L. Rudakov

Three dimensional character of whistler turbulence

It is shown that the dominant nonlinear effect makes the evolution of whistler turbulence essentially three dimensional in character. Induced nonlinear scattering due to slow density perturbation resulting from ponderomotive force triggers energy flux toward lower frequency. Anisotropic wave vector spectrum is generated by large angle scatterings from thermal plasma particles, in which the wave propagation angle is substantially altered but the frequency spectrum changes a little. As a consequence, the wave vector spectrum does not indicate the trajectory of the energy flux. There can be conversion of quasielectrostatic waves into electromagnetic waves with large group velocity, enabling convection of energy away from the region. We use a two-dimensional electromagnetic particle-in-cell model with the ambient magnetic field out of the simulation plane to generate the essential three-dimensional nonlinear effects.

Weak turbulence in the magnetosphere: Formation of whistler wave cavity by nonlinear scattering

We consider the weak turbulence of whistler waves in the in low-β inner magnetosphere of the earth. Whistler waves, originating in the ionosphere, propagate radially outward and can trigger nonlinear induced scattering by thermal electrons provided the wave energy density is large enough. Nonlinear scattering can substantially change the direction of the wave vector of whistler waves and hence the direction of energy flux with only a small change in the frequency. A portion of whistler waves return to the ionosphere with a smaller perpendicular wave vector resulting in diminished linear damping and enhanced ability to pitch-angle scatter trapped electrons. In addition, a portion of the scattered wave packets can be reflected near the ionosphere back into the magnetosphere. Through multiple nonlinear scatterings and ionospheric reflections a long-lived wave cavity containing turbulent whistler waves can be formed with the appropriate properties to efficiently pitch-angle scatter trapped electrons. The prim...

Current multiplier to improve generator-to-load coupling for pulse-power generators

Modem pulse-power generators allow obtaining multi - Mega-Ampere currents with sub-microsecond rise-times in useful loads. However, at least during a portion of the rise-time, the load inductance, Ld, can be several times smaller than that of the generator, L0, limiting the energy transfer efficiency. We define a relatively simple circuit modification, which improves the generator-to-load coupling, multiplying the load current in the case of interest, where L0>;>;Ld. The suggested intermediate connection operates similarly to an N:1 transformer. In its simplest configuration, for N=2, the proposed current multiplier requires an additional volume having high self-inductance, L, connected through a single convolute to both generator and load. The modified hardware allows, having possibly one additional unknown, the convolute, and without changing the current rise-time very much an increase of the load current by the factor of Id/I0=2L/(L+Ld), where Io represents the generator current and L can be easily made much greater than Ld. The benefit of the scheme usage is illustrated in simple circuit simulations for two types of potential applications with static, Ld = const, and dynamic, Ld(t), loads.

Quasilinear evolution of plasma distribution functions and consequences on wave spectrum and perpendicular ion heating in the turbulent solar wind

The measured spectrum of kinetic Alfven wave fluctuations in the turbulent solar wind plasma is used to calculate the quasi-linear evolution of the initially stable electron and ion distribution functions. The resulting ion distribution function is found to be unstable to electromagnetic left hand polarized ion cyclotron-Alfven waves as well as right hand polarized magnetosonic-whistler waves. These waves can pitch angle scatter the ion super-thermal velocity component to provide perpendicular ion heating. Additionally, right hand polarized waves transfer some part of kinetic Alfven wave flux to whistler waves.The measured spectrum of kinetic Alfven wave fluctuations in the turbulent solar wind plasma is used to calculate the electron and ion distribution functions resulting from quasi-linear diffusion. The modified ion distribution function is found to be unstable to long wavelength electromagnetic ion cyclotron waves. These waves pitch angle scatter the parallel ion velocity into perpendicular velocity which effectively increases the perpendicular ion temperature.

