Neutral Collisions Research Articles

The Omnitrap Platform: A Versatile Segmented Linear Ion Trap for Multidimensional Multiple-Stage Tandem Mass Spectrometry.

Multidimensional multiple-stage tandem processing of ions is demonstrated successfully in a novel segmented linear ion trap. The enhanced performance is enabled by incorporating the entire range of ion activation methods into a single platform in a highly dynamic fashion. The ion activation network comprises external injection of reagent ions, radical neutral species, photons, electrons, and collisions with neutrals. Axial segmentation of the two-dimensional trapping field provides access to a unique functionality landscape through a system of purpose-designed regions for processing ions with maximum flexibility. Design aspects of the segmented linear ion trap, termed the Omnitrap platform, are highlighted, and motion of ions trapped by rectangular waveforms is investigated experimentally by mapping the stability diagram, tracing secular frequencies, and exploring different isolation techniques. All fragmentation methods incorporated in the Omnitrap platform involving radical chemistry are shown to provide complete sequence coverage for partially unfolded ubiquitin. Three-stage (MS3) tandem mass spectrometry experiments combining collision-induced dissociation of radical ions produced by electron meta-ionization and further involving two intermediate steps of ion isolation and accumulation are performed with high efficiency, producing information rich spectra with signal-to-noise levels comparable to those obtained in a two-stage (MS2) experiment. The advanced capabilities of the Omnitrap platform to provide in-depth top-down MSn characterization of proteins are portrayed. Performance is further enhanced by connecting the Omnitrap platform to an Orbitrap mass analyzer, while successful integration with time-of-flight analyzers has already been demonstrated.

Hybrid simulation of Alfvén wave parametric decay instability in a laboratory relevant plasma

Large-amplitude Alfvén waves are subject to parametric decays, which can have important consequences in space, astrophysical, and fusion plasmas. Although the Alfvén wave parametric decay instability (PDI) was predicted decades ago, observational evidence is limited, stimulating considerable interest in laboratory demonstration of the instability and associated numerical modeling. Here, we report an important step toward direct hybrid simulation of the Alfvén wave PDI in a laboratory plasma, using antenna-like wave injection of a circularly polarized wave and realistic wave-plasma parameters. Considering collisionless damping, we identify the threshold Alfvén wave amplitudes and frequencies required for triggering the instability. These threshold behaviors are corroborated by simple theoretical analysis, considering the balance between PDI growth and ion Landau damping and PDI development in a bounded plasma. Other effects not included in the present model such as finite transverse wave scale and ion–neutral collisions are briefly discussed. These hybrid simulations promise to be a useful tool for investigating laboratory Alfvén wave dynamics and may provide guidance for future laboratory demonstration of the PDI process.

Sheath expansion effect of double flush mounted probe in weakly ionized plasma

Sheath expansion is a distinctive feature of the double flush mounted probe because of the embedded configuration. Previously, the sheath expansion effect was usually neglected in weakly ionized plasma dominated by collisions between charged particles and neutrals. In this work, we investigated the sheath expansion effect of the double flush mounted probe in weakly ionized plasma. Results indicate that measurements using the double flush mounted probe were also influenced to a certain extent by the sheath expansion effect in weakly ionized plasma. To eliminate the influence, an empirical analytical formula has been presented to eliminate the influence of sheath expansion. In addition, a fitting curve is given based on experimental data, which indicates that sheath expansion should be considered in processing the measured data when the plasma pressure is lower than 200 Pa. In summary, this work indicates that the ion–neutral collision is a crucial factor that affects sheath expansion in addition to the radius parameter and probes' bias, which can be extended to double flush mounted probe diagnostics in collisional plasma such as the reentry plasma sheath and high-powered plasma thruster.

Floating potential method using a thermionic emissive probe including an ionizing and collisional presheath

This article studies the interaction of an argon plasma with an emissive probe considering the effect of both ionization and ion–neutral collisions. The floating potential is determined from the I–V characteristic curves as a function of the probe temperature, neutral gas pressure and plasma electron temperature. This potential increases with increasing probe temperature until reaching the plasma potential, exceeding the saturation value previously indicated by other authors. Finally, a relationship between the plasma electron temperature and the probe temperature at which the floating potential reaches the plasma potential is shown, demonstrating that these probes can be used for diagnosis of the plasma electron temperature.

