Observation of transition from a reactive-medium instability to an inverse Landau damping in a beam-plasma system

The wave excited by a coaxial probe in a beam-plasma system usually propagates as two waves which are independent of each other, i.e., a damped Trivelpiece mode of the plasma and a growing space charge wave of the beam (referred to a reactive-medium instability). While, when the beam velocity is equal to or slightly larger than the phase velocity of the plasma wave, the inverse Landau damping of the wave is observed, instead of the damped Trivelpiece mode.     

Observations of nonlinear Landau damping in an electron beam-plasma system

The nonlinear Landau damping of space charge waves of an electron beam into Trivelpiece modes by nonlinear interaction with plasma electrons has been observed experimentally in an electron beam-plasma system.     

Thermal effects in the dissipative instability of the electron beam-plasma systems

The effects of the thermal motion of the charged particles in the dissipative instability of the under and over-limiting currents of a relativistic electron beam in a fully magnetized beam-plasma waveguide is investigated. It is shown that by increasing the temperature of the plasma electrons, the resonant frequency of the waveguide slightly increases and the growth rates of the instability development decreases. In addition, an increase of the temperature of the plasma electron can change the dissipative hydrodynamic instability to the collisionless kinetic instability. Furthermore, the dissipative instability of the overlimiting electron beam is shown to be more sensitive with respect to the electron plasma temperature compared to the underlimiting electron beam case.     

The limitation of an explosive instability of two counter-streaming electron beams

Second-order perturbation calculation shows that an explosive instability of three resonantly interacting waves is limited and converted into a multi-periodic process by non-linear terms of the same order as those that destabilize the waves in the first-order approximation.     

Return current instability and its effects on beam-plasma system

The return current induced in a plasma by a relativisitc electron beam generates a new electron-ion two-stream instability (return current instability). Although the effect of these currents on the beam-plasma e-e instability is negligible, there exists a range of wave numbers which is unstable only to return current (RC) instability and not to e-e instability. The electromagnetic waves propagating along the direction of the external magnetic field, in which the plasma is immersed, are stabilized by these currents but the e.m. waves with frequencies,ω ²≪Ω _e ² ≪ω _pe ² (Ω _e andω _pe being cyclotron and plasma frequency for the electrons of the plasma respectively) propagating transverse to the magnetic field get destabilized. Heuristic estimates of plasma heating, due to RC instability and due to decay of ion-acoustic turbulence generated by the return current, are made. The fastest time scale on which the return current delivers energy to the plasma due to the scattering of ion-sound waves by the electrons can be ∼ω _pi ⁻¹ (ω _pi being the plasma frequency for the ions).     

Stochastic heating of ions in a modulated electron beam-plasma system

A modulation of an electron beam density at frequencies lower than an ion cyclotron frequency enhanced the low frequency instability of broad band and heated plasma ions in a beam-plasma system. The heating of ions is explained by the stochastic process of a weakly turbulent plasma.     

Nonlinear saturation of hot beam-plasma instability

The wave-particle interactions in a beam-plasma system in the presence of finite but small thermal motions of the particles are investigated in the linear as well as nonlinear regime. During the linear growth, the thermal motions are found to have a stabilizing effect. The nonlinear evolution is studied by using the Perturbed Orbit Formalism. Due to the thermal motions the nonlinear saturation of growth is found to take place at a level lower than that of the cold case. A detailed study of the energy balance shows that nonresonant particles pick up a bigger fraction of the energy lost by the streaming motion of the beam, thus leading to more efficient ‘heating’.     

Off-peak saturation effects of beam-plasma instability

For beam-plasma instability in the absence of a magnetic field, hybrid simulations produce almost identical results to those of particle simulations (both agree with linear and nonlinear analyses), but with much reduced computing cost and noise level. With the verification by simulations, it is shown that the saturation level is very strongly influenced by the discrete wave number spectrum, sometimes giving different, by an order of magnitude, results by choosing slightly different off-peak (in growth rate curve) parameters, which must be considered in most simulations (whether hybrid or particle or whether magnetized or unmagnetized)     

Measurements of the dispersion relation in an ion beam-plasma system

Measurements of the dispersion relation and of the damping rate in an ion beam-plasma system are performed. Near the exciter three normal modes, the ion acoustic, slow and fast ion-beam modes exist. Agreement between experiment and theory is found.     

Microwave emission and beam propagation measurements in ahigh-power relativistic electron beam-plasma system

Microwave emission was measured from a system consisting of an unmagnetized plasma and a propagating electron beam. A 93-cm² velvet cathode, with an anode-cathode gap of 5.9 cm, injects the electron current into the plasma through an aluminized Mylar anode. Measurements were made of the diode voltage and current in the 6-μV water dielectric accelerator and net current through the beam-plasma system. The unmagnetized plasma is produced by a 90-μs, 90-Å current pulse, emitted from a thermionic LaB₆ electron source, that preionizes argon fill in a 1-m-long, 15-cm-diameter Lucite tube. A microwave spectrometer detects the radio-frequency output in the 2-18, 18-26, and 26-47 GHz bands, filters, and then separates into narrower subbands. The emission takes place in two distinct phases. The 2-GHz output rises promptly with the current pulse and then decays. At 6-GHz and above, a low-level microwave prepulse appears simultaneously with the 2-6 GHz output. This output rises sharply 25 ns after the current pulse begins and includes frequencies out to and beyond 40 GHz. The radio-frequency output falls off before the current pulse ends. The microwave intensity decays monotonically with frequency     

Many-wave explosive instability

For special initial phase relations and amplitude values intermediate asymptotic state solutions are obtained to the problem of four interacting triplets of waves yielding explosive instability.     

Relativistic electron beams and beam-plasma interaction

The interaction of a (relativistic) electron beam with a plasma is discussed. In the introduction the history of the field and related topics are reviewed. Next the momentum space distribution of a relativistic electron beam (REB) is treated. Then, taking into account the kinetic properties of a REB, linear wave dispersion in a REB-plasma system is surveyed. Dispersion diagrams are calculated and presented for a number of representative sets of parameters. Finally a discussion is given of the dispersive properties of unstable waves. Criteria are developed to discriminate between the various types of possible non-linear behaviour of the waves as they grow to their saturation level.     

The dispersion relation of the charge and current compensated relativistic electron beam-plasma system

Analysing the general dispersion relation numerically, the unstable regions of the relativistic electron-beam plasma system were determined. The external parameters were varied to ensure more effective instability excitations. The full charge and current compensation presumptions lead to new synchronism predictions. The return current's slow space charge wave and return current's slow cyclotron wave are synchronous with the plasma ion wave.     

Dissipative instability under weak beam-plasma coupling in finite magnetic field

Beam-plasma interaction is considered in a model of plasma microwave generator: cylindrical waveguide with thin annular plasma and spatially separated thin annular e-beam. Finite external longitudinal magnetic field and dissipation are assumed. Such a configuration is favorable to trigger a new type of dissipative beam instability with more critical, as compared to conventional, inverse proportional dependence of the growth rate on the dissipation.     

Saturation of the relavistic beam-plasma instability for arbitrary density ratios

The nonlinear saturation level W_s of the one-dimensional beam-plasma instability was calculated as a function of beam energy γ₀ for all values of beam/plasma density ratio η. For η ? 0.01, W_sdecreases with increasing γ₀.