Energetic Neutrals Research Articles

Material erosion at the vessel walls of future fusion devices

For a next-step fusion device the erosion at the vessel walls during normal operation caused by the bombardment with different species from the plasma is estimated. The wall fluxes are calculated for a given ‘burning’ plasma with the computer code B2-EIRENE. With the known sputtering yields the erosion rates for vessel walls made out of Be, C, Fe, Mo or W, are calculated. The erosion by ions is found to be of the same order of magnitude as the erosion by energetic neutrals. The erosion has minima around the area of gas feed and maxima close by and at the bottom near the baffles of the divertor. The calculated total erosion is of the order of 10 21 to 10 22 atom/s for low Z materials, and a factor 10–20 lower for high Z materials, such as W, resulting in about 5 kg per full-day of operation or 1 t/y.

Plasma molding over surface topography: Simulation of ion flow, and energy and angular distributions over steps in RF high-density plasmas

A two-dimensional fluid/Monte Carlo (MC) simulation model was developed to study plasma molding over surface topography. The radio frequency (RF) sheath potential evolution, ion density, and flux profiles over the surface were predicted with a self-consistent fluid simulation. The trajectories of ions and energetic neutrals (resulting by ion neutralization on surfaces or charge exchange collisions in the gas phase) were then followed with a MC simulation. In this paper, ion flow, energy and angular distributions of ions, and energetic neutrals bombarding an otherwise planar surface with a step are reported. The step height was comparable to the sheath thickness for the RF high-density plasma considered. As one approaches the step sidewall, the ion flux decreases, the ion energy distribution narrows, and the ion impact angle increases drastically. The ion impact angle at the foot of the step scales with the ratio of sheath thickness to step height. The energetic neutral flux is found to be comparable to the ion flux on the horizontal surface near the step sidewall. Simulation results are in good agreement with experimental data on ion flux and ion energy and angular distributions near the step.

Contribution of the Different Erosion Processes to Material Release from the Vessel Walls of Fusion Devices during Plasma Operation

In high temperature plasma experiments several processes contribute to erosion and loss of material from the vessel walls. This material may enter the plasma edge and the central plasma where it acts as impurities. It will finally be re-deposited at other wall areas. These erosion processes are: • evaporation due to heating of wall areas. At very high power deposition evaporation may become very large, which has been named “blooming”. Large evaporation and melting at some areas of the vessel wall surface may occur during heat pulses, as observed in plasma devices during plasma disruptions. • At tips on the vessel walls and/or hot spots on the plasma exposed solid surfaces electrical arcs between the plasma and the vessel wall may ignite. They cause the release of ions, atoms and small metal droplets, or of carbon dust particles. • Finally, atoms from the vessel walls are removed by physical and chemical sputtering caused by the bombardment of the vessel walls with ions as well as energetic neutral hydrogen atoms from the boundary plasma. All these processes have been, and are, observed in today's plasma experiments. Evaporation can in principle be controlled by very effective cooling of the wall tiles, arcing is reduced by very stable plasma operation, and sputtering by ions can be reduced by operating with a cold plasma in front of the vessel walls. However, sputtering by energetic neutrals, which impinge on all areas of the vessel walls, is likely to be the most critical process because ions lost from the plasma recycle as neutrals or have to be refuelled by neutrals leading to the charge exchange processes in the plasma. In order to quantify the wall erosion, “Materials Factors” (MF) have been introduced in the following for the different erosion processes.

Edge ion heating and parametric decay during injection of ion cyclotron resonance frequency power on the Alcator C-Mod tokamak

A population of suprathermal ions has been observed in the scrape off layer during injection of power in the ion cyclotron frequency range on the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. These ions generated energetic neutrals which were measured with a charge exchange neutral particle analyzer. In the hydrogen minority heating regime, fast particles were generated only when the injected power was above 500 kW. Fast particles were observed at a low level of injected power when the rf was a multiple of the ion cyclotron frequency in the scrape off layer. The power deposited in the edge was always less than five percent of the injected power and did not increase impurity levels. The theoretical threshold of parametric decay instabilities for decay of the pump wave in the antenna near field into an ion Bernstein wave and an ion cyclotron quasimode matches the observed threshold for fast ion production under a variety of conditions.

Advanced Fluid Information. Plasma Molding over Surface Topography.

