Showing papers in "Physics of fluids. B, Plasma physics in 1991"
TL;DR: The modified plasma dispersion function (MPDF) as mentioned in this paper is based on the generalized Lorentzian (kappa) particle distribution function, and a comprehensive set of graphs of the real and imaginary parts of the MPDF is presented.
Abstract: The modified plasma dispersion function (MPDF), based on the generalized Lorentzian (kappa) particle distribution function, is introduced, and a comprehensive set of graphs of the real and imaginary parts of the MPDF is presented. For any positive integral value of kappa, MPDF is calculated in closed form as a finite series. It is demonstrated how the MPDF approaches the plasma dispersion function in the limit as kappa yields infinity, a result to be expected since the kappa distribution function formally approaches the Maxwellian as kappa yields infinity. It is concluded that the MPDF can provide a tool in studying microinstabilities in plasmas when the particle distribution function is not only the standard generalized Lorentzian, but also of the Lorentzian type, including the loss-cone, bi-Lorentzian, and product bi-Lorentzian distributions.
683 citations
TL;DR: In this paper, the mechanism for the generation of mean poloidal flow by turbulence is identified and elucidated, and two methods of calculating poloidal acceleration are given and shown to yield predictions which agree.
Abstract: The mechanism for generation of mean poloidal flow by turbulence is identified and elucidated. Two methods of calculating poloidal flow acceleration are given and shown to yield predictions which agree. These methods link flow generation to the quasilinear radial current or the Reynolds stress 〈VrVθ〉. It is shown that poloidal acceleration will occur if the turbulence supports radially propagating waves and if radial gradients in the turbulent Reynolds stress and wave energy density flux are present. In practice, these conditions are met in the tokamak edge region when waves propagate through the outermost closed flux surface or when convection cells with large radial correlation length are situated in steep gradient regions. The possible impact of these results on the theory of the L→H transition is discussed.
422 citations
TL;DR: In this article, the Hirshman and Sigmar moment approach was used to derive explicit expressions for the poloidal and toroidal rotation speeds of the primary ion and impurity species.
Abstract: Explicit expressions for the neoclassical poloidal and toroidal rotation speeds of primary ion and impurity species are derived via the Hirshman and Sigmar moment approach. The rotation speeds of the primary ion can be significantly different from those of impurities in various interesting cases. The rapid increase of impurity poloidal rotation in the edge region of H‐mode discharges in tokamaks can be explained by a rapid steepening of the primary ion pressure gradient. Depending on ion collisionality, the poloidal rotation speed of the primary ions at the edge can be quite small and the flow direction may be opposite to that of the impurities. This may cast considerable doubts on current L to H bifurcation models based on primary ion poloidal rotation only. Also, the difference between the toroidal rotation velocities of primary ions and impurities is not negligible in various cases. In Ohmic plasmas, the parallel electric field induces a large impurity toroidal rotation close to the magnetic axis, which seems to agree with experimental observations. In the ion banana and plateau regime, there can be non‐negligible disparities between primary ion and impurity toroidal rotation velocities due to the ion density and temperature gradients. Detailed analytic expressions for the primary ion and impurity rotation speeds are presented, and the methodology for generalization to the case of several impurity species is also presented for future numerical evaluation.
329 citations
TL;DR: In this paper, the penetration of a helical magnetic perturbation into a rotating tokamak plasma is investigated and it is shown that unless the frequency of the imposed perturbations matches closely to one of the natural mode frequencies, reconnection at the rational surface is suppressed by a large factor.
Abstract: The penetration of a helical magnetic perturbation into a rotating tokamak plasma is investigated. In the linear regime, it is found that unless the frequency of the imposed perturbation matches closely to one of the natural mode frequencies, reconnection at the rational surface is suppressed by a large factor. In order to deal with the problem in the nonlinear regime a theory of propagating, constant‐ψ magnetic islands is developed. This theory is valid provided the island width greatly exceeds any microscopic scale length (but still remains small compared with the minor radius), and the magnetic Reynolds number of the plasma is sufficiently large. An island width evolution equation is obtained which, in addition to the usual Rutherford term, contains a stabilizing term due ultimately to the inertia of the plasma flow pattern set up around the propagating island. A complete solution is presented for the case where the island and its associated flow pattern are steady. In the nonlinear regime, a fairly sh...
