Vacuum Discharge Research Articles

Measurement of plasma electron energy distribution functions by Langmuir probe using a modified AC modulation method

Abstract In the diagnosis of vacuum discharge plasma using the Langmuir probe, the electron energy distribution function(EEDF) is closely related to the second derivative of current to voltage (d2I/dV2) on the Langmuir probe. While d2I/dV2 is very sensitive to probe noise, its accuracy directly affects the measurement results of the electron energy distribution function. In this paper, a modified AC modulation method is proposed to improve the measurement accuracy of the electron energy distribution function. First, two small AC signals of different amplitudes are modulated on the measured DC scanning voltage of the Langmuir probe, and the probe current signal is measured. Then, the fundamental amplitudes of the two AC signals are corrected by spectral analysis and using the all-phase Fast Fourier Transformation - Fast Fourier Transformation(apFFT-FFT) amplitude correction algorithm. Then the accurate first-order derivative of the probe current to voltage (dI/dV) is obtained using the AC modulation derivative correction algorithm, and finally the electron energy distribution function is obtained by numerically differentiating dI/dV to obtain d2I/dV2. The method of this paper is compared with the traditional data processing method through experiments, and the results show the superiority and effectiveness of the method.

Methodology for Testing Selected Parameters of Low-Current Vacuum Electric Arc

This article presents the author’s methodology for testing selected parameters of a low-current vacuum arc, implemented using an innovative test stand based on a vacuum discharge chamber with a contact system mounted inside. In order to verify the validity of the adopted research methodology, as well as the correctness of the operation of the developed laboratory bench, measurements and calculations were made, among other things, of the energy and burning time of the vacuum arc, depending on selected factors, such as pressure and the delay time of the contact opening, calculated from the “passage through zero” of the sinusoid of the current flowing through the system. The tests were performed at 230 V and a current of 5 A for two pressure values: p1 = 1.00 × 105 Pa (atmospheric pressure) and p2 = 4.00 × 10−3 Pa (high vacuum environment). It was found that the vacuum insulation technique allows a significant reduction in the value of the arc energy and the burning time of the arc. It was also observed that in the case of a high vacuum environment, the ignition of the vacuum arc occurs after a time equal to about 3 ms from the “passage through zero” of the current flowing through the system. Below this value, the phenomenon did not occur. The results obtained provide an opportunity for the design and manufacturing of vacuum switchgear, where there is the prospect of reducing the negative effects associated with the arc burning process in the contact gap.

Numerical study on the effects of discharge parameters on vacuum arc plasma

Studying the characteristics of vacuum arc plasma within a wide range of discharge parameters is vital for its applications. In this study, specific parameters of vacuum discharge, such as cathode temperature ranging from 6000 to 8000K , emitted electron temperature spanning from 0.69 to 2.5eV , and arc burning voltage within the range of 16−30V , have been selected to analyze the effects of these parameters on the cathode erosion, cathode potential drop (CPD), and ion velocity. A validated one-dimensional spherical PIC–DSMC cathode spot model is employed to simulate the generation and expansion of vacuum arc plasma. The results indicate that increasing the cathode temperature and the emitted electron temperature enhance the cathode eroded mass. However, this mass diminishes as the arc voltage increases due to more Cu ions and Cu atoms returning to the cathode under high CPD and charge exchange collisions, respectively. The cathode current density remains nearly constant within the explored range of cathode temperature, but increases with shifting the emitted electron temperature and arc voltage. A higher cathode current density, coupled with the positive cathode plasma resistance, induces a higher CPD that significantly surpasses the cathode sheath voltage. Furthermore, a higher ion velocity is observed at increased arc voltage due to the higher cathode current density, which generates stronger electron friction for accelerating ions. These findings could be valuable in designing a suitable arc voltage for vacuum electric thrusters to enhance thrust.

Suppression of multipacting growth in a superconducting spoke resonator by the use of higher order modes

Abstract Multipacting is a vacuum discharge phenomenon that constrains the performance of accelerator structures and various microwave devices significantly. The different ways to mitigate multipacting discharge have been a central research goal in Accelerator Physics & Technologies for a prolonged period. In this work, both Monte-Carlo and particle-in-cell (PIC) simulations show that the multipactor avalanche can be mitigated, by the use of Higher Order Modes (HOMs). The intuition behind the approach of using HOMs to suppress multipacting has been explained first. Then, the multipacting suppression has been demonstrated in a simple 1-D metallic gap, when it is exposed to a second carrier frequency in addition to fundamental rf mode, with the help of a Monte-Carlo simulation code. Finally, a detailed PIC simulation has been carried out using CST Microwave studio, to investigate the effect of various HOMs on the multipactor growth in a superconducting spoke cavity. It is observed that a slight excitation of some particular modes can drastically diminish the multipactor growth. Simulation has been done first without any HOMs, to calculate the growth rate and also to spot the discharge location. Next, among various HOMs (more specifically, quadrupole modes), only those have been selected, whose magnetic field locations coincide with the multipacting prone zone of the cavity. In a simulation study, the presence of these preselected quadrupole modes has been found to exhibit a substantial suppression effect on the multipacting growth of the cavity.

