Low Energy Neutrals Research Articles

Complementary interstellar detections from the heliotail

The heliosphere is a protective shield around the solar system created by the Sun’s interaction with the local interstellar medium (LISM) through the solar wind, transients, and interplanetary magnetic field. The shape of the heliosphere is directly linked with interactions with the surrounding LISM, in turn affecting the space environment within the heliosphere. Understanding the shape of the heliosphere, the LISM properties, and their interactions is critical for understanding the impacts within the solar system and for understanding other astrospheres. Understanding the shape of the heliosphere requires an understanding of the heliotail, as the shape is highly dependent upon the heliotail and its LISM interactions. The heliotail additionally presents an opportunity for more direct in situ measurement of interstellar particles from within the heliosphere, given the likelihood of magnetic reconnection and turbulent mixing between the LISM and the heliotail. Measurements in the heliotail should be made of pickup ions, energetic neutral atoms, low energy neutrals, and cosmic rays, as well as interstellar ions that may be injected into the heliosphere through processes such as magnetic reconnection, which can create a direct magnetic link from the LISM into the heliosphere. The Interstellar Probe mission is an ideal opportunity for measurement either along a trajectory passing through the heliotail, via the flank, or by use of a pair of spacecraft that explore the heliosphere both tailward and noseward to yield a more complete picture of the shape of the heliosphere and to help us better understand its interactions with the LISM.

Low-Energy Plasma Source for Clean Vacuum Environments: EUV Lithography and Optical Mirrors Cleaning

Plasma cleaning of extreme ultra-violet (EUV) optics for the semiconductor industry requires atomic-level precision. Low-energy ions and neutrals can be highly beneficial for this purpose. However, ion energies in many industrial capacitively or inductively coupled plasmas may be too high for atomic precision processing so that ions can cause sputtering and re-deposition of materials and produce vacuum system contamination. We discuss two sources that create low-energy ions: a capacitively coupled (CCP) plasma source operating at several tens of MHz and an electron beam gas ionization source. The goal here is to minimize the contamination by limiting the ion impact energy to a few tens of electron-volts. Ion energy and flux measurements on a grounded surface are characterized with a compact retarding field ion spectrometer. Plasma-induced contamination is quantified using X-ray photo-electron spectroscopy (XPS). Low ion energy plasma sources introducing little surface contamination may be interesting for cleaning and accelerated testing in EUV lithography (EUVL)-related research and for cleaning of front-end optical mirrors in fusion reactor diagnostics.

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).

Molecular dynamics simulations of Si etching in Cl- and Br-based plasmas: Cl+ and Br+ ion incidence in the presence of Cl and Br neutrals

Classical molecular dynamics (MD) simulations have been performed for Cl+ and Br+ ions incident on Si(100) surfaces with Cl and Br neutrals, respectively, to gain a better understanding of the ion-enhanced surface reaction kinetics during Si etching in Cl- and Br-based plasmas. The ions were incident normally on surfaces with translational energies in the range Ei = 20–500 eV, and low-energy neutrals of En = 0.01 eV were also incident normally thereon with the neutral-to-ion flux ratio in the range Γn0/Γi0 = 0–100, where an improved Stillinger--Weber potential form was employed for the interatomic potential concerned. The etch yields and thresholds presently simulated were in agreement with the experimental results previously reported for Si etching in Cl2 and Br2 plasmas as well as in Cl+, Cl2+, and Br+ beams, and the product stoichiometry simulated was consistent with that observed during Ar+ beam incidence on Si in Cl2. Moreover, the surface coverage of halogen atoms, halogenated layer thickness, surface stoichiometry, and depth profile of surface products simulated for Γn0/Γi0 = 100 were in excellent agreement with the observations depending on Ei reported for Si etching in Cl2 plasmas. The MD also indicated that the yield, coverage, and surface layer thickness are smaller in Si/Br than in Si/Cl system, while the percentage of higher halogenated species in product and surface stoichiometries is larger in Si/Br. The MD further indicated that in both systems, the translational energy distributions of products and halogen adsorbates desorbed from surfaces are approximated by two Maxwellians of temperature T1 ≈ 2500 K and T2 ≈ 7000–40 000 K. These energy distributions are discussed in terms of the desorption or evaporation from hot spots formed through chemically enhanced physical sputtering and physically enhanced chemical sputtering, which have so far been speculated to both occur in the ion-enhanced surface reaction kinetics of plasma etching.

