Propagation Of Space Charge Waves Research Articles

Stability analysis of Hasegawa space-charge waves in a plasma waveguide with collisional ion beam

The dispersion relation for the Hasegawa space-charge wave propagating in a cylindrical waveguide dusty plasma containing collision-dominated ion stream is derived by using the fluid equations and the Poisson equation which lead to a Bessel equation. The solution of Bessel equation is null at the boundary and then the roots of the Bessel function would characterize the property of space-charge wave propagation. We have found that the Hasegawa space-charge wave can be excited for a large axial wave number. The growth rate of excitation increases as the order of the roots of the Bessel function increases. The growth rate decreases with an increase of the radius of cylindrical waveguide as well as with an increase of the collision frequency. We found that the disturbance of wave can be damped only for small wave numbers.

Whimsicality of multi-mode Hasegawa space-charge waves in a complex plasma containing collision-dominated electrons and streaming ions

The influence of collision-dominated electrons on multi-mode Hasegawa space-charge waves are investigated in a complex plasma containing streaming ions. The dispersion relation for the multi-mode Hasegawa space-charge wave propagating in a cylindrical waveguide filled with dusty plasma containing collision-dominated electrons and streaming ions is derived by using the fluid equations and Poisson’s equation which lead to a Bessel equation. By the boundary condition, the roots of the Bessel function would characterize the property of space-charge wave propagation. It is found that two solutions exist for wave frequency, which are affected by the radius of waveguide and the roots of the Bessel function. The damping and growing modes are found to be enhanced by an increase of the radius. However, an increase of electron collision frequency would suppress the damping and the growing modes of the propagating space-charge wave in a cylindrical waveguide plasma.

The University of Maryland Electron Ring Program

The University of Maryland Electron Ring (UMER) is a unique machine that uses scaled electron beams at nonrelativistic energies (10keV) to inexpensively model GeV beams of heavy ions over long path lengths (kilometers of transport distance). The UMER beam parameters correspond to space charge tune depressions, at injection, adjustable in the range of 0.14–0.85. Although a ring, many of the intense beam studies on UMER are applicable to linacs. This paper reviews the UMER program, which contains experimental, computational, and theoretical components. We outline the research areas of interest, recent accomplishments, and future plans, emphasizing the relevance to heavy ion drivers. Specific topics include longitudinal induction focusing and beam manipulations; generation and propagation of space charge waves, including large-amplitude solitons; bunch end interpenetration and observation of a multi-stream instability; beam halo studies; and diagnostic development.

A new non-invasive approach for the measurement of ion beam energy

A non-invasive approach for the measurement of the kinetic energy of space-charge dominated ion beams is proposed based on measurements of beam current signals at two different positions. We found that the evolution of space-charge waves, which are generated by weak voltage variations in induction accelerator gaps, can be computed precisely by Fourier analysis of the current signals. The beam energy profile can then be derived from the current signals including the effect of transient space-charge waves. Through analytic studies of the space-charge wave propagation, confirmed by 3-D PIC simulations, we show that the current signals are remarkably sensitive to the weak voltage variations.

Orientational dependencies of space-charge waves under the action of a magnetic field in GaAs:Cr

Space-charge wave (SCW) excitation is studied under the influence of an external magnetic field in GaAs:Cr depending on the orientational angle of the exciting oscillating sinusoidal interference pattern to the external electric field. It was found that the tilt of the SCW propagation direction induced by the magnetic field results in a shift of the orientational dependence curve. This feature allows determining the deviation angle of charge carriers’ trajectory and their sign. A quantitative theoretical explanation of the effects is proposed and a theoretical expression for the SCW ac-current is given.

Numerical simulations of space charge waves in InP films and microwave frequency conversion under negative differential conductivity

Numerical simulations of amplification and propagation of space charge waves in InP films is investigated theoretically. A microwave frequency conversion using the negative differential conductivity phenomenon is carried out when the harmonics of the input signal are generated. An increment in the amplification is observed in n-InP films at essentially higher frequencies f<70 GHz, when compared with n-GaAs films f<44 GHz. This work provides a way to achieve a frequency conversion and amplification of micrometer and millimeter waves.

