Low-density Beam Research Articles

Deceleration of fast ion rarefied beam due to Cherenkov interaction with ion-acoustic waves.

The deceleration of a low-density beam of ions in plasma with developed ion-acoustic turbulence arising in strong electric field is described. The time and length of beam deceleration along and across the anisotropy axis of the wave number distribution of ion-acoustic waves are found. It is shown to what extent an increase in the strength of the electric field that generates turbulence is accompanied by a decrease in the time and length of braking. As the beam propagates along the anisotropy axis, its velocity decreases to approximately the velocity of ion sound, and the direction of propagation does not change. When the beam is decelerated with an initial velocity across the anisotropy axis, a velocity component appears along the anisotropy axis during deceleration, which results in the beam deflection from the initial direction. In this case, the modulus of the beam velocity at the end of deceleration is close to the ion sound velocity.

Transverse emittance measurement in 2D and 4D performed on a Low Energy Beam Transport line: benchmarking and data analysis

2D and 4D transverse phase-space of a low-energy ion-beam is measured with two of the most common emittance scanners. The article covers the description of the installation, the setup, the settings, the experiment and the benchmark of the two emittance meters. We compare the results from three series of measurements and present the advantages and drawbacks of the two systems. Coupling between phase-space planes, correlations and mitigation of deleterious effects are discussed. The influence of background noise and aberrations of trace-space figures on emittance measurements and RMS calculations is highlighted, especially for low density beams and halos. A new data analysis method using noise reduction, filtering, and reconstruction of the emittance figure is described. Finally, some basic concepts of phase-space theory and application to beam transport are recalled.

Influence of ablation on energy deposition in polymer material under irradiation of intense pulsed ion beam

Short-pulse length and high-power density, intense pulsed ion beam (IPIB) has been widely studied in material processing during past decades. Ablation effect plays a great role in the interaction between IPIB and material and may affect the energy deposition of IPIB, thus further influencing the beam application and diagnostics. Therefore, the investigation of ablation effect on energy deposition of IPIB in the irradiated material is of great significance for its applications and diagnostic techniques. In this work, experiments on the IPIB irradiation are carried out on the BIPPAB-450 accelerator at Beihang University. Its maximum accelerating voltage is 450 kV, peak current density is 150 A/cm<sup>2</sup>, energy density is 1.5–1.8 J/cm<sup>2</sup> and pulse duration (FWHM) is 80 ns. Polymer materials which have low thermal conductivity, low decomposition temperature and thus yield to ablation under low beam density, such as polycarbonate (PC), polyvinyl chloride (PVC) and polymethyl methacrylate (PMMA), are chosen in the present research. The 304 stainless steel is used for calorimetric beam diagnostics and comparative analysis. Energy deposition in polymer material and 304 stainless steel are obtained by high infrared imaging diagnostics. It is revealed that the distributions of energy deposition in these two kinds of materials differ from each other obviously. The highest energy density deposited in the 304 stainless steel appears in the center of the irradiated area where focused is the beam with a higher energy density. However, the central energy density in polymer material turns out to be lower than the surrounding area, indicating that a large portion of the ion beam is prevented from reaching the target. Meanwhile, the simulation based on the finite element method is carried out for the thermal filed distribution and evolution under the IPIB irradiation. The simulation result indicates that the strong ablation can be generated on the target surface since the highest temperature caused by IPIB irradiation is much higher than its decomposition temperature. According to the results of experiments and simulation, the polymer material can start to be ablated at the initial stage of IPIB irradiation which will consume partial energy and the products of ablation may act as shielding to block the energy deposition in the same pulse.

The influence of magnetic fields on the wake field and stopping power of an ion-beam pulse in plasmas

We performed two-dimensional particle-in-cell simulations to investigate how a magnetic field affects the wake field and stopping power of an ion-beam pulse moving in plasmas. The corresponding density of plasma electrons is investigated. At a weak magnetic field, the wakes exhibit typical V-shaped cone structures. As the magnetic field strengthens, the wakes spread and lose their typical V-shaped structures. At a sufficiently strong magnetic field, the wakes exhibit conversed V-shaped structures. Additionally, strengthening the magnetic field reduces the stopping power in regions of low and high beam density. However, the influence of the magnetic field becomes complicated in regions of moderate beam density. The stopping power increases in a weak magnetic field, but it decreases in a strong magnetic field. At high beam density and moderate magnetic field, two low-density channels of plasma electrons appear on both sides of the incident beam pulse trajectory. This is because electrons near the beam pulses will be attracted and move along with the beam pulses, while other electrons nearby are restricted by the magnetic field and cannot fill the gap.