250 citations
TL;DR: In this article, the L to H transition in the DIII-D tokamak is associated with two clear signatures: edge density fluctuations are abruptly suppressed (in ≊100 μsec), while the edge poloidal rotation velocity vθ increases, implying that the radial electric field Er becomes more negative.
Abstract: The L to H transition in the DIII‐D tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (IAEA, Vienna, 1987), Vol. I, p. 159] is associated with two clear signatures: edge density fluctuations are abruptly suppressed (in ≊100 μsec), while the edge poloidal rotation velocity vθ increases, implying that the radial electric field Er becomes more negative. Detailed new spectroscopic profile measurements show that the changes in vθ and Er generate a region of sheared electric field and poloidal flow of width ≊3–5 cm. This region develops simultaneously with, and has the same spatial extent as, the edge fluctuation suppression zone as measured using a reflectometer system. Furthermore, the radial extent of the shear and fluctuation suppression zones encompass the location of the H‐mode edge transport barrier. These observations are consistent with recent theoretical models of the L–H transition, and a comparison with these theories is presented. Data are also presented on the evolution of edge parameters and density fluctuations after the transition: the shear and fluctuation suppression layers are maintained for the duration of the quiescent H‐mode phase, while relative density fluctuation levels decrease and interior plasma confinement gradually improves. Precursors to several different types of edge localized mode (ELMs) are also discussed.
225 citations
TL;DR: In this article, a second-order solution for multimode classical Rayleigh-Taylor growth was proposed to show that the initial amplitude information in weakly nonlinear phase is not dominated by nonlinear effects.
Abstract: For many cases of interest to inertial fusion, growth of Rayleigh–Taylor and other hydrodynamic instabilities is such that the perturbations remain linear or weakly nonlinear. The transition to nonlinearity is studied via a second‐order solution for multimode classical Rayleigh–Taylor growth. The second‐order solution shows how classical Rayleigh–Taylor systems forget initial amplitude information in the weakly nonlinear phase. Stabilized growth relevant to inertial fusion is qualitatively different, and initial amplitudes are not dominated by nonlinear effects. In all systems with a full spectrum of modes, nonlinear effects begin when mode amplitudes reach about 1/Lk2, for modes of wave number k and system size L.
197 citations
TL;DR: In this paper, the effects of ion collisionality on the plasma sheath are revealed by a two-fluid model using a power law dependence on ion energy, and exact numerical solutions of the model are used to determine the collisional dependence of the sheath width and the ion impact energy at the wall.
Abstract: The effects of ion collisionality on the plasma sheath are revealed by a two‐fluid model. In contrast to previous work, the ion–neutral collision cross section is modeled using a power law dependence on ion energy. Exact numerical solutions of the model are used to determine the collisional dependence of the sheath width and the ion impact energy at the wall. Approximate analytical solutions appropriate for the collisionless and collisionally dominated regimes are derived. These approximate solutions are used to find the amount of collisionality at the center of the transition regime separating the collisionless and collisional regimes. For the constant ion mean‐free‐path case, the center of the transition regime for the sheath width is at a sheath width of five mean‐free paths. The center of the transition regime for the ion impact energy is at a sheath width of about one‐half of a mean‐free path.
191 citations
TL;DR: In this article, the ion temperature gradient-driven instability is considered and the saturation of a single eddy is modeled by a simple nonlinear equation, and it is shown that eddies that are elongated in the direction of the temperature gradient are the most unstable and have the highest saturation amplitudes.
Abstract: The ion‐temperature‐gradient‐driven instability is considered in this paper. Physical pictures are presented to clarify the nature of the instability. The saturation of a single eddy is modeled by a simple nonlinear equation. It is shown that eddies that are elongated in the direction of the temperature gradient are the most unstable and have the highest saturation amplitudes. In a sheared magnetic field, such elongated eddies twist with the field lines. This structure is shown to be an alternative to the usual Fourier mode picture in which the mode is localized around the surface where k∥ =0. These elongated twisting eddies, which are an integral part of the ‘‘ballooning mode’’ structure, could survive in a torus. The elongated eddies are shown to be unstable to secondary instabilities that are driven by the large gradients in the long eddy. It is argued that the ‘‘mixing length’’ is affected by this nonlinear process, and is unlikely to be a linear eigenmode width.