Numerical simulations of a two-fluid jet at a magnetic null point in a solar arcade

ABSTRACT We study the formation and evolution of jets in the solar atmosphere using numerical simulations of partially ionized plasma. The two-fluid magnetohydrodynamic equations with ion+electron and neutral hydrogen components are used in two-dimensional Cartesian geometry. Numerical simulations show that a localized non-linear Gaussian pulse of ion and neutral pressures initially launched from the magnetic null point of a potential arcade located below the transition region quickly develops into a shock due to the decrease of density with height. The shock propagates upwards into the solar corona and lifts the cold and dense chromospheric plasma behind in the form of a collimated jet with an inverted-Y shape. The inverted-Y shape of jets is connected with the topology of a magnetic null point. The pulse also excites a non-linear wake in the chromosphere, which leads to quasi-periodic secondary shocks. The secondary shocks lift the chromospheric plasma upwards and create quasi-periodic jets in the lower corona. Ion and neutral fluids show generally similar behaviour, but their relative velocity is higher near the upper part of the jets, which leads to enhanced temperature or heating due to ion–neutral collisions. Simulations of jets with an inverted-Y shape and their heating may explain the properties of some jets observed in the solar atmosphere.

How sheath properties change with gas pressure: modeling and simulation

Particle-in-cell simulations are used to study how neutral pressure influences plasma properties at the sheath edge. The high rate of ion–neutral collisions at pressures above several mTorr are found to cause a decrease in the ion velocity at the sheath edge (collisional Bohm criterion), a decrease in the edge-to-center density ratio (h l factor), and an increase in the sheath width and sheath potential drop. A comparison with existing analytic models generally indicates favorable agreement, but with some distinctions. One is that models for the h l factor need to be made consistent with the collisional Bohm criterion. With this and similar corrections, a comprehensive fluid-based model of the plasma boundary transition is constructed that compares well with the simulation results.

Self-consistent formation and steady-state characterization of trapped high-energy electron clouds in the presence of a neutral gas background

This study considers the self-consistent formation and dynamics of electron clouds interacting with a background neutral gas through elastic and inelastic (ionization) collisions in coaxial geometries similar to gyrotron electron guns. These clouds remain axially trapped as the result of crossed magnetic field lines and electric equipotential lines creating potential wells similar to those used in Penning traps. Contrary to standard Penning traps, in this study, we consider a strong externally applied radial electric field which is of the same order as that of the space-charge field. In particular, the combination of coaxial geometry, strong radial electric fields, and electron collisions with the residual neutral gas (RNG) present in the chamber induce non-negligible radial particle transport and ionization. In this paper, the dynamics of the cloud density and currents resulting from electron–neutral collisions are studied using a 2D3V particle-in-cell code. Simulation results and parametric scans are hereby presented. Finally, a fluid model is derived to explain and predict the cloud peak density and peak radial current depending on the externally applied electric and magnetic fields, and on the RNG pressure.

Electrode Cooling and Microwave Absorption by Phase Change Fluid Discharge Plasma

It is hard to realize the high microwave absorption property and the low operating temperature in a gas discharge plasma simultaneously, due to the electron number density, and electron neutral collision frequency physically needed for the former is a typical severe heating scenario for the latter. In this article, the phase change process is proposed to be introduced into the controlled area of the discharge plasma system to increase collision frequency and decrease the discharging electrode temperature at the same time. An ac high-voltage power-driven quasi-periodic array of the phase change fluid discharge (PCFD) device is investigated. The most apparent tendency of the PCFD plasma is the improved diffusivity and uniformity. At the same time, the temperature of the electrode and its vicinity, where the phase change material is reserved, is lower than the ambient temperature. Compared with the controlled discharge state of the low gaseous pressure dielectric barrier discharge (L-DBD) plasma without phase change process, the absorption attenuation in the <inline-formula> <tex-math notation="LaTeX">$X$ </tex-math></inline-formula>-band is enhanced in all tested polarization angles, which driving strategy could control. Analyses show that the increased microwave absorption, frequency shift, and bandwidth broadening mainly result from the phase change process and its influences on the plasma discharge.

A study of neutral collisions and viscous force on the formation of astronomical objects including with QMHD fluid model

A theoretical study has been undertaken on the influence of neutral collision frequency and viscous force on Jeans instability in radiative magnetized quantum plasma. A study was conducted using linearized quantum magneto-hydrodynamic equations. A general dispersion relation is acquired employing the normal mode analysis technique and the dispersion relation is discussed for longitudinal and transverse propagation. The altered general dispersion relation is derived and discussed to acquire Jean's state of instability by containing the magnetic field and quantum correction. The numerical findings are provided to demonstrate the impact of these changes on the rate of expansion of Jeans instability. The figure captions include too many details in this paper which represent that all considered parameters have a stabilizing or destabilizing effect on the growth rate of instability. The present study is suitable to understand the phenomena of acoustic waves in astronomical space plasma science.

