Voltage Spark Discharge Research Articles

Experimental Investigation and Mechanism Analysis on Rock Damage by High Voltage Spark Discharge in Water: Effect of Electrical Conductivity

High voltage spark discharge (HVSD) could generate strong pressure waves that can be combined with a rotary drill bit to improve the penetration rate in unconventional oil and gas drilling. However, there has been little investigation of the effect of electrical conductivity on rock damage and the fragmentation mechanism caused by HVSD. Therefore, we conducted experiments to destroy cement mortar, a rock-like material, in water with five conductivity levels, from 0.5 mS/cm to 20 mS/cm. We measured the discharge parameters, such as breakdown voltage, breakdown delay time, and electrical energy loss, and investigated the damage mechanism from stress waves propagation using X-ray computed tomography. Our study then analyzed the influence of conductivity on the surface damage of the sample by the pore size distribution and the cumulative pore area, as well as studied the dependence of internal damage on conductivity by through-transmission ultrasonic inspection technique. The results indicated that the increase in electrical conductivity decreased the breakdown voltage and breakdown delay time and increased the energy loss, which led to a reduction in the magnitude of the pressure wave and, ultimately, reduced the sample damage. It is worth mentioning that the relationship between the sample damage and electrical conductivity is non-linear, showing a two-stage pattern. The findings suggest that stress waves induced by the pressure waves play a significant role in sample damage where pores and two types of tensile cracks are the main failure features. Compressive stresses close horizontal cracks inside the sample and propagate vertical cracks, forming the tensile cracks-I. Tensile stresses generated at the sample–water interface due to the reflection of stress waves produce the tensile cracks-II. Our study is the first to investigate the relationship between rock damage and electrical conductivity, providing insights to guide the design of drilling tools based on HVSD.

Effect of CO2 dilution on forced ignition and development of CH4/air ignition kernels

Reliable ignition of reactant mixtures containing combustion products is important to the function of many practical combustion devices. Prior work has considered the influence of CO2 on ignition delay times and laminar flame speeds. This work examines the effects of dilution with CO2 on forced ignition and development of ignition kernels in CH4/air mixtures. Ignition kernels were initiated by a high voltage spark discharge in CH4/air/diluent mixtures with a bulk velocity of 5 m/s and with 0.7 ≤ Φ ≤ 0.8. Ignition probabilities and kernel growth rates were determined from radiation intensity measurements collected using a Telops MS-IR FAST infrared camera. Nitrogen was used as an inert in a subset of conditions to facilitate identifying changes in chemistry associated with dilution by CO2. Chemistry effects of diluents on ignition were isolated by examining undiluted, N2 diluted, and CO2 diluted flows with equivalent adiabatic flame temperatures. Dilution with CO2 reduced the probability of ignition relative to undiluted and N2 diluted conditions with the same adiabatic flame temperature. Ignition kernels diluted with CO2 also tended to be smaller than those initiated in both undiluted and N2 diluted mixtures, with reduced growth rates.

Effects of plate electrode materials on hydrogen production by pulsed discharge in ethanol solution

Hydrogen production from an ethanol solution by pulsed high voltage spark discharge was optimized by varying the material of plate electrode. It is the first time that metal work function has been used in hydrogen production by plasma reforming. With low work function metal plate electrode, both energy efficiency and hydrogen yield can be increased. The flow rate and the percentage concentration of hydrogen were achieved 1.3L/min and 75% respectively while discharging with zinc plate electrode for hydrogen production, which is much better than traditional stainless steel electrodes at the same conditions. The analysis of emission spectra was also accomplished in this work. All the intensity of existing spectral lines showed a decline with higher work function metal as plate electrode, and H may be the key to the process of hydrogen production. Additionally, electron temperature and density were also estimated. Both of which were increased with lower work function metal plate electrodes. The electron temperature and density can reach 24,000K, 7.5×1018cm−3 respectively.

Characteristics of hydrogen produced by pulsed discharge in ethanol solution

Hydrogen production from ethanol solution by pulsed high voltage spark discharge was investigated in this work. It is the first time that pulsed high voltage spark discharge in ethanol solution has been used for hydrogen production in the existing documents. A needle–plate reactor for on-board hydrogen production was also designed, which is small-scale and portable for automobiles. By optimizing the reaction conditions, the flow rate of hydrogen can reach 422mL/min at peak voltage=30kV, discharge frequency=30Hz, electrode distance=15mm, initial ethanol concentration=50%. It was also found that energy consumption of this method is about 1.69kWh/m3, which is better than most existing methods.

