Distribution Of Neutral Particles Research Articles

Simulation of charge-exchange induced NBI losses on EAST

The neutral beam injection is widely adopted in tokamaks as a key heating tool, playing a crucial role in generating burning plasmas. However, the loss of beam ions can damage the first wall and reduce the heating efficiency, resulting in failure to maintain steady-state conditions. In this work, the effect of neutral particles in the edge on fast ions generated by NBI in the Experimental Advanced Superconducting Tokamak (EAST) device is studied using the particle tracer code (PTC). The poloidal distribution of neutral particles is calculated by edge plasma simulation code SOLPS-ITER. In this simulation, four beam lines in EAST are considered: co-current tangential (co-tang), co-current perpendicular (co-perp), counter-current tangential (ctr-tang) and counter-current perpendicular (ctr-perp). It is shown that, in the absence of neutral particles, the loss fraction of ctr-injection is considerably higher than that of the co-injection. When considering the neutral particles, it is found that the ctr-perp injection demonstrates a significant variation in particles loss fraction (ranging from 18.56% to 25.42%) compared to the other three injection configurations. In terms of the loss fraction induced by neutral particles, ctr-injection exceeds co-injection, and perpendicular configuration exceeds tangential configuration. Furthermore, the difference of charge exchange ratios of three different energy (full energy, half energy, one third energy) of the four injections can be attributed to variations in the poloidal trajectories associated with each of these injections. Moreover, approximately half of fast ions which undergo neutralization directly lose to the first wall while the rest re-enter the bulk plasma and re-ionize. Except for the ctr-tang injection, the reionization ions from the other three injections exhibit effective confinement.

Functional optimization for a Beam Driven Plasma Neutralizer in DEMO Neutral Beam Injector

The Beam Driven Plasma Neutralizer (BDPN) has been proposed as a more efficient alternative to the gas neutralizer for negative-ion based Neutral Beam Injection (NNBI). In this paper we model the performance of an entire NNBI beamline with a BDPN. We simultaneously consider all the relevant physics and engineering aspects, the most important being the plasma density and degree of ionization inside the BDPN as a function of its geometry and feed gas flow, the geometrical transmission of the beamline, the dependence of the neutral gas distribution in the beamline on the geometry of the beamline components and gas flows, and the species evolution of the extracted D− beam through this neutral and charged particle distribution. Furthermore, we calculate the heat loads expected on the BDPN parts and on the NBI components located downstream of it and study the effect of the magnetic cusp field across the BDPN entrance on beamline transmission. While our results constitute an optimization only under the applied boundary conditions, we find that the beamline with a BDPN increases the system’s wall plug efficiency by about 13% to 0.34 from the 0.30 estimated for a gas neutralizer.

Comparison of the influence of 2D and 3D geometry of the main chamber on plasma parameters in the SOL of ASDEX Upgrade

In this paper the influence of toroidally asymmetric wall features on plasma solutions for ASDEX Upgrade is investigated by using the 3D scrape-off-layer simulation code EMC3-EIRENE. A comparison of simulation results in a 2D case with a toroidally symmetric first wall and divertor and a 3D case that differs from the 2D setup by including the 3D structure of the poloidal rib-limiters on the low field side of ASDEX Upgrade, highlights notable differences in the main chamber neutral particle distributions, ionisation sources and plasma flow patterns. Both neutral particle distribution and ionisation sources extend poloidally further upwards at the outer mid-plane in the 3D case and the plasma flow is globally influenced by the 3D wall features. Both simulations are conducted with identical input parameters to isolate the influence of wall geometry from other factors. By analysing the transport of neutrals from different poloidal locations it was possible to explain the observed discrepancies by different transport paths for recycled neutrals from the divertor region, only accessible in the 3D version of the wall geometry. Together with observed differences in fall-off lengths for plasma flow and electron temperature at the outer mid-plane, presented results are of key importance for interpreting global impurity migration experiments.

Estimation of parameters of radio-frequency ion injector with an additional magnetostatic field

The paper presents the results of theoretical study for the influence of an additional magnetostatic field applied to the radio-frequency discharge region on the operating parameters of radio-frequency ion injector, taking into account the shape of its discharge chamber. The integral characteristics are calculated for three injector configurations: with a cylindrical, conical, and hemispherical discharge chamber. For calculations, we used a mathematical model, in which the charged particle behavior in plasma is described by relations close to magnetohydrodynamic, while the neutral particle distribution is determined using an approximate analytical solution obtained when considering a one-dimensional problem of gas flow in a tube. Simulation was carried out in the COMSOL Multiphysics software package. According to the calculation results, an additional magnetostatic field can improve the ion injector performance with any discharge chamber configuration. The greatest operation efficiency increase was observed with a cylindrical discharge chamber. However, according to calculations, the best performance can be obtained if a hemispherical chamber is used.

