Interaction of an ion bunch with a plasma slab

Charge neutralization of a short ion bunch passing through a plasma slab is studied by means of numerical simulation. It is shown that a fraction of plasma electrons are trapped by the bunch under the action of the collective charge separation field. The accelerated electrons generated in this process excite beam?plasma instability, thereby violating the trapping conditions. The process of electron trapping is also strongly affected by the high-frequency electric field caused by plasma oscillations at the slab boundaries. It is examined how the degree of charge neutralization depends on the parameters of the bunch and plasma slab.     

Effect of a strong monochromatic electromagnetic wave with circular polarization on one-dimensional wake waves in plasma

A study is made of the excitation of wake waves by a one-dimensional bunch of charged particles in an electron plasma in the presence of an intense monochromatic pump wave with circular polarization. In the main state (in the absence of a bunch), the interaction between a pump wave and a plasma is described by the Maxwell equations and the nonlinear relativistic hydrodynamic equations for a cold plasma. The excitation of linear waves by a one-dimensional bunch is investigated against a cold plasma background. It is shown that, in a certain range of the parameter values of the bunch, pump wave, and plasma, the amplitude of the excited transverse waves grows as the energy of the bunch particles increases until the relativistic factor of the bunch reaches a certain threshold value above which the transverse wave amplitude becomes essentially independent of the bunch particle energy and grows as the intensity and frequency of the pump wave increase. The amplitude and wavelength of the longitudinal field, which is shown to depend weakly on the energy of the bunch particles, grows with increasing the pump wave intensity.     

Excitation of growing wake waves by an electron bunch in a plasma in an intense pump wave

A study is made of the excitation of wake waves by a one-dimensional electron bunch in an electron plasma in the presence of an intense monochromatic pump wave with circular polarization. In the main state (in the absence of a bunch), the interaction between a pump wave and a plasma is described by Maxwell's equations and the nonlinear relativistic hydrodynamic equations for a cold plasma. The excitation of linear waves by a one-dimensional bunch is investigated against a cold plasma background. It is shown that, in a certain range of parameter values of the bunch, pump wave, and plasma, the excitation is resonant in character and the amplitude of the excited wake waves increases with distance from the bunch.     

Study of nonneutral plasma storage in a magnetic trap with a rotating electric field at the lepta facility

Results from experimental studies of plasma storage in a Penning-Malmberg trap at the LEPTA facility are presented. The number of stored particles is found to increase substantially when using the so-called “rotating wall” method, in which a transverse rotating electric field generated by a cylindrical segmented electrode cut into four pairs is applied to the plasma storage region. The conditions of transverse compression of the plasma bunch under the action of the rotating field and buffer gas are studied. The optimal storage parameters are determined for these experimental conditions. Mechanisms of the action of the rotating field and buffer gas on the process of plasma storage are discussed.     

Experimental study of the evaporation and expansion of a solid pellet in a plasma heated by an electron beam

Results are presented from experiments on the injection of solid pellets into a plasma heated by an electron beam in the GOL-3 device. For this purpose, two pellet injectors were installed in the device. The target plasma with a density of ～10¹⁵ cm^?3 was produced in a solenoid with a field of 4.8 T and was heated by a highpower electron beam with an electron energy of ～1 MeV, a duration of ～7 s, and a total energy of 120–150 kJ. Before heating, the pellet was injected into the center of the plasma column transversely to the magnetic field. The injection point was located at a distance of 6.5 or 2 m from the input magnetic mirror. Polyethylene pellets with a mass of 0.1–1 mg and lithium-deuteride pellets with a mass of 0.02–0.5 mg were used. A few microseconds after the electron beam starts to be injected into the plasma, a dense plasma bunch is formed. In the initial stage of expansion, the plasma bunch remains spherically symmetric. The plasma at the periphery of the bunch is then heated and becomes magnetized. Next, the dense plasma expands along the magnetic field with a velocity on the order of 300 km/s. A comparison of the measured parameters with calculations by a hydrodynamic model shows that, in order to provide such a high expansion velocity, the total energy density deposited in the pellet must be ～1 kJ/cm². This value substantially exceeds the energy density yielded by the target plasma; i.e., the energy is concentrated across the magnetic field onto a dense plasma bunch produced from the evaporated particle.     