A two-dimensional fluid/Monte Carlo simulation model was developed to study plasma molding over surface topography. The plasma sheath evolution and potential distribution over the surface were predicted with a self consistent fluid simulation. The trajectories of ions and energetic neutrals were then followed with a Monte Carlo simulation. Energy and angular distributions of ions and energetic neutrals bombarding an otherwise planar target with a step are reported. As one approaches the step, the ion flux decreases and the ion impact angle increases drastically. For a time invariant sheath (DC case), the ion energy distributions (IED) remain relatively unaffected. When the plasma sheath oscillates at radio frequencies, the IED narrows, while the ion angular distribution becomes broader as one approaches the step

Hybrid model for the calculation of ion distribution functions behind a direct current or radio frequency driven plasma boundary sheath

A hybrid fluid dynamic/kinetic model is presented which describes the sheath and the presheath regions of dc or rf driven low pressure gas discharges in a realistic and self-consistent way. The model assumes an infinite extended sheath parallel to the electrode, allowing a one-dimensional spatial description. It provides for the presence of multiple positive ion species and their collisional interactions with the neutral background, and takes into account the possibility of a nonharmonic modulation of the sheath potential and the application of an external dc bias; in this work, the model is applied to a two-species capacitively coupled argon and oxygen plasma. The input required by the model consists of the fluxes of the incoming ions, of the modulating current, and of the pressure, the composition, and the temperature of the background gas. On output, the model provides the values of the electric field and of the particle densities within the sheath and the presheath, the total voltage drop across the sheath, and also the energetically and angularly resolved distributions of the positive ions and the energetic neutrals which impinge the material substrate at the boundary. In general, the model is able to treat dc discharges as well as capacitively and/or inductively coupled rf discharges, it thus covers most of the plasmas used in very large scale integration microelectronics manufacturing and other surface modification techniques. Using the model, studies of the energy distributions of the incoming ions have been performed for a wide range of parameters, and the effects of varying process conditions have been investigated. At low and intermediate pressures (p<50 mTorr), the distribution functions of rf driven discharges exhibit a characteristic bimodal structure; this structure disappears with increasing pressure as ion-neutral collisions become significant. A comparison of calculated ion energy distributions with experimental measurements on capacitively coupled argon and oxygen discharges shows excellent quantitative agreement. In addition to the ion energy distribution, the angular distributions of the incident ions at various energies are also discussed as a function of the neutral gas pressure. It turns out that the details of the angular distribution not only depend on the field structure of the sheath itself but also on that of the presheath. The results of the presented model are therefore more reliable than those of previous models which restricted themselves to the sheath region. This high physical accuracy of the presented model, together with its flexibility and its high execution speed, allows its use as a tool for technology-oriented computer-aided design in the microelectronics industry.

Anisotropic radiation damage by charge exchange neutrals under tokamak discharges in TRIAM-1M

Thin foil specimens were exposed to tokamak discharges in TRIAM-1M. As a result of the microstructural observation, accumulated radiation damage (dislocation loops) due to charge exchange (CX) neutrals were observed only in the specimens pointing to the lower half of plasma and almost no damage in the specimens pointing to the upper side. These results indicate that the lower half of the plasma is the major source for energetic CX neutrals. This anisotropy can be explained by the effect of the gradient B drift of high-energy ions trapped in a toroidal ripple. The mean flux was estimated to 4.2×10 17 particles m −2 s −1 sr −1 from the lower side by using the areal density (A.D.) of dislocation loops. The depth distribution of the dislocation loops shows a significant contribution of the high-energy CX neutrals. The energetic CX neutrals will give a great impact on material degradations.

Plasma–surface interaction effects during high ion temperature long pulse experiments in TRIAM-1M

TRIAM-1M tokamak is very suitable for studies on plasma material interaction because of its very long duration plasma discharges. We have investigated the damage and modification of plasma facing materials by exposing to the plasma in the scrape-off layer (SOL) of this machine and review the recent results in this paper. Dense dislocation loops were formed in Mo and W by exposing to the plasma for a few 10 min. This damage was caused by energetic charge-exchange hydrogen neutrals generated mainly in the lower half of the plasma. The mean flux of responsible neutrals for the damage was estimated to be 3×10 17 m −2 s −1 sr −1 . Plasma facing surfaces were covered by an impurity deposit consisting of very fine crystalline grains (about 1 nm in size), whose crystal structure is not normal body-centered-cubic (bcc) structure but face-centered-cubic (fcc) structure. It was found that retention of hydrogen in these peculiar materials is much stronger than `normal' Mo.

