Volume 11, Issue 9, September 2004
 LETTERS


Properties of fluid deuterium under doubleshock compression to several Mbar
View Description Hide DescriptionThe compressibility of fluid deuterium up to several Mbar has been probed using laserdriven shock waves reflected from a quartz anvil. Combining highprecision shock velocity measurements with the doubleshock technique, where differences in equation of state (EOS) models are magnified, has allowed better discrimination between theoretical predictions in the secondshock regime. Doubleshock results are in agreement with the stiffer EOS models—which exhibit roughly fourfold singleshock compression—for initial shocks up to and above , but diverge from these predictions in between. Softer EOS models—which exhibit sixfold singleshock compression at —overestimate the reshock pressure for the entire range under study.

Criteria for second stability for ballooning modes in stellarators
View Description Hide DescriptionAn expression determining how variations in the pressure gradient and average magnetic shear affect ballooning stability for a stellarator equilibrium is presented. The procedure for determining the marginal stability boundaries, for each field line, depends only on the equilibrium and a single ballooning eigenfunction calculation. This information is sufficient to determine if increasing pressure gradient is stabilizing or destabilizing and to predict whether the configuration possesses a second stable region.

Collisional transport in a low aspect ratio tokamak
View Description Hide DescriptionCalculations of collisional diffusivities in toroidal magnetic plasma confinementdevices order the toroidal gyroradius to be small relative to the poloidal gyroradius, i.e., , where and . This ordering is central to what is usually referred to as neoclassical transport theory. This ordering is incorrect at low aspect ratio (with aspect ratio , where is the major radius of the torus and is the minor radius), where it can be the case that . The correction to the test particle diffusivities is numerically calculated by comparing the diffusivities as determined by a full orbit code (which we refer to as omniclassical diffusion) with those from a gyroaveraged orbit code (neoclassical diffusion), and then corroborated by an analytic calculation. The omniclassical diffusion can be up to 2.5 times the calculated neoclassical value. The implications of this work for the analysis of collisional transport in low aspect ratio devices are discussed.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Quantum collective approach to the thermodynamic properties of degenerate plasma
View Description Hide DescriptionThermodynamic functions of a system of partially degenerate electrons and strongly coupled ions are derived from first principles. A quantum collective approach is developed to analyze nonidealities inherent to very high density plasma. The model considers the electron oscillations(plasmons) and ion oscillations (ion sound waves) as quasiparticles sharing the energy of the system. Statistical thermodynamic calculations lead to simple, analytical expressions for internal energy as well as an equation of state. A dispersion relation for the high frequency branch is introduced to take into account the partial degeneracy state and thereby to quantify temperature finiteness effect on thermodynamic properties of very dense plasma. The present results are in good quantitative agreement with the existing models and represent a significant improvement over previous calculations which are based mainly on numerical experiments. More physical insight is explicitly stated presently which makes a contribution to the theoretical knowledge of coupled degenerate plasma for thermonuclear fusion as well as of astrophysical interests.
 Nonlinear Phenomena, Turbulence, Transport

Absolute and convective instabilities of parallel propagating circularly polarized Alfvén waves: Decay instability
View Description Hide DescriptionThe absolute and convective instabilities of circularly polarized Alfvén waves propagating along an ambient magnetic field are studied. The approximation of ideal magnetohydrodynamics is used. The analysis is restricted to the decay instability that occurs when the sound speed is smaller than the Alfvén speed. In addition, it is assumed that the amplitude of an unstable Alfvén wave (pump wave) is small. This assumption allows us to study the problem analytically using expansions in power series with respect to . It is shown that there are quantities, and , such that the pump wave is absolutely unstable in a reference frame moving with velocity with respect to the rest plasma if . If either or , then the pump wave is convectively unstable. The expressions for and are found. The signaling problem is studied in a reference frame where the pump wave is convectively unstable. It is shown that spatially amplifying waves exist only when the signaling frequency is in two narrow symmetric frequency bands with the widths of the order of . The implication of the obtained results on the interpretation of observational data obtained in space missions is discussed. It is shown that circularly polarized Alfvén waves propagating in the solar wind are convectively unstable in a reference frame of any spacecraft moving with the velocity not exceeding a few tens of km/s in the solar reference frame. The spatial amplification scale of these waves is very large, of the order of In view of these results it is not surprising at all that evidence of the decay instability of Alfvén waves in the solar wind is sparse.
 Ionospheric, Solarsystem, and Astrophysical Plasmas