171 citations
TL;DR: Quasihelically symmetric stellarators as mentioned in this paper are nonaxisymmetric, toroidal configurations in which the strength of the magnetic field depends on only one angular coordinate, instead of two, within the constant pressure surfaces.
Abstract: Quasihelically symmetric stellarators are nonaxisymmetric, toroidal configurations in which the strength of the magnetic field depends on only one angular coordinate, instead of two, within the constant pressure surfaces. The confinement properties of these configurations are similar to those of axisymmetric tokamaks. Although stellarators with exact quasihelical symmetry are proven not to exist, good approximations can be found.
170 citations
TL;DR: In this article, a bifurcation in the thermal confinement of tokamaks, which resembles the L• to H•mode transition, is shown to follow from properties of edge turbulence recently derived by Biglari et al. and the standard neoclassical theory of poloidal rotation.
Abstract: A bifurcation in the thermal confinement of tokamaks, which resembles the L‐ to H‐mode transition, is shown to follow from properties of edge turbulence recently derived by Biglari et al. [Phys. Fluids B 2, 1 (1990)], and the standard neoclassical theory of poloidal rotation. The temperature profiles develop a pedestal at the plasma edge, and the poloidal rotation near the edge is considerably increased, when the heating power exceeds a critical value. The energy confinement time is a discontinuous function of increasing heating power, but is continuous for decreasing power (power hysteresis). Critical values of density and magnetic field are found, which must be exceeded in order for the bifurcation to occur. The scaling of the power threshold with density, magnetic field, and ion mass is similar to what is found experimentally.
147 citations
TL;DR: In this article, the effects of ion-ion collisions on the long wavelength limit of the ion modes are discussed and a Monte-Carlo scheme for the inclusion of ion collisions, in which ions can undergo Coulomb collisional dynamical friction, velocity space diffusion, and random walk of guiding centers, is constructed.
Abstract: Ion‐temperature‐gradient‐driven modes in the presence of ion–ion collisions in a toroidal geometry with trapped ions have been studied by using a one‐and‐one‐half‐dimensional (11/2‐D) linearized gyrokinetic particle simulation code in the electrostatic limit. The purpose of the investigation is to try to understand the physics of flat density discharges, in order to test the marginal stability hypothesis. Results giving threshold conditions of LTi/R0, and linear growth rates and mode frequencies over all wavelengths for the collisionless ion‐temperature‐gradient‐driven modes are obtained. The behavior of ion‐temperature‐gradient‐driven instabilities in the transition from slab to toroidal geometry, with trapped ions, is shown. A Monte‐Carlo scheme for the inclusion of ion–ion collisions, in which ions can undergo Coulomb collisional dynamical friction, velocity space diffusion, and random walk of guiding centers, has been constructed. The effects of ion–ion collisions on the long wavelength limit of the ion modes is discussed.
TL;DR: In this paper, the authors analyzed the Landau damping of electrostatic Langmuir waves and ion-acoustic waves in a hot, isotropic, nonmagnetized, generalized Lorentzian plasma using the modified plasma dispersion function.
Abstract: The Landau damping of electrostatic Langmuir waves and ion-acoustic waves in a hot, isotropic, nonmagnetized, generalized Lorentzian plasma is analyzed using the modified plasma dispersion function. Numerical solutions for the real and imaginary parts of the wave frequency omega sub 0 - (i)(gamma) have been obtained as a function of the normalized wave number (k)(lambda sub D), where lambda sub D is the electron Debye length. For both particle distributions the electrostatic modes are found to be strongly damped at short wavelengths. At long wavelengths, this damping becomes less severe, but the attenuation of Langmuir waves is much stronger for a generalized Lorentzian plasma than for a Maxwellian plasma. It is concluded that Landau damping of ion-acoustic waves is only slightly affected by the presence of a high energy tail, but is strongly dependent on the ion temperature.