Characterization of neutral particle fluxes from ICWC and ECWC plasmas in the TOMAS facility

Electron- (ECWC) and ion- (ICWC) cyclotron wall conditioning are essential means for controlled fusion to modify the surface state of plasma-facing components in order to reduce impurity generation and fuel accumulation in the wall. Development of ECWC and ICWC requires characterization of neutral particle fluxes generated in discharges, because neutrals enhance the homogeneity of the conditioning, which may contribute to remote or shadowed areas, especially in the presence of a permanent magnetic field (e.g. W7-X, ITER). A time-of-flight neutral particle analyzer (ToF-NPA) with 4.07 m flight distance is employed to measure time- and energy-resolved low energetic (<1 keV) neutral particle distributions. The ToF-NPA setup tested at the EXTRAP T2R reversed field pinch was installed at the TOMAS toroidal plasma facility to determine low energy neutral particle fluxes while investigating the impact of the gas pressure in the instrument and compatibility with low count rates during EC- and ICWC discharges. TOMAS has a major radius of 0.78 m and provides various plasma operation conditions: toroidal magnetic field up to 0.12 T, EC frequency 2.45 GHz with the power of 0.6–6 kW, IC frequency of 10–50 MHz with the power of up to 6 kW. Early results on the characterization of three phases (EC only, EC + IC, and IC only) of hydrogen discharges demonstrate: (i) the low energy (10–725 eV) neutrals distribution has been determined by the NPA system, (ii) the mixed EC + IC phase produces the highest population of neutral particles, while the EC only provides one order of magnitude lower rate, (iii) the neutrals produced in IC only have higher average energy (28 eV) than EC only (7 eV) and EC + IC (16 eV).

Probe researches of glow discharge in argon

The gas-discharge media with an inverse distribution of neutral and charged particles are of great interest for science and practical applications. A striking example of this is the laser operation. In the second half of the last century, a number of articles used to consider the possibility of forming an inverse electron distribution function (EDF), resulting in negative plasma conductivity. By analogy with the inverse population of excited states in lasers, such a medium will amplify electromagnetic waves. One of the first papers was the work [1]. The most complete theoretical study was presented in [2]. It is shown that with an inverse EDF in the low-energy part in gases with a Ramsauer minimum of the elastic scattering cross section stationary absolute negative conductivity will be formed.

Modeling q95 windows for the suppression of edge localized modes by resonant magnetic perturbations in the DIII-D tokamak

A toroidal asymptotic matching model of the response of a tokamak plasma to a static resonant magnetic perturbation (RMP) is used to simulate the n = 3 RMP-induced edge-localized-mode-suppression windows in q95 that are evident when the plasma current is slowly ramped in DIII-D discharge #145380. All quantities employed in the simulation are derived from experimental measurements, apart from the neutral particle data. Three cases are considered. In the first case, the natural frequencies of tearing modes resonant in the plasma are determined by the ion flows at the corresponding resonant surfaces, which is the prediction of nonlinear tearing mode theory. In the second case, the natural frequencies are determined by the local E×B velocities at the resonant surfaces. In the third case, the natural frequencies are determined by the electron flows at the resonant surfaces, which is the prediction of linear tearing mode theory. The second case gives the best agreement between the simulations and the experimental observations. The first and third cases only lead to partial agreement between the simulations and the observations. In the first case, the lack of complete agreement may be a consequence of using an inaccurate assumption for the neutral particle distribution in the pedestal. In the third case, the lack of complete agreement is probably due to the fact that the response of a tokamak plasma to an RMP is not accurately described by linear tearing mode theory.

