap distance between 1.0 and 2.5 mm. The argon DBD was excited by a 3 kHz sinusoidal voltage with a fixed discharge power between 1.72 and 1.76 W for each gap distance. Voltage and current waveforms and Lissajous figures were acquired to investigate the electrical characteristics of the DBD. Compared with fixing the discharge energy, a higher decolorization rate was obtained with shorter gap distance. Therefore, shortening the gap distance could lead to increased decolorization efficiency of the indigo carmine solution. The mechanism of higher efficiency with shorter gap distance is discussed on the basis of the electrical characteristics and possible plasma chemical reactions in the argon DBD on the solution surface.

Plasma Polymerization in a Nitrogen/Ethanol Dielectric Barrier Discharge: A Parameter Study

Plasma polymerization experiments are typically conducted by sustaining a non-thermal plasma in a gas flow containing monomer molecules. Recently, it was however observed that plasma polymerization can also occur in a nitrogen/2% ethanol vapor dielectric barrier discharge operated at medium pressure (5.0 kPa). To obtain a better understanding of this peculiar plasma polymerization behavior, a detailed parameter study is conducted in this work. The influence of plasma exposure time at fixed discharge power on the coating properties is investigated as well as the influence of discharge power applying a fixed energy density. Results reveal that at a discharge power ≥ 3.0 W, very hydrophilic (WCA values < 11°) coatings are deposited possessing N/C ratios in the range 32–38% and O/C ratios in the range 21–40%. XPS analysis shows that the coatings mainly consist out of amides, amines, imines and nitril/isonitrile groups. FTIR results confirm this conclusion and also show that the C≡N groups are mainly present as isonitriles. With increasing discharge power, a small decrease in O/C ratio is observed combined with an increasing amount of isonitriles. The investigated treatment times also do not affect the surface chemical composition nor the surface wettability suggesting excellent in-depth coating homogeneity. The plasma exposure time does however strongly affect the coating thickness: a linear increase in coating thickness with plasma exposure time is observed in this work. The obtained results thus prove that a nitrogen/ethanol vapor DBD operated at medium pressure is capable of depositing highly hydrophilic nitrogen- and oxygen-rich deposits, which can have applications in different research areas.

Effects of Arc Discharge Mode on the Efficiency of Biogas Reforming in an AC-Pulsed Arc Plasma System

The effects of arc discharge mode on biogas reforming performance in an AC-pulsed arc plasma system were investigated for different pulse rising times. The discharge mode changed from the re-strike arc mode to constant arc discharge mode on increasing the pulse rising time from 2 to 5 μs at a fixed discharge power. The length of the arc column periodically changed under re-strike mode, while the arc column was always longer under constant arc mode. The production yield of synthesis gas and the energy efficiency significantly improved when the arc discharge was operated under constant arc mode. The conversion rates of CH4 and CO2 increased from 15 to 80 and 12 to 27%, respectively. Under constant arc mode, the production yield of synthesis gas was 6.5 times higher than that under re-strike arc mode. In addition, the energy cost for synthesis gas production decreased remarkably from 52 to 9 kJ/mol. This is because the biogas reforming reactions, including thermal cracking, dry reforming, and partial oxidation, proceeded effectively in an area of the arc plasma, wherein the gas could react in the high-enthalpy region for a duration sufficient to ensure efficient reforming reactions. Therefore, the arc discharge mode significantly affected the chemical reactions and energy efficiency of biogas reforming.

Removal of low-concentration benzene in indoor air with plasma-MnO2 catalysis system

Non-thermal plasma (NTP) and combined plasma-MnO2 catalytic (CPMC) air cleaners were tested for removal of low-concentration benzene in air. Both air cleaners were made of stainless steel needle matrix plate and used DC corona discharger. The effects of discharge power and relative humidity (RH) on benzene removal efficiency were investigated in a closed chamber. The intermediate products produced in purification processes were analyzed using gas chromatography-mass spectrometer (GC-MS). The concentrations of discharge byproducts and CO2 selectivity produced in both processes were also compared. It was found that the benzene removal efficiency increased with discharge power in both systems; With the increase of RH in air, benzene removal efficiency firstly increased and then decreased in NTP while it gradually decreased in CPMC. For a fixed discharge power of 9 W and RH of 20% in CPMC, the conversion of benzene increased from 82.9% to 89.6%, the CO2 selectivity increased from 38% to 80%, the concentration of O3 decreased from 25.3 ppm to 1.3 ppm, and NO2 formation decreased from 234 ppm to 25.7 ppm, compared with NTP.