Temperature dependence of the neutralization of low energy K+ ions scattered from Au nanoclusters on SiO2

Neutralization during the scattering of 2.5keV K+ from Au nanoclusters on SiO2/Si(111) is investigated as a function of post-deposition surface temperature. Small nanoclusters are formed by deposition at low temperature (115K), as evidenced by a larger neutralization probability for singly scattered K+ as compared to scattering from Au metal. The neutralization probability in scattering from small clusters is found to reversibly decrease with increasing surface temperature. This contradicts the general understanding of charge exchange for alkali low energy ion scattering, however, which predicts that higher temperatures should increase the neutralization. Temperature-dependent changes in surface composition or structure are ruled out as causes, while additional experiments are proposed to resolve the discrepancy.

On the ionization and dissociative photoionization of iodomethane: a definitive experimental enthalpy of formation of CH3I

The dissociative photoionization onset energy of the CH(3)I --> CH(3)(+) + I reaction was studied at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source (SLS) using a new imaging photoelectron photoion coincidence (iPEPICO) apparatus operating with a photon resolution of 2 meV and a threshold electron kinetic energy resolution of about 1 meV. Three previous attempts at establishing this value accurately, namely a pulsed field ionization (PFI)-PEPICO measurement, ab initio calculations and a mass-analyzed threshold ionization (MATI) experiment, in which the onset energy was bracketed by state-selected excitation to vibrationally excited (2)A(1) A states of the parent ion, have yielded contradictory results. It is shown that dimers and adducts formed in the supersonic molecular beam affected the PFI-PEPICO onset energy. The room temperature iPEPICO experiment yields an accurate 0 K onset of 12.248 +/- 0.003 eV, from which we derive a Delta(f)H(o)(298 K)(CH(3)I) = 15.23 +/- 0.3 kJ mol(-1), and the C-I bond energy in CH(3)I is 232.4 +/- 0.4 kJ mol(-1). The room temperature breakdown diagram shows a fine structure that corresponds to the threshold photoelectron spectrum (TPES) of the A state. Low internal energy neutrals seem to be preferentially ionized in the A state when compared with the X state, and A state peaks in the TPES are Stark-shifted as a function of the DC field, whereas the dissociative photoionization of X state ions is not affected. This suggests that there are different competing mechanisms at play to produce ions in the A state vs. ions in the X state. The competition between field ionization and autoionization in CH(3)I is compared with that in Ar, N(2) and in the H-atom loss energy region in CH(4)(+). The binding energies of the neutral and ionic Ar-CH(3)I clusters were found to be 26 and 66 meV, respectively.

Time-of-flight spectroscopy of the energy distribution of laser-ablated atoms and ions

The growth of ultrathin films, deposited by laser ablation, crucially depends on the energy of the ablated species. Therefore, a time-of-flight (TOF) spectrometer has been constructed and measurements have been carried out in order to determine the energy distribution of laser-ablated Fe and Pt atoms and ions in the plasma created by nanosecond pulses of a frequency-doubled neodymium doped yttrium aluminum garnet laser. The experiments have been performed in ultrahigh vacuum at relatively low laser power. For measuring the spectra of the neutrals, a cross-beam electron source for postionization and electric as well as magnetic fields for repelling the ions are employed. Nevertheless, measurements of neutral particles are restricted to low plasma densities due to electrostatic shielding within the plasma, leading to an inefficient deflection of charged particles by electrostatic and magnetic fields. Test measurements have been performed by utilizing the TOF spectrometer as a pressure gauge and also by chopping the electron beam, running the TOF spectrometer as a residual gas mass spectrometer. The spectra of the laser-ablated plasmas have shown plasma conditions with a Debye length of approximately 10(-4) m, densities of 10(15)-10(16) m(-3) and ion energies up to 150 eV. Neutral spectra have shown an unexpectedly low fraction of neutrals (10(-3)-10(-4)) and hyperthermal energies up to several 10 eV, possibly contributed by recombination of ions and electrons in the plasma. Even though gas spectra had demonstrated the expected sensitivity of the TOF spectrometer for low-energy neutrals, no thermally evaporated neutral atoms could be found.