Enhancement of an E parallel B type neutral particle analyzer with high time resolution in the Large Helical Device

In order to study fast ion confinement and transport mechanisms both in quiescent plasmas and in the presence of magneto-hydrodynamic (MHD) activity, an electric field parallel to the magnetic field type neutral particle analyzer (E||B-NPA) was installed using Micro-Channel Plates (MCPs) in the Large Helical Device (LHD). The E||B-NPA is an established diagnostic used to measure the energy distribution of charge exchange neutral particles escaping from plasmas corresponding to hydrogen, deuterium, and tritium, respectively. The LHD deuterium experimental campaign started in 2017. To obtain more details of MHD behavior, we improved on time resolution performance of the E||B-NPA using a high-speed Pre-Amplifier and Discriminator (PAD) and Latching Scaler Module (LSM). For the last LHD experimental campaign, the E||B-NPA was installed on nearly the equatorial plane of the vacuum chamber to measure the passing energetic particles generated by tangentially injected neutral beams. As the results of the E||B-NPA commissioning, it is confirmed that the E||B-NPA successfully detected the energetic particles and obtained the energy distributions corresponding to hydrogen. Also, the effect of the neutron to E||B-NPA is estimated using the other neutron diagnostics during the experiments.

Electromagnetic dipole moment and time reversal invariance violating interactions of high energy short-lived particles in bent and straight crystals

A particle, which moves in a crystal experiences weak interactions with electrons and nuclei along with electromagnetic interaction. Measuring the polarization vector and the angular distribution of charged and neutral particles scattered by axes (planes) of an unbent (straight) crystal enables to obtain restrictions for the electromagnetic dipole moments value and for magnitudes of constants describing T-odd ($CP$-odd) interactions beyond the standard model. Spin precession of channeled particles in bent crystals at the LHC gives a unique possibility for measurement of constants determining T-odd ($CP$-odd) violating interactions of short-lived baryons and $\ensuremath{\tau}$-leptons with electrons and nuclei (nucleons), similarly to the possibility of measuring particles' electric and magnetic moments. Methods to separate P-odd spin rotation from the MDM-caused and T-odd spin rotations are discussed.

Icy giant planet exploration: Are entry probes essential?

“Flyby, orbit, land” has been the guiding philosophy of planetary exploration. This systematic approach has been highly successful in addressing the most fundamental questions of the origin, evolution, and habitability of the solar system. For the giant planets, entry probes take the place of “landers”, since the “land” of the gas and icy giant planets, their solid core, lies some tens of thousands of kilometers beneath their cloud tops, hence impractical to reach. On the other hand, during a planet's accretionary heating phase the volatiles trapped in the core material would have been released, forming the atmosphere, together with the most volatile of the gases, hydrogen, helium and neon, that were captured gravitationally when the core became massive enough. Those atmospheric volatiles would thus be accessible by entry probes deployed to relatively shallow depths in the upper troposphere, allowing the determination of the abundances and isotopes of at least the most critical of the “heavy elements” (mass greater than helium). The heavy elements are key constraints to planetary formation and evolution models [1]. Entry probes are essential to retrieve their abundances, which are feasible only in situ at probe depths. That was the rationale behind NASA's 1995 Galileo probe mission at Jupiter and the Saturn Probe mission in NASA's New Frontier 4 candidate list of science themes. Saturn probe proposals to ESA's Cosmic Vision Program are similarly inspired. In 2015, NASA commissioned an Ice Giant Planets Study to recommend a comprehensive set of science goals and objectives, and further to develop potential mission architectures for accomplishing them in the 2023–2033 decade. The most highly rated mission from that study is an orbiter with probe to either Uranus or Neptune [2]. While remote sensing observations from the orbiter will yield the composition, structure and the distribution of neutral and charged particles in the magnetosphere and the upper atmosphere, the entry probe will determine the abundances and isotopic ratios of the noble gases (He, Ne, Ar, Kr, Xe), H, C, and possibly N and S. The noble gases, in particular, are key to discriminating among formation models, and their values from the probe entry location would represent global values as they are unaffected by meteorology, dynamics or chemistry. We will elaborate on these issues and then briefly discuss possible scenario/s for a mission to the icy giant planets, with particular focus on entry probes.

Numerical analysis of time-of-flight distributions of neutral particles for pulsed laser ablation of binary substances into vacuum

Direct Monte Carlo simulation of the laser-induced plume expansion under pulsed laser evaporation of binary substances into vacuum has been performed. Analysis of separation of the mixture components has been carried out for a wide range of the problem parameters: the mass ratio, the number of evaporated monolayers, and the initial mixture concentration. Special attention has been paid to examination of the mixture separation and changing the mixture concentration for particles passing through the time-of-flight detector located on the normal to the evaporating surface. Universal dependences of the energy and concentration ratios of species passing through the time-of-flight detector on the number of evaporated monolayers have been obtained. It is shown that these dependences can be used for analysis of any experimental data independently on the concentration ratio. The obtained relations well correlate with known numerical and experimental data on gas phase separation under pulsed evaporation into vacuum. The obtained results allow to completely determine the conditions at the evaporating surface (the surface temperature, the number of evaporated monolayers, and the initial mixture composition) from experimentally measured time-of-flight distributions of neutral particles.