Development and Application of SONIC Divertor Simulation Code to Power Exhaust Design of Japanese DEMO Divertor

An integrated divertor simulation code, SONIC, has been developed in order to predict a self-consistent transport solution of the plasma, neutral and impurities in the scrape-off layer (SOL) and divertor. SONIC code has contributed to determining the divertor design and power handling scenarios for the Japanese (JA) fusion demonstration (DEMO) reactor. Radiative cooling scenario of Ar impurity seeding and the divertor performance have been demonstrated to evaluate the power exhaust scenarios with Psep = 230–290 MW. The simulation identified the decay length of the total parallel heat flux profile as being broader than the electron one, because of the ion convective transport from the outer divertor to the upstream SOL, produced by the plasma flow reversal. The flow reversal also reduced the impurity retention in the outer divertor, which may produce the partial detachment. Divertor operation margin of key power exhaust parameters to satisfy the peak qtarget ≤ 10 MWm−2 was determined in the low nesep of 2 − 3 × 1019 m−3 under severe conditions such as reducing radiation loss fraction, i.e., f*raddiv = (Pradsol + Praddiv)/Psep and diffusion coefficients (χ and D). The divertor geometry and reference parameters (f*raddiv ~ 0.8, χ = 1 m2s−1, D = 0.3 m2s−1) were consistent with the low nesep operation of the JA DEMO concepts. For either severe assumption of f*raddiv ~ 0.7 or χ and D to their half values, higher nesep operation was required. In addition, recent investigations of physics models (temperature-gradient force on impurity, photon transport, neutral–neutral collision) under the DEMO relevant SOL and divertor condition are presented.

Emission from HCN and CH3OH in comets

Aims. The aim of this work is to characterise HCN and CH3OH emission from recent comets. Methods. We used the Onsala 20-m telescope to search for millimetre transitions of HCN towards a sample of 11 recent and mostly bright comets in the period from December 2016 to November 2019. Also, CH3OH was searched for in two comets. The HCN sample includes the interstellar comet 2I/Borisov. For the short-period comet 46P/Wirtanen, we were able to monitor the variation of HCN emission over a time-span of about one month. We performed radiative transfer modelling for the observed molecular emission by also including time-dependent effects due to the outgassing of molecules. Results. HCN was detected in six comets. Two of these are short-period comets and four are long-period. Six methanol transitions were detected in 46P/Wirtanen, enabling us to determine the gas kinetic temperature. From the observations, we determined the molecular production rates using time-dependent radiative transfer modelling. For five comets, we were able to determine that the HCN mixing ratios lie near 0.1% using contemporary water production rates, ${Q_{{{\rm{H}}_2}{\rm{O}}}}$, taken from other studies. This HCN mixing ratio was also found to be typical in our monitoring observations of 46P/Wirtanen but here we notice deviations of up to 0.2% on a daily timescale which could indicate short-time changes in outgassing activity. From our radiative transfer modelling of cometary comae, we find that time-dependent effects on the HCN level populations are of the order of 5–15% when ${Q_{{{\rm{H}}_2}{\rm{O}}}}$ is around 2 × 1028 mol s−1. The effects may be stronger for comets with lower ${Q_{{{\rm{H}}_2}{\rm{O}}}}$. The exact details of the time-dependent effects depend on the amount of neutral and electron collisions, radiative pumping, and molecular parameters such as the spontaneous rate coefficient.

Impact of azimuthal forcing on the Brillouin limit in a collisional two-species Ohkawa filter

This paper investigates the physics of plasma separation in a two species rotating collisional Ohkawa filter, when the source of rotation is an orbital angular momentum carrying wave. The electric field is treated self-consistently with ion and electron radial motion. The injection of angular momentum causes radial currents leading to charge penetration and electric field build up. The electric field varies until an equilibrium with the friction forces is reached. Both collisions with neutrals and Coulomb collisions are considered. In the case where the electric field is driven by the resonant wave, there is no collisional breakdown of the Brillouin limit [Rax et al., Phys. Plasmas 22, 092101 (2015)]; on the contrary, the maximum achievable electric field decreases when the collision frequency is increased. When two species are present, one that undergoes the wave forcing while the second does not interact with the wave, we find the following: the first species is confined, while the second species can be expelled or confined depending on the charge to mass ratio and the collisionalities. Assuming equal charge numbers, if the second species is the heavy one, it is always expelled, which is a standard result. When the second species is the light one, it can also be expelled in the common case where neutral collisions dominate over Coulomb collisions, which constitutes a new result.

