On the charge density modulation in the bunch by the interaction with electromagnetic wave

The efficiency of the charge density modulation of an electron bunch in the field of a linearly polarized monochromatic electromagnetic wave propagating oppositely to the bunch motion is considered. It is shown that as a result of the bunch-wave interaction the coherently radiating bunches can be obtained.     

Possibility of the charge density modulation of an electron bunch in the electromagnetic wave field

We consider the problem of modulation of the electron density in a linear bunch after its interaction with a linearly polarized, monochromatic electromagnetic wave, as well as an opportunity to observe this effect. It is shown that one can achieve significant values of the modulation depth at high intensities of the electromagnetic wave.     

Emittance-exchange-based high harmonic generation scheme for a short-wavelength free electron laser

Generation of short-wavelength radiation by a free-electron laser using up-frequency conversion of an electron bunch density modulation is currently an area of active research. We propose a new scheme for producing the longitudinal electron bunch density modulation similar to the recently proposed echo-enabled harmonic generation but based on an emittance exchange beam line and a multislit mask. Beam line analysis and start-to-end simulation are presented.     

Optimal energy-phase correlation for superradiance of intense electron bunches

Investigated is the optimal energy-phase correlation for superradiance of an intense electron bunch in an undulator field. It is noted that the superradiant emission saturates due to the bunch self-field at an energy proportional to the total charge in the bunch.     

新型金属丝靶电子源实验研究

电子束在基础科学研究、工农业生产和医疗领域发挥了重要作用。提出了一种新型的电子源技术方案:高功率激光脉冲轰击金属丝靶,可以产生大量能量在百keV量级的热电子,一部分热电子在丝靶表面自生电磁场的作用下沿着丝靶运动,丝靶后方可以获得指向性良好的电子束。实验上成功在金、钨和铜丝靶后方获得了电子束团,测量了束团束斑、电荷量和能谱。铜丝靶单发实验收集到的电子束团总电荷量可达3 nC,能量分布在0～240 keV区间内,能谱在100 keV附近呈现峰值。提出了微波压缩方案,设计了2腔微波聚束腔,利用ASTRA对微波腔压缩过程进行了模拟计算。结果显示,可以将电荷量1 nC、长度55 ps的束团压缩至27 ps,满足后续微波加速器对电子源的要求。     

The stimulated Breit-Wheeler process as a source of background <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> pairs at the international linear collider

Passage of beamstrahlung photons through the bunch fields at the interaction point of the ILC determines background pair production. The number of background pairs per bunch crossing due to the Breit-Wheeler, Bethe-Heitler and Landau-Lifshitz processes is well-known. However, the Breit-Wheeler process also takes place in and is modified by the bunch fields. A full QED calculation of this stimulated Breit-Wheeler process reveals cross-section resonances due to the virtual particle reaching the mass shell. The one-loop electron self-energy in the bunch field is also calculated and included as a radiative correction. The bunch field is considered to be a constant crossed electromagnetic field with associated bunch field photons. Resonance is found to occur whenever the energy of contributed bunch field photons is equal to the beamstrahlung photon energy. The stimulated Breit-Wheeler cross-section exceeds the ordinary Breit-Wheeler cross-section by several orders of magnitude and a significantly different pair background may result.      

Laser seeding of the storage-ring microbunching instability for high-power coherent terahertz radiation

We report the first observation of laser seeding of the storage-ring microbunching instability. Above a threshold bunch current, the interaction of the beam and its radiation results in a coherent instability, observed as a series of stochastic bursts of coherent synchrotron radiation (CSR) at terahertz frequencies initiated by fluctuations in the beam density. We have observed that this effect can be seeded by imprinting an initial density modulation on the beam by means of laser "slicing." In such a situation, most of the bursts of CSR become synchronous with the pulses of the modulating laser and their average intensity scales exponentially with the current per bunch. We present detailed experimental observations of the seeding effect and a model of the phenomenon. This seeding mechanism also creates potential applications as a high-power source of CSR at terahertz frequencies.     

Visualizing the charge order and topological defects in an overdoped （Bi,Pb）2Sr2CuO6+x superconductor

Electronic charge order is a symmetry breaking state in high-Tc cuprate superconductors. In scanning tunneling microscopy, the detected charge-order-induced modulation is an electronic response of the charge order. For an overdoped(Bi,Pb)2Sr2CuO6+x sample, we apply scanning tunneling microscopy to explore local properties of the charge order. The ordering wavevector is nondispersive with energy, which can be confirmed and determined. By extracting its order-parameter field, we identify dislocations in the stripe structure of the electronic modulation, which correspond to topological defects with an integer winding number of ±1. Through differential conductance maps over a series of reduced energies, the development of different response of the charge order is observed and a spatial evolution of topological defects is detected. The intensity of charge-order-induced modulation increases with energy and reaches its maximum when approaching the pseudogap energy. In this evolution, the topological defects decrease in density and migrate in space. Furthermore, we observe appearance and disappearance of closely spaced pairs of defects as energy changes. Our experimental results could inspire further studies of the charge order in both high-Tccuprate superconductors and other charge density wave materials.     