Trapping of electrons and acceleration of the electron bunch in a wake wave

The process of electron trapping by a wake wave excited by a laser pulse in a plasma channel in the case where the electron bunches are injected into the vicinity of the maximum of the wakefield potential at a velocity lower than the wave phase velocity is considered. The mechanism for the formation of a compact electron bunch in the trapping region when only the electrons of the injected bunch that are trapped in the focusing phase mainly undergo the subsequent acceleration in the wakefield is analyzed. The influence of the spatial dimensions of the injected bunch and its energy spread on the length of the trapped electron bunch and the fraction of trapped electrons is studied analytically and numerically. For electron bunches with different ratios of their spatial dimensions to the characteristic dimensions of the wake wave, the influence of the injection energy on the parameters of the high-energy electron bunch trapped and accelerated in the wake-field is studied.     

Nonlinear properties of two-dimensional wake waves excited by relativistic electron bunches in plasma

The properties of a nonlinear plasma wake wave excited by an axially symmetric relativistic electron bunch are studied. It is shown that the nonlinear dependence of the wake wavelength on the transverse coordinate leads to distortion of the phase front of the wake wave and to steepening and oscillations of the transverse profile of the wakefield. The magnetic field of the wake wave is nonzero and oscillates at a frequency higher than the plasma electron frequency. Because of nonlinearity, the amplitude of the excited wake wave changes with distance from the bunch. The increase in nonlinearity leads to the development of turbulence and chaotization of the wakefield and results in the switching-on of the thermal effects and plasma heating.     

Differential Response to Water Balance and Bunch Load Generates Diversity of Bunch Production Profiles Among Oil Palm Crosses (Elaeis guineensis)

Oil palm (Elaeis guineensis Jacq.) produces bunches throughout the year, following annual cycles marked by a peak season, with genetic diversity regarding the regularity of the annual profile of bunch production. The aim of this study was to understand this diversity among a set of oil palm crosses. We hypothesized that this diversity originated from differential responses to water balance and bunch load. Seven crosses with different production cycles were studied during 6 years in Benin, where dry seasons are marked. Phenological stages of phytomer development were recorded from leaf appearance to bunch harvest. Inequality in the distribution over the months of the year of phytomers at each stage was quantified using the Gini coefficient. We found that annual variations in the rate of early abortions, sex ratio, time between leaf opening and appearance of female inflorescence (AFI) and time between AFI and flowering strongly contributed to the diversity in production profile among crosses. To a lesser extent, annual variations in the time of bunch maturation also generated diversity in the production profile. Sex ratio was positively correlated with water balance and negatively with bunch load when leaves were around axil number -25 (approximately 29 months before harvest). Early abortions were positively correlated with bunch load when leaves were at axil number 10 (approximately 9 months before harvest). Correlations varied among crosses, indicating differential responses of crosses to variations in water balance and bunch load, which eventually created significant diversity among crosses regarding the regularity of bunch production profiles.     

Dynamics of an Electron Bunch Accelerated by a Wakefield

The energy characteristics of an electron bunch accelerated by a wakefield are largely determined by the initial bunch dimensions. Present-day injectors are still incapable of ensuring the initial spatial parameters of the bunches required for their acceleration without increasing the energy spread of the bunch electrons. In connection with this, the possibility is studied of improving the energy characteristics of an accelerated bunch by precompressing it in the longitudinal direction in the stage of trapping by a wakefield. Analytic formulas are derived that describe the one-dimensional dynamics of the spatial and energy characteristics of a short (much shorter than the wakefield wavelength) electron bunch in both the trapping and acceleration stages. The analytical results obtained are shown to agree fairly well with the results from one-dimensional and three-dimensional simulations, provided that the electrons are injected into the region that is optimum for acceleration. The possibility is discussed of forming compressed bunches so as to ensure the high quality of the bunch in the course of its acceleration to high energies.     