Collective effects on the wakefield and stopping power of an ion beam pulse in plasmas

A two-dimensional (2D) particle-in-cell simulation is carried out to study the collective effects on the wakefield and stopping power for a hydrogen ion beam pulse propagation in hydrogen plasmas. The dependence of collective effects on the beam velocity and density is obtained and discussed. For the beam velocity, it is found that the collective effects have the strongest impact on the wakefield as well as the stopping power in the case of the intermediate beam velocities, in which the stopping power is also the largest. For the beam density, it is found that at low beam densities, the collective contribution to the stopping power increase linearly with the increase of the beam density, which corresponds well to the results calculated using the dielectric theory. However, at high beam densities, our results show that after reaching a maximum value, the collective contribution to the stopping power starts to decrease significantly with the increase of the beam density. Besides, at high beam densities, the wakefield loses typical V-shaped cone structures, and the wavelength of the oscillation wakefield increases as the beam density increases.

Non-stationary Luminescence as a Probe of Charged Species

We have developed a new approach – non-stationary luminescence (NsL), for probing charged species in irradiated solids. This original two-stage technique is based on some kind of “pump-probe” experiments with controlled in situ “injection” of electrons via their release from the traps stimulated by the sample heating. The ionic species of interest are first generated by an intense electron beam. The species produced are then probed by recombination luminescence under low-density beam to minimize production of new charged species. The “probing” – detection of NsL spectra, is performed under gradual heating of the irradiated sample in order to release electrons from progressively deeper traps and initiate the recombination with positively charged species. The approach developed is demonstrated by examples D2-doped solid Xe and solid N2.

Surface Effects Under Visible Irradiation and Heat Treatment on the Phase Stability of γ-Fe2O3 Nanoparticles and γ-Fe2O3 −SiO2 Core–Shell Nanostructures

The structural evolution of γ-Fe2O3 (maghemite) in bare nanoparticles and in core/shell γ-Fe2O3/SiO2 systems was studied as a function of laser irradiation and heat treatment by the combined use of Raman spectroscopy, transmission electron microscopy, X-ray diffraction. The study was addressed to deepen understanding the driving mechanisms at the basis of the maghemite to hematite (α-Fe2O3) phase transition and to understand the possible correlation with surface/defects states. In the bare system, phase transformation was obtained with very low beam density powers (less than 2 mW at 632.8 nm focalized with a conventional 10× microscope objective) and the threshold for transition further decreases in vacuum conditions, suggesting that the phase transformation can be achieved even without thermal assistance. On the contrary, phase transformation cannot be obtained by light irradiation in a γ-Fe2O3/SiO2 core/shell system, but it can be induced by heat treatment at very high temperature (1100 °C). Fe2O3 nanoparticles at high temperature can diffuse inside the silica matrix forming aggregates with the α phase and increased size. The key role of the particle surface is discussed and a physical mechanism for the nucleation of hematite crystallites from the bonding of neighboring maghemite nanoparticles through hydrate defect states is proposed.

Quasi-phase-matched acceleration of electrons in a corrugated plasma channel

A laser pulse propagating in a corrugated plasma channel is composed of spatial harmonics whose phase velocities can be subluminal. The phase velocity of a spatial harmonic can be matched to the speed of a relativistic electron resulting in direct acceleration by the guided laser field in a plasma waveguide and linear energy gain over the interaction length. Here we examine the fully self-consistent interaction of the laser pulse and electron beam using particle-in-cell (PIC) simulations. For low electron beam densities, we find that the ponderomotive force of the laser pulse pushes plasma channel electrons towards the propagation axis, which deflects the beam electrons. When the beam density is high, the space charge force of the beam drives the channel electrons off axis, providing collimation of the beam. In addition, we consider a ramped density profile for lowering the threshold energy for trapping in a subluminal spatial harmonic. By using a density ramp, the trapping energy for a normalized vector potential of ${a}_{0}=0.1$ is reduced from a relativistic factor ${\ensuremath{\gamma}}_{0}=170$ to ${\ensuremath{\gamma}}_{0}=20$.

Nonlinear theory of coupled waves exemplified by beam-plasma instability

The nonlinear stage of instability of an annular electron beam spatially separated from an annular plasma is investigated. The equations describing coupled waves for an arbitrary ratio between the beam and plasma densities are derived. It is shown that instability saturates at distances on the order of several inverse spatial growth rates. The saturation is caused by relativistic nonlinearity, generation of the second harmonic, and low-frequency modulation of the electromagnetic field. At larger distances, resonant generation of low-frequency beam oscillations becomes a dominant factor. In the case of a low-density beam, an expression for the maximum power of the generated plasma wave is obtained in an explicit form.