Time-dependent simulations of the global polar wind

It has been clearly established that there is a substantial outflow of ionospheric plasma from the Earth's ionosphere in both the northern and southern polar regions. The outflow consists of both light thermal ions (H + and He +) and an array of energized ions (NO +, O 2 +, N 2 +, O +, N +, He +, and H +). If the outflow is driven by thermal pressure gradients in the ionosphere, the outflow is called the “classical” polar wind. On the other hand, if the outflow is driven by energization processes either in the auroral oval or at high altitudes in the polar cap, the outflow is called the “generalized” polar wind. In both cases, the field-aligned outflow occurs in conjunction with magnetospheric convection, which causes the plasma to drift into and out of the sunlit hemisphere, cusp, polar cap, nocturnal auroral oval, and main trough. Because the field-aligned and horizontal motion are both important, three-dimensional (3-D) time-dependent models of the ionosphere–polar wind system are needed to properly describe the flow. Also, as the plasma executes field-aligned and horizontal motion, charge exchange reactions of H + and O + with the background neutrals (H and O) act to produce low-energy neutrals that flow in all directions (the neutral polar wind). This review presents recent simulations of the “global” ionosphere–polar wind system, including the classical, generalized, and neutral polar winds. The emphasis is on displaying the 3-D and dynamical character of the polar wind.

Optical emission spectroscopy in an inverted cylindrical magnetron plasma

Optical emission spectroscopy was performed on a novel inverted cylindrical magnetron (ICM) sputtering system. This ICM technique offers the advantage of depositing insulating films such as alumina. Initial data were acquired for two conditions: titanium targets with argon (85 sccm) at 0.27 Pa and 6.5 kW; aluminum targets with argon (85 sccm) also at 0.27 Pa and 6.5 kW. For both plasma conditions, emission scans were acquired from 400 to 825 nm. After calibrating our spectroscopy system's optical response, we generated electronic Boltzmann plots using argon lines to evaluate the relative population of five specific excited states. From the Boltzmann plots, we calculated a Boltzmann temperature for each condition. Results indicate a higher temperature (5200 K) for the Ti conditions than for the Al conditions (2100 K), though the population of the argon excited states was higher for the Al conditions. These results are generally consistent with a two-temperature electron energy distribution function where the high energy electrons (responsible for the initial excitation of argon ground state atoms) have more energy for the Al plasma (producing more excited states) but where the low energy electrons and/or neutrals and ions (responsible for the relative distribution of excited states which are close in energy) have lower energy (producing a lower Boltzmann temperature).

Angular resolved energy distributions of low energy light ions reflected from a polycrystalline Mo surface

We have developed an experimental system to study systematically the interaction of low energy beams with materials. The angular distributions and energy spectra of positive and negative ions reflected from a polycrystalline Mo surface bombarded by a beam of low energy (1–3keV) ions and neutrals of H, H2, H3, He, and O have been measured with this system. The experimental results show that the charge state of outgoing particles yields a pronounced difference in the reflection coefficient and some weaker effect upon the energy distribution. The reflected beams usually showed maximum intensities at around the angle of mirror reflection of the incident beams. A carbon coated Mo sample prepared in a separate plasma chamber was also tested.

Microscopic damage of metals exposed to the helium discharges in TRIAM-1M tokamak and its impact on hydrogen recycling process

High-energy charge-exchange particles bombarding the plasma-facing wall may cause not only surface sputtering but also damage the materials inside due to their rather high energy. In the case of burning plasma, we should take into account the effects of helium because it is well known that helium atoms have much stronger effects on material damage than hydrogen atoms. In this paper, therefore, microscopic damage of metals exposed to long pulse discharges of helium plasma in TRIAM-1M was studied and the impact on the hydrogen recycling process was discussed by comparing it with helium ion irradiation experiments. Considerably larger amounts of dislocation loops and very dense fine bubbles were formed by the irradiation of rather low-energy charge-exchange neutrals of helium. According to the irradiation experiments with low-energy helium ions, formation of dense helium bubbles drastically enhances hydrogen trapping and makes the desorption difficult. These results indicate that the hydrogen recycling phenomenon during the burning plasma discharge must be quite different from that of the hydrogen plasma discharge experiments.