Electrostatic perturbations in partially ionized plasma with the effects of ionization and recombination
View Description Hide DescriptionThe behavior of the electrostatic ion acoustic mode in a partially ionized plasma is studied in the presence of collisions which involve processes of ionization and recombination, taking into account the dynamics of the neutrals caused by elastic and inelastic collisions with ions. The application of the model to space plasmas, which are usually subject to gravity, is discussed in detail. A dispersion equation which includes the effects of ionization and recombination is derived and the stability/instability conditions are discussed. Parameters applicable to a region of the upper solar chromosphere are used and the increment of the ion sound wave is calculated yielding an unstable ion sound wave for wavelengths larger than .
 Lowtemperature Plasmas, Plasma Applications, Plasma Sources, Sheaths

Characterization of the resonant electromagnetic mode in helicon discharges
View Description Hide DescriptionThis paper presents experimental evidence for the existence of a resonant electromagnetic mode in a helicon plasma, and characterizes its resonant frequency, energy absorption rate, and contribution to the overall power balance. An experimental method developed to detect and characterize the plasma impedance is introduced. Power flow analysis shows that the resonant electromagnetic mode is the dominant mechanism driving helicon plasma discharges in cylindrical geometry. The weakly damped wavefields are absorbed in a short distance, indicating that the wavefields may be trapped in a potential well created by a density gradient.
 Inertially Confined Plasmas, Dense Plasmas, Equations of State

Stimulated Raman scattering of a laser beam in a plasma with azimuthal magnetic field
View Description Hide DescriptionA strong azimuthal magnetic field localizes the lower hybrid waves radially in laser produced plasmas. The laser pump parametrically excites a lower hybrid wave and a backscattered electromagnetic sideband wave. The density perturbation due to the lower hybrid wave couples with the oscillatory velocity of electrons due to the pump wave, to produce a nonlinear current driving the sideband. The pump and sideband waves exert a ponderomotive force on electrons driving the lower hybrid wave. The local effects reduce the growth rate of stimulated Raman scattering. The fundamental radial eigenmode of the lower hybrid wave is the maximally growing mode. The scattering process can be used as a diagnostic for the azimuthal magnetic field.
 Lowtemperature Plasmas, Plasma Applications, Plasma Sources, Sheaths

Effects of electron inertia in capacitively coupled radio frequency discharges
View Description Hide DescriptionThe effects of the electron inertia on the plasma and sheathdynamics in capacitively coupled rf discharges with frequency are investigated (here, and are the rf frequency and bulk ion plasma frequency, respectively). It is found that the effects of the electron inertia on the plasma density and ion velocity in the quasineutral region depend on the ratio of the amplitudes of the discharge current and ion current (here, is the unit charge, is the plasma density at center, and is the ion sound speed). If the ratio is small so that (here, and are ion and electron masses, respectively), the ion and timeaveraged electron densities, ion velocity, and electric fields are little affected by the electron inertia. Otherwise, the effects of the electron inertia are significant. It is also shown that the assumption that the electrons obey the Boltzmann distribution in the sheath is invalid when the electron flux flowing to the electrode is significant.
 Nonlinear Phenomena, Turbulence, Transport

Relativistic quasilinear diffusion in axisymmetric magnetic geometry for arbitraryfrequency electromagnetic fluctuations
View Description Hide DescriptionA relativistic bounceaveraged quasilinear diffusion equation is derived to describe stochastic particle transport associated with arbitraryfrequency electromagnetic fluctuations in a nonuniform magnetized plasma. Expressions for the elements of a relativistic quasilinear diffusiontensor are calculated explicitly for magnetically trapped particle distributions in axisymmetric magnetic geometry in terms of gyrodriftbounce waveparticle resonances. The resonances can destroy any one of the three invariants of the unperturbed guidingcenter Hamiltonian dynamics.
 Magnetically Confined Plasmas, Heating, Confinement