Gold charge state distributions in highly ionized, low-density beam plasmas

We present a systematic study of Au charge state distributions (CSDs) from low density, nonlocal thermodynamic equilibrium plasmas created in the Livermore electron beam ion traps (EBIT-I and EBIT-II). X-ray emission from Ni-like to Kr-like Au ions has been recorded from monoenergetic electron beam plasmas having E(beam)=2.66, 2.92, 3.53, and 4.54 keV, and the CSDs of the beam plasmas have been inferred by fitting the collisionally excited line transitions and radiative recombination emission features with synthetic spectra. We have modeled the beam plasmas using a collisional-radiative code with various treatments of the atomic structure for the complex M- and N-shell ions and find that only models with extensive doubly excited states can properly account for the dielectronic recombination (DR) channels that control the CSDs. This finding would be unremarkable for plasmas with thermal electron distributions, where many such states are sampled, and the importance of DR is well established. But in an EBIT source, the beam is resonant with only a subset of such states having spectator electrons in orbitals with high principal quantum number n (8≤n≤20). The inclusion of such states in the model was also necessary to obtain agreement with observed stabilizing transitions in the x-ray spectra.

Classical and quantum description of electron trapping during the Cherenkov beam instability in plasma

A nonlinear quantum theory of the Cherenkov instability of a nonrelativistic monoenergetic electron beam in a cold plasma is constructed. It is shown that the instability of a low-density beam is almost purely quantum in nature and results from the emission of one quantum of a plasma wave—a plasmon—by the beam electrons. The number of emitted (and absorbed) plasmons increases with beam density, so, in the limit of high-density beams, the instability becomes a classical Cherenkov beam instability in plasma. Some analytic solutions and estimates are found, detailed numerical results are obtained, and the evolution of the quantum distribution function of the beam electrons in different regimes of the beam instability is investigated.

Formation of (Xe2H)* centers in solid Xe via recombination: nonstationary luminescence and internal electron emission

The formation of excimers (Xe2H)* in solid Xe doped with molecular hydrogen under irradiation by an electron beam is studied using the original two-stage technique of nonstationary (NS) cathodoluminescence (CL) in combination with current activation spectroscopy—thermally stimulated exoelectron emission (TSEE). Charged species are generated using a high-density electron beam. The species produced are probed with a low-density beam with sample gradually heated. The near-UV emission of the (Xe2H)* is used to monitor the neutralization process. It is found that the temperature behavior of the NS CL band of (Xe2H)* clearly correlates with the yield of TSEE measured after identical pre-irradiation of the sample. The fingerprints of the thermally stimulated detrapping of electrons—«internal electron emission»—-in the spectrum of NS CL point to the essential role of the neutralization reaction in the stability of protons solvated by rare-gas atoms.

Generation of quasi‐isotropic electron population during nonlinear beam‐plasma interaction

The nonlinear interaction of a single or a bidirectional electron beam and a background plasma is analyzed on the basis of electrostatic weak turbulence theory. It is found that for a sufficiently high electron beam density, the nonlinear interaction produces quasi‐isotropic electron population. This is in contrast to our previous finding in which a relatively low electron beam density was adopted, and for which complete isotropization was not achieved. The present finding may thus provide a possible explanation for the observed isotropic solar wind electron distribution within the context of electrostatic nonlinear theory involving Langmuir and ion‐sound turbulence, without the resorting to additional mechanisms such as the pitch angle scattering by electromagnetic whistler turbulence.

WEIBEL, TWO-STREAM, FILAMENTATION, OBLIQUE, BELL, BUNEMAN...WHICH ONE GROWS FASTER?

Many competing linear instabilities are likely to occur in astrophysical settings, and it is important to assess which one grows faster for a given situation. An analytical model including the main beam plasma instabilities is developed. The full 3D dielectric tensor is thus explained for a cold relativistic electron beam passing through a cold plasma, accounting for a guiding magnetic field, a return electronic current and moving protons. Considering any orientations of the wave vector allows to retrieve the most unstable mode for any parameters set. An unified description of the Filamentation (Weibel), Two-Stream, Buneman, Bell instabilities (and more) is thus provided, allowing for the exact determination of their hierarchy in terms of the system parameters. For relevance to both real situations and PIC simulations, the electron-to-proton mass ratio is treated as a parameter, and numerical calculations are conducted with two different values, namely 1/1836 and 1/100. In the system parameters phase space, the shape of the domains governed by each kind of instability is far from being trivial. For low density beams, the ultra-magnetized regime tends to be governed by either the Two-Stream or the Buneman instabilities. For beam densities equalling the plasma one, up to four kinds of modes are likely to play a role, depending of the beam Lorentz factor. In some regions of the system parameters phase space, the dominant mode may vary with the electron-to-proton mass ratio. Application is made to Solar Flares, Intergalactic Streams and Relativistic shocks physics.