TL;DR: In this article, the BBGKY hierarchy was used to describe an ensemble of plasmas characterized by guiding center electrons and stationary ions, and the recombination rate was determined by considering only the initial cascade through states of electron ion pairs with separation of order b. These pairs may be described as guiding center atoms since the dynamics is classical and treatable with the guiding center drift approximation.
Abstract: The three‐body recombination rate is calculated for an ion introduced into a magnetically confined, weakly correlated, and cryogenic pure electron plasma. The plasma is strongly magnetized in the sense that the cyclotron radius for an electron rce=(kBTe/me)1/2/Ωce is small compared to the classical distance of closest approach b=e2/kBTe, where Te is the electron temperature and Ωce=eB/mec is the electron cyclotron frequency. Since the recombination rate is controlled by a kinetic bottleneck a few kBTe below ionization, the rate may be determined by considering only the initial cascade through states of electron‐ion pairs with separation of order b. These pairs may be described as guiding center atoms since the dynamics is classical and treatable with the guiding center drift approximation. In this paper, an ensemble of plasmas characterized by guiding center electrons and stationary ions is described with the BBGKY hierarchy. Under the assumption of weak electron correlation, the hierarchy is reduced to a...
TL;DR: In this paper, the stability of ballooning modes in the presence of sheared toroidal flows is investigated, and a new stability criterion is derived that explicitly demonstrates that flow shear is stabilizing at constant centrifugal force gradient.
Abstract: The stability of ballooning modes in the presence of sheared toroidal flows is investigated. The eigenmodes are shown to be related by a Fourier transformation to the nonexponentially growing Floquet solutions found by Cooper [Plasma Phys. Controlled Fusion 30, 1805 (1988)]. It is further shown that the problem cannot be reduced further than to a two‐dimensional partial differential equation. Next, the generalized ballooning equation is solved analytically for a circular tokamak equilibrium with sonic flows, but with a small rotation shear compared to the sound speed. With this ordering, the centrifugal forces are comparable to the pressure gradient forces driving the instability, but coupling of the mode with the sound wave is avoided. A new stability criterion is derived that explicitly demonstrates that flow shear is stabilizing at constant centrifugal force gradient.
TL;DR: In this paper, a nonlocal heat flow model based on numerical simulations of the decay of linearized thermal waves using the electron Fokker-Planck code spark is presented.
Abstract: A brief review of nonlocal heat‐flow models is presented. Numerical difficulties associated with their implementation, as recently demonstrated by Prasad and Kershaw [Phys. Fluids B 1, 2430 (1989)], are discussed and a simple solution is proposed. A new nonlocal heat‐flow formula is developed, based on numerical simulations of the decay of linearized thermal waves, using the electron Fokker–Planck code spark. The formula is tested by modeling the full implosion of a CH shell driven by 351 nm laser irradiation. Results are shown to be in good agreement with spark simulations.
TL;DR: In this paper, it was shown that noncircularity of tokamak flux surfaces leads to frequency gaps in the magnetohydrodynamic Alfven continuum and discrete modes having macroscopic structure are shown to exist and have many common features with toroidicity induced Alfven eigenmodes.
Abstract: It is shown that noncircularity of tokamak flux surfaces leads to frequency gaps in the magnetohydrodynamic Alfven continuum. Within these gaps discrete modes having macroscopic structure are shown to exist and have many common features with toroidicity induced Alfven eigenmodes. The present work focuses on ellipticity. Since κ−1>e in many tokamaks the ellipticity induced Alfven eigenmode may indeed be a more robust mode. The most global mode couples the m=1, n=1 and m=3, n=1 ‘‘cylindrical’’ eigenmodes. The region of strong coupling occurs at the q(r)=2 surface and the width of the coupling region is finite and of order (κ−1)a. Furthermore, for typical limiter q(r) profiles satisfying 1≲q≲3, the dominant mode harmonics do not intersect the continuum Alfven spectrum.
TL;DR: In this article, the cross section for transition scattering of electromagnetic waves on charged dust particles in a plasma is calculated, extending the results of a previous paper [J. Plasma Phys. 42, 429] where the case of longitudinal waves has been considered.