Acceleration of electron bunches injected into a wake wave

The process of trapping and acceleration of nonmonoenergetic electron bunches by a wake wave excited by a laser pulse in a plasma channel is investigated. The electrons are injected into the vicinity of the maximum of the wakefield potential with a velocity lower than the wave phase velocity. The study is aimed at utilizing specific features of a wakefield with substantially overlapped focusing and accelerating phases for achieving monoenergetic electron acceleration. Conditions are found under which electrons in a finite-length nonmonoenergetic bunch are accelerated to high energies, while the energy spread between them is minimal. The effect of energy grouping of electrons makes it possible to obtain compact high-energy electron bunches with a small energy spread during laser plasma acceleration.     

Generation of transition radiation in the form of electromagnetic surface waves by electron bunches

The generation of transition radiation in the form of electromagnetic surface waves by a nonrelativistic electron bunch as it crosses the vacuum-semiconductor interface or a thin semiconductor plate in vacuum is investigated. A study is made of a bunch that has the shape of an ellipsoid of revolution, with a uniform charge density distribution over its volume, and moves along the normal to the interface. When the energy dissipation in the semiconductor is taken into account, the spectrum of the transition radiation emitted in the form of surface waves comprises a peak whose width is comparable to its mean frequency. It is shown that, in each of the two cases under consideration, the generation efficiency, defined as the ratio of the radiated energy to the kinetic energy of the bunch electrons, is maximum for a bunch of certain dimensions. The dependence of the radiated energy and of the generation efficiency on the thickness of a thin semiconductor plate is investigated for given bunch dimensions. It is found that the corresponding dependences have a maximum, which can be explained as being due the competition between the two effects: as the plate thickness increases, on the one hand, the region where the radiation is generated becomes larger, so that the radiation power increases, and, on the other hand, the dissipative energy losses become higher.     

Acceleration of an electron bunch injected in front of a laser pulse generating an accelerating wake wave

A study is made of a promising method for injecting an electron bunch into an accelerating laser-plasma system. A bunch is injected ahead of the front of a laser pulse generating a wake wave that propagates in a direction collinear with the pulse and has a velocity lower than the pulse group velocity. The influence of the initial nonmonoenergetic character of the bunch on its trapping and acceleration is investigated. By appropriately choosing the laser pulse parameters and the bunch injection energy, it is possible to create such conditions for the trapping of an initially nonmonoenergetic bunch by the wake wave that, over a certain acceleration distance, there will be no energy spread of the bunch due to its initial nonmonoenergetic character, a circumstance that allows compact electron bunches to be accelerated to high energies, with a minimum energy spread.     

Study of the development of relativistic plasma bunches in a long mirror trap by optical and X-ray imaging and numerical simulations

The work presents experimental results demonstrating the feasibility of autoresonance acceleration of electrons in a long mirror trap with a reverse magnetic field. It is shown that gyromagnetic autoresonance results in the formation of a plasma bunch with average electron energy of several hundred keV, which is confined for a long time in the trap. The results of computer simulations of the regime of reverse gyromagnetic autoresonance agree well with the experimental data.     

Ion-acoustic “Houston's horse” effect

The structure of an ion-acoustic forerunner excited by a shock wave in a weakly ionized plasma is studied. It is shown that, when the shock velocity exceeds the ion-acoustic speed, a soliton bunch is produced at the perturbation front. The increase in the shock velocity to a certain critical value is accompanied by an increase in the soliton amplitude. A further increase in velocity leads to an explosive-like collapse of the bunch, which results in a decrease in the medium resistance. This phenomenon is analogous to the “Houston's horse” effect in narrow-channel hydrodynamics.     

Acceleration of dense electron bunches at the front of a high-power electromagnetic wave

The acceleration of dense electron bunches (e.g., those produced by the ionization of thin films) at the front of a high-power electromagnetic wave in vacuum is considered. It is shown that the reaction force of the intrinsic radiation of a bunch can play a significant role in the acceleration process because it gives rise to an additional accelerating force acting on the bunch and to forces that compress the bunch in the longitudinal direction. As a result, all of the bunch electrons can be synchronously accelerated during the first several half-periods of the external electromagnetic field.     