On the Frequency Distribution of Neutral Particles from Low-Energy Strong Interactions

The rejection of the contamination, or background, from low-energy strong interactions at hadron collider experiments is a topic that has received significant attention in the field of particle physics. This article builds on a particle-level view of collision events, in line with recently proposed subtraction methods. While conventional techniques in the field usually concentrate on probability distributions, our study is, to our knowledge, the first attempt at estimating the frequency distribution of background particles across the kinematic space inside individual collision events. In fact, while the probability distribution can generally be estimated given a model of low-energy strong interactions, the corresponding frequency distribution inside a single event typically deviates from the average and cannot be predicted a priori. We present preliminary results in this direction and establish a connection between our technique and the particle weighting methods that have been the subject of recent investigation at the Large Hadron Collider.

Forward hadron production in ultra-peripheral proton–heavy-ion collisions at the LHC and RHIC

We discuss hadron production in the forward rapidity region in ultraperipheral proton-lead collisions at the LHC and proton-gold collisions at RHIC. Our discussion is based on the Monte Carlo simulations of the interactions of virtual photons emitted by a fast moving nucleus with a proton beam. We simulate the virtual photon flux with the STARLIGHT event generator and then particle production with the SOPHIA, DPMJET, and PYTHIA event generators. We show the rapidity distributions of charged and neutral particles, and the momentum distributions of neutral pions and neutrons at forward rapidities. According to the Monte Carlo simulations, we find large cross sections of ultraperipheral collisions for particle production especially in the very forward region, leading to substantial background contributions to investigations of collective nuclear effects and spin physics. Finally we can distinguish between proton-nucleus inelastic interactions and ultraperipheral collisions with additional requirements of either of the charged particles at midrapidity and a certain level of activities at negative forward rapidity.

Sizing of neutral sub 3 nm tungsten oxide clusters using Airmodus Particle Size Magnifier

Measurement of the size distributions of neutral sub 3nm aerosol particles is technically a challenging task. Here we present laboratory experiments on sizing of neutral tungsten oxide clusters with two independent methods: Airmodus A11 Particle Size Magnifier (PSM) and a combination of a high resolution differential mobility analyzer (DMA) and an electrometer. The test particles were generated with a hot wire generator, and their mass and composition were measured with a high resolution mass spectrometer. The particles were shown to be (WO3)x up to 12,000amu or approximately 2nm. As particle counters are usually calibrated with electrically charged particles but often measure mostly neutral particles, we quantified the enhancement due to charge to the PSM cut-off diameter to be approximately 0.1nm. To compare directly the two methods, charged fraction of the sample particles and effect of organic impurity ions to the measured concentrations were estimated, and the PSM was shown to measure similar size classified concentrations as the combination of DMA and electrometer. The results confirm that with a sound characterization of the particle source and the particle counter, sub 3nm neutral particle distributions can be measured with good precision with a method based on supersaturation scanning.

Investigation on Microcosmic Characteristics of Trichel Pulse in Bar-Plate DC Negative Corona Discharge Based on a Novel Simulation Model

An improved multi-component two-dimensional hybrid model is presented for the simulation of Trichel pulse corona discharge in dry air (O2:N2=1:4). The model is based on the plasma hydrodynamics and the chemical models, in which 12 species (e, O, O2, O3, O2+, O4+, O2-, O-, N2, N2+, N4+, and N2O2+) and 27 collision reactions between them are considered. In addition, the photoionization and secondary electron emission effect are also incorporated within the model as well. The simulation is established with a bar-plate electrode configuration with an inter-electrode gap of 3.3 mm where the negative DC voltage applied to the bar is 5.0 kV, the pressure in air discharge is fixed at 1.0 atm, and the gas temperature is assumed to be a constant (300 K). Based on the novel model, a few key microcosmic characteristics such as electric field distribution, net charge density distribution, electron density distribution at 5 typical time points during a process of Trichel pulse are discussed emphatically. What is more, the investigation on electron generation and disappearance rates, as well as positive ion, negative ion and neutral particle distribution characteristic along the axis of symmetry in the later Trichel pulse cycle are also detailed. The obtained results will provide valuable insights to the physical mechanism of negative corona discharge in air

Perpendicular and tangential angularly resolved multi-sight neutral particle analyzer system in LHD

The particle loss in a helical plasma can be found by measuring the angular distribution of the energetic neutral particles using the angularly resolved multi-sightline neutral particle analyzer (ARMS). In ARMS, the AXUV detector with 20 segments usually used as UV monitor is utilized as the particle detector. Two ARMSs, which are installed with perpendicular and tangential views are operated for measuring the real time neutral particle distribution and investigating the particle loss.