Abstract: The cross section for transition scattering of electromagnetic waves on charged dust particles in a plasma is calculated, extending the results of a previous paper [J. Plasma Phys. 42, 429 (1989)] where the case of longitudinal waves has been considered. For the case of nonlinear screening of the charged dust by the plasma particles (i.e., ‖eφ0/Te‖ ≫ 1, where φ0 is the dust grain surface potential and Te is the electron plasma temperature), numerical and analytical results are presented, showing a significant enhancement, proportional to the square of the grain surface charge, in the cross section with respect to scattering by free electrons. The effect is independent of the sign of the charge for wavelengths larger than the Debye length.
TL;DR: In this article, the numerical investigation of the Orszag-Tang vortex system in compressible magnetofluids with embedded supersonic regions is continued, with initial average Mach numbers M=1.0 and 1.5 and β=10/3 with Lundquist numbers S=50, 100, or 200.
Abstract: The numerical investigation of Orszag–Tang vortex system in compressible magnetofluids continues, this time using initial conditions with embedded supersonic regions. The simulations have initial average Mach numbers M=1.0 and 1.5 and β=10/3 with Lundquist numbers S=50, 100, or 200. Depending on the particular set of parameters, the numerical grid contains 2562 or 5122 collocation points. The behavior of the system differs significantly from that found previously for the incompressible and subsonic analogs. Shocks form at the downstream boundaries of the embedded supersonic regions outside the central magnetic X point and produce strong local current sheets that dissipate appreciable magnetic energy. Reconnection at the central X point, which dominates the incompressible and subsonic systems, peaks later and has a smaller impact as M increases from 0.6 to 1.5. Reconnection becomes significant only after shocks reach the central region, compressing the weak current sheet there. Similarly, the correlation between the momentum and magnetic field begins significant growth later than in subsonic and incompressible flows. The shocks bound large compression regions, which dominate the wave‐number spectra of autocorrelations in mass density, velocity, and magnetic field. The normalized spectral amplitude of the cross helicity is almost zero over the middle and upper portions of the wave‐number domain, unlike the incompressible and subsonic flows. The thermal and magnetic pressures are anticorrelated over a wide wave‐number range during the earlier portion of the calculations, consistent with the presence of quasistationary structures bounded by shocks.
Abstract: The confinement and stability properties of toroidal plasma equilibria are largely determined by the strength of the magnetic field expressed in terms of Boozer coordinates Constraints due to the toroidicity of the configuration and the divergence‐free property of the magnetic field limit the freedom in the Taylor series expansion of the magnetic field strength Such field strength constraints limit the variety of desirable three‐dimensional plasma equilibria that are available to the fusion program
TL;DR: In this article, the propagation and damping of ion-acoustic waves were investigated in a Q-machine plasma consisting of K(+) positive ions, SF6(-) negative ions, and electrons.
Abstract: Propagation and damping of ion-acoustic waves have been investigated in a Q-machine plasma consisting of K(+) positive ions, SF6(-) negative ions, and electrons. The phase velocity of the ion-acoustic 'fast' mode increases with increasing epsilon, the concentration of negative ions. The wave damping decreases with increasing epsilon, and nearly disappears, for the highest wave frequencies investigated, when epsilon is more than about 0.9. Both results are in agreement with predictions from Vlasov theory.
TL;DR: In this paper, the problem of the propagation of a harmonic temperature perturbation in a plasma with both diffusive and nondiffusive energy transport is addressed, where the energy flux is modeled by two (radially varying) effective coefficients for the diffusive energy sinks and of cylindrical geometry are taken into account.
Abstract: The problem of the propagation of a harmonic temperature perturbation in a plasma with both diffusive and nondiffusive energy transport is addressed. The energy flux is modeled by two (radially varying) effective coefficients for the diffusive and nondiffusive transport, and the effects of perturbed energy sinks and of cylindrical geometry are taken into account. A simple, local relationship is found between the two transport coefficients and the gradients of the phase and amplitude of the temperature perturbation. This relationship can be used for the interpretation of heating modulation experiments, provided data at different modulation frequencies are available. Since a harmonic density perturbation in a plasma follows a similar linearized transport equation, a similar model can be applied also to density modulation experiments.