Electron–neutral collisions effects on Langmuir probe in the lower E-region ionosphere

We present the first set of particle-in-cell simulations including Monte Carlo collisions between charged and neutral particles used to simulate a cylindrical Langmuir probe in the electron saturation regime with a collisional electron sheath. We use a setup focused on the E-region ionosphere; however, the results of these simulations are analyzed in a general sense using dimensionless values. We find that the electron currents get enhanced as the collision frequency for electrons increases and the values of λe/λD→1, where λe is the electron mean free path and λD is the electron Debye length. In addition, we apply the simulation results to a sounding rocket experiment and show how we can correct the currents for the Investigation of Cusp Irregularities-4 sounding rocket due to collisions while it flies through the E-region.

Methanol Negative Ion Fragmentation Probed in Electron Transfer Experiments.

In this contribution, we report a novel comprehensive investigation on negative ion formation from electron transfer processes mediated by neutral potassium atom collisions with neutral methanol molecules employing experimental and theoretical methodologies. Methanol collision-induced fragmentation yielding anion formation has been obtained by time-of-flight mass spectrometry in the wide energy range of 19 to 275 eV in the lab frame. The negative ions formed in such a collision process have been assigned to CH3O-, OH-, and O-, with a strong energy dependence especially at lower collision energies. The most intense fragment anions in the whole energy range investigated have been assigned to OH- and CH3O-. Additionally, the potassium cation energy loss spectrum in the forward scattering direction at 205 eV impact energy has revealed several features, where the two main electronic states accessible during the collision events have vertical electron affinities of -8.26 ± 0.20 and -10.36 ± 0.2 eV. Quantum chemical calculations have been performed for the lowest-lying unoccupied molecular orbitals of methanol in the presence of a potassium atom, lending strong support to the experimental findings.

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.

Benchmark experiments of the power law parametrization of the effective ion collecting area of a planar Langmuir probe in low temperature plasmas

For unmagnetized low temperature Ar plasmas with plasma density ranging from 3 × 108 to 1010 cm−3 and an electron temperature of ∼1 eV, the expansion of the ion collecting area of a double-sided planar Langmuir probe with respect to probe bias is experimentally investigated, through a systematic scan of plasma parameters. In accordance with many existing numerical studies, the ion collecting area is found to follow a power law for a sufficiently negative probe bias. Within our experimental conditions, the power law coefficient and exponent have been parameterized as a function of the normalized probe radius and compared with numerical results where qualitatively comparable features are identified. However, numerical results underestimate the power law coefficient while the exponent is overestimated. Our experimental measurements also confirm that ion–neutral collisions play a role in determining the expanded ion collecting area, thus changing values of the power law coefficient and exponent. This work suggests that a power law fit to the ion collecting area must be performed solely based on experimentally obtained data rather than using empirical formulae from simulation results since material and cleanness of the probe, type of working gas, and neutral pressure may also affect the expansion of the ion collecting area, factors which are difficult to model in a numerical simulation. A proper scheme of analyzing an I–V characteristic of a Langmuir probe based on a power law fit is also presented.

Low-energy electron beam generation in inductively coupled plasma via a DC biased grid

Low-energy electron beam generation using a DC biased grid was investigated in an inductively coupled plasma (ICP). The electron beam was measured in argon gas at various pressures, ICP source powers, and substrate voltages (V sub). At a low ICP source power (50 W), an electron beam was generated even at small values of V sub (10 V), however at a high ICP source power (200 W), an electron beam was only generated when a higher voltage (30 V) was applied due to the short sheath thickness on the grid surface. The sheath on the grid surface is an important factor for generating electron beams because low-energy electrons are blocked. If the sheath thickness to small, a high voltage should be applied to generate an electron beam, as accelerate regions cannot exist without the sheath. At high pressure, since electrons experience numerous neutral collisions, a high substrate voltage is needed to generate an electron beam. However, if the applied substrate voltage becomes too high (40 V) at high pressure, high-energy electrons result in secondary plasma under the grid. Therefore, maintaining a low pressure and low ICP source power is important for generating electron beams.