Influence of a Gas Injection Direction on the Reforming Performance in a Bipolar Pulse-Driven Plasma Reactor Operating at Atmospheric Pressure

Hydrogen production from methane reforming using the partial oxidation reaction is carried out using a bipolar pulse-driven plasma reactor operating at atmospheric pressure. The influence of axial and radial injections of CH4 and O2 on the reforming performance is investigated in conjunction with the spatial distribution of their spectral emissions by varying the O2/C ratio at a fixed discharge power. With increasing the O2/C ratio, the axial injection achieves the better conversion and efficiency. The measurement of spectral emissions reveals that the higher efficiency of axial injection is caused by its efficient use of oxidation reaction heat.

Role of Duty Ratio in Diamond Growth by Pulsed DC-Bias Enhanced Hot Filament Chemical Vapour Deposition

In this study, the role of the pulse duty ratio was investigatedduring the deposition of diamond films in a hot filament chemicalvapour deposition reactor with a pulsed-dc biased substratepositively relative to the hot filaments. The voltage-currentcharacteristics showed that the discharge current rose with theincrease of biasing voltage, which was modified by the duty ratio.Before deposition, two approaches were adopted for the pre-treatmentof the silicon substrates, respectively, and the substrates werescratched by diamond paste or seeded by diamond powders using theso-called `soft dry polished' technique. Diamond films weredeposited under a fixed discharge power by changing the duty ratios.In the first group with scratched substrates, it was found thatunder a high duty ratio the diamond grew slowly with quite poornucleation, while in the second case a high duty ratio induced ahigh deposition rate and good diamond quality. Reactive hydrocarbonspecies with high energy are essential for the initial nucleationprocess, which is more effectively achieved at a high biasingvoltage in the condition of a low duty ratio. In the film growthprocess, the large discharge current at a high duty ratio representsan increased concentration of electrons and reactive species aswell, promoting the growth of diamond films.

Driving frequency effect on the electron energy distribution function in capacitive discharge under constant discharge power condition

A modern trend of VHF driven plasma sources in semiconductor processing stimulates a lot of studies concerning the driving frequency effect on plasma parameters in a capacitive discharge. In spite of abundant studies, the validation and application of these results in industrial plasma processing are still questionable because these studies were performed under a fixed rf voltage condition or an assumption of Maxwellian electron energy distribution, while the fixed discharge power condition and non-Maxwellian distribution are typical in industrial plasma processing. To resolve this problem, the authors investigated the driving frequency effect on plasma parameters (electron density and temperature) under the fixed discharge power condition by measuring the electron energy distribution functions, which are the most important factor in chemical reactions during the plasma processing. A remarkable result was observed—as the driving frequency increases, the electron temperature increases and the electron density remains almost constant or decreases, which is opposite to what previous studies have suggested. This result can be tentatively explained as a result of an enhancement of collisional heating in the bulk plasma with driving frequency under the fixed discharge power condition.

Reduction of nitrogen oxide in N2 by NH3 using intermittent dielectric barrier discharge

NO in N2 gas was removed by injecting ammonia radicals, which were externally generated by flowing the NH3 gas diluted with Ar gas through dielectric barrier discharge with a one-cycle sinusoidal-wave power source. The NO reduction for changes in both the applied voltage and the repetition rate was well correlated with the discharge power, which was proportional to the total discharge time per unit time. There was an optimum NH3 concentration in the narrow concentration window for the energy efficiency of NO reduction at a fixed discharge power. The maximum energy efficiency was obtained at small values of the NH3 concentration and the discharge power. The low NH3 concentration effectively increased the energy efficiency by drastically decreasing the discharge-firing voltage.