TL;DR: In this article, low-frequency, flute-type electrostatic fluctuations propagating across a strong, homogeneous magnetic field are studied experimentally and the importance of large structures for the turbulent plasma diffusion is discussed.
Abstract: Low‐frequency, flute‐type electrostatic fluctuations propagating across a strong, homogeneous magnetic field are studied experimentally. The fluctuations are generated by the Kelvin–Helmholtz instability. The presence of relatively long‐lived vortexlike structures in a background of wide‐band turbulent fluctuations is demonstrated by a conditional sampling technique. Depending on plasma parameters, the dominant structures can appear as monopole or multipole vortices, dipole vortices in particular. The importance of large structures for the turbulent plasma diffusion is discussed. A statistical analysis of the randomly varying plasma flux is presented.
TL;DR: In this article, the expansion of a two-electron population, collisionless plasma into vacuum has been examined in detail, and a new plasma source creating dense, pulsed discharge plasma with a low background pressure (P not greater than 2 x 10 to the -6th torr) was developed to perform in situ measurements of the temporal and spatial plasma evolution during its expansion into vacuum.
Abstract: The expansion of a two-electron-population, collisionless plasma into vacuum has been examined in detail. Plasma density, plasma potential, electric field, and particle disribution functions have been measured in situ. It is demonstrated that the presence of a low-pressure (P not less than 2 x 10 to the -5th torr) background neutral gas modifies the expansion of the plasma. A new plasma source creating dense, pulsed discharge plasma with a low background pressure (P not greater than 2 x 10 to the -6th torr) has been developed to perform in situ measurements of the temporal and spatial plasma evolution during its expansion into vacuum.
TL;DR: The nonlinear evolution of resistive pressure gradient driven turbulence with diamagnetic effects included generates a dc electric field (poloidal velocity) through the convective nonlinearity in the momentum balance equation as mentioned in this paper.
Abstract: The nonlinear evolution of resistive pressure‐gradient‐driven turbulence with diamagnetic effects included generates a dc electric field (poloidal velocity) through the convective nonlinearity in the momentum balance equation. This radial electric field has a strong shear and contributes to the saturation of the turbulence; its effect on the saturation level of turbulence is more important than the change of the time and length scales of the modes by the direct ω* effects.
TL;DR: In this paper, a simple decay model based on absolute equilibrium theory is found to predict a mapping of initial onto time asymptotic states, and to accurately describe the long time behavior of the runs when magnetic helicity is present.
Abstract: The time asymptotic behavior of a Galerkin model of 3D magnetohydrodynamics (MHD) has been interpreted using the selective decay and dynamic alignment relaxation theories. A large number of simulations has been performed that scan a parameter space defined by the rugged ideal invariants, including energy, cross helicity, and magnetic helicity. It is concluded that time asymptotic state can be interpreted as a relaxation to minimum energy. A simple decay model, based on absolute equilibrium theory, is found to predict a mapping of initial onto time asymptotic states, and to accurately describe the long time behavior of the runs when magnetic helicity is present. Attention is also given to two processes, operating on time scales shorter than selective decay and dynamic alignment, in which the ratio of kinetic to magnetic energy relaxes to values 0(1). The faster of the two processes takes states initially dominant in magnetic energy to a state of near-equipartition between kinetic and magnetic energy through power law growth of kinetic energy. The other process takes states initially dominant in kinetic energy to the near-equipartitioned state through exponential growth of magnetic energy.
TL;DR: In this article, a new process of preferential strong heating of positrons through the ion synchrotron maser instability in positron-electron-proton magnetized plasmas is investigated using particle-in-cell simulations.
Abstract: A new process of the preferential strong heating of positrons through the ion synchrotron maser instability in positron-electron-proton magnetized plasmas is investigated using particle-in-cell simulations. It is shown that the positrons form a nonthermal power-law-like energy distribution via their gyroresonant interaction with the extraordinary modes emitted by the ions. It is noted that this process may be of significance in connection with the shock excitation of nonthermal synchrotron radiation from astrophysical systems powered by relativistic outflows from compact central objects, e.g., supernova remnants powered by pulsars and jets from active galactic nuclei.