Nanorods of Metallic Bismuth and Antimony by the Impulse Plasma in Liquid

A new synthesis method of nanomaterials using the impulse plasma in liquid by the low voltage spark discharge is presented. The bismuth and antimony nanopowders were for the first synthesized by electric discharge method in liquid. The impulse plasma in liquid enables us to quench from plasma state, by which we can synthesize nanomaterials, nanorods, etc. In addition, the applied power is 100 times smaller than those of arc discharge. The possibility of obtaining nanorods of metallic bismuth and antimony by the impulse plasma in liquid between two electrodes from corresponding metal is presented.

Synthesis Method of Nanomaterials by Pulsed Plasma in Liquid

A new synthesis method of nanomaterials using pulsed plasma in liquid by the low voltage spark discharge is presented. The fullerene C60 and TiO nanopowder were for the first time synthesized by electric discharge method in liquid. The purity of C60 was >99%, which is much higher than those by the conventional arc plasma in inert gas methods (less than 80% C60 and 20% C70 and other fullerenes). Copper nanoparticles prepared by this method were smaller than those by arc method by a factor of >5. The pulsed plasma in liquid enables us to quench from plasma state, by which we can synthesize nanomaterials, metastable materials, etc. In addition, the applied power is 100 times smaller than those of arc discharge.

Hydrodynamic modelling of transient cavities in fluids generated by high voltage spark discharges

Application of a voltage pulse having a rise time of tens of nanoseconds to electrodes immersed in water results in streamer development and the formation of a highly conductive plasma channel between the electrodes. The electrical resistance of such channels decreases rapidly from a few ohms to a few tens of milliohms due to Joule heating resulting from the high current which flows through the plasma. The dynamics of the plasma resistance depend on the parameters of the discharge circuit and the medium in which the discharge takes place. The resistance of the channel reaches a minimum value approximately at the moment of the peak current for under-damped current oscillations. During the resistance collapse, the pressure inside the channel rises to several GPa, causing a rapid expansion of the channel which forms a cavity in the liquid resulting in a high power ultrasound pulse. The cavity expands to a maximum size which is dependent on the circuit driving the discharge and the properties of the plasma discharge channel. The cavity then collapses producing a second acoustic pulse. In this paper the dynamic resistance of the spark channel is described using a phenomenological model based on the plasma channel energy balance equation used by Braginskii. The model which links the hydrodynamic characteristics of the channel and the resulting cavity with the parameters of the electric driving circuit allows the development of the plasma channel and cavity to be predicted. The peak high-power ultrasound (HPU) pressures calculated using this approach are compared with the pressure values estimated by an analytical model which uses a constant value of the spark channel resistance derived from experimental data. Comparisons are also made with direct measurements of HPU output made using a Pinducer sensor. Although the model is based on a phenomenological description of the plasma channel dynamics and its resistance and requires the value of the spark constant, the results obtained using this approach provide a reasonable agreement with experimental measurements and could therefore be used for the estimation of HPU pulse characteristics in practical applications of spark discharges in water.

Raman scattering study of photoluminescent spark-processed porous InP

Raman scattering has been used to study porous InP (001) samples prepared by the application of high voltage spark discharges in air and argon atmospheres. For photoluminescent material, Raman scattering as well as Normaski microscopy of a transversally cut sample, (011) face, show the existence of two very distinct zones, that lie at different depths: a superficial luminescent region constituted mostly by In 2O 3 and InPO 4 oxides, and a second adjacent deeper zone formed by damaged InP. These results highlight the role that the oxidation plays in this material as source of the visible luminescence that the material emits when excited with UV radiation. The deepest region shows InP-like vibrational behavior with broad longitudinal optical (LO) and transversal optical (TO) phonon bands. A fit of the observed lineshapes of the TO and LO modes using the spatial correlation model provides an estimate of the size of the crystalline regions ( L∼30 Å).

