Lighter Ion Species Research Articles

Sputtering yield reduction for nano-columnar W surfaces under D ion irradiation

This study investigates the sputtering properties of nano-columnar tungsten surfaces under 2 keV D2+ irradiation. It was conducted both by performing experiments in a highly sensitive quartz crystal microbalance setup as well as by numerical methods using two simulation codes called SPRAY and SDTrimSP-3D. A key question was whether the strong sputtering yield reduction observed in previous studies under 2 keV Ar+ irradiation is maintained when using a much lighter ion species like deuterium. For the latter, a substantially larger projected range in the material has to be expected. Therefore, a total confinement of the ion-solid interaction within the W nano-columns cannot be assumed a priori. However, both the experiments and the numerical simulations showed in good agreement that the geometrically induced sputtering yield reduction is still observed to the same extent despite the increased range of ions in the material. Thus, a potential application of such nano-columnar tungsten surfaces as plasma facing components in future nuclear fusion devices is not affected.

Nonlinear kinetic simulation study of the ion–ion streaming instability in single- and multi-ion species plasmas

The nonlinear evolution of the ion–ion streaming instability (IISI) is studied using numerical techniques novel to this problem that afford direct insight into the evolution of the particle distributions of each species. During the linear phase of the instability, we demonstrate quantitative agreement with linear kinetic theory. Subsequently, the electrostatic field generated by the IISI causes ring-like velocity distributions of ions to form that are both heated and slowed to varying degrees relative to their initial flows. Due to variation in the trapping conditions for ion species of differing charge-to-mass ratio, when flows of multiple species interact, the nonlinear evolution of each species can be starkly different: we show a case where a lighter ion species is completely stopped by a heavier ion species via the IISI alone (i.e., without collisions) and, for the first time, demonstrate how the IISI can introduce a relative flow between ion species that initially have the same flow velocities, thereby separating them.

Ion species stratification within strong shocks in two-ion plasmas

Strong collisional shocks in multi-ion plasmas are featured in many environments, with Inertial Confinement Fusion (ICF) experiments being one prominent example. Recent work [Keenan et al., Phys. Rev. E 96, 053203 (2017)] answered in detail a number of outstanding questions concerning the kinetic structure of steady-state, planar plasma shocks, e.g., the shock width scaling by the Mach number, M. However, it did not discuss shock-driven ion-species stratification (e.g., relative concentration modification and temperature separation). These are important effects since many recent ICF experiments have evaded explanation by standard, single-fluid, radiation-hydrodynamic (rad-hydro) numerical simulations, and shock-driven fuel stratification likely contributes to this discrepancy. Employing the state-of-the-art Vlasov-Fokker-Planck code, iFP, along with multi-ion hydro simulations and semi-analytics, we quantify the ion stratification by planar shocks with the arbitrary Mach number and the relative species concentration for two-ion plasmas in terms of ion mass and charge ratios. In particular, for strong shocks, we find that the structure of the ion temperature separation has a nearly universal character across ion mass and charge ratios. Additionally, we find that the shock fronts are enriched with the lighter ion species and the enrichment scales as M4 for M ≫ 1.

Multiple ion species plasmas with thermal ions in an oblique magnetic field

Using a three-fluid model, the combined effects of an oblique magnetic field and finite temperature of positive ion species on the characteristics of the sheath region of multi-component plasmas are investigated numerically. It is assumed that the ion species are singly charged and have different masses. In the presence of an external magnetic field, it is shown that the density distribution of positive ion species (especially the lighter ion species) begins to fluctuate and does not decrease monotonically towards the wall. Also, it is shown that by increasing the magnetic field, the amplitude of fluctuation increases and its position moves towards the sheath edge. Moreover, it is illustrated that the presence of the magnetic field affects the sheath width and by increasing the magnetic field, the sheath width decreases. In addition, the results show that in the presence of the magnetic field, the increase of temperature of positive ion species has an infinitesimal effect on the sheath width and density distribution of positive ion species.

Laser-induced fluorescence measurements of argon ion velocities near the sheath boundary of an argon–xenon plasma

The Bohm sheath criterion in single- and two-ion species plasma is studied with laser-induced fluorescence using a diode laser. Xenon is added to a low pressure unmagnetized dc hot filament argon discharge confined by surface multidipole magnetic fields. The Ar II transition at 668.614 nm is adopted for optical pumping to detect the fluorescence from the plasma and to measure the argon ion velocity distribution functions with respect to positions relative to a negatively biased boundary plate. The structures of the plasma sheath and presheath are measured by an emissive probe. The ion concentrations of the two-species in the bulk plasma are calculated from ion acoustic wave experiments. Results are compared with previous experiments of Ar–He plasmas in which the argon ions were the heavier ion species. Unlike the previous results, the argon speed is slower than its own Bohm velocity near the sheath–presheath boundary in the Ar–Xe plasma where argon ions are the lighter ion species. We argue that this result is consistent with the behaviour of the helium ion required by the generalized Bohm criterion in the previous experiments with Ar–He plasmas. Further, our results suggest that the measured argon ion speed approaches the ion sound speed of the system.

Low-frequency electromagnetic waves in a partially ionized multi-component magnetoplasma

The dispersion properties of low-frequency electromagnetic waves in a partially ionized multi-ion dusty magnetoplasma containing electrons, ions of two different species, charged dust particles and neutrals have been investigated. It has been found that the combined effects of stationary charged dust particles and an additional ion component introduce a new cut-off frequency, and that the polarities of dust particles and the second ion-species significantly modify this new as well as the existing forbidden frequency bands. It has also been observed that the collisions of lighter ion species significantly reduce or completely eliminate both the new and existing forbidden frequency bands, while those of heavier ions reduce or completely eliminate the new forbidden frequency band, but enhance the existing forbidden frequency band.