Tritium retention of plasma facing components in tokamaks

The areal distribution of tritium retention in tiles from TEXTOR, TFTR, JT-60U and JET has been measured via the imaging plate technique and the results are discussed from the perspective of carbon–hydrogen chemistry. It is found that the observed tritium distribution clearly shows asymmetries in poloidal and toroidal directions and also reflects the local temperature history of the analyzed tiles. We show the first clear evidence of the loss of high energy tritons by toroidal magnetic field ripple. We distinguish three different contributions to tritium retention in tokamaks with carbon plasma facing components: high energy tritons escaping from the core plasma, low energy ions and neutrals from the edge plasma, and molecular tritium from gas fueling. These components are retained at different depths and with different concentrations. Tritium from the edge plasma dominates the retained inventory but could be reduced if the surface temperature was higher. We propose tokamak operation with plasma facing components above 1000 K as a possible way to reduce the tritium inventory.

On the potential for direct or MHD conversion of power from a novel plasma source to electricity for microdistributed power applications

The generation of electricity using direct electrostatic and magnetohydrodynamic (MHD) conversion of the plasma-particle energy of small to midsize chemically assisted microwave or glow discharge plasma (CA-plasma) power sources in the range of a few hundred Watts to several tens of kilowatts for microdistributed commercial applications (e.g., household, automotive, light industry, and space-based power) is studied for the first time. In the determination of the effect of plasma parameters on conversion efficiency, careful attention was paid to the unique plasma conditions of low-pressure, low-ionization fraction, and nonthermal ion energies that are much greater than that of the thermal ions of traditional MHD but much lower than those of a fully ionized plasma typically generated for fusion experiments. The density of plasma ions and neutrals and their cross sections for processes such as charge exchange were also considered. The most important parameters were found to be charged-particle density and energy, as well as the large inventory of neutral gas atoms and molecules. Momentum and charge exchange of plasma ions with the large background fraction of neutrals represents a limitation to conversion efficiency. Two conversion technologies were examined in some detail. We considered the possibility of converting a CA-plasma using adaptations of a member of the broad category of electromagnetic direct converters previously developed for recovery and conversion of the high energy particles lost from tandem mirror and magnetically confined plasmas, and an MHD converter previously developed for conversion of high pressure combustion gases to electricity. While it was found that both conversion techniques performed well under ideal conditions for conversion of plasma to electricity showing conversion efficiencies of >50%, the tight coupling of plasma cell and converter, size limitations, particle energy, and the substantial inventory of relatively low energy neutrals may substantially reduce the efficacy of direct electrostatic converters under these conditions. However, MHD conversion of CA-plasmas appears feasible at /spl sim/50% efficiency with a simple compact design.

Study of quasi-resonant neutralization of low energy H + ion colliding with rare-gas solid surfaces in terms of hole localization and delocalization

It is useful to consider the electron capture by H + ions in terms of the hole capture by targets. Although Ar, Kr and Xe rare-gas solids are the closed-electron-shell material, the valence band is formed because of a weak covalent bonding among neighboring atoms. The electrons in the valence band are immobile, but the hole captured by target atoms from H + ion can dissipate. If the hole dissipates into the bulk before being recaptured by neutralized H, the ion survival probability is significantly decreased. The time scale of this hole dissipation can compete with the time scale of collisional interaction in the impact energy range of 10–500 eV. As a result, the neutralization probability strongly depends on the collision velocity and the valence-band width. The hole delocalization effect can explain the experimental observations: significantly smaller ion survival probability for Kr and Xe solid surfaces than that for a Ar solid surface, and the related phenomenon.

Process damage assessment of a low energy inductively coupled plasma-based neutral source

In this article, we report process damage studies using a low energy inductively coupled plasma-based neutral stream source. Low energy neutrals are generated by the surface reflection neutralization method. Quasistatic capacitance-voltage measurement results of this work demonstrate that this low energy neutral source, which provides controllable fast neutrals for cleaning applications, induces much less damage than a pulsed inductively coupled plasma source. Most of the neutral process damage is caused by ultraviolet photons escaping from the plasma source zone. The process-induced damage by fast neutrals increases as the reflector bias is lowered. The fast neutral process damage also increases with the rf power. Unlike the neutral process-induced damage, the damage induced by pulsed plasmas is found to be a function of both the gas composition and rf pulsed frequencies.

