Ions In Collisions Research Articles

Simulation of surface flashover probability distribution with gas-solid charge transport/multiplication coupling of epoxy resin

<p indent="0mm">The insulation failure of key equipment in power systems caused by gas-solid interface flashover of basin insulators is a key scientific problem that must be solved urgently. The time and space scales of gas electron multiplication and solid-surface charge evolution span multiple orders of magnitude, resulting in an unclear physical mechanism of the gas-solid coupling interface discharge of a basin insulator. A gas-solid coupling surface flashover model was established considering three junction point charge injection, solid surface charge transport, and gas collision ionization. The violent collision ionization in the gas caused by electrons emitted from the solid was taken as the surface flashover criterion. The key parameters characterizing surface charge transport and gas collision ionization were extracted based on gas-solid coupling surface flashover, and a model considering random variables was established to simulate the flashover voltage distribution of epoxy resin. The flashover voltage obeys the Weibull distribution when the charge transport parameters are Gaussian random variables, and the simulation results are consistent with the experimental results. By comparing the simulation and experimental results, it is found that the characteristic flashover voltage and distribution parameter are mainly controlled by the gas and solid media, respectively, and each charge transport random variable has a different effect on the distribution shape parameter. The Weibull distribution of flashover voltage consistent with the experimental results can be obtained by adjusting the standard deviation of charge transport random variables. This research explains the relationship between the Weibull distribution of flashover voltage and the charge transport and gas collision ionization parameters, providing an effective method to simulate the probability distribution characteristics of flashover voltage, which can also be used to evaluate the failure probability of the gas-solid interface under a certain voltage.

Thermal characteristic and spatial morphology between electrode and phase changing ice during de-icing process of dielectric barrier discharge and critical behavior of the surface charge density

Dielectric barrier discharge (DBD) plasma recently has been demonstrated as an effective in situ heating methodology in anti-ice and de-ice applications of the aircraft. However, the thermal stability of this special heat source during the interactions between the plasma and the ice block may pose serious concerns. In this paper, it is found that when the ice blocks of different characteristic locations covered the DBD device with alternative current (AC) power, two different kinds of interaction processes can be resulted: First, the concentration of the heat flow through the local enhancement of the streamers between the electrode and the ice, long-lasting for the glaze ice ended with the partial evaporation of the melted ice block with larger separations, or short-lived intermittent for the rime ice until the boundary is beyond the reach. Second, sparks, very likely to be generated between the electrode and the ice block when the separation and electric field meet the transition conditions, resulting in the fast evaporation of the melted ice block and the severe aging or on-sight burning of the devices, even at the operation voltage safe for the AC-DBD plasma without ice blocks. It is emphasized that the coexistence or the number of the ice blocks of different separations does not alter such behavior. Supported by the calculation of the streamer-to-spark transition criterion, and affected by ice phase change, it is argued that the de-icing AC-DBD plasma is a strong nonlinear heat source, resulting from the surface charge accumulation, melting of the ice blocks, and the enhancement of ionization collision frequency due to the mixing of the evaporated water molecules. A new critical condition, the critical surface charge density was proposed for the initiation of spark, ρsc: Densities higher than ρsc will lead to the catastrophic spark, while the lower will lead to the selective concentration of the heat flow for the faster de-icing process. Due to the random distribution nature of the electrode-to-ice separations during the real-world practice of an AC-DBD heat source for de-icing, the existence of ρsc may be under further scrutiny and considered as a crucial factor for the stable operation of the plasma heat sources for de-icing.

Plasma–neutral coupling allows electrostatic ion cyclotron waves to propagate below ion cyclotron frequency

The effect of ion–neutral collisions on the propagation characteristics of electrostatic ion cyclotron (EIC) waves in a partially ionized plasma is investigated. The dispersion relation of EIC waves is derived using a fluid model taking neutral dynamics into account. The propagation properties of EIC modes, including the damping factor, are examined for various ionization degrees and collision frequencies, which determine the momentum transferred from ions to neutral particles. It is found that the motion of neutral particles driven by plasma–neutral coupling leads to an increase in the effective ion mass, and consequently, EIC waves can propagate even below the ion cyclotron frequency. In a hot neutral gas, the gas-thermal mode can also propagate as well as the EIC mode. The possibility of observing in the laboratory and the Earth's ionosphere is discussed.