Acousto-optic modulation in magnetised diffusive semiconductors

The modulation of an intense electromagnetic beam induced by the acousto-optic (AO) effect has been analysed in a transversely magnetised semiconductor-plasma medium. The effect of carrier diffusion on the threshold field and gain profile of the modulated wave has been extremely investigated using coupled mode theory. The origin of the AO interaction is assumed to lie in the induced nonlinear diffusion current density of the medium. By considering the modulation process as a four wave parametric interaction an expression for effective third-order AO susceptibility describing the phenomena has been deduced. The modulation is greatly modified by propagation characteristics such as dispersion and diffraction due to dielectric relaxation of the acoustic mode. The threshold pump field and the steady state growth rates are estimated from the effective third-order polarisation in the plasma medium. Analytical estimation reveals that in the presence of enhanced diffusion due to excess charge carriers the modulated beam can be effectively amplified in a dispersionless acoustic wave regime. The presence of an external dc magnetic field is found to be favourable for the onset of diffusion induced modulational amplification of the modulated wave in heavily doped regime. Received 5 November 2001     

Longitudinal Single Bunch Instability Caused by Wake Field of Electron Cloud

The electron cloud accumulated in the vicinity of positron beam generates longitudinal electric field during the passage of bunch. The longitudinal interaction between bunch and electron cloud can lead to the distortion of the bunch shape. We use a simple analytic formula to calculate the longitudinal electric field due to electron cloud. Based on the longitudinal wake field, the macro-particle tracking method is used to simulate the variation of bunch longitudinal profile in different electron cloud densities and the simulation also shows that the synchrotron oscillation tune is slightly shifted by the wake field. By comparing the simulation results and the analytical estimation from potential distortion theory, the longitudinal wake field from electron cloud can be seen as a potential well effect.     

Ponderomotive effects in intense pumping wave action on electron and plasma bunches

Ponderomotive effects that arise when an intense plane pumping wave acts on low-concentration electron and plasma bunches are theoretically studied within the framework of a one-dimensional model. Using the Lagrange variables, an electron (plasma) bunch under the action of a pumping field can be represented as a gas comprising macroparticles with ponderomotive and Coulomb interactions. The ponderomotive force at small interparticle distances is attractive, that is, directed oppositely to the Coulomb force; it cannot, however, completely balance the latter. The constructed model is used to study superradiance, which arises when an intense pumping wave acts on an extended electron bunch. Radiation is then scattered in the form of narrow pulses whose amplitude is proportional to the total number of particles in the bunch. In addition, we describe acceleration of a neutral plasma layer, narrow on the wavelength scale, in the field of an intense wave and radiation field-induced partial contraction of an electron bunch with an incompletely compensated charge.     

Anharmonic pulses due to transition radiation

The possibility of using the transition radiation of an electron bunch to generate nonstationary anharmonic pulses in free space and in a dispersive medium is demonstrated. It is shown that the transition radiation of the bunch gives rise to an infinite train of nonstationary anharmonic pulses described by solutions to the Klein-Gordon equation. The polarization of the resulting field at its leading edge appears to be different from the field polarization at the interface. The leading edge becomes steeper with time. It is shown that the bunch density longitudinal distribution and the parameters of the dispersive medium can be determined from the values of the fields and their derivatives near the leading edge of the resulting signal.     

Halo formation and emittance growth of positron beams in plasmas

An ultrarelativistic 28.5 GeV, 700-microm-long positron bunch is focused near the entrance of a 1.4-m-long plasma with a density n(e) between approximately equal to 10(13) and approximately equal to 5 x 10(14) cm(-3). Partial neutralization of the bunch space charge by the mobile plasma electrons results in a reduction in transverse size by a factor of approximately equal to 3 in the high emittance plane of the beam approximately equal to 1 m downstream from the plasma exit. As n(e) increases, the formation of a beam halo containing approximately 40% of the total charge is observed, indicating that the plasma focusing force is nonlinear. Numerical simulations confirm these observations. The bunch with an incoming transverse size ratio of approximately 3 and emittance ratio of approximately 5 suffers emittance growth and exits the plasma with approximately equal sizes and emittances.     

