Two-ion Species Plasma Research Articles

Proton acceleration from picosecond-laser interaction with a hydrocarbon target

As an intense picosecond laser pulse irradiates a hydrocarbon target, the protons therein can be accelerated by the radiation pressure as well as the sheath field behind the target. We investigate the effect of the laser and hydrocarbon target parameters on proton acceleration with two/three-dimensional particle-in-cell simulations. It is found that the resulting two-ion species plasma can generate a multiple peaked charge-separation field that accelerates the protons. In particular, a smaller carbon-to-hydrogen ratio, as well as the thinner and/or lower density of the target, leads to a larger sheath field and thus proton beams with a larger cutoff energy and smoother energy spectrum. These results may be useful in achieving high-flux quasi-monoenergetic proton beams by properly designing the hydrocarbon target.

Propagation of shear Alfvén waves in a two-ion plasma and application as a diagnostic for the ion density ratio

In this paper, we propose an efficient diagnostic technique for determining spatially resolved measurements of the ion density ratio in a magnetized two-ion species plasma. Shear Alfvén waves were injected into a mixed helium–neon plasma using a magnetic loop antenna, for frequencies spanning the ion cyclotron regime. Two distinct propagation bands are observed, bounded by $\omega < \varOmega _\textrm {Ne}$ and $\omega _{ii} < \omega < \varOmega _\textrm {He}$ , where $\omega _{ii}$ is the ion–ion hybrid cutoff frequency and $\varOmega _\textrm {He}$ and $\varOmega _\textrm {Ne}$ are the helium and neon cyclotron frequencies, respectively. A theoretical analysis of the cutoff frequency was performed and shows it to be largely unaffected by kinetic electron effects and collisionality, although it can deviate significantly from $\omega _{{ii}}$ in the presence of warm ions due to ion finite Larmor radius effects. A new diagnostic technique and accompanying algorithm was developed in which the measured parallel wavenumber $k_\parallel$ is numerically fit to the predicted inertial Alfvén wave dispersion in order to resolve the local ion density ratio. A major advantage of this algorithm is that it only requires a measurement of $k_\parallel$ and the background magnetic field in order to be employed. This diagnostic was tested on the Large Plasma Device at UCLA and was successful in yielding radially localized measurements of the ion density ratio.

Drift mode in a bounded plasma having two-ion species

The drift wave is investigated in a two-ion species plasma in several different cases. The global drift mode is studied in a plasma bounded in a cylinder having Gaussian density profile corresponding to different poloidal wavenumbers. The frequency of the mode becomes a little larger when it is investigated without including the ion cyclotron wave dynamics. The effect of magnetic shear on the wave propagation along the density gradient is studied in a Cartesian geometry assuming absorbing boundary. It is found that the wave amplitude is reduced when two-ion species are present (with the same concentration) compared to pure electron-ion plasma.

Exact Solution for the Generalized Bohm Criterion in a Two-Ion-Species Plasma

For a weakly collisional two-ion species plasma, it is shown that the minimum phase velocity of ion acoustic waves (IAWs) at the sheath-presheath boundary is equal to twice the phase velocity in the bulk plasma. This condition provides a theoretical basis for the experimental results that each ion species leaves the plasma with a drift velocity equal to the IAW phase velocity in the bulk plasma [D. Lee et al., Appl. Phys. Lett. 91, 041505 (2007)10.1063/1.2760149]. It is shown that this result is a consequence of the generalized Bohm criterion and fluid expressions for the IAW phase velocities.

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.

Radio-frequency heating of sloshing ions in a straight field line mirror

A scenario to sustain a sloshing ion population with radio-frequency heating in a newly proposed mirror device, the straight field line mirror, is examined. The possibilities of ion cyclotron heating in two-ion species plasma have been analyzed and a scheme with longitudinal wave conversion and fundamental harmonic heating of deuterium ions in tritium plasma has been investigated. This scheme provides efficient ion heating for high deuterium “minority” concentration without substantial conversion to slow waves and heating of the electrons. Numerical calculations carried out for a reactor-scale device show that conversion of the fast magnetosonic wave to the fast Alfvén wave occurs. For reasons of strong cyclotron absorption of the fast Alfvén wave, only a small portion of the wave energy transits through the cyclotron layer and penetrates to the central part of the trap. The power deposition is peaked at the plasma core. The amount of deposited power does not depend sensitively on the parameters of the discharge. The study uses numerical three-dimensional calculations for the time-harmonic boundary problem for Maxwell’s equations. For radio-frequency heating in this scheme, a simple efficient strap antenna is proposed. It has low Q antenna and operates in the regime of global resonance overlapping.