Isotope effects in an electron cyclotron resonance ion source inmixtures of 15N/14N

Isotope effects in an electron cyclotron resonance ion source areinvestigated in mixtures of 15N and 14N. The ratio of the extractedcurrents I(15Nq + )/I(14Nq + ) increases with increasingcharge state q. This so-called `isotope anomaly' proves to be dependent onboth the mixture ratio of the gases as well as the microwave power fed intothe discharge. It is shown that the often-utilized `ion cooling' model cannotsolely explain the observed isotope effects as long as the temperature isassumed to be equal for all ion species. Since we observed the occurrence oflow-frequency noise in correlation with the isotope anomaly, we suggest thatthe ions are additionally heated by ion Landau damping. Due to the massdependence of this process, the lighter ion species will be heatedpredominantly. This results in an enhanced transport of lighter ions out ofthe plasma and a better confinement for the heavier component, which,consequently, may be more effectively ionized to higher charges states.

Charged particle motion inside the retarding potential analyzer

One of the Ionospheric Plasma and Electrodynamics Instrument (IPEI) payload onboard the Republic of China satellite 1 (ROCSAT-1) spacecraft is a Retarding Potential Analyzer (RPA) to measure the ionospheric plasma composition, temperature, and the ram velocity of the flow. A three-dimensional computer model has been developed to simulate the ion and electron motions inside the RPA of the IPEI payload. The dynamical changes in the charged particles’ motions corresponding to different electric potential profiles inside the RPA detector resulted in some distinctive measuring characteristics of the RPA detector, which show discrepancies in the measured of currents at the collector plate between the analytical model and the computer simulation. The important discoveries in the computer model are: (a) Ion transparency cannot be represented by the physical grid sizes in the detector. The alignments of the grid wires will also affect the ion transparency. The grid planes should be aligned for the maximum optical transparency to detect light ion fluxes. (b) Cut-off energy for protons is not as sharp as indicated in the analytical model. The contamination in the collector current from lighter ion species is possible. (c) Electron contamination at the collector plate is noticed if the mesh sizes are larger than the electron Debye length.

On accuracy of RPA measurements of ion fluxes

One of the IPEI (Ionospheric Plasma and Electrodynamics Instrument) payload onboard the ROCSAT-1 spacecraft is a RPA detector to measure the ionospheric plasma species and temperature. A 3-D computer model has been developed to simulate the ion and electron motions inside the RPA. The results are compared with the analytical solutions in the 1-D theoretical model. Overall agreement between the two approaches is good. However, the detailed dynamical behavior of the charged particles' motions in response to the changes in the electric potential profiles inside the RPA detector, explains the causes of the physical discrepancies between the analytical model and the computer simulation. The important discoveries in the computer model are: (a) Ion transparency can not be represented by the physical grid sizes in the detector. The alignments of the meshed-wire planes will affect the ion transparency. (b) Cut off energy for protons is not as sharp as indicated in the analytical model. The contamination in the collector current from lighter ion species is possible. (c) Electron contamination in the collector plate is noticed if the mesh sizes are larger than the electron Debye length.

Laboratory experiments on ion waves associated with parallel electric fields

We have performed laboratory experiments to study the property of waves along a quasi‐static parallel electric field in a plasma column. The parallel electric field is setup by allowing two different sources of plasmas to interpenetrate each other. Since all ions accelerated by the electric field will gain the same energy across the potential drop, the lighter ion species will acquire higher velocities. The wave modes associated with a single ion species (Ar+) and two ion species (He+ and Ar+) are quite different. In the plasma with only one species, the fast mode of the ion‐electron acoustic waves is observed in our experiment. This mode is always damped, and the results are analyzed by the theory. When the plasma consists of two ion species, the linear theory of ion acoustic waves indicates that the velocity difference between two ion species of different masses can excite ion‐ion acoustic waves via the two‐stream instability. Experimentally, unstable waves generated by the costreaming He+ and Ar+ ions are found to exist. By actively launching waves near the unstable frequencies at the plasmas, we observe that the waves grow at a certain range of distance. The results are quantitatively explained with the kinetic dispersion relations for the ion acoustic waves.

Ion Beam Assisted Deposition of Thin Carbonaceous Films: II . Adhesion Characteristics in Aqueous and Cathodically Delaminating Conditions

The adhesion characteristics of thin (ca. 160Å) carbonaceous films produced by ion beam assisted deposition (IBAD) on steel were tested by the tape peel test under dry, wet, and cathodically delaminating conditions. It has been proposed that the degree of bonding established in IBAD films is dependent on the amount of energy transferred in recoil processes at the interface. To test this, ion beams of various gases (H, He, Ne, , ) were accelerated (30 keV) and directed into a monomeric target gas (1,3 butadiene) held in front of the steel substrate. For purposes of comparison, adhesion tests were also performed on a series of conventional marine coatings (epoxy‐polyamide, polyurethane, vinyl). The work of adhesion of each film/substrate combination was calculated and compared to the adhesion data. The adhesion of ion assisted coatings under all conditions tested was found to be many orders of magnitude greater than conventional coatings and did not correlate with surface energy or work of adhesion calculations. The adhesion performance of conventional coatings, on the other hand, did correlate with surface energy data. Although the adhesion of the ion assisted coatings is attributed to an atomically mixed interface created by recoil processes, electronic damage due to “stitching” phenomenon may also contribute to enhanced film bonding when irradiated by lighter ion species.