On the Contribution of a Cluster Target to Generation of the DD Neutrons in a Nanosecond Vacuum Discharge

On the Contribution of a Cluster Target to Generation of the DD Neutrons in a Nanosecond Vacuum Discharge

Small-sized bright point-like source of picosecond soft x-ray pulses based on a high-voltage vacuum discharge

A small-sized soft x-ray (energy ≤ 1 keV) source based on a high-voltage vacuum discharge with a current of 20 kA and an ultrashort pulse duration of about 10 ps has been developed. It was shown that the radiation was emitted by a micropinch, which was formed in the discharge plasma and had a size of the order of a micrometer. In a single shot, an image of the test object was obtained with a resolution of several micrometers.

Thermionic Vacuum Discharges for Thin Film Depositions

The thermionic vacuum discharge method is very effective in that the films obtained using this technology are characterised by a very high degree of adhesion, density and purity because the deposition technique is carried out in high, very high or, if possible, in ultra-very high vacuum conditions with no gas present. When the substrate is placed in vacuum, no heat transfer particles are present, the substrate being heated only by the ion incident on the surface. This advantage recommends the TVD method for deposits on plastics or other thermally sensitive materials. Additionally, this slow heat transfer reduces energy loss, making the deposition method industrially competitive. The paper aims to present theoretical aspects of this type of discharge, compared to typical or more popular plasmas but also to present the achievements of this method and its utility in the thin films production, layers that have specific imposed properties. The practical depositions and applications presented are in the nuclear fusion-related material science and also for obtaining materials for granular structures, used as magneto-resistive coatings.

Vacuum discharge analysis of CNT field cathode using a computerized field projector

The mechanism of vacuum discharge of a multitip field cathode based on carbon nanotubes has been studied. The analysis was carried out using a multichannel field emission information collection system, a computerized field projector, and a program for recording and cyclic playback of the experimental results. The discharge under study occurred in the regime of a relatively stable emission current without an increase in the external voltage level. The time dependences of the local currents of all individual emission sites registered on the cathode surface were obtained. It is shown that the discharge consists of several successive phases, the key of which are an explosion in the region of one of the most active carbon nanotubes and a plasma torch above it. The effect of the discharge on neighboring nanotubes, which is associated with both the destruction of emission sites and their appearance, is also described.

Vacuum Arcs on Solar Array Surfaces

Arcs on the solar arrays appear to be the most plausible source for the electromagnetic radiation observed with the Arecibo radio telescope. In the laboratory, electrostatic discharges (arcs) on solar array coupons were studied in a simulated geosynchronous equatorial orbit environment. Arc current pulses were recorded with a high time resolution (up to 0.2 ns). Spectral properties of these pulses demonstrated no monotonic behavior but rather separated peaks within the frequency range of 100–1000 MHz. Electromagnetic radiation from these arcs was registered and recorded with a Yagi antenna 8 m distant from the arc site, and a high-speed oscilloscope. Analysis of the spectra resulted in the discovery of well-separated “lines.” Evaluation of arc current spectra and radio-frequency spectra revealed statistically significant patterns in the frequency domain. Measured radio fluxes and theoretical estimates of the power of electromagnetic radiation from arcs on spacecraft solar arrays strongly argue in favor of the arcing hypothesis for the source of this radiation. These results have enabled considerable progress to be made in understanding the physical nature of the generation of electromagnetic radiation by transient vacuum discharges.

Effects of the electromagnetic power coupling on vacuum breakdown

Vacuum breakdown is a key factor limiting the performance of applications that utilize vacuum for insulation. The surface electric field has been regarded in previous studies as the only parameter that determines the occurrence of a vacuum breakdown, for a given surface condition. However, several recent works indicated that the ultimate limit for a vacuum breakdown also depends strongly on the electromagnetic power available in the system to be delivered at the breakdown site. In this work, we study this dependence both experimentally using a pulsed DC system and by numerical simulations using a particle-in-cell (PIC) model. By varying the circuit impedance which includes resistance and capacitance, we controllably limit the power coupled to the vacuum discharge gap. The experimental results show that the breakdown voltage increases with increasing impedance, i.e. with increasing circuit resistance and decreasing capacitance; The PIC results showed that a minimum current is required besides the critical voltage during the initiation of the breakdown plasma, and the breakdown voltage is defined by the circuit impedance and the critical power loaded to the gap just before the breakdown, which is in agreement with the experimental results.