Impurity leakage and radiative cooling in the first nitrogen and neon seeding study in the closed DIII-D SAS configuration

A comparative study of nitrogen versus neon has been carried out to analyze the impact of the two radiative species on power dissipation, SOL impurity distribution, divertor and pedestal characteristics. The experimental results show that N remains compressed in the divertor, thereby providing high radiative losses without affecting the pedestal profiles and displacing carbon as dominant radiator. Neon, instead, radiates more upstream than N thus reducing the power flux through the separatrix leading to a reduced ELM frequency and compression in the divertor. A significant amount of neon is measured in the plasma core leading to a steeper density gradient. The different behavior between the two impurities is confirmed by SOLPS-ITER modeling which for the first time at DIII-D includes multiple impurity species and a treatment of full drifts, currents and neutral–neutral collisions. The impurity transport in the SOL is studied in terms of the parallel momentum balance showing that N is mostly retained in the divertor whereas Ne leaks out consistent with its higher ionization potential and longer mean free path. This is also in agreement with the enrichment factor calculations which indicate lower divertor enrichment for neon. The strong ionization source characterizing the SAS divertor causes a reversal of the main ions and impurity flows. The flow reversal together with plasma drifts and the effect of the thermal force contribute significantly in the shift of the impurity stagnation point affecting impurity leakage. This work provides a demonstration of the impurity leakage mechanism in a closed divertor structure and the consequent impact on pedestal. Since carbon is an intrinsic radiator at DIII-D, in this paper we have also demonstrated the different role of carbon in the N vs Ne seeded cases both in the experiments and in the numerical modeling. Carbon contributes more when neon seeding is injected compared to when nitrogen is used. Finally, the results highlight the importance of accompanying experimental studies with numerical modeling of plasma flows, drifts and ionization profile to determine the details of the SOL impurity transport as the latter may vary with changes in divertor regime and geometry. In the cases presented here, plasma drifts and flow reversal caused by high level of closure in the slot upper divertor at DIII-D play an important role in the underlined mechanism.

Robust measurements of the electron-neutral collision frequency using plasma impedance probes

Local measurement of ionospheric parameters such as electron density, electron neutral collision frequency, neutral density, and the background temperature has a great benefit to improve the empirical models and validate remote observations of the lower ionosphere. One of the well-established instruments that have been used on rocket payloads and satellites to study the dynamic and temporal evolution of the ionospheric plasma is the plasma impedance probe (PIP). The PIP consists of an electrically short antenna that has a radiating part when placed in a plasma environment. It has been shown that the variation of characteristic features of the antenna input impedance is modified by local plasma parameters which can be used to infer ionospheric parameters. This paper aims at combining different aspects of the PIP characteristic impedance curve in order to make the measurements of local plasma parameters robust. The idea of using two different antenna types on the same payload is investigated. The input phase information along with the input impedance amplitude is employed for the first time to examine the idea of using two antenna types as PIP on the same mission for exact ionospheric plasma and neutral density observations. The unique impedance characteristic curves of dipole and patch antennas are shown to be able to enhance the accuracy of the observations.

Correlation of Changes in Electronic and Device Properties in Organic Photovoltaic with Exposure to Air

AbstractIn polymer‐based organic photovoltaic cells, the capability of charge extraction/injection of molybdenum trioxide (MoO3) is demonstrated as interface layer for polymer bulk heterojunction (BHJ). The present work determines the dipole formed at the MoO3 and conjugated poly [2,3‐bis (3‐octyloxyphenyl) quinoxaline‐5,8‐diyl‐alt‐thiophene‐2,5‐diyl] (TQ1):(6,6)‐Phenyl C71 butyric acid methyl ester (PC71BM) bulk heterojunction interface. This dipole has a major influence on the charge transport across the TQ1:PC71BM interface. The formation of the dipole is determined with electron spectroscopy and the depth profiling technique neutral impact collision ion scattering spectroscopy. Metastable induced electron and UV‐photoelectron spectroscopy is applied to determine the enclosed surface coverage of minimum 1.5 nm MoO3 and a maximized dipole strength of 2.4 eV with &gt;2 nm MoO3 deposition on BHJ. It is also demonstrated that the air exposure reduces the dipole at the interface and leads to a decrease in cell performance providing evidence that the dipole formed at the interface is beneficial for the functioning of photovoltaic cells. The results indicate that the reduction in dipole might not be the only mechanism leading to cell performance degradation. The findings of the present work have relevance for other BHJ systems because MoO3 as high work function material is likely to interact with other organic materials in a similar way.