Stabilization of line tied resistive wall kink modes with rotating walls
View Description Hide DescriptionA method suggested by Gimblett [ C. G. Gimblett, Plasma Phys. Controlled Fusion, 31 2183 (1989) ] for stabilizing resistive wall modes by using a rotating double wall configuration is applied to a line tied screw pinch equilibrium. The line tied boundary conditions provide an additional stabilizing mechanism relative to instabilities present in periodic cylinders that limits ideal kink instability to only be present when for modes. With differentially rotating walls, resistive wall modes can be stabilized with values less than unity. For a given equilibrium, there exists an optimal spacing between a stationary and a rotating wall that minimizes the critical wall rotation frequency for stabilization.
 Nonlinear Phenomena, Turbulence, Transport

Asymptotic freezeout of the perturbations generated inside a corrugated rarefaction wave
View Description Hide DescriptionBased on previous work [J. G. Wouchuk and R. Carretero, Phys. Plasmas 10, 4237 (2003)], the conditions of asymptotic freezeout of the ripples at the tail of a corrugated rarefaction wave are analyzed. The precise location of the freezingout regions in the space of preshock parameters is tried, and an efficient algorithm for their determination is given. It is seen that asymptotic freezeout can only happen for gases that have an isentropic exponent . It is shown that the late time freezeout of the ripple perturbations is correlated to the initial tangential velocity profile (at ) inside the expansion fan.
 Basic Plasma Phenomena, Waves, Instabilities

Spatial mode structures of electrostatic drift waves in a collisional cylindrical helicon plasma
View Description Hide DescriptionIn a cylindrical helicon plasma, mode structures of coherent drift waves are studied in the poloidal plane, the plane perpendicular to the ambient magnetic field. The mode structures rotate with a constant angular velocity in the direction of the electron diamagnetic drift and show significant radial bending. The experimental observations are compared with numerical solutions of a linear nonlocal cylindrical model for drift waves [Ellis et al., Plasma Phys. 22, 113 (1980)]. In the numerical model, a transition to bended mode structures is found if the plasma collisionality is increased. This finding proves that the experimentally observed bended mode structures are the result of high electron collisionality.
 Nonlinear Phenomena, Turbulence, Transport

Transport coefficients in relativistic collisionless plasmas
View Description Hide DescriptionThe computation of the relativistic transport coefficients in collisionless plasmas is presented. The stationary relativistic Vlasov equation is analytically solved for perturbed plasmas with respect to the global equilibrium defined by the Maxwell–Boltzmann–Jüttnerdistribution function. The explicit expression of the distribution function is derived and the generalized collisionless transport coefficients are deduced for arbitrary plasma temperature. It is found that the relativistic effects tend to increase the value of the transport coefficients. In particular, in ultrarelativistic regimes the temperature anisotropy reaches its maximum value.
 Inertially Confined Plasmas, Dense Plasmas, Equations of State

Design of a cryogenic ignition capsule for the National Ignition Facility
View Description Hide DescriptionOptimized performance of a capsule intended to produce ignition on the National Ignition Facility [J. A. Paisner, J. D. Boyes, S. A. Kumpan, W. H. Lowdermilk, and M. S. Sorem, Laser Focus World 30, 75 (1994)] is presented. Performance is optimized, for a isotropic drive on a beryllium(copper) ablator, by varying the ablator outside radius, ablator thickness, the concentration of copperdopant in the ablator, and the fuel thickness, while keeping the absorbed energy fixed. Dopant concentration is constrained to be uniform in the ablator. The drive shock timing is adjusted to produce a low entropy implosion for each set of dimensions. The absorbed energy is kept fixed at , which results in the ablator outside radius remaining practically constant, about . For capsule geometry near that resulting in optimal implosion yield, the absorbed energy depends only slightly on the ablator or fuel thickness. The parameter space of capsule dimensions was searched for central vapor densities of 0.3 and . Despite the detailed optimization, it is found that the capsule is notably more unstable than comparable capsules with a graded dopant in the ablator, as reported in previous literature.
 Magnetically Confined Plasmas, Heating, Confinement

