Effect Of Ponderomotive Force Research Articles

Ponderomotive force effects in the vicinity of a radio-frequency probe in a plasma

The electric field excited by a probe (antenna) exerts a radio-frequency pressure on the plasma surrounding the probe. Two effects are discussed: the redistribution of the static plasma density and potential, resulting in the formation of the r.f. sheath; and a frequency mixing in the case of excitation at two or multiple frequencies. The antenna may be excited either by a r.f. generator or by an incident wave field, in which case the ponderomotive pressure is due to the scattered field. It is shown that an electric antenna receiving two high frequency waves responds to a beat frequency, and the amplitude of this response is estimated. The amplitude is enhanced resonantly around the plasma frequency. This effect should be taken into account in any interpretation of the electric antenna measurements in the space plasma.

Onset of laser light filamentation in preformed plasmas

The onset of laser beam filamentation in a preformed nonuniform plasma is investigated by using a two-dimensional computer code in which time-dependent plasma hydrodynamics and heat transport effects are accounted for. Laser beam propagation is modeled using a paraxial wave approximation—refraction, diffraction, absorption, and ponderomotive force effects are accounted for. The simulations reveal details by which multifilamentary plasma and laser beam structures evolve in the region near the critical density surface. For the particular case of a 1.05 μm laser beam with an intensity of 1015 W/cm2 incident on a long scale length preformed plasma, a consistent treatment of the ponderomotive forces and of the plasma dynamics is found to be essential in determining the evolution of the filamentary structures.

Ion-cyclotron-frequency stabilization of internal kink mode and sawtooth oscillations in tokamaks.

It is proposed that the ponderomotive force due to applied ion-cyclotron resonance-frequency waves can stabilize the internal kink mode in tokamaks. The sufficient stability criterion is derived and the necessary power estimated. It is concluded that at the rf power level, present in the Joint European Torus experiment, the ponderomotive force effects are significant and may be responsible for the modification of the sawtooth activity observed in recent experiments.

Ponderomotive force effect on plasma density profile in the underdense region

The flux transport effect on the radiation-induced plasma density profile and ablative flow outside the resonance layer in steady state in one dimensional spherical geometry studied. The numerical predictions conclude the existence of plateau-like plasma density profile rather than overdense bump formation in the underdense plasma. The plasma density profile, though retaining its shape, becomes more pronounced as the heat flux is increased. However, the length of the plasma density plateau is found to be quite small when compared with the isothermal case.

Soliton Modes in an Unstable Plasma

A synergetic approach is applied to the study of nonlinear evolution of unstable waves in an electron beam plasma. When the beam velocity is sufficiently large compared with the thermal velocity of electrons, the system is linearly unstable against electrostatic perturba· tions of large wavelengths, but stabilized in its nonlinear stage by the effect of ponderomotive force of high·frequency fields. The results show that the amplitude of unstable plane wave behaves such as ~sech rt as a function of time t, where r is the linear growth rate, never growing up infinitely. It is also shown that the plane wave is unstable for long wavelength modulation and is stabilized by emission of a series of envelope solitons.

Excitation of electrostatic fluctuations by thermal modulation of whistlers

It is shown that a large‐amplitude field‐aligned whistler wave can excite nonducted whistler sidebands and nonresonant low‐frequency electrostatic collisional gradient drift modes. The effects of the ponderomotive force as well as the heating nonlinearities are included in the study of the modulational instabilities. For this case it is found that the heating nonlinearity dominates over the ponderomotive force effects, provided that the doppler‐shifted frequency of the collisional modes is smaller than the electron collision frequency. Our investigation can be relevant in understanding why enhanced density irregularities appear in the presence of electromagnetic heater waves in the lower part of the ionosphere.

Filamentation of an annular laser beam in a short wavelength laser produced plasma

We report on the first observations of periodic structures caused by the filamentation instability. These structures were observed on 0.35 μm laser irradiated planar target experiments. Comparison of these observations with self-focusing theory indicates that even though thermal filamentation may initiate the instability, ponderomotive force effects eventually dominate the actual filamentation process.

Suppression of flute instability in an axisymmetric double-cusp plasma by RF ponderomotive force

An analysis is made of the stabilizing effect of a radial ponderomotive force on the interchange mode in a double-cusp mirror plasma which has unfavourable curvature on internal flux surfaces. A cavity mode in the long central cell region is set up by exciting electromagnetic ion cyclotron waves and it provides the radial force which opposes the curvature drive. Application of this method to RFC-XX shows that the ICRH power needed for stabilization is comparable to that needed for heating.

Strong electromagnetic waves in a magnetized relativistic electron-positron plasma

It is shown that in a strongly magnetized relativistic electron-positron plasma, strongly localized large amplitude circularly polarized electromagnetic wave pulses exist. The localization is due to relativistic mass variation as well as ponderomotive force effects. Three types of pulses are found analytically: the sharply spiked pulse in a strongly magnetized cold plasma, the smooth pulse in a week magnetized warm plasma, and the moderately spiked pulse for a weakly magnetized cold plasma. The physical mechanisms giving rise to these pulses are distinct for each case. Possible implications of our investigation to pulsar radiation are discussed.

Filamentation instability in magneto plasmas

It is shown that the filamentation of ordinary-mode pumps propagating at an arbitrary angle with respect to the magnetic field has to be magnetic field-aligned, but not for extraordinary-mode pumps. A general dispersion relation is derived including the effects of magnetic field and collisions, and the nonlinear effects of ponderomotive force, thermal focusing force, and the beating currents. Threshold field and growth rates are obtained and compared to the results of the unmagnetized and collisionless case. Applications of these results to ionospheric modifications are discussed.