Ionization waves (striations) in a low-current plasma column revisited with kinetic and fluid models

A one-dimensional particle-in-cell Monte Carlo collisions method has been used to model the development and propagation of ionization waves in neon and argon positive columns. Low-current conditions are considered, that is, conditions where stepwise ionization or Coulomb collisions are negligible (linear ionization rate). This self-consistent model describes the development of self-excited moving striations, reproduces many of the well-known experimental characteristics (wavelength, spatial resonances, potential drop over one striation, and electron “bunching” effect) of the ionization waves called p, r, and s waves in the literature, and sheds light on their physical properties and on the mechanisms responsible for their existence. These are the first fully kinetic self-consistent simulations over a large range of conditions reproducing the development of p, r, and s ionization waves. Although the spatial resonances and the detailed properties of the striations in the nonlinear regime are of kinetic nature, the conditions of existence of the instability can be obtained and understood from a linear stability analysis of a three-moment set of quasi-neutral fluid equations where the electron transport coefficients are expressed as a function of electron temperature and are obtained from solutions of a 0D Boltzmann equation. An essential aspect of the instability leading to the development of these striations is the non-Maxwellian nature of the electron energy distribution function in the uniform electric field prior to the instability onset, resulting in an electron diffusion coefficient in space much larger than the energy diffusion coefficient.

Theoretical and experimental studies on the captured electron population probability of hydrogen-like O and N ions in collision with Al surface

The study of the interaction between highly charged ions and solid surfaces not only has great significance for basic scientific research such as atomic physics, astrophysics, and high energy density physics but also has promising application prospects in biomedicine, nanotechnology, surface analysis, and microelectronics. In this paper, the intermediate Rydberg states formed during highly charged O&lt;sup&gt;7+&lt;/sup&gt; and N&lt;sup&gt;6+&lt;/sup&gt; ions incident on Al surface are studied theoretically by using the two-state vector model. Both the probability of electron capture into different Rydberg states (&lt;i&gt;n&lt;sub&gt;A&lt;/sub&gt;&lt;/i&gt; = 2 − 7) and the most probable neutralization distances are given. The calculation shows that the larger principal quantum number &lt;i&gt;n&lt;sub&gt;A&lt;/sub&gt;&lt;/i&gt; is relevant to smaller probability. Therefore, the X-rays emitted by O&lt;sup&gt;7+&lt;/sup&gt; and N&lt;sup&gt;6+&lt;/sup&gt; ions incident on the Al surface come mainly from the de-excitation of the smaller &lt;i&gt;n&lt;sub&gt;A&lt;/sub&gt;&lt;/i&gt; to the ground state. In order to confirm the calculations, we measured the X-ray emission spectra of O&lt;sup&gt;7+&lt;/sup&gt; and N&lt;sup&gt;6+&lt;/sup&gt; ions in collisions with the Al surface in the energy range of 3-20 keV/q. The experiments were performed at an ECR ion source located in Institute of modern physics. We also calculated the transition energies (n&lt;i&gt;p&lt;/i&gt;-1&lt;i&gt;s&lt;/i&gt;) from different high Rydberg states to the ground state by using the FAC code. The center of the measured &lt;i&gt;K&lt;/i&gt; X-ray peak is close to the calculated transition energy from the principal quantum number n=2 to n=1, it is consistent with our results obtained by the two-state vector model as well. In addition, we found the experimental &lt;i&gt;K&lt;/i&gt; X-ray yield for O&lt;sup&gt;7+&lt;/sup&gt; ions incidence at lower energy collisions is almost the same with N&lt;sup&gt;6+&lt;/sup&gt; ions, but larger at higher energy collisions. When the ion incident kinetic energy is low, the X-ray emission is mainly owing to the decay of “above the surface”hollow atoms. Because of the small difference in the critical distances for the capture of electrons by O&lt;sup&gt;7+&lt;/sup&gt; and N&lt;sup&gt;6+&lt;/sup&gt; to form hollow atoms, the X-ray yields produced in both cases are almost the same at low energy collisions. In contrast, as increasing the incident energy, the ions have a long-range in the target, so the contribution from the decay of “above the surface” and “below the surface”hollow atoms need to be considered at the same time.

InGaAs/InP SAGCM avalanche photodiode with a heterojunction multiplication layer

In this paper, the InGaAs/InP separate absorption, grading, charge, and multiplication avalanche photodiode (SAGCM-APD) with a heterojunction multiplication layer and a hybrid absorption layer has been simulated. Due to the effect of the heterojunction multiplication layer on avalanche photodiode collision ionization, compared with the traditional structure, the break-down voltage and the punch-through voltage of the structure with a heterojunction multiplication layer was reduced by 2 V. By optimizing the thickness of the heterojunction multiplication layer, the electrical performance of the device has been enhanced. The avalanche gain was 193.37, which is much higher than the conventional structure (51.65). Besides, the thickness of InP in the heterojunction multiplication layer affects the capacitance characteristics of the device.