Anomalous cosmic rays and the generation of energetic neutrals in the region beyond the termination shock

Some characteristics of the neutral hydrogen flux of 55-80 keV detected by CELIAS/HSTOF on board SOHO (in particular, the intensity peaks during the periods when the instrument was directed towards the anti-apex of the local interstellar medium) suggest that the observed atoms may be created in the distant heliosphere, beyond the termination shock, from the anomalous cosmic ray protons which become neutralized by charge-exchange with the background gas. The theoretical models on which this conclusion was first based were very simple and missed some important features. Here we report about the first study of transport of low energy anomalous cosmic rays beyond the solar-wind termination shock employing a model of the heliosphere treating self-consistently the flows of plasma and neutral atoms and including the pressure of cosmic rays. The energetic neutral-atom (ENA) flux created by charge-exchange is calculated and compared with the observations of CELIAS/HSTOF. The viability of using ENAs as a means of imaging the structure of the termination shock, the distant heliosphere and the very local interstellar medium is discussed. We also include some results which may be interesting in connection to possible observations by INCA/Cassini and HENA/IMAGE.

Neutral velocity distribution at a negatively biased electrode in a collisional ion sheath

This paper reports on the planar one-dimensional theoretical estimation of neutral velocity distribution at a negatively biased electrode. The pressure is chosen to be high (∼few Torr) so that the ion sheath is collisional with charge exchange being the dominant collision mechanism. This collision leads to a distribution of ion velocities at the electrode and also generates a large number of energetic neutrals inside the ion sheath that bombards the electrode. Depending on the place of their generation inside the ion sheath, the neutrals have a distribution of velocities at the electrode. The theory indicates that the peak of the neutral velocity distribution shifts to lower velocities as pressure increases. It also indicates that in such collisional ion sheaths, the neutrals deposit more momentum and kinetic energy on the negatively biased electrode than the ions.

Optical and structural properties of highly c-axis oriented aluminum nitride prepared by sputter-deposition in pure nitrogen

Nanocrystalline aluminum nitride films (70–1400 nm thick) were prepared by rf-magnetron sputtering from an Al-target at a substrate temperature of 540 K. For the first time, the influence of the pressure on the film properties was investigated for sputter-deposition in pure nitrogen-atmosphere. The pressure was varied in the range from 0.15 to 4.0 Pa. The deposition rate decreases monotonically from 0.35 to 0.17 nm/s with pressure. Films of 300 nm thickness show at 1 Pa nitrogen pressure a maximum of the crystallite size (in direction parallel to the film normal), which amounts to 30 nm. Highly strained crystallites grow at pressures below 0.5 Pa. The films are (excepted sputtering above 2 Pa) in general highly c-axis oriented. It is found that the crystallites are of a cylinder-like shape with axis orientation parallel to the film normal and a roughly constant length-to-diameter ratio of 4. The optical properties static refractive index and optical Tauc gap depend weakly on the pressure and are on average 2.0 and 5.0 eV, respectively. The bombardment of the growing AlN by energetic nitrogen neutrals originating from reflection at the target, which was simulated by TRIM.SP Monte-Carlo calculations, is suggested to be the general origin of the preferred c-axis growth of sputter-deposited films.

Effect of target material on deposition and properties of metal-containing DLC (Me-DLC) coatings

Metal containing diamond-like carbon (Me-DLC) coatings were prepared by magnetron sputter deposition using tungsten, tungsten carbide, niobium and titanium as target materials. An essential parameter for the process characterization is the target voltage. The substrate heating during the film growth depends on the target material. For the tungsten target, the contribution of energetic neutrals to the heat flux is quite high. From the viewpoint of resistance to abrasive wear, the best properties were found for W-DLC. The Nb-DLC coatings have an intermediate wear rate but offer the lowest friction coefficients and a high performance under adhesive wear loading. All Me-DLC coatings investigated in this study exhibit clearly higher wear rates than metal-free DLC films prepared by a radio frequency glow discharge technique. Metallic intermediate layers allow the preparation of coatings with a very good adhesion.

