Weak electron beams in inhomogeneous plasma
Inhomogeneous plasma containing weak electron beam using Vlasov equation, noting wave-wave coupling effect
Baker, J. C.↗
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Results for “INHOMOGENEOUS PLASMA”
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Inhomogeneous plasma oscillations in streaming plasmas
Inhomogeneous plasma oscillations in counterstreaming plasmas analyzed for electric fields and resonant frequencies from Vlasov equation
Baker, J. C.↗
Effect of plasma inhomogeneity on plasma wakefield acceleration driven by long bunches
Effects of plasma inhomogeneity on self-modulating proton bunches and accelerated electrons were studied numerically. The main effect is the change of the wakefield wavelength which results in phase shifts and loss of accelerated particles. This effect imposes severe constraints on density uniformity in plasma wakefield accelerators driven by long particle bunches. The transverse two stream instability that transforms the long bunch into a train of micro-bunches is less sensitive to density inhomogeneity than are the accelerated particles. The bunch freely passes through increased density regions and interacts with reduced density regions.
43 PARTICLE ACCELERATORS↗
Phase mixing of upper hybrid oscillations in a cold inhomogeneous plasma placed in an inhomogeneous magnetic field
We study phase mixing/wave breaking phenomena of upper hybrid modes in a cold inhomogeneous plasma placed in an inhomogeneous magnetic field. Inhomogeneities both in the background ion density and magnetic field profile are treated as periodic in space but independent in time. The Lagrangian fluid description is employed to obtain an exact solution of this fully nonlinear problem. It is demonstrated that the upper hybrid modes, excited by an initial local charge imbalance, break via phase mixing, induced by the inhomogeneities. It is also shown that it is possible to avoid phase mixing in excited upper hybrid oscillations in an inhomogeneous plasma containing a finite amplitude ion density fluctuation. The choice of external magnetic field is shown to have a key role in avoiding phase mixing in such oscillations. The relevance of our investigation regarding the particle acceleration in an inhomogeneous plasma has also been discussed.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Two-dimensional studies of relativistic electron beam plasma instabilities in an inhomogeneous plasma
Relativistic electron beam propagation in plasma is fraught with several micro instabilities like two stream, filamentation, etc., in plasma. This results in severe limitation of the electron transport through a plasma medium. Recently, however, there has been an experimental demonstration of improved transport of Mega Ampere of electron currents (generated by the interaction of intense laser with solid target) in a carbon nanotube structured solid target [G. Chatterjee et al., Phys. Rev. Lett. 108, 235005 (2012)]. This then suggests that the inhomogeneous plasma (created by the ionization of carbon nanotube structured target) helps in containing the growth of the beam plasma instabilities. This manuscript addresses this issue with the help of a detailed analytical study and 2-D Particle-In-Cell simulations. The study conclusively demonstrates that the growth rate of the dominant instability in the 2-D geometry decreases when the plasma density is chosen to be inhomogeneous, provided the scale length 1/k{sub s} of the inhomogeneous plasma is less than the typical plasma skin depth (c/ω{sub 0}) scale. At such small scale lengths channelization of currents is also observed in simulation.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Laser pulse driven terahertz generation via resonant transition radiation in inhomogeneous plasmas
An intense, short laser pulse propagating across a plasma boundary ponderomotively drives THz radiation. Full format PIC simulations and theoretical analysis are conducted to investigate the properties of this radiation. Simulation results which show the THz emission originates in regions of varying density and covers a broad spectrum with maximum frequency close to the maximum plasma frequency. In the case of a sharp vacuum-plasma boundary, the radiation is generated symmetrically at the plasma entrance and exit, and its properties are independent of plasma density when the density exceeds a characteristic value determined by the product of the plasma frequency and the laser pulse duration. For a diffuse vacuum-plasma boundary, the emission from the plasma entrance and exit is asymmetric: increasing and decreasing density ramps enhance and diminish the radiated energy, respectively. Enhancements by a factor of 50 are found and simulations show that a 1.66 J, 50 fs driver pulse can generate ∼400 μJ of THz radiation in a 1.2 mm increasing density ramp. We present a model that attributes this effect to a plasma resonance process in the density ramp. The results from the model match those of the simulations for ramp lengths less than 600 μm. For longer ramps for which simulations are too time consuming, the model shows that the amount of radiation reaches a maximum at a ramp length determined by collisional absorption.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Channeling of multikilojoule high-intensity laser beams in an inhomogeneous plasma