Experimental study of large-amplitude perturbations in space-charge dominated beams

Detailed experimental measurements are presented concerning the propagation of space-charge waves of varying amplitudes in an intense, charged-particle beam. A short perturbation to the density profile is applied at the electron gun, and both current and mean energy profiles are measured at two locations downstream. The measurements are compared to predictions of a linear 1D cold-fluid model, and self-consistent particle-in-cell simulations. For sufficiently small perturbation amplitudes, the experiment, simulation, and 1D theory agree. For larger amplitudes, the simulation begins to diverge from theoretical predictions due to nonlinear effects. Experimental observations for large-amplitude perturbations differ markedly from either theory or simulation. With the aid of simulations with mismatched and misaligned beams, this departure of experiments from predictions is demonstrated to be caused by the loss of beam current due to scraping aided by the larger radius of the perturbation.

Nonstationary effects of the space charge in semiconductor structures

A study of nonstationary effects of space charge in semiconductor structures, such as nonlinear wave interactions in active media operating in the microwave and millimeter wave range, is presented in this paper. Also, an exhaustive analysis of different models describing the propagation of space charge waves is carried out. Furthermore, we have concluded that the most appropriate nonlocal model to describe the space charge wave propagation in thin films possessing negative differential conductivity is the detailed balance model.

Aperture effects and mismatch oscillations in an intense electron beam

When an electron beam is apertured, the transmitted beam current is the integral of the incident beam current density over the aperture area. Space charge forces generally cause an increase in incident beam current to result in an increase in incident beam spot size. Under certain circumstances, the spot size will increase faster than the current, resulting in a decrease in current extracted from the aperture. When using a gridded electron gun, this can give rise to negative transconductance. Here, this effect is explored in the case of an intense beam propagating in a uniform focusing channel. It is shown that proper placement of the aperture can decouple the current extracted from the aperture from fluctuations in the source current, thereby isolating systems downstream of the aperture from upstream current fluctuations. It is also shown that apertures can serve to alter longitudinal space charge wave propagation by changing the relative contribution of velocity and current modulation present in the beam.

Numerical simulations of propagation of SCWs in strained Si/SiGe heterostructures at 4.2 and 77 K

The negative differential conductance phenomenon that occurs in strained Si/SiGe hetrostructures at low temperatures, have been used to perform numerical simulations of the propagation and amplification of space charge waves (SCWs). The 2D numerical simulations indicate that amplification of SCWs on the surface of the heterostructure may occur up to frequencies of 40 GHz when cooled down to 77 K, and up to 44 GHz at 4.2 K.

On Quantum Corrections to Space Charge Waves in Silicon

An analysis that incorporates quantum corrections and the thermal conductivity term into the classical hydrodynamic model of the propagation of space-charge waves in silicon is presented. From numerical simulations we have seen that for frequencies f 8THz both quantum corrections and thermoconductivity are equally important. These results suggest accurate simulations of ultra-small device require the thermal conductivity term to be included in the model.

Two-dimensional simulations of amplification of space charge waves in a strained Si/SiGe heterostructure at 77 K

We theoretically investigated the propagation and amplification of space charge waves on the surface of a strained Si/SiGe heterostructure at 77K, using the negative differential conductance phenomenon. In this work, we also have done a comparison between the n-GaAs thin film and strained Si/SiGe heterostructure with respect to the propagation of space charge waves. We have obtained results in 2D of propagation and amplification of space charge waves in a strained Si/SiGe heterostructure until for frequencies f<40GHz.

Modifications to Harmonic Current Bunching of Electron Beams From RF Cavities Due to Radial Boundary Conditions

Harmonic current bunching of an electron beam after it passes through a standing-wave RF cavity is well known, with analytic solutions in both the transversely infinite space-charge regime and the ballistic regime. The Wallander formula is often used between these regimes for finite-sized beams, with pretty good accuracy. These formulas describe simple space-charge modes that are launched by the RF cavity, and assume a single space-charge reduction factor. However, upon close examination these formulas do not provide the complete story. Additional space-charge modes are introduced by the radial boundary conditions, and the actual total harmonic current is modified somewhat from the simple formulas, and includes beating effects between the different modes, even in the small-signal, or linear, regime. This beating leads to mode mixing and eventual damping, which also affects the long-term propagation of longitudinal space-charge waves in accelerator structures.

Chaotic motion of space charge wave fronts in semiconductors under time-independent voltage bias.