Effects of unique ion chemistry on thin-film growth by plasma-surface interactions.

Plasma processing is a standard industrial method for the modification of material surfaces and the deposition of thin films. Polyatomic ions and neutrals larger than a triatomic play a critical role in plasma-induced surface chemistry, especially in the deposition of polymeric films from fluorocarbon plasmas. In this paper, low energy CF3+ and C3F5+ ions are used to modify a polystyrene surface. Experimental and computational studies are combined to quantify the effect of the unique chemistry and structure of the incident ions on the result of ion-polymer collisions. C3F5+ ions are more effective at growing films than CF3+, both at similar energy/atom of approximately 6 eV/atom and similar total kinetic energies of 25 and 50 eV. The composition of the films grown experimentally also varies with both the structure and kinetic energy of the incident ion. Both C3F5+ and CF3+ should be thought of as covalently bound polyatomic precursors or fragments that can react and become incorporated within the polystyrene surface, rather than merely donating F atoms. The size and structure of the ions affect polymer film formation via differing chemical structure, reactivity, sticking probabilities, and energy transfer to the surface. The different reactivity of these two ions with the polymer surface supports the argument that larger species contribute to the deposition of polymeric films from fluorocarbon plasmas. These results indicate that complete understanding and accurate computer modeling of plasma-surface modification requires accurate measurement of the identities, number densities, and kinetic energies of higher mass ions and energetic neutrals.

Polar CEPPAD/IPS energetic neutral atom (ENA) images of a substorm injection

The CEPPAD Imaging Proton Spectrometer on the POLAR spacecraft has proven to perform very well as an Energetic Neutral (ENA) atom imager, despite the fact that it was designed primarily for measuring energetic ions in-situ. ENAs emitted from the ring current can be detected during storm- as well as quiet-time conditions and can be monitored continuously for many hours at a time when Polar is situated in the polar cap. In addition, we are able to routinely detect ‘bursts’ of ENA emissions in response to substorm-associated ion injections. In this paper, we present ENA images of a single such event together with global auroral imager data from the POLAR VIS instrument. LANL geosynchronous energetic particle data, and ground magnetic Pi2 data in order to establish that such bursts are indeed caused by substorm injections.

Effect of nitrogen species and energy on the crystallinity and luminescent properties of GaN films grown by the reactive neutral beam-assisted evaporation process

The low-temperature growth of GaN on (0 0 0 1) sapphire substrates was studied using excited neutral nitrogen molecules as an approach to avoid ion damage in the ion beam assisted evaporation process. The excited neutral nitrogen molecules (N 2 ∗), together with nitrogen ions (N 2 +), were produced by electron impact in the discharge chamber encased in the ion source. To selectively extract N 2 ∗, the single grid configuration was replaced by a grounded triple grid configuration, through which the flux of ionic species to the growing film surface was significantly reduced. The crystallinity and luminescent properties of GaN films were greatly improved due to the increasing ratio flux of excited neutral/ion in the triple-grid configuration. TEM results inferred that the bombardment of ion beam on the growing GaN surface can induce a significant residual stress and more defects, compared to the excited neutral beam. The effect of the non-ionic nitrogen species on the film crystallinity is discussed in the aspect of kinetic energy. As the discharge voltage of the ion source increased, more energetic neutrals were generated by the charge transfer collision between nitrogen ions and thermal neutrals, and degraded the properties of GaN film.

Sun–Earth geometry, geomagnetic activity, and planetary F2 layer ion density: Part I: Signatures of magnetic reconnection

Our previous quantitative analyses have shown that geomagnetic activity and planetary ion density of the F2 layer of the ionosphere seem to share the same parent cause, the solar wind, whose entry into geospace is controlled by the Sun–Earth geometry. The thrust of this paper is four fold: (a) to establish the reality of this not clearly recognized connection, (b) to demonstrate that geomagnetic activity varies seasonally with three separate and independent components, viz. a semiannual, an annual and a Sun–Earth-distance determined component, all of which can be accurately derived from solar–terrestrial geometry alone, (c) to evaluate the contribution of each of these components which, taken together, appear to represent the steady-state signatures of the mechanism of magnetopause reconnection, and (d) to highlight the fact that the currently used planetary geomagnetic indices are deficient and therefore need to be revised. Since detailed understanding of the precise mechanism of the entry of solar wind energy into geospace is still lacking, no mechanism is suggested to show how solar wind energy is transported to the F2 layer (including low and equatorial latitudes). Magnetospheric electric fields, precipitation of energetic neutrals produced through charge exchange reactions with ions in the ring current and radiation belt particles, Joule heating, etc., may all be involved, but the energy for all such processes still comes from the solar wind. Apart from the three components of the reconnection mechanism mentioned above, a steady component due to the viscous interaction mechanism should also be present.