Sheaths: More complicated than you think

Sheaths in low temperature collisionless and weakly collisional plasmas are often viewed as simple examples of nonlinear physics. How well do we understand them? Closer examination indicates that they are far from simple. Moreover, many predicted sheath properties have not been experimentally verified and even the appropriate “Bohm velocity” for often encountered two-ion species plasma is unknown. In addition, a variety of sheathlike structures, e.g., double layers, can exist, and many two- and three-dimensional sheath effects have not been considered. Experimental studies of sheaths and presheaths in weakly collisional plasmas are described. A key diagnostic is emissive probes operated in the “limit of zero emission.” Emissive probes provide a sensitive diagnostic of plasma potential with a resolution approaching 0.1V and a spatial resolution of 0.1cm. Combined with planar Langmuir probes and laser-induced fluorescence, they have been used to investigate a wide variety of sheath, presheath, and sheathlike structures. Our experiments have provided some answers but have also raised more questions.

Magnetodynamics of a multicomponent (dusty) plasma. I. Rotation waves near low-frequency cutoff in a homogeneous medium

By considering one of the species of a two-ion species plasma to be massive charged dust particles, the origin of the low-frequency cutoff in a multispecies plasma is found to be a fluid rotation of the light component with a characteristic frequency, Ωr=(ZnH∕ne)ΩL, where nH and ne are densities of the heavy component and electrons, respectively, Z is the charge state of the heavy component, and ΩL is the light ion cyclotron frequency. A fluid resonance at ω=Ωr modifies the properties of plasma oscillations in the vicinity of the cutoff and gives rise to the rotation waves in a manner similar to the cyclotron waves. The presence of a heavy component leads to the nonlinear Schrödinger equation for the system. The nonlinear dynamics of such a system consists of a combination of strong and weak turbulence with strong turbulence dominating in the long wavelength and the weak turbulence dominating in the short wavelength.

Localized global ICRF eigenmodes and conversion zones in a two-ion species tokamak

The location of the slow - fast wave conversion zones is analysed for a tokamak in ICRF. The obtained dispersion relation which determines the surfaces of wave conversion describes the cases of weak and strong mode coupling as well as the intermediate one. Using the same approach, the localized global eigenmodes are studied. With respect to their nature, they are separated into two types. The first is the TLE - TAE-like eigenmode. Like the TAE it is the result of mode coupling. The second one is the single-mode eigenmodes. This type is represented by two eigenmodes. One is the extension of the GAE (global Alfvén eigenmode) to the two-ion species plasma. The other one is OHE (original hybrid eigenmode) which exists in the case of strong mode coupling. The field structure and dispersion of these waves are discussed.

Gyrokinetic theory of perpendicular cyclotron resonance in a nonuniformly magnetized plasma

The extension of gyrokinetic theory to arbitrary frequencies by Chen and Tsai [Phys. Fluids 26, 141 (1983); Plasma Phys. 25, 349 (1983)] is used to study cyclotron absorption in a straight magnetic field with a perpendicular, linear gradient in strength. The analysis includes the effects of magnetic field variation across the Larmor orbit and is restricted to propagation perpendicular to the field. It yields the following results for propagation into the field gradient. The standard optical depths for the fundamental O-mode and second harmonic X-mode resonances are obtained from the absorption profiles given in this paper, without invoking relativistic mass variation [see also Antonsen and Manheimer, Phys. Fluids 21, 2295 (1978)]. The compressional Alfvén wave is shown to undergo perpendicular cyclotron damping at the fundamental minority resonance in a two-ion species plasma and at second harmonic resonance in a single-ion species plasma. Ion Bernstein waves propagating into the second harmonic resonance are no longer unattenuated, but are increasingly damped as they approach the resonance. It is shown how the kinetic power flow affects absorption profiles, yielding information previously obtainable only from full-wave theory. In all cases, the perpendicular cyclotron damping arises from the inclusion of magnetic field variation across the Larmor orbit.

A theory of fast-wave absorption, transmission, and reflection in the ion cyclotron range of frequencies

A second-order differential equation for the fast wave propagating in a hot, two-ion species plasma is obtained. This second-order approximation is obtained unambiguously and allows the wave amplitude to be identified with one of the electric field components. The approximation is based on replacing the coupling to the ion-Bernstein wave by a localized perturbation of the fast wave. For the case of perpendicular propagation, the second-order equation reduces to Budden’s equation giving the well-known transmission coefficient for both two-ion hybrid and second-harmonic resonance. The equation includes the effect of simultaneous minority fundamental and majority second-harmonic cyclotron damping. The solutions of the second-order equation as a function of n∥ give absorption transmission and reflection coefficients that agree well with the results based on models giving higher-order differential equations and solved by means of much more complex numerical codes.