Ponderomotive Force Effects due to a Circularly Polarized Electromagnetic Wave

AbstractThe modifications in the time‐averaged plasma particle distribution functions in the presence of a large amplitude electromagnetic wave are studied. It is shown that the ponderomotive force is responsible for tail formation, non‐zero particle currents and heat fluxes. Explicit results, which describe those effects, are derived.

Ponderomotive force effects on slow-wave coupling

Localized plasma density depressions are observed to form near a multi-ring slow-wave structure when the value of the nonlinearity parameter, s = ω2pe‖Ez‖2/8πω2nκT, is of order unity. Consequent changes in the wave propagation and coupling efficiency are reported. For large enough values of s, the coupling efficiency may be reduced by 50% from the linear value.

THE TWO-DIMENSIONAL NONLINEAR EFFECTS OF PONDEROMOTIVE FORCE OF LOWER HYBRID WAVE

We discussed the nonlinear effects of ponderomotive force, which is exerted by powerful LH waves with large electrical fields (|E|～1.5 kV/cm). An obvious nonlinear nonhomogeneity of density in the direction parallel to B0 has been found. It might be serious enough to influence the propagation of the long waves (such as waves with N‖～1). As for the direction perpendicular to B0,if there is reflection inside the plasma (for example, from the region close to the limiter of Tokamak), a series of density cavities might appear along the path of LH wave transmission, and the picture of plasma displacement is no longer suitable.

Effect of ponderomotive forces on wave dispersion and second-harmonic light emissions in laser-produced plasmas

Effect of ponderomotive forces on wave dispersion and second-harmonic light emissions in laser-produced plasmas

Stochastic Plasma Heating by a Large-Amplitude Standing Wave

Strong heating of a laboratory plasma is experimentally observed in the presence of a standing wave when the wave amplitude exceeds a certain threshold. This threshold amplitude is absolutely measured from observations of the ponderomotive force effects and compared to the theoretically expected value for the onset of large-scale stochasticity.

Simulation of lower-hybrid heating in a nonuniform plasma slab

A computer simulation study of the excitation of lower hybrid waves by external sources and the associated plasma heating is presented. A plasma slab with a nonuniform density profile is modeled with a two and one-half dimensional electrostatic particle code. Both a finite-length electrostatic antenna and a single wave exciter are considered. The frequency is chosen so that a wave conversion layer exists in the plasma. It is found that a narrow sheath is formed at the plasma edge and the dominant effect is surface electron heating within that region. Ponderomotive force effects as well as the nonlinear generation of harmonics and subharmonics also occur. A low amplitude lower hybrid wave is observed to be excited by the sources and propatates to the wave conversion layer where it is absorbed.

Nonlinear converging resonance cones

The trajectory modification of a focused resonance cone due to ponderomotive force effects was investigated in the Princeton L-3 device. A single ring pulsed at f0∼50 MHz (flh<<fo<<fpe, fce ) launches a converging resonance cone in an argon plasma with ne∼109–1010, B∼400 G–1 kG, and Te∼1–3 eV. At high driving power the strong localized electric fields modify the density through the ponderomotive force on the electrons, leading to a modification of the resonance cone trajectory. The cone is seen to bend in toward the axis and the focus moves toward the ring in agreement with the recent theory. When the rf is modulated, the density perturbation at the focus propagates radially outward and excites ion-acoustic waves. The wave dispersion relation was verified in He+, Ne+, Ar+, and Kr+ plasmas. Electron heating was observed which is consistent with collisional heating.

Energy flow and absorption along resonance cones near the lower hybrid

Macroscopic and microscopic forms of the energy conservation theorem for quasi-static waves are applied to determine the approximate power flux density and rate of energy absorption along the resonance cones near the lower hybrid resonance, both for the incoming cone and the outgoing converted ion thermal cone. It is found that for the incoming cone excited by a point gap source, there is almost as much power flux and absorption along each of the first few secondary peaks as along the main peak, so that the channel along which most of the energy flow and absorption occurs extends the width of several peaks of the cone. On the outgoing cone, the power flow and absorption is more concentrated on the main peaks than for the incoming cone. Nonlinear ponderomotive force effects are found to be more important, in general, for this cone than the incoming cone unless substantial absorption occurs in the mode conversion region. An asymptotic relationship is found connecting the power absorbed to the power flux and the spatial damping rate of the potential of the resonance cones.

Excitation of lower hybrid waves in a finite plasma

A computer simulation study of the launching of lower hybrid waves by external sources and their propagation in a finite but homogeneous plasma slab is presented. The study makes use of a two and one-half dimensional electrostatic particle code developed for simulation of bounded plasma. A number of antenna configurations are considered: a point source, two finite-length capacitor plates, and a phased array of capacitor plates. When the oscillating frequency does not match a bounded plasma resonance, one can observe resonance cones, energy absorption at the plasma surface, ion cyclotron modulation of the source, and energetic ions, depending on the parameters chosen. The excitation of a bounded plasma resonance is also considered. The nonlinear evolution of the resonance shows that wave-particle interactions and ponderomotive force effects play an important role. The possibility of controlling the electron temperature profile is discussed.

Re-examination of strongly flux limited thermal conduction in laser produced plasmas

Laser interaction experiments on plastic foils at 1.06 μm and 10.6 μm wavelengths have been re-examined using the LASNEX hydrodynamics code modified to include ponderomotive force effects and realistic hot electron generation. The data for a wide range of experiments have been accurately modeled without invoking an anomalous limit on the electron heat flux.