Influence of different O2/H2O ratios on He atmospheric pressure plasma jet impinging on a dielectric surface

A two-dimensional (2D) axisymmetric fluid model is built to investigate the effect of different O2 and H2O admixture on the plasma dynamics and the distribution of reactive species in He atmospheric pressure plasma jet (APPJ). The increase of O2:H2O ratio slows down both the intensity and the propagation speed of the ionization wave. Due to the decrease of both H2O ionization rate and H2O Penning ionization as well as the stronger electronegativity of O2, the increase of O2:H2O ratio results in a significant reduction of electron density in the APPJ, which restricts the occurrence of electron collision ionization reactions and inhibits the propagation of plasma. The excitation energy loss of O2 is not the reason for the weakening of the plasma ionization wave. The densities of O2 +, O− and O2 − increase with the rise of O2 admixture while H2O+ decreases due to the decrease of electron density and H2O concentration. OH− density is affected by both the increase of O− and the decrease of H2O so it shows a peak in the case of O2:H2O = 7:3. O is mainly produced by the excitation reactions and the electron recombination reaction (e + O2 + → 2O), which is directly related to the O2 concentration. OH is mainly produced by e + H2O → e + H + OH so the OH density decreases due to the decrease of electron density and H2O concentration with the increase of O2:H2O ratio. On the dielectric surface, when the propagation of the streamer extinguishes, O flux shows an upward trend while the OH flux decreases, and the propagation distance of O and OH decreases with the increase of O2:H2O ratio.

Electron spectra for twisted electron collisions

Ionization collisions have important consequences in many physical phenomena, and the mechanism that leads to ionization is not universal. Double differential cross sections (DDCSs) are often used to identify ionization mechanisms because they exhibit features that distinguish close collisions from grazing collisions. In the angular DDCS, a sharp peak indicates ionization through a close binary collision, while a broad angular distribution points to a grazing collision. In the DDCS energy spectrum, electrons ejected through a binary encounter collision result in a peak at an energy predicted from momentum conservation. These insights into ionization processes are well-established for plane wave projectiles. However, the recent development of sculpted particle wave packets reopens the question of how ionization occurs for these new particle wave forms. We present theoretical DDCSs for (e, 2e) ionization of atomic hydrogen for electron vortex projectiles. Our results predict that the ionization mechanism for vortex projectiles is similar to that of non-vortex projectiles, but that the projectile’s momentum uncertainty causes noticeable changes to the shape and magnitude of the vortex DDCSs. Specifically, there is a broadening and splitting of the angular DDCS peak for vortex projectiles, and an increase in the cross section for high energy ejected electrons.

Proton-helium collisions at intermediate energies: Singly differential ionization cross sections

We investigate the four-body proton-helium scattering problem using the two-center wave-packet convergent close-coupling approach. The approach uses a correlated two-electron wave function for the helium target. The continuum is discretized using wave-packet pseudostates. Calculations of ionization cross sections differential in the electron emission energy, in the emission angle, as well as in the scattered-projectile angle are performed in the intermediate energy region where coupling between various channels and electron-electron correlation effects are important. The results agree well with experimental data, where available. Moreover, our calculations reveal an interesting interplay between direct ionization and electron capture into the continuum. In particular, we demonstrate that the ionization cross section differential in the angle of the ejected electron is dominated by electron capture into the continuum for ejection into small angles, while ejection into large angles is purely due to direct ionization. It is concluded that the two-center wave-packet convergent close-coupling approach can provide accurate singly differential cross sections for ionization in proton-helium collisions. For comparison, a recently developed method that reduces the target to an effective single-electron system is also used. Somewhat unexpectedly, the results of the effective one-electron method exhibit a very good level of agreement with the full two-electron ones for all three singly differential cross sections. Therefore, we also conclude that, at least for the purpose of calculating the singly differential cross sections for single ionization of helium, an effective one-electron treatment of the target suffices.