Whistler Wave Resonances in Laboratory Plasma

Standing whistler wave patterns have been investigated in the Naval Research Laboratorys Space Physics Simulation Chamber. In the original experimental configuration, partial reflection of the antenna-launched whistler waves from the chamber end boundaries occurs, setting up a combination of standing and traveling waves. By controlling the axial magnetic field strength profile, cyclotron absorption of the whistler waves can be induced before reflection occurs, leaving only the forward propagating waves. By comparing standing-wave amplitudes to that when the wave is prevented from reflecting, cavity Qs in excess of 30 have been observed. Under uniform axial magnetic field conditions, the addition of planar conducting grids across the vacuum chamber cross section at the ends of the plasma column provides improved reflecting surfaces and corresponding increases in the value of Q.

Magnetodynamics of a multicomponent (dusty) plasma. II. Magnetic drift waves in an inhomogeneous medium

The electromagnetic oscillations near the low-frequency cutoff (the rotation waves) and their stability and nonlinear state, analyzed in the companion paper [G. Ganguli and L. Rudakov, Phys. Plasmas 12, 042110 (2005)] for a homogeneous plasma, are generalized to include inhomogeneity in the equilibrium density and magnetic field. It is shown that in the forbidden frequency band below the cutoff the magnetic drift waves appear, which can exist even in a cold plasma. The magnetic drift wave and nonlinear structures associated with it are analyzed and their relevance to astrophysical plasmas are discussed. It is found that in an inhomogeneous plasma the rotation wave packets can couple to the magnetic drift waves. The behavior of such structures is governed by nonlinear Schrodinger equation. The spatial scale of the nonlinear structures due to the magnetic drift waves in astrophysical plasmas such as dense molecular clouds, which are the regions of star creation, is estimated to be around 10 a.u.

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 Schrodinger 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.

Laboratory studies of nonlinear whistler wave processes in the Van Allen radiation belts

Important nonlinear wave-wave and wave-particle interactions that occur in the Earths Van Allen radiation belts are investigated in a laboratory experiment. Predominantly electrostatic waves in the whistler branch are launched that propagate near the resonance cone with measured wave normal angle greater than 85°. When the pump amplitude exceeds a threshold ∼5×10−6 times the background magnetic field, wave power at frequencies below the pump frequency is observed at wave normal angles (∼55°). The scattered wave has a perpendicular wavelength that is nearly an order of magnitude larger than that of the pump wave. Occasionally, the parametric decay of a lower hybrid wave into a magnetosonic wave and a whistler wave is simultaneously observed with a threshold of δB/B0∼7×10−7.

Experimental characterization of nonlinear processes of whistler branch waves

Experiments in the Space Physics Simulation Chamber at the Naval Research Laboratory isolated and characterized important nonlinear wave-wave and wave-particle interactions that can occur in the Earths Van Allen radiation belts by launching predominantly electrostatic waves in the intermediate frequency range with wave normal angle greater than 85° and measuring the nonlinearly generated electromagnetic scattered waves. The scattered waves have a perpendicular wavelength that is nearly an order of magnitude larger than that of the pump wave. Calculations of scattering efficiency from experimental measurements demonstrate that the scattering efficiency is inversely proportional to the damping rate and trends towards unity as the damping rate approaches zero. Signatures of both wave-wave and wave-particle scatterings are also observed in the triggered emission process in which a launched wave resonant with a counter-propagating electron beam generates a large amplitude chirped whistler wave. The possibilit...

Generation of electromagnetic waves in the very low frequency band by velocity gradient

It is shown that a magnetized plasma layer with a velocity gradient in the flow perpendicular to the ambient magnetic field is unstable to waves in the Very Low Frequency band that spans the ion and electron gyrofrequencies. The waves are formally electromagnetic. However, depending on wave vector k¯=kc/ωpe (normalized by the electron skin depth) and the obliqueness, k⊥/k||, where k⊥,|| are wave vectors perpendicular and parallel to the magnetic field, the waves are closer to electrostatic in nature when k¯≫1 and k⊥≫k|| and electromagnetic otherwise. Inhomogeneous transverse flows are generated in plasma that contains a static electric field perpendicular to the magnetic field, a configuration that may naturally arise in the boundary layer between plasmas of different characteristics.