A standard drift-diffusion model of space charge wave propagation in semiconductors has been studied numerically and analytically under dc voltage bias. For sufficiently long samples, appropriate contact resistivity, and applied voltage-such that the sample is biased in a regime of negative differential resistance-we find chaos in the propagation of nonlinear fronts (charge monopoles of alternating sign) of electric field. The chaos is always low dimensional, but has a complex spatial structure; this behavior can be interpreted using a finite-dimensional asymptotic model in which the front (charge monopole) positions and the electrical current are the only dynamical variables.

Effects of wiggler and axial guide fields on wave propagation in a free-electron laser

An analysis of the propagation of space-charge and electromagnetic waves through combined helical wiggler and axial guide magnetic fields is presented. A convenient method using complex basis vectors is used to derive a dispersion relation for coupled electrostatic and electromagnetic waves. Numerical results for space-charge waves differ markedly from the conventional approximation. For example, the effective electron-density factor Φ is double valued, becomes complex in some parameter regimes and large in others, and does not become singular near the stability limit for group I orbits. A numerical study of right circularly polarized electromagnetic waves also shows interesting effects of the combined wiggler and guide fields on the dispersion.

Observation of solitary waves in an ion-beam–plasma system

Propagation of nonlinear space-charge waves in an ion-beam–plasma system is investigated in a double-plasma device. The velocity of the beam is high enough to avoid ion–ion instability. The density ratio of the beam to the plasma is kept high ([les ]0.6), which makes the maximum amplitude of solitary waves large. The measured velocity and width of the compressional solitary waves of the fast and the slow beam mode are compared with predictions of the pseudopotential method in which the temperatures of beam and plasma ions are taken into consideration. Reasonable agreement is obtained between the experimental and theoretical results.

Longitudinal space-charge waves and instabilities in intense beams

Space-charge effects and collective behavior play an important role in the dynamics of intense beams for accelerator applications. One of the research issues is the longitudinal space-charge waves and instabilities in high current accelerators. At the University of Maryland, an experimental program has been conducted to systematically study space-charge waves and longitudinal instabilities in space-charge dominated electron beams. This includes the generation of localized space-charge waves in an active device; the propagation and end reflection of space-charge waves in a conducting transport channel; and the longitudinal instabilities in a resistive-wall channel. This paper reports and discuss our experimental results.

Space-charge waves in a coaxial plasma waveguide

The propagation of space-charge waves through a coaxial waveguide containing an annular plasma in an axial magnetic field is investigated. Both plasma and cyclotron types of waves are analyzed in the electrostatic approximation. Equations for the determination of the dispersion relations are derived from the Poisson equation and the electron continuity and momentum transfer equations. A numerical study of the dispersion curves for azimuthally symmetrical waves is presented. A significant departure from the dispersion characteristics of a cylindrical plasma waveguide are found to occur unless the inner radius of the waveguide is small compared to the outer radius.

Beam space-charge modes in a free-electron laser

Propagation of space-charge waves in a Raman free-electron laser is analyzed. The device under consideration consists of a cylindrical metallic waveguide filled with a relativistic electron beam which passes through a helical wiggler magnetic field and a uniform axial magnetic field. A solution of the fluid equations and Gauss’s law is developed in the form of truncated Fourier and Fourier–Bessel series. The dispersion relation is derived and the combined effects of the waveguide boundary and the wiggler and axial magnetic fields on cyclotron-like waves are studied numerically. In the limit of infinite waveguide radius, the dispersion relation yields a well-known expression in which the combined wiggler and axial fields determine an effective plasma frequency. In the limit of zero wiggler field, it reduces to the dispersion relation for axisymmetric space-charge waves which, in the beam frame, are purely electrostatic.

A small signal theory of gyro-devices using the concept of space-charge waves

The linear theory of the interaction between a thin annular beam of gyrating electrons and a traveling electromagnetic wave is developed. This theory is very close in its methodology to the classical small signal theory of O- and M-type devices. Just this fact makes it possible to consider a number of issues in the theory of gyro-devices, namely, space-charge wave propagation in drifting electron beams, noise and parametric phenomena, effects of input signal suppression (Kompfner dip) in gyro-traveling-wave-tubes, and gyro-backward-wave-amplifiers.