TL;DR: In this article, a nonequilibrium model is developed for the prediction of two-dimensional flow, electron and heavy particle temperatures, and number density distributions in cascaded arcs of monatomic gases.
Abstract: A nonequilibrium model is developed for the prediction of two‐dimensional flow, electron and heavy particle temperatures, and number density distributions in cascaded arcs of monatomic gases. The system of strongly coupled elliptic partial differential equations describing plasma flow is solved by a numerical method based on a control volume with a nonstaggered numerical grid. The model is applied for the computation of both stagnation and flowing argon arc plasmas. The results show that the plasma in stagnation arcs is nearly in local thermal equilibrium (LTE), except very close to the wall, whereas fast flowing arc plasmas exhibit a significant degree of nonequilibrium, both close to the wall and in the inlet region. The results of the calculations are in satisfactory agreement with experimental data, both for the cases of stagnation and flowing argon cascaded arc plasmas.
TL;DR: In this article, the authors investigated the impact of the initial shock structure stemming from the early temporal phase of an induced spatial incoherence (ISI)smoothed laser beam and showed that foam-like layers, multiple laser wavelengths, and shallow angles of incidence can sharply reduce the perturbation level.
Abstract: One of the critical elements for high‐gain target designs is the high degree of symmetry that must be maintained in the implosion process. The induced spatial incoherence (ISI) concept has some promise for reducing ablation pressure nonuniformities to ≊1%. The ISI method produces a spatial irradiance profile that undergoes large random fluctuations on picosecond time scales but is smooth on long time scales. The ability of the ISI method to produce a nearly uniform ablation pressure is contingent on both temporal smoothing and thermal diffusion. In the start‐up phase of a shaped reactorlike laser pulse, the target is directly illuminated by the laser light and thermal diffusion is not effective at smoothing residual nonuniformities in the laser beam. During this period in the laser pulse, the target response is dominated by the initial shock generated by the laser pulse and the results indicate that this first shock can be the determining factor in the success or failure of the implosion process. The results of numerical simulations of several target/laser pulse designs which were investigated in an attempt to mitigate the impact of the initial shock structure stemming from the early temporal phase of an ISI‐smoothed laser beam are presented. It is shown that ‘‘foamlike’’ layers, multiple laser wavelengths, and shallow angles of incidence can sharply reduce the perturbation level stemming from the first shock.
TL;DR: In this article, the authors describe the propagation of ion acoustic solitons that propagate in a positive ion-negative ion plasma by launching them from a solid metal disk to which a small negative step voltage [Δφ≊−2(Te/e) V] is applied.
Abstract: Experiments on the propagation of ion acoustic solitons that propagate in a positive ion–negative ion plasma are described. The solitons are launched from a solid metal disk to which a small negative step voltage [Δφ≊−2(Te/e) V] is applied. The mechanism for the excitation of the soliton, the identification of the fast and the slow modes in such a plasma, and the observation of a transient sheath are presented.
TL;DR: In this paper, a heavy ion beam probe has been used to measure the plasma space potential profiles in the tokamak and the Ohmic discharges studied were perturbed by externally produced resonant magnetic fields.
Abstract: A heavy ion beam probe has been used to measure the plasma space potential profiles in the tokamak TEXT [Nucl. Fusion Technol. 1, 479 (1981)]. The Ohmic discharges studied were perturbed by externally produced resonant magnetic fields (an ergodic magnetic limiter or EML). Without these perturbations the plasma central potential is generally consistent with the value calculated from radial ion momentum balance, using experimental values of density and ion temperature and assuming a neoclassical poloidal rotation velocity. Exceptions to the agreement are found when operating with reduced plasma parameters. Possible reasons for this discrepancy are explored, in particular, the effects of intrinsic magnetic field fluctuations, and modifications to the self‐consistent radial electric sheath. With the application of the EML fields the edge electric field and potential increase during periods of magnetic island overlap. A test particle calculation of electron transport shows increases in diffusivity also occur during periods of magnetic island overlap. These calculated changes in diffusivity are interpreted in terms of a stochastic layer width, which is itself used to predict a potential change for comparison with the experimental results.