Charge Exchange Neutrals Research Articles

Interaction of charge exchange neutrals with the main chamber walls of plasma machines

The fluxes and energy distributions of the charge exchange (CX) neutrals as measured at specific locationsat the tokamak ASDEX Upgrade (AUG) and the stellarator W7-AS at Garching are discussed as a function of the dischargeconditions. For the evaluation of the plasma-wall interaction, knowledge of the energetic neutral fluxes andtheir energy and angular distributions at all poloidal and toroidal locations are required. For AUG these are obtainedfrom B2-EIRENE computer simulations, taking the experimental results into account. The CX fluxes and the shapes ofthe spectra vary greatly around a poloidal cross-section. This has a strong effect on the wall erosion bysputtering and hydrogen isotope implantation into the vessel walls. The sputtering of the actual carbon wall, andthe effect on possible wall materials, such as tungsten, beryllium, TiC and SiC, are discussed. The depthdistribution of the CX neutrals implanted into the walls is calculated for two cases using the TRIMSP code.Saturation, release by glow discharges and permanent retention of the implanted hydrogen isotopes are discussed.This is important for the expected tritium content of the vessel walls of a future D-T fusion device.

Transport Barriers Formation and Subneoclassical Energy Fluxes: A Unified Theoretical Approach

We show that ``subneoclassical'' heat fluxes are rigorous and straightforward consequences of the revisited neoclassical theory---including finite Larmor radius (FLR) and inertia effects---that we published earlier [Phys. Plasmas 1, 619 (1994)]. The formation of sheared radial electric field profiles displaying most of the characteristics observed in the transport barriers associated with $L\ensuremath{-}\mathrm{to}\ensuremath{-}H$ transitions is another consequence of this first principles theory. The threshold condition results from a competition between rotation spin-up related to FLR and friction by charge-exchange neutrals.

Impacts of charge-exchange neutrals on degradation of plasma-facing materials

This paper reviews the recent data from the PISCES linear plasma facility and the TRIAM-1M tokamak, demonstrating the impacts of energetic charge-exchange particles on surface erosion and bulk damage. High energy neutrals can result in significant surface erosion via sputtering even for tungsten. For the erosion of tungsten it has been emphasized in the PISCES data that oxygen-containing plasma impurities dominate the overall erosion behavior at energies below the sputtering thresholds for hydrogenic species. Radiation damage has been observed in plasma-facing materials in TRIAM-1M due to charge-exchange hydrogen neutrals. The accumulation of dislocation loops has been observed in metals subjected to the bombardment of charge-exchange hydrogen neutrals with energy up to keV. The flux was on the order of 10 18 H atoms/m 2/s, which is comparable with the ITER first wall condition. Severe embrittlement was observed in tungsten after long-term exposure in TRIAM-1M. Due to their strong interaction with lattice defects, helium atoms cause more profound effects on material properties than hydrogen. Defect accumulation by keV helium ions induces significant hardening and embrittlement even at high temperatures. As such, bombardment of energetic neutrals of hydrogen and helium can result in unrecoverable damage in plasma-facing materials and degrade even material bulk properties.

Diagnostic method for radial electric fields in Tokamaks by the observation of ripple-trapped ions

Weakly collisional ions trapped in the toroidal field ripples at the outer plasma edge of a Tokamak can be prevented from escaping the plasma due to grad B drift by a counteracting radial electric field. This leads to an increase in the density of ripple-trapped ions, which can be monitored by the analysis of charge exchange neutrals. The minimum radial electric field Er necessary to confine ions with energy E and charge q (q=−1: charge of the electron) is Er=−E/(q×R), where R is the major radius at the measuring point. Slowing-down ions from neutral injection are usually in the right energy range to be sufficiently collisionless in the plasma edge and show confinement by radial electric fields in the range of tens of kV/m. The density of banana ions is almost unaffected by the radial electric field indicating that the change in the fluxes of ripple-trapped ions is, indeed, caused by the radial electric field and not by direct changes in plasma parameters. Single particle calculations for real ASDEX Upgrade geometry and magnetic fields but with an assumed electric field profile give a rule of thumb estimate of the electric field strength. They also make plausible the reason why the charge exchange flux may react in less than 100 μs to an abrupt onset of a radial electric field with a halfwidth of at least the Larmor diameter of the particles in question and thus make this measurement an interesting tool for the investigation of the edge radial electric field.