Effect of Pulp Cell Number and Assimilate Availability on Dry Matter Accumulation Rate in a Banana Fruit [ Musa sp. AAA group 'Grande Naine' (Cavendish subgroup)]

Fruit position on the bunch (inflorescence) is an importantpart of variability in banana fruit weight at harvest, as fruitsat the bottom of the bunch (distal fruits) are approx. 40% smallerthan those at the top (proximal fruits). In this study, therespective roles of cell number and cell filling rate in thedevelopment of pulp dry weight are estimated. To this end, thesource/sink ratio in the plant was altered at different stagesof fruit development. Leaf shading (reducing resource availability),bunch bagging (increasing sink activity by increasing fruittemperature), and bunch trimming (decreasing sink size by fruitpruning), applied once cell division had finished, showed thatthe pulp filling rate depends on resource availability. Bunchbagging and bunch trimming were also carried out before theend of cell division to estimate the role of pulp cell numberin the development of pulp dry weight. A sampling method wascalibrated to evaluate pulp cell number from the digestion ofa fixed portion of the pulp in a solution of chromic and nitricacids. A relationship was found between pulp cell number andfruit length at the end of cell division. It was observed thatpulp cell number is a determining factor in pulp dry weightvariability within a bunch. On the other hand, the cell fillingrate was identical for all fruits in the bunch and was influencedby the source/sink ratio. A Michaelis-Menten relationship wasinvoked to relate the cell filling rate in a bunch to the source/sinkratio during bunch filling. Copyright 2001 Annals of BotanyCompany Banana fruit, Musa sp., fruit growth, cell number, cell filling rate, source/sink ratio, temperature     

葡萄的He-Ne激光诱变初探

以北醇(二倍体)及龙宝、黑臭林、伊豆锦、先锋(四倍体)二年生植株为材料,用He—Ne激光(或等寓子束)辐照其腋芽诱变效果的初探。He—Ne激光束的功率为6—12.5mw,功率密度为180—50mw/cm~2,辐照时间5—40分钟,产生等离子束的电流强度为60mA,处理时间为5—20秒。He—Ne激光处理时间以15—30分钟为好,一次连续40分钟时芽枯死,剂量合适时,芽、幼叶、成叶、枝条和果穗形态变化明显。具体处理参数与芽的性质。萌动状态及品种有关。等离子束辐照时不易超过5秒钟。He—Ne激光处理后,幼叶、成叶、叶裂加深、重锯齿缘,节间缩短25％,果穗整齐,付穗减小或消失,落花落果轻,不感染霜霉病或得病较轻。     

Pulsations of a bunch of charged particles in a Penning trap

A method has been developed for calculating the gas-dynamic parameters of the initial stage of pulsations of a nonuniform bunch of charged particles in a Penning trap. An analysis is made of the arguments given by V.A. Syrovoy in his comments [Plasma Phys. Rep. 29, 92 (2003)] on the interpretation of the solution in the form of a uniform bunch that was derived earlier by the author.     

Design of an experiment on wakefield acceleration on the VEPP-5 injection complex

Relativistic beams produced by the VEPP-5 injection complex (Budker Institute of Nuclear Physics, Siberian Division, Russian Academy of Sciences) can be used to generate plasma waves with a longitudinal electric field of 1 GV/m. A part of the electron (or positron) driver bunch is accelerated by this field over a distance of up to 1 m. The main advantage of the proposed design over the previous wakefield acceleration experiments is the beam preparation system capable of compressing bunches to a length of σ_z = 0.1 mm in the longitudinal direction and producing an optimal longitudinal profile of the beam density. The main parameters of the planned device are as follows: the electron energy at the entrance to the plasma is 510 MeV, the number of particles in the bunch is 2 × 10¹⁰, the plasma density is up to 10¹⁶ cm^?3, the number of accelerated particles is up to 3 × 10⁹, and their energy spread is less than 10%. The physical project of the experiment is presented, and the results of computer simulations of the beam-plasma interaction are described.     

Computer tomography imaging of fast plasmachemical processes

Results are presented from experimental studies of the interaction of a high-enthalpy methane plasma bunch with gaseous methane in a plasmachemical reactor. The interaction of the plasma flow with the rest gas was visualized by using streak imaging and computer tomography. Tomography was applied for the first time to reconstruct the spatial structure and dynamics of the reagent zones in the microsecond range by the maximum entropy method. The reagent zones were identified from the emission of atomic hydrogen (the H_α line) and molecular carbon (the Swan bands). The spatiotemporal behavior of the reagent zones was determined, and their relation to the shock-wave structure of the plasma flow was examined.