Performance Simulation of a 5 kW hall Thruster

Hall thruster is a kind of plasma optics device, which is used mainly in space propulsion. To simulate the discharge process of plasma and the performance of a 5 kW hall thruster, a two-dimensional PIC-MCC model in the R-Z plane is built. In the model, the anomalous diffusion of the electrons including Bohm diffusion and near-wall conduction is modeled. The Bohm diffusion is modeled by using a Brownian motion instead of the Bohm collision method and the near-wall conduction is modeled by a secondary electron emission model. In addition to the elastic, excitation, and ionization collisions between electrons and neutral atoms, the Coulomb collisions are included. The plasma discharge process including the transient oscillation and steady state oscillation is well reproduced. First, the influence of the discharge voltage and magnetic field on the steady state oscillation is simulated. The oscillation amplitude increases as the discharge voltage gets larger at first, and then decreases. While the oscillation amplitude decreases as the magnetic field gets stronger at first, and then increases. Later, the influence of the discharge voltage and mass flow rate on the performance of the thruster is simulated. When the mass flow rate is constant, the total efficiency initially increases with the discharge voltage, reaches the maximum at 600 V, and then declined. When the discharge voltage is constant, the total efficiency increases as the mass flow rate rises from 10 to 15 mg/s. Finally, a comparison between simulated and experimental performance reveals that the largest deviation is within 15%, thereby indirectly validating the accuracy of the model.

Particle-in-cell simulations of low-pressure air plasma generated by pulsed x rays

When a photon pulse illuminates a cavity, photoelectrons are emitted from the surface of the cavity, thereby producing the system-generated electromagnetic pulse (SGEMP). Previously, simulations on the SGEMP in a low-pressure environment using a swarm model showed poor applicability to describe the dynamics of photoelectrons and plasmas. In this work, a 3D electromagnetic particle-in-cell with the Monte Carlo collisions model was constructed to investigate the effect of low-pressure air (0–500 mTorr) on the cavity SGEMP response. To model air plasma created by high-energy (keV) photoelectron flows, six kinds of particles (electrons, N2, O2, N2+, O2+, and O2−), as well as the elastic, ionization, attachment, and excitation collisions, were included in the model. The results showed that the peak electric field was in the order of 106 V/m. The peak electric field decreased with increasing air pressure because of dissipation of the space charge barrier (SCB) owing to the generation of secondary electron-ions. The dissipation of the SCB allowed more axially moving photoelectrons, so the fraction of transmitted current and the peak magnetic field increased. The energy conversion between the charged particles and the electromagnetic fields is discussed. Approximately 9% of the photoelectron energy was converted to electromagnetic energy for the SGEMP in vacuum. When the gas pressure increased, the electromagnetic energy conversion rate decreased. The energy relaxation of secondary electrons played an important role in the evolution of the plasma density. To validate the established simulation model, the calculated current was compared with those in the benchmark experiments.

Effective single-electron treatment of ion collisions with multielectron targets without using the independent-event model

In ion-atom collisions involving multielectron targets the role of inner-shell electrons becomes important with increasing collision energy. Usually, processes involving the outer-shell and inner-shell electrons are treated using an independent-event model. However, the independent-event model becomes impractical when the number of shells is more than two. We develop an effective single-electron approach to ion collisions with multielectron atomic targets that overcomes this difficulty by treating all the atomic electrons on an equal footing. The approach allows the calculation of single-ionization and single-electron transfer cross sections by including the excitation of any one of the inner- or outer-shell target electrons. Accordingly, the approach does not differentiate which one of the many target electrons is captured or ionized. This is a unique feature of the proposed approach. The ground-state wave function for the target atom obtained in the multiconfiguration Hartree-Fock approach is used to calculate the probability density for the whole atom. The latter is then averaged over the spatial coordinates and spin variables of all of the target electrons except for the position of one electron from the nucleus. The obtained single-electron probability density is then used to derive a pseudopotential describing the interaction of one electron with an effective field produced by the target nucleus and the other electrons. The procedure reduces the many-body Schr\"odinger equation governing the collision system effectively into a three-body one. The reduced Schr\"odinger equation is solved using the two-center wave-packet convergent close-coupling approach. As an illustrative example we calculate integrated cross sections for single-electron transfer and ionization in proton collisions with lithium, sodium, and potassium. The results obtained agree very well with available experimental data for electron transfer. However, we find significant discrepancies with experiment for single ionization of Na and K, which warrant further experimental and theoretical investigations.