Neutral diffusion and anomalous effects on ion flow shear

Bulk ion flow can be modified by ion–neutral interactions in the edge region of tokamaks where neutral atoms are abundant. In this region, the standard neoclassical expression for the ion flow is not consistent with the experimental observations. Previous work in the plateau regime [Valanju et al., Phys. Fluids B 4, 2675 (1992)] suggests that the discrepancy is caused mainly by momentum loss due to ion–neutral charge exchange. In the present work we show that in the absence of neutral diffusion, charge exchange cannot affect the neoclassical transport of the ions significantly if the neutral mean-free path is short and the neutral density is low compared to the plasma density. However, the inclusion of neutral diffusion and anomalous processes in the plateau regime can substantially modify parallel ion flow inside the separatrix as determined by the flux surface averaged momentum constraint. Moreover, the local shear in the flow may be determined by a balance between neutral diffusion and anomalous effects which may then modify the turbulent transport.

Transport-Property Measurements in the Plume of an SPT-100 Hall Thruster

To understand the interaction between the SPT-100 thruster and the spacecraft, accurate knowledge of exhaust plume properties must be obtained. This investigation utilized a suite of interrelated diagnostic probes in an effort to provide detailed knowledge of the exhaust plume on a molecular kinetic level. Probes used for the investigation included a retarding potential analyzer, a planar Faraday probe, an emissive probe, both totaland radiant-heat- ux probes, as well as a newly developed neutral particle  ux probe. Combining the knowledge gained from these probes it was possible to quantify the transport of mass, energy, and charge within the plume at 0.5and 1.0-m radius from the thruster exit. Among the phenomena uncovered were a high-energy ionic annulus surrounding a narrow low-energy core within the plume at 0.5-m radius from the thruster exit. This core structure was Ž lled in at 1.0-m radius. Additionally, a population of energetic charge-exchange neutrals was found within 25 deg of the thruster centerline along with the corresponding low-energy charge-exchange ions attributed to plume/background gas interactions.

The analysis of background gas heating in direct current sputtering discharges via particle simulation

A particle-in-cell/Monte Carlo numerical model has been developed to simulate a direct current discharge self-consistently with the motion and thermalization of both energetic charge-exchange neutrals and sputtered cathode atoms. In the model the charged particle motions are considered in a self-consistent electric field. A one-dimensional glow discharge in Ar has been simulated for the cases of Al and Cu cathode. The background argon gas heating has been predicted, with the temperature rise being larger for the case of Cu cathode which is characterized by a higher sputtering yield than Al. The balance of power input into the gas due to the energetic neutrals, sputtered atoms, and ions is analyzed. The dominant contribution is from energetic neutrals. Comparison of the calculated fluxes of these three species at the cathode surface shows a great contribution of the energetic neutrals into sputtering of the cathode material. The effect of applied voltage has been also investigated. Lastly, the influence of the gas heating on discharge characteristics is discussed.

Divertor plasma studies on DIII-D: experiment and modelling

In a magnetically diverted tokamak, the scrape-off layer (SOL) and divertor plasma separates the first wall from the core plasma, intercepting impurities generated at the wall before they reach the core plasma. The divertor plasma can also serve to spread the heat and particle flux over a large area of divertor structure wall using impurity radiation and neutral charge exchange, thus reducing peak heat and particle fluxes at the divertor strike plate. Such a reduction will be required in the next generation of tokamaks, for without it the divertor engineering requirements are very demanding. To successfully demonstrate a radiative divertor, a highly radiative condition with significant volume recombination must be achieved in the divertor, while maintaining a low impurity content in the core plasma. Divertor plasma properties are determined by a complex interaction of classical parallel transport, anomalous perpendicular transport, impurity transport and radiation, and plasma - wall interaction. In this paper we will describe a set of experiments on DIII-D designed to provide detailed two-dimensional documentation of the divertor and SOL plasma. Measurements have been made in operating modes where the plasma is attached to the divertor strike plate and in highly radiating cases where the plasma is detached from the divertor strike plate. We will also discuss the results of experiments designed to influence the distribution of impurities in the plasma using enhanced SOL plasma flow. Extensive modelling efforts will be described which are successfully reproducing attached plasma conditions and are helping to elucidate the important plasma and atomic physics involved in the detachment process.

Charge exchange neutral particle analysis on ICRF-heated plasmas in the GAMMA10 tandem mirror

The ion energy balance in the central cell of the GAMMA10 tandem mirror is investigated for ICRF-heated plasmas on the basis of ion temperatures determined from charge exchange neutral particle measurements. The radial profiles of perpendicularly heated ions are measured using a neutral particle energy analyser (NPA). An experiment on ICRF power modulation is performed in order to evaluate the energy confinement. The resultant temporal change in the ion temperature is analysed from the charge exchange neutrals and central cell diamagnetism. A time evolution code of ion energy balance based on the radial profiles of measured plasma parameters is developed to predict the temporal behaviour of the ion stored energy and the calculated results are compared with the measurements.