Convective transport in the scrapeofflayer by nonthermalized spinning blobs
View Description Hide DescriptionIn this paper, twodimensional blob models of convective transport in the scrapeofflayer (SOL) are generalized to include the internal temperature profile of the blob. This generalization provides a mechanism for blob internal spin and enables consideration of SOL energy transport. Solutions with aligned density and temperature contours satisfy the resulting “hot blob” equations and are considered here. It is shown that spin increases blob coherence, prevents the formation of extended radial streamers or fingers, reduces the radial convectionvelocity due to mixing and mitigation of the curvatureinduced charge polarization, and provides a new mechanism for poloidal motion of the blob. Additionally, spinning blobs are shown to survive as coherent objects in the presence of weak externally sheared flows, and have blob speeds that depend on the sign of the spin relative to the external sheared flow. The work provides strong motivation for investigating the physics of parallel disconnected blobs, and the relationship of spin and disconnection physics to edge localized mode propagation and the density limit.
 Inertially Confined Plasmas, Dense Plasmas, Equations of State

Selffocusing of electromagnetic beams in collisional plasmas with nonlinear absorption
View Description Hide DescriptionIn this paper the formalism of selffocusing of electromagnetic waves is extended to include nonlinear absorption by the medium. A complex eikonal has been employed, which does not need any approximation about the relative magnitudes of the real and imaginary parts of the dielectric constant or their dependence on the irradiance of the beam. The specific case of collisional plasmas has been considered as an application of the theory. It is seen that the nonlinearity in absorption tends to cancel the effect of divergence on account of diffraction. The dependence of the beam width and attenuation on distance of propagation has been illustrated for specific cases. The relevance of the investigation to radio wave propagation has also been pointed out.
 Magnetically Confined Plasmas, Heating, Confinement

Edge transport and the lowtohigh transition in tokamaks with Dshaped magnetic flux surfaces
View Description Hide DescriptionNonlinear, threedimensional numerical simulations of finite driftballooning turbulence in the edge of tokamaks with Dshaped magnetic fluxsurfaces are presented. The simulations are based on the reduced Braginskii equations. The turbulenttransport in tokamaks with Dshaped magnetic fluxsurfaces is directly compared to that in tokamaks with circular magnetic fluxsurfaces. The energy flux can be extremely different depending on the shape of the magnetic fluxsurfaces. While a tokamak with circular flux surfaces exhibits exceedingly poor confinement, an otherwise identical tokamak with Dshaped flux surfaces is in the high (H) mode where the transport rate is small. The magnitude of the energy flux in these two cases differs by a multiplicative factor larger than 1000.
 Inertially Confined Plasmas, Dense Plasmas, Equations of State

Inferring the capsuleabsorbed radiation energy from experimental hohlraum radiation temperature
View Description Hide DescriptionFor indirect laser fusion, the implosion is driven by the fusion capsuleabsorbed radiation energy emitted by laserproduced plasma. However, the absorbed energy could hardly be directly measured experimentally and usually would require numerical simulation. This paper puts forward a method by which the capsuleabsorbed radiation energy can be inferred from the measured timedependent radiation temperature. In the method, the capusleabsorbed radiation energy is seen as an effective radiation energy loss and should be reflected in the experimental radiation temperature. Furthermore, it is not necessary to know what materials the capsule is made of or how it is constructed.
 Ionospheric, Solarsystem, and Astrophysical Plasmas

Macroscale instability of the ion shell distribution function in the divergent solar wind
View Description Hide DescriptionAs a result of cyclotron interaction with Alfvén waves propagating from the sun, pitch angle diffusion of resonant particles takes place and a shelllike distribution function of resonant ions is formed at each distance from the sun. Stability of the solar wind ion shelllike distribution function with respect to excitation of waves at larger distances is addressed. It is shown in linear approximation, that in the case when the phase velocity of Alfvén waves decreases with distance, ions with shell distribution excite outward propagating Alfvén waves with smaller phase velocities when they advance to larger distances. The nonlinear dynamics of the wave spectrum as well as the evolution of the ion distribution function are studied. The characteristic spectrum at the highfrequency edge of the magnetohydrodynamic fluctuations is explained.