Dependence on the atmosphere of preparation of the luminescence of spark processed porous GaAs

We report on the preparation of photoluminescent porous GaAs by the application of high voltage spark discharges in atmospheres of pure oxygen, pure nitrogen, and in mixed N2:O2 ratios of 4:1 and 1:4. The spark-processed porous (spp) samples were characterized by the observation of their visible photoluminescence (PL) when illuminated with UV monochromatic radiation. Some differences are observed in the initial PL spectra of the spp-GaAs according to the atmosphere of preparation under similar sparking and time conditions. The PL consists of two dominant bands, a yellow-green band between ∼2.2 and 2.6 eV and a blue-violet band, centered at 3.1 eV. Comparison with Raman and PL results from As2O3 and As2O5 indicates that the PL in the spp-GaAs is produced by the formation of these compounds by exposure to oxygen during the preparation. This is reinforced by energy dispersive x-ray spectroscopy measurements that indicate that the spp-GaAs is always oxidized, even when prepared under a nitrogen flow. The blue-UV emission at 3.1 eV suggests that we cannot rule out confinement as a contributing mechanism for this PL. Raman spectra indicate that for samples prepared in pure nitrogen, the resulting material consists of amorphous As and GaAs, and the cubic form of As2O3, arsenolite. PL and Raman indicate that there exists an increasing degree of amorphization of the resultant material with the introduction of nitrogen in the preparation atmosphere.

Processing of porous GaAs at low frequency sparking

We report on the preparation of photoluminescent porous GaAs by the application of high voltage spark discharges at low repetition rates (20 Hz) in air and in argon atmospheres. The spark-processed porous (spp) samples were characterized by the observation of their visible photoluminescence (PL) when illuminated with UV monochromatic radiation. In contrast to previous work on spp-GaAs at high sparking frequencies we find that the PL of samples prepared at low sparking frequency is highly reproducible from sample to sample. Important differences are observed in the initial PL spectra of the spp-GaAs according to the atmosphere of preparation under similar conditions. After prolonged air exposure both the spp-GaAs prepared in air and in argon show two similar broad peaks at energy positions 2.5 and 3.1 eV. Raman results indicate that the PL might not be associated to any size dependent mechanism. We present evidence that oxygen compounds formed by the exposure to air of the samples play a role in the PL excited in the spp-GaAs. This is reinforced by x-ray photoelectronic spectroscopy measurements that indicate that the spp-GaAs is covered by an oxidized film.

放電プラズマ·電磁界を応用した生物学·農学的研究 4.高電圧放電による雑草除去

In an attempt to replace chemicals for weed control, high voltage spark discharge has been applied. With the application of high voltage, discharge takes place, and current flows through the stem and root. Microscopic observation indicates that cells are damaged. The electrical resistance of the damage plant’s stems and roots decreased significantly. Several different types of apparatus were constructed, and field test results show the effectiveness of electrical discharge for weed control.

Luminescence of spark processed porous InP

We report on the preparation of photoluminescent porous InP by the application of high voltage spark discharges in air and in argon atmospheres. The spark-processed porous (spp) samples were characterized by the observation of their visible photoluminescence (PL) when illuminated with UV monochromatic radiation. Important differences are observed in the PL spectra of the spp-InP according to the atmosphere of preparation under similar conditions. Both the spp-InP prepared in air and in argon after air exposure show three similar broad peaks at energy positions 1.85, 2.43 and 3.1 eV. The PL in the spp-InP can be understood as originating from quantum confinement effects of the InP radioactive regions in crystallites of nanometric dimensions, as result of the analysis with the well known model in the literature of the recombination of an electron-hole pair interacting inside a nanometric size sphere. The average size of the radiative active InP core in these crystallites is of the order of 42 Å. We present evidence that oxygen plays a crucial role in the PL excited in the spp-InP. This is reinforced by XPS measurements that indicate that the spp-InP crystallites are covered by an extra oxidized shell of at least 30 Å in thickness.

Electron density profiles in single spark discharges

Quadratic Stark broadening of Ar(I) emission lines was used to measure the spatial distribution of the electron concentration in individual high voltage spark discharges. Corrections were made for ion broadening asymmetry and the instrument transfer function. Within the measurement uncertainty, the spatial distribution of the electron concentration did not vary from spark-to-spark. The uncertainty was limited by the imprecision in knowledge of the electron temperature, which was estimated using a LTE model.