Charge-exchange soft X-ray emission of highly charged ions with inclusion of multiple-electron capture

ABSTRACT Charge exchange has been recognized as a primary source of soft X-ray emission in many astrophysical outflow environments, including cometary and planetary exospheres impacted by the solar wind. Some models have been set up by using different data collections of charge-exchange cross-sections. However, multiple-electron transfer has not been included in these models. In this paper, we set up a charge-exchange model with the inclusion of double-electron capture (DEC), and make a detailed investigation of this process on X-ray emissions of highly charged carbon, nitrogen, oxygen, and neon ions by using available experimental cross-sections. We also study the effect of different n-selective cross-sections on soft X-ray emission by using available experimental n-distributions. This work reveals that DEC enhancement on line intensity is linearly proportional to the ratio of ion abundance in the solar wind. It is more obvious for soft X-rays from carbon ions (C4+) in collision with CO2, and the enhancement on line intensity can be up to 53 per cent with typical ion abundances [Advanced Composition Explorer (ACE)] in the solar wind. The synthetic spectra with parameters from the Ulysses mission for the solar wind reveal velocity dependence, target dependence, as well as the non-negligible contribution from the DEC.

Integrated total and state-selective cross sections for bare beryllium ion collisions with atomic hydrogen

We apply the two-centre wave-packet convergent close-coupling approach to calculate integrated total and state-selective cross sections for electron capture and ionisation in bare beryllium ion collisions with atomic hydrogen. This is done in the energy region between 1 keV/u to 1 MeV/u. Good agreement was achieved in comparison to previous theoretical works for the total electron capture cross section across all energies, however the total ionisation cross section shows disagreement with preceding calculations. We present accurate n and ℓ-resolved (n and ℓ are the principal and the angular momentum quantum numbers of the final state in electron capture processes, respectively) cross sections required for plasma impurity diagnostics where there have been discrepancies between different approaches for n > 5. Otherwise, excellent agreement between our results and previous calculations of state-selective cross sections for electron-capture is observed for a wide number of different n. The calculations have been performed on a fine energy mesh to identify oscillatory structures in the n- and nl-resolved capture cross sections in the low-energy region. Our results show pronounced oscillations in the cross sections for n ⩾ 5 below 20 keV/u.

Modelling of carbon nanotube film based temperature sensor: thermal emission and gas discharge

The carbon nanotube (CNT) film based ionized temperature sensor is sensitive to gas temperature, and shows good sensitivity compared with other temperature sensors. But it is still unclear about the effect of CNT film on thermal emission and gas discharge at different temperatures. In this article, we established a gas discharge model of the CNT film temperature sensor. Field assisted thermal emission is simulated at the tip of CNTs by analysing the field enhancement effect and effective work function. Ionization collision, excitation, recombination collision, Penning ionization and quenching of argon are considered in order to obtain the interaction of various particles at different temperature. The current density-temperature characteristic of the temperature sensor was obtained at 24–80 V. The increase of the working voltage is helpful to improve the output current and sensitivity of the temperature sensor. Response time of the sensor will not change in the temperature range of 293–373 K. However, the change of temperature will affect the current density, secondary electron emission and reaction rate. In addition, the sensor has different temperature sensitivity in argon and helium. The above simulation results are helpful to understand the role of CNT film and temperature sensitivity of the ionized sensor. It can also be used to study and improve the sensitivity of this type of sensor.

Simulation of DC surface flashover of epoxy composites in compressed nitrogen

To date, numerical simulation techniques for surface flashover are still under development. In this work, a DC surface flashover numerical simulation model is constructed based on a gas–solid coupling surface flashover theoretical model with a multilayered structure at the gas–solid interface. Considering the effects of solid, gas, and gas–solid interaction on surface flashover, bipolar charge transport in the solid surface layer, collision ionization in the gas phase layer, secondary electron emission, and gas adsorption in the gas surface layer are combined to calculate the surface flashover voltage. By initializing model parameters, surface charge transport dependent dc surface flashover voltages of epoxy composites in compressed nitrogen are calculated. The results indicate that the surface flashover voltage increases with surface deep trap level, deep trap density, shallow trap density, and carrier mobility; however, surface flashover voltage decreases with surface shallow trap level and surface charge density. To further investigate the effects of surface trap on surface flashover, a “U-shaped” curve is constructed to describe the relationship between surface flashover voltage and surface trap level by the simulation method which shows good agreement with experimental results. The simulation indicates surface flashover voltage of epoxy composites is influenced by surface deep and shallow traps in the solid surface layer—shallow traps mainly influence surface charge dissipation, while deep traps mainly influence electron emission on the solid surface. The value of Ptr/Pde is crucial for the dominating surface trap in surface flashover.