Stripping and cleaning of photoresist using low energy neutrals

Experimental studies of a broad-area hyperthermal neutral stream source for photoresist cleaning are reported. Low energy neutrals are generated from a high-density inductively coupled plasma by the surface reflection neutralization method. Rates of removal of photoresist polymers such as polymethyl methacrylate as functions of the hyperthermal translational energy and background neutral pressure are reported. The results demonstrate this low energy neutral source provides controllable fast neutrals for cleaning applications. Ex situ and in situ measurements yield typical removal rates of about 10 Å/s without stream collimation. The removal rates at increasing pressures show a trade-off between creating higher density plasma, leading to a greater initial neutral flux and attenuation of neutrals due to collisions. These observations are in good agreement with previous Monte Carlo simulations. The removal rate increases linearly with rf power and is nearly constant as the neutral energy is varied by varying the reflector bias. One consequence of neutral directionality in collimated stream applications is the presence of unremoved carbon compounds on the sidewalls as indicated by angle-resolved x-ray photoelectron spectroscopy.

Trajectory-dependent neutralization of low energy Li+ scattered from alkali adsorbates on Ni(111).

Strongly trajectory-dependent neutralization is observed for low energy ${\mathrm{Li}}^{+}$ scattered from Cs adsorbed on Ni(111). For near glancing-exit scattering, the ion yield as a function of Cs coverage shows a pronounced peak and this behavior can be described well by a resonant charge-exchange model using the average work function. However, the yield is much smaller for near normal-exit scattering and the neutralization is mainly determined by the local electrostatic potential. Trajectory-independent, complete neutralization is observed at high coverages.

The low-energy neutral fluxes and their impurity production at the walls of ASDEX

With the low-energy neutral particle analyser LENA at ASDEX the density dependence of the neutral flux, its mean energy and its impurity production at the walls has been investigated for ohmic hydrogen and deuterium discharges. For hydrogen a large increase of the neutral flux with increasing density was observed, while the mean energy of the neutrals decreases. This holds for Hydrogen over the whole range of densities. At constant density the low energy neutral flux increases with the safety factor q. This is in contrast to the high energy flux as measured with the charge exchange analyser. The flux increase of low energy neutrals can be correlated with the increased decay length of the electron density for higher q-values. For deuterium the same behavior up to densities of n ̄ e ∼- 3.5 × 10 13 cm −3 was observed. For higher densities the flux behaves different corresponding to the different confinement regimes. Only in the regime of saturated ohmic confinement, known so far, the flux increases further with increasing n ̄ e ,. With the operational conditions of the new divertor configuration in ASDEX a further increase of τ e with n ̄ e up to the density limit was observed (improved ohmic confinement). In this regime the neutral flux saturates with increasing n ̄ e . In spite of the reduced impurity production at the walls, Z eff increases in this case.

Application of diffusion theory to neutral atom transport in fusion plasmas

It is found that the energy dependent diffusion theory provides excellent accuracy in the modelling of transport of neutral atoms in fusion plasmas. Two reasons in particular explain the good accuracy. First, while the plasma is optically thick for low energy neutrals, it is optically thin for high energy neutrals and the diffusion theory with Marshak boundary conditions gives accurate results for an optically thin medium, even for small values of c, the ratio of the scattering cross-section to the total cross-section. Second, the effective value of c at low energy is very close to 1 because of the downscattering via collisions of high energy neutrals. The first reason is proven computationally and theoretically by solving the transport equation in a power series in c and solving the diffusion equation with ‘general’ Marshak boundary conditions. The second reason is established numerically by comparing the results from a one-dimensional, general geometry, multigroup diffusion theory code, written for this purpose, with the results obtained using the transport code ANISN. Earlier studies comparing one-speed diffusion and transport theory indicated that the diffusion theory would be inaccurate. A detailed analysis shows that this conclusion is limited to a very specific case. Surprisingly, for a very wide range of conditions and when energy dependence is included, the diffusion theory is highly accurate.

Detection of low energy neutrals by a channel electron multiplier

A method for determining relative channel electron multiplier (CEM) detection efficiencies for low energy neutrals using the technique of direct recoiling is described. The method uses time-of-flight analysis of low-energy atoms recoiling from an energetic ion/surface collision. The system used for demonstration is 1–10 keV Ne + scattering from a polyimide resin surface, producing H and C recoils. The behavior of the CEM detection efficiency γ can be divided into two regions: A flat plateau region is observed at high velocity while below a specific velocity, ν H = 4.4 × 10 7 and ν C = 3.2 × 10 7 cm/s, γ decreases linearly with velocity with slope dγ dν = 6.4 × 10 −8 s/cm . This behavior is discussed in terms of secondary electron emission by a kinetic ejection process.