Oscillating Plasmas for Proton- Boron Fusion in Miniature Vacuum Discharge

Earlier, the experiments on the aneutronic proton-boron (pB) fusion in a miniature nanosecond vacuum discharge (NVD) with oscillatory plasma confinement and correspondent α particles yield were presented. In this work, we consider some specific features of oscillatory confinement as a relatively new type of plasma confinement for fusion. Particle-in-cell (PiC) simulations of pB fusion processes have shown that the plasma in NVD, and especially on the discharge axis, is in a state close to a quasineutral one, which is rather different from the conditions in the well-known scheme of periodically oscillating plasma spheres (POPSs) suggested earlier for fusion. Apparently, small-scale oscillations in NVD are a mechanism of resonant ion heating, unlike coherent compressions in the original POPS scheme. Nevertheless, the favorable scaling of the fusion power in NVD turns out to be close to the POPS fusion but differs significantly both in the compression ratio and in the values of the parameter of quasineutrality. In addition, unlike the POPS scheme, PiC simulation reveals that the distribution functions of protons and boron ions in NVD are non-Maxwellian. Therefore, we have an aneutronic pB synthesis in a nonequilibrium plasma remaining “nonignited” on the discharge axis.

Electrically induced light emission via nanoscale vacuum discharge in nanogap electrodes without luminescent molecules

Understanding the mechanisms of electrically induced light emission in nanoscale junctions is crucial in molecular-scale optoelectronics. In this study, we investigated the light emission from nanogap electrodes with an insulating and nonluminescent material (polystyrene). Electrically induced light emission was observed over a wide wavelength range of 400–600 nm. The threshold voltages for the light emission ranged from 3 to 15 V, depending on the device, indicating that the electric field induced at the nanogap, rather than the applied potential, is the driving force for the light emission. The intensity of the light emission varies exponentially with the current with an exponent higher than 1.6, and the light emission is driven by a thermally activated current. These characteristics are inconsistent with the mechanisms reported for solid-state nanogap devices. We conclude that light emission in the nanogap electrodes is attributed to nanoscale vacuum discharge with metal plasma. The light emission from this mechanism can conflict with the other mechanisms ever reported since it occurs at such low voltages. The results also suggest intrinsic instability in molecular-scale electronic devices based on nanogap electrodes because the vacuum discharge is accompanied by the plasticization of the electrode.

Guest Editorial Special Issue on Electrical Discharges in Vacuum

This Special Issue contains the expanded version of selected papers presented at the 29th International Symposium on Discharges and Electrical Insulation in Vacuum (ISDEIV). The Symposium was scheduled to take place in June 2020 in Padua, Italy, but due to the Covid-19 pandemic, it was held on September 26–30, 2021, in virtual mode. The Symposium was hosted by the Consorzio RFX, Padua, which is a research infrastructure whose mission is the research on fusion science, implemented through the participation to the European Fusion Program, through the development and exploitation of RFX—the largest Reversed Field Pinch Fusion machine—and through the development and exploitation of the Neutral Beam Test Facility (NBTF)—the prototype of the Neutral Beam Injectors for ITER. Members of the Consorzio RFX are the Italian National Research Council (CNR), the National Agency for Energy and Sustainable Development (ENEA), the National Institute on Nuclear Physics (INFN), and Padova University. The Conference was attended remotely by 167 participants from 17 different countries.

Interaction between microwave and dielectric surface discharge in vacuum and low-pressure gas

The interaction between high-power microwave and dielectric surface discharge in vacuum and low-pressure gas is investigated by using an electromagnetic particle-in-cell–Monte Carlo collision model. Maxwell equations are solved by the finite-difference time-domain method combined with the boundary condition between the total and scattered field. The simulation results show that the transmission power loss is small and mainly attributed to the absorption of surface discharge, when the secondary electron multipactor reaches a steady state in vacuum. The simulated value of transmission power loss in vacuum is in good agreement with the experimental data. At a low pressure, the multipactor is the main source of electrons in the initial stage of discharge. After the multipactor reaches a steady state, the ionization leads to a significant increase in the number density of plasma near the dielectric surface. The absorbed power of plasma is greater than the reflected power in the initial stage of discharge, but with the increase of time, the latter becomes larger and even close to the power of incident wave. As the pressure increases, the transmission power decays faster due to the increase of ionization rate. When the microwave field near the dielectric surface decays significantly at a low pressure, the steady state of multipactor disappears, and the peak of plasma number density is near the surface, but not closest to the surface.