Enceladus plume variability and the neutral gas densities in Saturn's magnetosphere

Neutral particle dominance over charged particles in Saturn's magnetosphere was evident prior to Cassini's arrival at Saturn in 2004. The observation of active plumes emanating from the south pole of Enceladus suggests that this small moon is likely to be the principal source of neutrals in Saturn's magnetosphere. Cassini has flown through the plumes on several occasions, and the resulting data imply the source rate is variable (∼1027 to 1028 water molecules/s). Here we use Cassini plasma spectrometer and Cassini magnetospheric imaging instrument observations to update neutral particle lifetimes and then use the most recent processed versions of Cassini ion neutral mass spectrometer observations made during encounters E2, E3, and E5 to constrain a 3‐D multispecies neutral particle model. This procedure improves constraints on the plume source rate, ejection velocity, and plume divergence. We find that the plume source rate varies by at least a factor of 4 over the 7 month period considered. Additionally, we find that previous estimates of the plume source rates based on E2 observations are most likely overestimated because the background neutral torus has not been adequately account for. On the basis of these results, we discuss the implications of this variability on global neutral particle distributions.

Analysis of atmospheric neutral and charged molecular clusters in boreal forest using pulse-height CPC

Abstract. We measured the size distribution of atmospheric neutral and charged clusters and particles down to mobility diameter around 1.5 nm by applying pulse-height CPC technique at SMEAR II station in Hyytiälä, southern Finland during spring 2007 and May 2008. The concentration of molecular clusters smaller than 3 nm seems to be highly variable in boreal forest environment, the concentration varied typically between 500–50 000 cm−3. By comparing to concentrations measured with ion spectrometers, we conclude that ion clusters and neutral clusters produced by ion-ion recombination are usually not sufficient to explain all of the observed clusters; the median fraction of recombination products from all neutral clusters was 4.9%. Before and during most new particle formation events the cluster formation rate rose only slightly, or remained close to stable. Nocturnal formation of clusters was also frequently observed.

Numerical simulation of the Zeeman effect in neutral xenon from NIR diode-laser spectroscopy

We present a numerical method for simulating neutral xenon absorption spectra from diode-laser spectroscopy of the Zeeman-split 6S′[1/2]→6P′[1/2] line at 834.682 nm-air in a galvatron’s plasma. To simulate the spectrum, we apply a Voigt profile to a spectrum of σ-transition lines of even- and odd-numbered isotopes computed from anomalous Zeeman and nonlinear Zeeman hyperfine structure theories, respectively. Simulated spectra agree well with Zeeman-split spectra measured from 30 to 300 G. A commercial nonlinear least-squares solver (LSQNONLIN) returns field strengths and translational plasma kinetic temperatures that minimize the error between simulated and experimental spectra. This work is a preamble to computing magnetic field topology and the speed distribution of neutral xenon particles in the plume of a Hall thruster from diode laser-induced fluorescence.

Charge exchange ion number density distribution in Hall thruster plume

The charge exchange (CEX) ions by the collisions between energetic ions and slow neutral particles in a Hall thruster plume are thought a source of the ions coming back to the satellite. Therefore, mechanical plume shielding has been proposed to prevent the CEX ions from spreading out from near exit region. In order to optimize the shield length, CEX reactions in the plume were evaluated. Number density distributions of neutral particles were measured by the laser absorption spectroscopy (LAS) targeting the absorption line of XeI 823.16 nm (6s[3/2] 2 0 → 6p[3/2] 2). Ambient neutral gas effect in a test chamber was cancelled out using the measurements at two different back pressures. Measured distribution was compared with the Particle-in-Cell (PIC) and Direct Simulation Monte Carlo (DSMC) analyses. As a result, normalized mean outflow velocity of neutral particles v ¯ z / v th was identified at 0.6, and the computed CEX ion density was found to be decreased from the channel exit by 50% at 80 mm distance.