Gridded retarding pressure sensor for ion and neutral particle analysis in flowing plasmas

Although classical electrostatic plasma probes can provide detailed information regarding electrons and ions, these devices rely on charge-carrying particles both for energy filtering and for detection. Neutral particles are transparent to such diagnostics. In light of this a new probe design was developed to provide a measurement of the neutral particle flux component of a flowing plasma. The simple design adopted for this probe utilizes a set of electrostatic retarding grids to remove the charged particles from the flow. The neutral particles are sensed within the tube of an off-the-shelf hot cathode ionization gauge. Additionally, by varying the potential on the retarding grids, this same probe configuration can be used to measure the ion energy distribution. Preliminary data from this probe were taken in the exhaust plume of a Hall-current accelerator designed for space propulsion. This probe was found to be a very simple, accurate diagnostic tool for such use. By using this probe in combination with interrelated diagnostics the spatial distribution of energetic charge exchange neutrals was quantified in the plume of an SPT-100 thruster.

Erosion at the vessel walls of JET

Erosion at the JET vessel walls has been investigated by means of carbon long term samples (LTS), partly covered with 530 nm Al and 750 nm Ni as well as implanted with Mo markers in a depth of 380 nm. The samples were exposed to JET plasmas between April–June 1995 (about 940 discharges). Upon removal, the surface layers of the LTS were analyzed with MeV ion beam techniques. On the inner torus walls, net erosion is found with an average of about 1.2 × 10 15 Al- atoms/ cm 2 and 9.4 × 10 14 Ni- atoms/ cm 2 per JET discharge respectively. The erosion of carbon was > 3.9 × 10 15 C- atoms/ cm 2 per JET discharge. The erosion is due to sputtering by energetic charge-exchange neutrals emitted from the plasma during the discharge and He glow discharge cleaning between the discharges.

Impurity induced neutralization of megaelectronvolt energy protons in JET plasmas

A neutral particle analyser was deployed on JET for a measurement of the distribution function of megaelectronvolt energy protons driven by the high power ion cyclotron resonance frequency heating of deuterium plasmas in the hydrogen minority scheme. Unexpectedly, efficient neutralization of megaelectronvolt energy protons was observed in the plasma centre without recourse to injection of a beam of atoms to provide electron donors for charge exchange neutralization reactions. A model is presented that elucidates the role of carbon and beryllium, the main intrinsic impurities in JET plasmas, in this process and establishes charge exchange between hydrogen-like ions of the impurities and protons as the main neutralization process. A model calculation for deducing the proton energy distribution function f(Ep) from the measured hydrogen flux is described. The effects of uncertainties in neutralization cross-sections on the inferred f(Ep) are examined. The validity of this model of impurity induced neutralization (IIN) is tested by using it to describe the measured hydrogen flux in different conditions of plasma heating and fuelling. A crucial point in making these tests was to use measured local values of bare impurity ion densities; the required measurements were realized using charge exchange spectroscopy. Using IIN modelling and a procedure in which a known change in the density of deuterium atoms at the plasma centre was made by applying neutral beam injection, we have deduced the background thermal deuterium atom density at the plasma centre, which is an important new diagnostic result. Concerning future experiments, the model predicts that carbon and beryllium impurities will be major contributors to neutralization of hydrogenic ions (protons, deuterons and tritons) in ITER plasmas, for ion energies E<or=1 MeV/u. According to these estimates measurements of the distribution function, in the megaelectronvolt energy range, of ion cyclotron resonance frequency heating driven ions of hydrogen isotopes

Analysis of the Radial Profile of Ion Temperature in ICRF-Heated GAMMA10 Plasmas Including the Effects of Ion Finite Larmor Radius

This paper describes a modified method of analysis of the charge exchange neutrals for determination of the radial profile of ion temperature including the effect of the ion finite Larmor radius. It also includes the effect of the charge exchange reaction with neutral hydrogen molecules. The ICRF-heated plasmas in the central cell of the GAMMA10 tandem mirror were analyzed by taking into account these effects. The obtained energy spectra and the count-rate spatial profiles of charge exchange neutrals were consistent with the experimental results. The present method, thus, proved to be effective in determining the radial profile of ion temperature.

Interpretation of the molecular fluxes measured at the periphery of a magnetically confined plasma

A new instrument (“Variable Geometry Sniffer Probe” or VGSP), allowing one to sample and mass analyse atoms, ions or molecules moving in different directions and at different locations at the periphery of a plasma, has been built and used in plasma edge studies in the TdeV tokamak. Three different regimes of particle sampling have been identified. First, the VGSP can measure the fluxes of hydrogen and impurity molecules issuing from the walls. Second, it has the capability of detecting low energy charge-exchange and Franck-Condon neutrals. Finally, there is a parallel ion flux sampling regime, for which it is shown that both the connection lengths to the divertor plates and the X × B plasma flows induced by edge electric fields play major roles.