Cohesive acceleration and focusing of relativistic electrons in overdense plasma

We describe our studies of the generation of plasma wake fields by a relativistic electron bunch and of phasing between the longitudinal and transverse fields in the wake. The leading edge of the electron bunch excites a high-amplitude plasma wake inside the overdense plasma column, and the acceleration and focusing wake fields are probed by the bunch tail. By monitoring the dependence of the acceleration upon the plasma's density, we approached the beam-matching condition and achieved an energy gain of 0.6 MeV over the 17 mm plasma length, corresponding to an average acceleration gradient of 35 MeV/m. Wake-induced modulation in energy and angular divergence of the electron bunch are mapped within a wide range of plasma density. We confirm a theoretical prediction about the phase offset between the accelerating and focusing components of plasma wake.     

A Theory of the Self-Consistent and Space-Charge Effect in Hybrid-Magnicon Amplifier

A novel axially modulated, annular electron beam cusp-injected, slowly varied section opened cavity, hybrid-magnicon amplifier had been proposed I once. The hybrid-magnicon has wavelength up to millimeter wave bend. The while its non-self-consistent non-space-charge nonlinear numerical calculation method was discussed. But due to bunch effect is changing uniform distribution state of the annular electron beam in the driver cavity, and then dynamic force balances of the beam is changing. Therefore, besides the interaction force between electron beam and the rotating TM_n10 mode electromagnetic field also must consider the electromagnetic force of current density and the action force between the space-charges and the action force of the charge density grade, et al. effect. For some problems of theory, this report try discuss from Maxwell Equations.     

强外加电场与大调制度下光折变动力学光栅形成研究

强外加电场与大调制度在光折变效应的研究中已经得到了广泛应用。采用PDECOL算法,严格求解光折变带输运方程,得到外加电场时不同调制度下光折变晶体中随时间变化的空间电荷场、载流子浓度,并讨论了外加电场对它们的影响。通过将物质方程与耦合波方程联立数值求解,可得到光折变光栅形成过程中两波耦合增益系数以及光束条纹相位的变化。模拟结果表明,在强外加电场作用下,两束记录光之间的光强与相位耦合都得到了增强,而原有的解析式忽视了强外加电场与大调制度对空间电荷场相位耦合的影响,此时不再适用。同时发现折射率光栅与记录光束条纹均发生弯曲,并不再保持平行。     

Weakly relativistic and space charge effects in interaction of high-power microwave with plasma

In the present work, numerical studies on the effects of weakly relativistic ponderomotive force and space charge in the nonlinear interaction of a high-power microwave beam with a plasma are carried out. It is shown that, the profiles of the electron density and dielectric permittivity contain high peaks, and modulation of wavelength occurs in electron density distribution by increasing the microwave energy flux. In addition, it is indicated that the profiles of the electric and magnetic fields in relativistic regime are lengthened more than non-relativistic regime by increasing the initial electron density and the relativistic effects cause the increase in oscillation wavelength of electron density, dielectric permittivity and space charge field, in comparison with the non-relativistic regime. Finally, the results of the research show that the steepening in electron density distributions and their oscillation wavelength are enhanced, when the relativistic effects appear.     

Bunch length measurement at Tsinghua Thomson scattering X-ray source

The length of electron beam from a photocathode RF gun is determined by a spectrometer, according to the relative energy spread induced by the bunch length during the acceleration in a linac. For a photocathode RF gun, different laser injected phase and beam charge are studied. The compression is changed for the different laser phases, as from 10° to 30°, and the bunch length is lengthened due to the strong longitudinal space charge force, caused by the increased charge.     

Electric convection of low-conductivity liquid in a horizontal capacitor in the presence of unipolar charge injection

Electric convection of low-conductivity liquid in a horizontal plane capacitor is analyzed with allowance for unipolar charge injection. Dynamics of charge transfer through stationary isothermal liquid in the presence of modulated electric field is studied. Effect of modulation amplitude and frequency on the spatiotemporal distribution of charge density and electric potential is considered. Nonlinear electric convection of nonisothermal low-conductivity liquid is studied in the presence of static electric field in a 2D system. Hysteresis transitions between two different (with respect to intensity) regimes of electric convection are analyzed.     

Electron beam emission from a diamond-amplifier cathode

The diamond amplifier (DA) is a new device for generating high-current, high-brightness electron beams. Our transmission-mode tests show that, with single-crystal, high-purity diamonds, the peak current density is greater than 400 mA/mm2, while its average density can be more than 100 mA/mm2. The gain of the primary electrons easily exceeds 200, and is independent of their density within the practical range of DA applications. We observed the electron emission. The maximum emission gain measured was 40, and the bunch charge was 50 pC/0.5 mm2. There was a 35% probability of the emission of an electron from the hydrogenated surface in our tests. We identified a mechanism of slow charging of the diamond due to thermal ionization of surface states that cancels the applied field within it. We also demonstrated that a hydrogenated diamond is extremely robust.