Observation of the Anomalous Increase of the Longitudinal Energy Spread in a Space-Charge-Dominated Electron Beam

We report a new experimental study of the growth of longitudinal energy spread in a space-charge-dominated electron beam, with a beam energy of several keV and beam current of approximately 100 mA. At relatively low beam densities, we measure growing energy spreads with distance along the transport channel, which are in remarkably good agreement with the theory of energy relaxation via Coulomb collisions. At higher beam densities, however, anomalous energy spreads exceeding the predictions of the relaxation theory are observed, which, we believe, could be caused by collective longitudinal-transverse instabilities observed in computer simulation studies. The onset of these instabilities occurs after several plasma periods according to calculations.

Excited Low Frequency Waves by Beam Plasma in Upstream of Collisionless Shock and Their Effect on Dissipation of Shock

由于很大一部分来自激波上游的粒子被激波面所反射,因此在准平行无碰撞激波的上游存在着等离子体束流.通过一维混合模拟方法,计算了束流密度较小(nb/n0=0.02)和较大(nb/n0=0.2)两种不同情形下的等离子体束流不稳定性.结果表明,在束流密度较小时,束流激发的主要是平行于背景磁场方向传播的右旋波,此波动只能对束流粒子产生影响,包括减速和加热.在束流密度较大时,束流可同时激发平行和反平行于背景磁场方向传播的右旋波,除能对束流粒子产生影响外,还可通过非共振作用加速和加热背景粒子.文中对准平行无碰撞激波耗散机制的影响也进行了讨论.

Anomalous Doppler Effect and Stimulated Cherenkov Effect in a Plasma Waveguide with a Thin-Walled Annular Electron Beam

A dispersion relation for the complex frequencies of the E modes excited by a thin-walled annular low-density beam in a cylindrical plasma waveguide is derived using the methods of perturbation theory. The cases of an annular and a uniform plasma filling are considered, and the corresponding wave growth rates are determined. A condition is obtained under which the primary mechanism for the excitation of the waveguide is the anomalous Doppler effect. The possibility is discussed of suppressing Cherenkov generation in a plasma resonator at the expense of the normal Doppler effect.

An inverse problem for the transport equation in the presence of a Riemannian metric

The stationary linear transport equation models the scattering and absorption of a low-density beam of neutrons as it passes through a body. In Euclidean space, to a first approximation, particles travel in straight lines. Here we study the analogous transport equation for particles in an ambient field described by a Riemannian metric where, again to first approximation, particles follow geodesics of the metric. We consider the problem of determining the scattering and absorption coefficients from knowledge of the albedo operator on the boundary of the domain. Under certain restrictions, the albedo operator is shown to determine the geodesic ray transform of the absorption coefficient; for simple manifolds this transform is invertible and so the coefficient itself is determined. In dimensions 3 or greater, we show that one may then obtain the collision (or scattering) kernel.

Parametric up-conversion of a trivelpiece–gould mode in a beam–plasma system

A large amplitude Trivelpiece–Gould (TG) mode, in a strongly magnetized beam–plasma system, parametrically couples to a beam space charge mode and a TG mode sideband. The density perturbation associated with the beam mode couples with the electron oscillatory velocity, due to the pump wave, to produce a nonlinear current, driving the sideband. The pump and the sideband waves exert a ponderomotive force on the electrons with a component parallel to the ambient magnetic field, driving the beam mode. For a pump wave having k0·v0b0/ω0 < 0, where ω0, k0 are the frequency and the wave number of the pump, and v0b0 is the beam velocity, the sideband is frequency upshifted. At low beam density (Compton regime) the growth rate of the parametric instability scales as two-thirds power of the pump amplitude, and one-third power of beam density. In the Raman regime, the growth rate scales as half power of beam density and linearly with pump amplitude. The background plasma has a destabilizing role on the instability.

Electron kinetic energy measurements from laser irradiation of clusters

We have measured electron kinetic energy spectra from the interaction of a low-density beam of noble-gas clusters with a high-intensity femtosecond laser pulse. Electron energies were measured both through their time of flight and through measurements of electron signals as a function of retarding voltages up to 7 kV. We have observed electrons with energies up to 6 keV and a sharp peak at an arrival time consistent with the detection of photons. The electrons had a broad angular distribution peaked along the polarization axis, while the sharp peak had an isotropic angular distribution. We attribute this sharp peak to uv, xuv, or x-ray photons formed as a result of the laser-cluster interaction and were unable to observe the two-lobed electron kinetic energy distribution reported by Shao et al. [Phys. Rev. Lett. 77, 3343 (1996)] with any combination of the laser and cluster parameters used.