Not Available
Ivancic, S.↗
Modelling of the beam-plasma interaction in a strongly inhomogeneous plasma
The interaction of a beam propagating in a inhomogeneous solar wind plasma is considered. The properties of the plasma waves are described by the one dimensional Zakharov's equation, the beam is modelled by means of particles moving in the electric fields of the Langmuir waves. We take into account the presence of high level density fluctuations that are known to be present in the solar wind. It is shown that when the level of density fluctuations is low, {delta}n/n{sub 0} Much-Less-Than 3k{sup 2}{lambda}{sub d}{sup 2}, the regime of beam relaxation is very similar to that occurring in a homogeneous plasma and can be described by the quasilinear (QL) equations. In this case, the relaxation length is very short and corresponds to that obtained using the QL approximation. On the contrary, when the level of density fluctuations overcomes some limit, i.e. {delta}n/n{sub 0}{>=}3k{sup 2}{lambda}{sub d}{sup 2}, where {delta}n/n{sub 0} Much-Less-Than 1, the plasma inhomogeneities crucially influence the process of relaxation. First, the linear wave growth becomes localized and clearly identifiable wave packets/clumps dominate the wave spectrum; this is associated with the kinematic properties of the waves' propagation and the wave-particle resonant interactions. Most of the wave packets grow in the regions of density gradients. The second important feature revealed by the beam relaxation consists in the generation of a tail of accelerated electrons with velocities V > V{sub b} exceeding the beam drift. The beam widens in both directions, toward lower velocities as well as higher velocities; the density of the accelerated electrons can reach more than 10 Division-Sign 20 % of the beam density and the energy flux carried by this population can become as large as 40% of the initial energy flux of the beam.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
PROBABILISTIC MODEL OF BEAM–PLASMA INTERACTION IN RANDOMLY INHOMOGENEOUS PLASMA
We propose a new model that describes beam–plasma interaction in the presence of random density fluctuations with a known probability distribution. We use the property that, for the given frequency, the probability distribution of the density fluctuations uniquely determines the probability distribution of the phase velocity of waves. We present the system as discrete and consisting of small, equal spatial intervals with a linear density profile. This approach allows one to estimate variations in wave energy density and particle velocity, depending on the density gradient on any small spatial interval. Because the characteristic time for the evolution of the electron distribution function and the wave energy is much longer than the time required for a single wave–particle resonant interaction over a small interval, we determine the description for the relaxation process in terms of averaged quantities. We derive a system of equations, similar to the quasi-linear approximation, with the conventional velocity diffusion coefficient D and the wave growth rate γ replaced by the average in phase space, by making use of the probability distribution for phase velocities and by assuming that the interaction in each interval is independent of previous interactions. Functions D and γ are completely determined by the distribution function for the amplitudes of the fluctuations. For the Gaussian distribution of the density fluctuations, we show that the relaxation process is determined by the ratio of beam velocity to plasma thermal velocity, the dispersion of the fluctuations, and the width of the beam in the velocity space.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Radiation from accelerated Alfven solitons in inhomogeneous plasmas
In a weakly inhomogeneous plasma, the large-amplitude Alfven waves propagating parallel to the ambient magnetic field are shown to evolve into accelerated Alfven solitons. Nonlinear interaction of the accelerated Alfven solitons with the Langmuir waves results in the emission of coherent radiations. Analytical expression for the power radiated per unit solid angle from a soliton is derived for two inhomogeneity profiles, namely the linear profile and the parabolic profile. For the case of uniform plasmas, the emission occurs via a decay-type process or resonant modes. In the presence of inhomogeneity, nonresonant modes provide a new channel for the emission of radiation. The power radiated per unit solid angle is computed for the parameters relevant to Comet Halley's plasma environment. For the nonresonant modes it is found to be several orders of magnitude higher than that for the case of resonant modes.