Three-dimensional neutral atom imaging of tokamak plasmas

Recent technological advances in neutral atom imaging of space plasmas can be applied to the next generation of long pulse, high density fusion experiments. In particular, the small size of these neutral atom imagers makes them ideal for measurements of charge exchange neutrals in the edge and divertor region of tokamaks. In the edge of tokamak plasmas, the lower energy neutrals are created locally and the higher energy neutrals are generated within a few mean free paths of the edge. This charge exchange neutral flux can be imaged and its energy spectrum analyzed. The energy spectrum measurements provide the radial information necessary for a three-dimensional (3D) image of the convolved ion and neutral populations.

Novel isotope exchange scenarios: investigation of particle recycling in the TdeV tokamak

The isotope ratio RI=D/(H+D) was measured during ohmic discharges in the magnetically diverted tokamak TdeV, using spectroscopy (Halpha and Dalpha ), gas analysis (H2, HD and D2), mass analysis of charge exchange neutrals, and detection of H and D in a collector probe, in the course of four different experimental phases: (I) 'dynamical' isotope switchover of the fuelling gas during a discharge, (II) shot-to-shot evolution with pure D2 fuelling, (III) same with walls conditioned by D2 glow discharge, and (IV) 'dynamical' switchover combined with divertor biasing and pumping. The different values obtained for RI are compared and analysed in terms of the particle recycling properties of the device. The dynamical scenario (phase I) revealed a lack of equilibrium between the edge and core isotopic composition that is an indication of the degree to which the particle flux is amplified by recycling in the vicinity of surfaces. The phase II results were fitted to a model which yielded an estimate of the magnitude of the wall inventory available for recycling. In phase III, the effectiveness of conditioning was assessed. Phase IV revealed an even more pronounced disequilibrium than phase I, and demonstrated the power of divertor biasing and pumping in exhausting the wall inventory and controlling the isotope ratio.

Imaging neutral particle detector

Abstract With the support of NASA's Innovative Research Program (IRP), we undertook the definition and initial development of an imaging neutral particle detector, a new instrument for global imaging of planetary magnetospheres. This instrument carries out remote sensing of magnetospheric plasmas by imaging and analyzing charge exchange neutrals, which are energetic neutral atoms (ENAs) created by charge exchange reactions between ambient neutrals and energetic ions. The imaging neutral particle detector can determine the composition of the charge exchange neutrals and therefore also the composition of the original hot plasma. Such an instrument, the Ion‐Neutral Camera of the Magnetospheric Imaging Instrument (MIMI), has been selected for flight on the NASA/ESA Cassini mission to Saturn and Titan, and a similar instrument is part of the baseline payload for the proposed Inner Magnetosphere Imager (IMI) mission to carry out remote sensing of Earth's magnetosphere. Remote sensing of charge exchange neutrals...

Rotation in Ohmically heated tokamaks: Experiment and theory

Comparison of experiments with theory leads to a reasonably complete understanding of impurity rotation in Ohmic plasmas in a tokamak. In the experiments, the components of the impurity rotation velocity were derived from measurements of the Doppler shift of carbon spectral emission. Neoclassical theory for rotation was evaluated using measured plasma profiles and compared with experiment. The comparisons suggest that plasma toroidal rotation is damped (but not quenched) by momentum loss due to ion–neutral charge exchange and that the damping can now be accurately predicted.

Design and calibration of the JET time of flight neutral particle analyzer with high noise rejection capability

This paper describes the design and calibration of a time-of-flight neutral particle analyzer used on the Joint European Torus (JET) tokamak in the diagnostics of plasma close to thermonuclear fusion conditions. The 15 channels of the analyzer can select energy and mass of charge exchange neutrals in the energy range from 0.5 to 250 keV. The analyzer has been calibrated for hydrogen, deuterium, and helium atoms. The utilization of the coincidence technique in the measurement of the time of flight of the detected neutrals also permits discrimination of the particle signal from a random background. Thus, the NPA operates successfully in the presence of the high neutron and gamma fluxes that are characteristic of the high-performance fusion plasmas produced in large tokamaks. The calibration data processing, the signal analysis, and the signal-to-noise evaluation are reported in detail.

Impurity rotation in hydrogen and helium plasmas

Rotation velocities were measured in two Ohmically heated discharges and compared to neoclassical predictions evaluated with measurements of Ti, ni, and Er. Hydrogen fueled one of the discharges, and helium was the principal fuel for the other. For both discharges, poloidal rotation is consistent with the momentum balance equation. In contrast, current neoclassical theory predicts the toroidal rotation velocity only in the case of negligible ion–neutral charge exchange and neutral ionization.