Stark electron density mapping in the high voltage spark discharge

The diagnostic utility of spatial and temporal electron density mapping in the analytical high voltage spark discharge using a charge coupled array detector (CCD) is critically evaluated. H β Stark broadening was utilized for electron density measurements where applicable. Quadratic Stark broadening of Ar II was employed in the discharge core, where hydrogen emission is weak. Peak electron density of 10 17 cm −3 was found by both probes. Coupling between electron temperature and computed electron concentration is quantitatively evaluated. Neither independent measurement of electron temperature nor Abel inversion is required for adequate understanding of the plasma dynamics as revealed by spatial electron density distributions. There is weak indication of a shock wave, previously reported only in Schlieren photographs, detaching from the spark channel approximately 37 μs after discharge triggering.

Electrode surface modifications and material transport into a high voltage vacuum spark discharge

The material eroded in a high voltage spark discharge was studied by scanning electron microscopy. It was found that the material collected near the spark discharge consisted mainly of very small particles, although some larger droplets were observed. Studies of the sparked electrode surface and of the deposited material by secondary ion mass spectrometry showed differences in their composition as compared to that of the solid bulk. The differences may be responsible for the different relative sensitivities observed in spark source mass spectrometry.

Comparative Complexity of Emission Spectra from ICP, dc Arc, and Spark Excitation Sources

A comparison of atomic emission spectra excited in high voltage spark and dc arc discharges and in an inductively coupled plasma revealed that the most complex spectra were emitted by the high voltage spark. The dc arc and the inductively coupled plasma yielded spectra of approximately equivalent complexity. These observations are not in accord with the impressions conveyed in the literature.

Analysis of the Effluent from a High Voltage Spark Discharge with a Microwave-Induced Plasma

A high voltage spark discharge was used to sample ferrous alloy samples in a point-to-plane configuration. An enclosed Petry spark stand was used so that the effluent from the spark could be put into a microwave discharge sustained at 2450 MHz. The emission from the eroded metals in the microwave discharge was monitored with a PARC Vidicon detector. This method of detection provided spectral simplification and improved signal-to-background ratios up to six-fold over the emission observed from the spark discharge. The emission from the microwave was shown to be linear with concentration for many lines observed.

Microscopic Investigations of the Material Eroded by a High Voltage Spark Discharge

The material eroded by a high voltage spark discharge in a point-to-plane configuration was investigated. Glass microscope slides were used to collect material close to the spark discharge and material which had traveled away from the spark. The effluent was also directed into a microwave discharge, and material from the spark which had traveled through this discharge was collected and inspected. The collected material was visually inspected with a light microscope and with a Scanning Electron Microscope (SEM). Using these microscopes, it was found that the material collected near the spark discharge consisted mainly of very small particles, although some larger droplets were also observed. As the material was collected farther from the spark, larger particles were observed. These were apparently aggregations of the smaller particles observed close to the spark. When these large particles passed through the microwave plasma, some were melted.

Thomson Scattering as a Diagnostic of Atmospheric Pressure Discharges

Thomson scattering is reviewed as a diagnostic technique for the determination of electron energy distributions and concentrations in a plasma. Both theoretical and experimental aspects are considered, and scattering spectra which would be expected for high voltage spark discharges and inductively coupled plasmas at atmospheric pressure are computed. A nonlinear method bearing the same relationship to Thomson scattering as CARS does to linear Raman spectroscopy is mentioned, and its properties are compared to the more common linear Thomson technique. Related plasma diagnostics involving high power laser scattering are also discussed.

Electron concentration measurement in the high voltage spark using stark broadening of H β emission

The applicability of Stark broadening of hydrogen atomic emission as a probe of electron concentrations in the high voltage spark discharge is critically evaluated. Instrumental limitations, broadening theory applicability and limitations due to discharge fluctuations are discussed. Comparison to literature time resolved, spatially resolved data indicates those regimes in which accurate electron concentration measurements can not be made using this technique. Suggestions are made for improvements in instrumentation which would allow faster throughput and more insight into discharge processes using Stark broadening as a diagnostic.