Optical Investigation of Excited Species Distribution in Low-Current Vacuum Discharges

It is well-known that when high currents flow through a vacuum gap, anode spots are formed on the anode surface. However, anode flare phenomena are also observed in low-current vacuum discharges. The objective of this article is to experimentally observe the spatial distribution of excited Cu atoms and ions generated from a single cathode spot (CS) and to interpret the mechanism of the anode flare at low currents (40–80 A). Optical emission spectroscopy (OES) and laser-induced fluorescence (LIF) techniques have been used for investigating low-current vacuum arc discharge confined to axial magnetic fields (AMFs). The effects of AMFs and arc currents on the axial distribution of the radiation intensity were investigated for different copper ionization states (Cu I and Cu II). The highest luminance was found near the electrodes for both excited neutral copper atoms and ions, and only weak spectral lines of excited Ni atoms were detected in the near-anode region. With increasing the external AMF, a bright “spot” appeared near the anode surface. In the visible light range, excited atoms were the main contributor to anode flare in low-current vacuum discharge. As measured by LIF, the density of copper atoms near the anode increased from 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10^{17}$ </tex-math></inline-formula> to 14 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10^{17}\,\,\text{m}^{-3}$ </tex-math></inline-formula> when the applied magnetic fields increased from 31 to 74 mT. The anode flare contains far more excited atoms from the cathode material than the anode material. They are the results of cathodic plasma expansion to reach the anode after charge neutralization and reflection on the anode surface. For the single CS, the observed fact that a tiny number of atoms leave the anode surface and participate in the anode flare is more likely the result of a sputtering effect rather than thermal evaporation by cathode electron stream.

Emission of anomalously hard x-ray radiation by a target upon exposition with an electron beam, ejected by a low-energy vacuum discharge with laser ignition

The characteristics of x-ray radiation of a target that is bombarded by a beam of accelerated electrons ejected by a low-power vacuum discharge with laser ignition are studied. It is shown that the maximum radiation energy exceeds the applied potential difference across the discharge gap by almost an order of magnitude and is inversely proportional to the mass of the cathode material ablated by laser radiation. The energy spectrum of x-ray radiation reconstructed from the radiation attenuation curve by filters-absorbers of various thicknesses showed that in the maximum acceleration mode the average energy of the main part of the spectrum is more than four times higher than the voltage across the discharge gap at the moment of beam emission. Possible mechanisms providing anomalous acceleration of electrons in the discharge are considered.

On the Plasma Quasineutrality under Oscillatory Confinement Based on a Nanosecond Vacuum Discharge

On the Plasma Quasineutrality under Oscillatory Confinement Based on a Nanosecond Vacuum Discharge

On Scaling of DD Fusion Power in a Nanosecond Vacuum Discharge

On Scaling of DD Fusion Power in a Nanosecond Vacuum Discharge

МОДЕЛЮВАННЯ ЕЛЕКТРОЕРОЗІЙНИХ ПРОЦЕСІВ НА ГРАФІТОВИХ ЕЛЕКТРОДАХ ПРИ ФОРМУВАННІ НАНОСТРУКТУР У ПЛАЗМОВОМУ СЕРЕДОВИЩІ

The theoretical model of erosion processes in electrode spots during vacuum discharge during nanostructure formation is developed in the work. In this model the sources and drains of heat in electrode spots are considered in detail. Thus, the heat flux density generated by ions taking into account the emission cooling, reverse electric current and Nottingham heat is taken into account for the cathode, and both positive anode and negative potential declines are considered in describing the surface heat source for the anode. The distribution of current density in the element of the considered electrode volume was considered in sufficient detail for describing the volumetric heat source. Heat transfer from plasma radiation and convective heat transfer, and heat dissipation due to the shift of the plasma evaporation front and its radiation were considered for a more comprehensive description of the heat balance equation. The heat flux due to the movement of the stain was also taken into account. Thanks to the proposed model, the temperature fields near the spots and the rate of evaporation of the material during the life of the spot were determined, which allowed to determine the erosion coefficient for the electrode spot. According to the proposed model, calculations were performed for graphite electrodes. The dependence of the erosion coefficient on the lifetime of the spot and the current density at the electrodes is obtained. It is determined that at current density of more than 1011 A / м2 the probability of material emission in the form of clusters increases, which will prevent the appearance of nanostructures. Dependences of the erosion coefficient on the lifetime were obtained for both stationary and moving spots. The obtained dependences indicate the significant decrease in the erosion coefficient with increasing velocity of the spots. The determined theoretical values of current density coincide in order with the experimental values. The model can be used to determine the critical values of technological parameters in obtaining nanostructures for different electrode materials.

Study of a Heterophase Medium Based on a Relaxing Plasma of a Diaphragm Discharge in Vacuum

Study of a Heterophase Medium Based on a Relaxing Plasma of a Diaphragm Discharge in Vacuum