High-rate deposition of biaxially textured yttria-stabilized zirconia by dual magnetron oblique sputtering

Biaxially textured yttria-stabilized zirconia thin films were deposited at high rates (≈3 μm/h) using a two-target reactive magnetron sputtering geometry. The two small-area magnetron targets were mounted at 45° or 56° with respect to, and on opposite sides of, the substrate normal. The films showed strong (001) out-of-plane texture. Azimuthal x-ray diffraction scans confirmed strong in-plane texture with a best (111)-peak full width at half maximum of 23°. The alignment was presumably due to directed impingement of sputtered atoms and/or energetic Ar neutrals reflected from the Zr–Y target surfaces. The film texture was not as strong when only one sputter source was used. Sputtering geometries useful for large-scale deposition of textured films are described.

Interaction of plasma facing materials for fusion devices with low energy hydrogen and helium particles

Radiation damage formed in metal specimens exposed to long pulse tokamak plasmas of TRIAM-1M was examined by transmission electron microscopy. By comparing these results with those of low energy hydrogen ion irradiation it was concluded that the charge exchange energetic neutrals of hydrogen emitted from the core plasma caused remarkable displacement damage. The flux of the neutrals in the energy range of 0.5–3 keV which were responsible for displacement damage, was estimated to be about 1.5–3 × 1018 H/m2/s. These energetic neutrals cause not only material degradation at the sub-surface region but also change bulk properties of plasma facing components in a plasma confinement device. Effect of helium plasma was also discussed with emphasis on very strong effects on damage accumulation. Damage by He is a serious issue of plasma facing materials.

Compositional variations of sputter deposited Ti/W barrier layers on substrates with pronounced surface topography

Sputter deposited Ti/W barrier layers have been found to be Ti deficient with respect to the target composition, which is attributed to the preferential resputtering of Ti from the deposited films by energetic neutrals or ions from the discharge. On the other hand, the sputtering yield of most materials is known to be strongly dependent on the angle of incidence of the bombarding species. Due to this angular dependence the resputtering rate of Ti/W films will also be a function of the local orientation of the surface. The substrates of the IC circuits onto which the Ti/W barrier layers are deposited normally possess a pronounced surface topography. The purpose of this work is to study the deviation of the expected Ti/W concentrations at sloped surfaces as a function of the energy of the bombarding species as well as the atom to ion flux arrival ratio. It is shown that the Ti concentration in the films as a result of the preferential sputtering does exhibit substantial concentration variations across sloped surfaces at constant discharge parameters. The experiments are done in an Ar rf discharge, using a Ti/W target with 50 at.% Ti and 50 at.% W. The films have been analysed by Rutherford Backscattering Spectrometry (RBS) and energy-dispersive X-ray Spectroscopy (EDS). The experimental results are discussed and compared with dynamic simulations using the Transport of Ions in Matter (TRIM) code, which indicate that the loss of Ti in the deposited films is primarily due to ballistic preferential sputtering effects.

Boronization in future devices – protecting layer against tritium and energetic neutrals

A thin boron film is attractive as a deuterium/tritium free wall, and as a protecting layer against impact of energetic charge-exchange neutrals in future fusion devices with long pulse operation. New experimental evidence is given for desorption of hydrogen isotopes from these films at relatively low temperature. Most hydrogen atoms in a boron-coated layer are re-emitted to the plasma side below 400°C without penetration into the substrate of stainless steel. The maintainability of a thin boron layer during a long pulse operation may be a problem. Boron atoms are hardly removed by pumping because their hydrides are easily disintegrated and redeposited. Gross migration of boron atoms inside the vessel is a concern. A condition required for avoiding the migration is discussed.