Ion Cyclotron Parametric Instability and Associated Ion Heating in a Two-Ion Species Plasma

Theoretical and experimental studies are presented on a parametric decay instability into two electrostatic ion cyclotron waves in a two-ion species plasma. Theoretically, by using a hot-electron plasma model, we generalize such a decay instability in the limits of long- and short-wavelength regions. The instability thresholds as well as the growth rates are predicted for these limiting cases. Experimentally, we modulate an electron beam near the sum of the ion cyclotron frequencies in an electron-beam-plasma system to produce an electric field in the radial direction. At high levels of beam modulation above a certain threshold, the beam pumps this instability. Associated ion heating takes place as the instability develops strongly. The instability threshold has been determined in a parameter space. The instability threshold predicted theoretically is compared with that obtained experimentally, and a quantitative agreement is obtained between them.

Transport of impure plasma with arbitrary toroidal rotation

A previous formulation of neoclassical transport for an axisymmetric plasma with arbitrary toroidal rotation is generalized to plasmas with impurity ions. Calculations in the banana regime for a two-ion species plasma for large aspect circular flux surfaces show significant enhancement of both particle and heat fluxes. The viscous part of the momentum flux remains small. A more detailed treatment of the electron equations reveals the importance of an assumption relating to the balancing of the toroidal torques on the electron fluids which is likely to require modification in practical situations.

Observation of ion-beam instability in a two-ion species plasma

The ion-beam instability in a two-ion species plasma is studied theoretically and experimentally, where the instability is caused by the interaction between the ion acoustic wave and the heavy-ion beam with the energy higher than that in a single ion plasma. The beam energies for the unstable wave depend on both the light ion concentration and the mass ratio of the heavy ion to the light one. The experimental results agree well with those calculated by the kinetic theory.

Observation of ion-beam instability in a two-ion species plasma

Observation of ion-beam instability in a two-ion species plasma

Wall Effects on the Propagation of Compressional Alfv�n Waves in a Cylindrical Plasma with Two-Ion Species

The dispersion relations for the compressional Alfven waves in a two-ion species plasma of deuterium and hydrogen are calculated for a configuration which includes a vacuum layer between the cylindrical plasma and the conducting wall. The presence of the vacuum layer strongly affects the propagation of the compressional Alfven wave, permitting some branches to propagate and penetrate the plasmacolumn over most frequencies in the ion-cyclotron range. Basic Alfven-wave propagation and heating experiments in two-ion species consequently should be possible using tokamak and mirror devces with minor radii smaller than the Alfven wavelength.

External excitation of ion cyclotron drift waves in a two-ion species plasma

Ion cyclotron drift waves propagating across a density gradient and a magnetic field have been excited externally in a two-ion species plasma, with its concentration ratio controlled. The measured dispersion relations agree with the theoretical predictions.

Development of a Plasma ion analyzer.

A Plasma ion analyzer for measuring mass, density and temperature of ions in a multi-ion species plasma has been developed. The analyzer is a simple and compact mass spectrometer where the magnetic field confining a plasma is used directly for the 180° magnetic deflection of accelerated ions. The numerical simulations of the analyzer characteristics provide the useful guiding principles for the data analysis as well as the optimum analyzer design. The test measurements of a two-ion species plasma has been made, and the results are in good agreement with the theoretical predictions and the spectroscopic measurements.

Ponderomotive force of an ion cyclotron wave in a two-ion species plasma

The ponderomotive force experienced by an ion in a left circularly polarized wave changes sign as the frequency increases from below to above the ion cyclotron frequency. If the wave frequency is chosen to lie between the ion cyclotron frequencies of two different isotopes, then the ponderomotive force tends to separate the two species in space. As a result, the index of refraction is modified. Self-consistent solutions of the nonlinear wave equation taking into account both effects are obtained analytically and numerically. The ratio of the densities of the two ions is proportional to exp[mE2(z)/4γ(1−γ)αB20kTi], where E(z) is the amplitude of the wave, B0 is the magnetic field, and Ti is the temperature of the ions, m=2m1m2/(m1+m2), α=(m1−m2)/(m1+m2), and γ=(ω−Ω2)/(Ω1−Ω2).

Eigenvalue formulation for linear modes in the homogeneous multiple-fluid plasma

The linearized equations of the multiple-fluid model, including an isotropic pressure law, of a plasma are cast into a concise form directly suited for the calculation of the full set of characteristic modes of the system. A fully ionized, homogeneous, drift-free, magnetized plasma having an arbitrary number of finite-temperature components is considered. For oscillations of the form exp[i (k⋅r−ωt)], the three separate problems of determining ω vs (k,ϑ), k vs (ω,ϑ), and kx vs (ω,kz) are treated by formulating the set of algebraic equations into matrix eigenvalue problems for which the eigenvalues are ω2, k2, and k2x, respectively. For each of the three cases, an example of the results of numerical evaluation of the eigenvalues is presented for an electron, two-ion species plasma. A method is described which may be used for calculating, directly within the eigensystem formulation, the group velocity of the characteristic modes.