Power deposition behavior of high-density transient hydrogen plasma on tungsten in Magnum-PSI

The lifetime of plasma-facing components (PFCs) will have a strong influence on the efficiency and viability of future fusion power plants. However, the PFCs suffer from thermal stresses and physical sputtering induced by edge-localized modes (ELMs). ELMs in future fusion devices are expected to occur with a high plasma density compared to current day devices such that coupling of recycling neutrals and plasma ions will be strong. Because of the scale hierarchy of future fusion devices compared to the present ones, the influence of this coupling is difficult to predict. Here, we investigate the ELM-like hydrogen plasma induced heat loads on tungsten in the linear device Magnum-PSI, producing ∼1 ms plasma pulses with electron densities up to 3.5 × 1021 m−3. A combination of time-resolved Thomson scattering and coherent Thomson scattering was used to acquire plasma parameters in front of the target. Moreover, a fast infrared camera coupled to finite element thermal analyses allowed to determine the deposited heat loads on the target. We found a significant inconsistency between the plasma power calculated with a conventional collisionless sheath model and the absorbed power by the target. Moreover, plasma stagnation upstream and plasma cooling downstream were observed during the pulses. The observations are explained based on ionization and elastic collisions between the recycling neutrals and plasma ions. The results highlight the impact of plasma-neutral interaction on the power deposition behavior of ELM-like hydrogen plasma on tungsten.

Coherent-control mechanisms of Penning and associative ionization in cold He*(2 3S)−He*(2 3S) reactive scattering

We explore coherent control of Penning and associative ionization in cold collisions of metastable He$^*({2}^3\text{S})$ atoms via the quantum interference between different states of the He$_2^*$ collision complex. By tuning the preparation coefficients of the initial atomic spin states, we can benefit from the quantum interference between molecular channels to maximize or minimize the cross sections for Penning and associative ionization. In particular, we find that we can enhance the ionization ratio by 30% in the cold regime. This work is significant for the coherent control of chemical reactions in the cold and ultracold regime.

Optical characterization of miniature flexible micro-tube plasma (FμTP) ionization source: A dielectric guided discharge

Miniature DBD has been obtaining more and more attention. A recently developed miniature flexible micro-tube plasma (FμTP) has proved to be an excellent soft ionization source for mass spectrometry and ion mobility spectrometry. In this paper, the optical characterization of the novel FμTP ionization source was carried out, aiming to cast light on the plasma propagation mechanism in one discharge cycle. To provide a clean environment for investigating the plasma propagation, the plasma was totally sustained in a capillary tube. It was found that in one discharge cycle, three discharge developments, namely two dielectric guided discharges in both positive and negative half cycle, and a negative glow discharge in the negative half cycle. Compared with the conventional dielectric barrier discharge, there is no formation of a dissociative plasma, which improves the soft ionization capacity of the present source. In addition to the whole light emission from 200 nm to 1100 nm, the lines related to the reactive species including He 706 nm, N2+ 391 nm, and O 777 nm were investigated spatially and temporally. Due to the effect of the attached charges, the plasma of the negative half cycle propagates faster than that of the positive half cycle. It was also found that the plasma in the positive half cycle propagates more discretely than that of the negative one, while the negative one has a larger emission intensity due to the overlap of the signal in both temporal and spatial scales. Further on, the electric field distribution was proved to be in Gaussian shape along the diameter of the capillary, while the negative one has a flat shape. By changing the concentration of N2 in He, the electron collision and penning ionization for generating N2+ were experimentally proved and identified. This study identifies the typical discharge process of the FμTP in one discharge cycle, indicating the plasma is indeed a dielectric guided discharge.

Experimental observation of the linear gain of back-illuminated ultraviolet avalanche photodiodes using a GaN/AlN periodically stacked structure

Semiconductor-based avalanche photodiodes (APDs) have the advantages of lower power and simpler fabrication of arrays compared with photomultiplier tubes. It is critical for weak-light imaging that the APD is operated under back illumination and with high linear gain. However, linear gain is fairly low for conventional PIN APDs. This paper presents a back-illuminated APD, whose multiplication layer is designed to be a GaN/AlN periodically stacked structure. The GaN/AlN heterostructure is introduced to enhance the ratio of ionization coefficient between electrons and holes, which proves to be helpful for increasing the linear gain. Under back illumination, the prototype device demonstrates a record-high external quantum efficiency (reflecting linear gain) of ∼5 × 102 at the reverse bias of 66 V. The device response spectra were also studied to further confirm that only the photo-excited electrons can trigger the collision ionization in the multiplication layer.