In situ investigation by energy dispersive X-ray diffraction (EDXRD) of the growth of magnetron sputtered ITO films

Using the new analytical technique of in situ energy dispersive X-ray diffraction (EDXRD) during magnetron sputtering, the phase and structure formation in tin-doped indium oxide films (ITO) has been investigated. Due to the parallel data collection and the high X-ray flux of a synchrotron radiation source, the measurement time of an EDXRD spectrum was only approximately 30 s, allowing the collection of 10–20 spectra during a deposition run. From the fluorescence lines of indium and tin the film composition as well as the film thickness can be determined. Pattern, position and width of the diffraction lines are used to derive the phases, the mechanical stress and grain size of the growing films. By varying the oxygen flow at a fixed discharge power, the phase formation during the deposition of In 1.8Sn 0.2O x with 0≤ x≤3.5 films was investigated. At low oxygen flows the metallic indium phase dominates. With increasing oxygen flow the indium crystallites become much smaller accompanied by a steep increase of the In 2O 3 phase. At a certain oxygen flow the crystallite size of the In 2O 3 phase exhibits a maximum connected with the lowest compressive film strain. This favorable crystalline structure of the films leads to the lowest film resistivities. At very high oxygen flows the crystallites become smaller and show an increase in compressive strain.

Surface temperature and thermal balance of probes immersed in high density plasma

The surface temperatures of thermal probes immersed in a low pressure inductively coupled argon discharge have been measured over a wide range of gas pressure. At a fixed discharge power, the measured temperature increases with gas pressure and decreases with increasing probe diameter. At a discharge power of 100 W, the surface temperature of a 0.4 mm diameter probe in the centre of the discharge ranges from C at 0.3 mTorr to C at 1 Torr. This temperature is considerably higher than the gas temperature. An analysis of the energy balance on a probe surface shows that plasma particle bombardment is the dominant heating process while radiation is the dominant cooling process. Probe temperatures found from an energy balance are in reasonable agreement with those measured in experiment.

Magnetic field distribution measurements in a low-pressure inductive discharge

The magnitude and relative phase of the rf magnetic flux density has been measured as a function of radial position using a magnetic (B-dot) probe in an inductively coupled low pressure argon discharge driven at 13.56 MHz. The spatial variation of the rf electric field and the discharge current density were determined from the probe measurements and are compared at gas pressures of 3, 30, and 300 mTorr for a fixed discharge power of 50 W and at discharge powers of 21, 50, and 103 W for a fixed gas pressure of 30 mTorr. In the active zone near the induction coil where the rf field is strong, the electron rf drift velocity derived from the B-dot data and Langmuir probe measurement was found to be considerably less than the electron thermal velocity even at the lowest argon pressure. For an axially symmetric inductive discharge, as is found in many applications, a two-dimensional magnetic probe measurement with space and phase resolution is necessary to correctly infer the rf electric field and the discharge current density distribution.

Critical evaluation of conditions of plasma polymerization

AbstractUnder conditions of plasma polymerization, we are dealing with the “reactive” or “self‐exhausting” rather than the “nonreactive” or “non‐self‐exhausting” gas phase (plasma). Therefore, many parameters that define the gas phase, such as system pressure and monomer flow rate, which are measured in the nonplasma state (before glow discharge is initiated), do not apply to a steady state of plasma, the conditions under which most of the studies on plasma polymerization are carried out. Consequently, information based on: (1) the polymer deposition rate measured at a fixed flow rate and discharge power, (2) the dependence of deposition rate on flow rate at fixed discharge power, or (3) the dependence of deposition rate on discharge power at fixed flow rate, does not provide meaningful data that can be used to compare the characteristic nature of various organic compounds in plasma polymerization. The significance and true meaning of experimental parameters applicable to conditions of plasma polymerization are discussed. The most important feature is that plasma polymerizations of various organic compounds should be compared at comparable levels of composite discharge power parameter W/FM, where W is discharge power, F is the monomer flow rate (given in moles), and M is the molecular weight of a monomer.