Lakhina, G. S.↗
Laser-driven electron acceleration in an inhomogeneous plasma channel
We study the laser-driven electron acceleration in a transversely inhomogeneous plasma channel. We find that, in inhomogeneous plasma channel, the developing of instability for electron acceleration and the electron energy gain can be controlled by adjusting the laser polarization angle and inhomogeneity of plasma channel. That is, we can short the accelerating length and enhance the energy gain in inhomogeneous plasma channel by adjusting the laser polarization angle and inhomogeneity of the plasma channel.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Convective amplification of a three-wave parametric instability in inhomogeneous plasma
The three-wave parametric instability in inhomogeneous plasma is revisited and the inconsistency of the convective amplification in previous literatures is resolved. By employing multiple methods, the amplification factor of the decay mode due to the inhomogeneity is confirmed as A=πγ{sub 0}{sup 2}/(v{sub g1x}v{sub g2x}κ{sup ′}), which is independent of the linear damping rate of the decay mode, where γ{sub 0} is the nonlinear growth rate in homogeneous plasma, v{sub g1x} and v{sub g2x} are the group velocities in the x direction of the two decay modes and κ{sup ′} is the derivative of the wavevector mismatch of the three coupled waves due to the plasma inhomogeneity.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Localization of linear kinetic Alfvén wave in an inhomogeneous plasma and generation of turbulence
This paper presents a model for the propagation of Kinetic Alfvén waves (KAWs) in inhomogeneous plasma when the inhomogeneity is transverse to the background magnetic field. The semi-analytical technique and numerical simulations have been performed to study the KAW dynamics when plasma inhomogeneity is incorporated in the dynamics. The model equations are solved in order to study the localization of KAW and their magnetic power spectrum which indicates the direct transfer of energy from lower to higher wave numbers. The inhomogeneity scale length plays a very important role in the turbulence generation and its level. The relevance of these investigations to space and laboratory plasmas has also been pointed out.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
On the asymptotic kinetic equation for inhomogeneous plasmas.
Asymptotic kinetic equation for inhomogeneous plasma compared to results for initial value problem and Bogoliubov method
ASYMPTOTIC FUNCTION↗
Propagation in a planar inhomogeneous plasma medium
Propagation in planar inhomogeneous plasma medium
Norgard, J. D.↗
Excitation of plasma resonances by a small pulsed dipole in a weakly inhomogeneous plasma
Plasma resonance excitation by small pulsed dipole in weakly inhomogenous plasma determined using WKB solutions and stationary phase approximations
Fejer, J. A.↗
Reflectionless passage of an electromagnetic wave through an inhomogeneous plasma layer
An exactly solvable model is used as a basis to study the reflectionless passage of a transverse electromagnetic wave through an inhomogeneous plasma containing large-amplitude, small-scale (subwave-length) structures (in particular, opaque regions) that cannot be correctly described by approximate methods. It is shown that, during the reflectionless passage of an electromagnetic wave, strong wave field splashes can occur in certain plasma sublayers. The nonuniform spatial plasma density profile is characterized by a number of free parameters describing the modulation depth of the dielectric function, the characteristic sizes of the structures and their number, the thickness of the inhomogeneous plasma region, and so on. Such plasma density structures are shown to be very diverse when, e.g., a wave that is incident from vacuum propagates without reflection through a plasma layer (wave barrier transillumination). With the cubic nonlinearity taken into account, a one-dimensional problem of the nonlinear transillumination of an inhomogeneous plasma can be solved exactly.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Electron density measurements of an inhomogeneous plasma using millimeter wave Fabry-Perot interferometers.
The electron density of a laboratory generated inhomogeneous plasma was measured using 60-90, GHz flat-plate and 150 GHz semi-confocal Fabry-Perot interferometers. The plasma was a negative glow-type and had a measured parabolic electron density profile. An analytical comparison between the derived equations of electron density for an assumed homogeneous plasma slab and an inhomogeneous plasma with parabolic distribution showed that the slab approximation was valid for such a profile. Comparison of measured values of electron density with those using X-band and K-band interferometers for the same plasma indicated good agreement.
Kendall, B. M.↗