Propagation of Plasma Bunches through a Transverse Magnetic Barrier

The injection of a plasma bunch into a multipolar trap can be applied to fill the trap with a plasma. The injection of the bunch into a tokamak-like trap can be considered an additional means for controlling the processes of plasma heating and fuel delivery to the central zone of a thermonuclear reactor. In both cases, the bunch is injected normally to the magnetic field of the trap. It has been shown theoretically, experimentally, and by numerical simulation that the depth of plasma bunch penetration into the magnetic field varies in direct proportion to the bunch energy and in inverse proportion to the magnetic pressure and the cross-sectional area of the plasma bunch. The data of this work allow researchers to estimate the values of plasma bunch parameters at which the bunch will be trapped. As a result, the process of plasma bunch trapping has been optimized.     

Broadband emission from a relativistic electron bunch in a semi-infinite waveguide

A study is made of the excitation of a transition radiation pulse during the injection of a charged particle bunch through the end metal wall into a semi-infinite cylindrical waveguide. Exact analytic expressions for the fields of a thin ring-shaped bunch are obtained in terms of the Lommel functions of two variables. The energy efficiency, power, and spectrum of radiation emitted from a finite-size charged bunch in a vacuum waveguide are calculated numerically with allowance for the multimode nature of the excited field. It is shown that, under certain conditions, the bunch can generate a short, high-intensity electromagnetic pulse with a broad frequency spectrum. The effect of various parameters of the charged bunch-waveguide system (such as the bunch current, bunch duration, and waveguide radius) on the generation efficiency of a transition radiation pulse is investigated.     

Excitation of a wake field by a relativistic electron bunch in a semi-infinite dielectric waveguide

A wake field excited by a relativistic electron bunch in a semi-infinite metal waveguide filled with a dielectric consists of the Vavilov-Cherenkov radiation, the “quenching”-wave field, and transient radiation, which interfere with each other. An exact analytic expression for the transient component of the field of a thin relativistic annular bunch is derived for the first time. The evolution of the space distribution of a field excited by a finite-size electron bunch is numerically calculated. The excitation of the wake field by a periodic train of electron bunches in a finite-length waveguide is studied.     

Coherent radiation from a beam accelerated in a microtrone

The coherent radiation of relativistic electrons that are accelerated in a microtrone and move through a beam-deflecting magnet is measured. Experiments show that the radiation intensity depends not only on the number of electrons but also on which region of the bunch the radiating electrons are extracted from. The measurements provide information on how the phase extent of the electron bunch varies over the cross section. Numerical calculations of the radiation intensity versus number of particles passing through the beam-deflecting magnet and of the bunch coherence factor versus transverse coordinate are in good agreement with the experiment.     

Bunch length measurement using a traveling wave RF deflector

RF deflectors can be used for bunch length measurement with high resolution. This paper describes a completed S-band traveling wave RF deflector and the bunch length measurement of the electron beam produced by the photocathode RF gun of the Shanghai DUV-FEL facility. This is the first time that such a transverse RF deflector has been developed and used to measure the bunch length of picosecond order in China. The deflector's VSWR is 1.06, the whole attenuation 0.5 dB, and the bandwidth 4.77 MHz for VSWR less than 1.1. With a laser pulse width of 8.5 ps, beam energy of 4.2 MeV, and bunch charge of 0.64 nC, the bunch lengths for different RF input power into the deflector were measured, and an averaged rms bunch length of 5.25 ps was obtained. A YAG crystal is used as a screen downstream of the deflector, with the calibrated value of 1 pix = 136 μm.     

Collective properties of a relativistic electron beam injected into a high intensity optical lattice

Behaviour of a relativistic electron bunch, injected and trapped in a high intensity optical lattice resulting from the interference of two laser beams is studied. The optical lattice modifies the phase space distribution of the electron bunch due to the trapping and compression of the electrons by a ponderomotive force. High-frequency longitudinal beam eigenmodes of the trapped electron bunch are described in the framework of fluid and kinetic models. Such beam oscillations are expected to play a pivotal role in a stimulated Raman scattering of laser beams on the electrons.     

Generation and time jitter of the loose soliton bunch in a passively mode-locked fiber laser

We report a regime of the loose soliton bunch in an erbium-doped passively mode-locked fiber laser. In this state,every soliton bunch consists of multiple pulses. The amount of multiple pulses inside the soliton bunch increase as the pump power rises. Moreover, the temporal average pulse-to-pulse separation decreases in general with the increase of the pump power. Further, the spatial-temporal sequences based on the dispersive Fourier transformation technique show that pulse-to-pulse interactions and time jitter can result in pulse forking inside the soliton bunch. Finally, we theoretically demonstrate the soliton bunch with different pulse-to-pulse separations.     

Electron bunch interaction with a monochromatic electromagnetic wave

A technique of modulation of the charge density in a bunch when it interacts with a linearly polarized laser beam is developed. Solution of the problem is based on the numerical method for determination of interaction time of different electrons in the bunch flying through the interaction region, with use of an integral of motion of an electron in the monochromatic electromagnetic field. It is shown that in the terahertz frequency range, at high values of the field strength, sufficiently harddriving modulation of the charge density in the bunch may be obtained. High efficiency of the bunch charge density modulation is demonstrated in plots for different electrodynamical parameters.     

Cerenkov radiation of a spatially extended charged bunch moving along the axis of a cylindrical channel in a transparent medium

Relations are obtained which determine the electromagnetic field produced by an axially symmetric bunch of charged particles moving along the axis of a cylindrical channel in a transparent medium. The problem of the Cerenkov radiation of such a bunch is investigated and relations are obtained for the intensity of this radiation.In conclusion the author thanks Professor A. A. Sokolov for discussing the results.     

On the catastrophes of a charged particle bunch

The mean particle density at time catastrophes of a particle bunch is estimated in terms of a self-consistent model that describes the injection of the charged cold fluid. Analytical solutions for electron bunch injection into steady-state and variable electric fields are found.     

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.      

单谐振腔束团长度监测器的设计与仿真

提出了一种利用单个谐振腔测量直线加速器束团长度的新方法。不同于传统的双谐振腔束团长度监测器，该方法仅需要使用单个谐振腔，省去了用于辅助测量的参考腔，从而简化了装置结构，节省了空间。针对国家同步辐射实验室基于可调谐红外激光的能源化学研究大型实验装置进行了监测器的物理设计。通过设定同轴探针的插入位置，解决了双模信号互相干扰的问题，利用微扰金属改变调节工作频率，克服了双模难以同时谐振的困难。利用CST软件进行建模和仿真。仿真结果表明，该监测器可实现2～5 ps束团长度的诊断，测量误差小于5%。     

负动量压缩因子磁聚焦结构在τ–粲工厂储存环设计中的应用

研究并提出了一个负动量压缩因子(α_p<0)τ–粲工厂储存环磁聚焦结构.对其束团拉伸的计算表明:在设计流强,束团长度将比自然束长短,束团长度不会拉伸.     

Fully coherent x-ray pulses from a regenerative-amplifier free-electron laser

We propose and analyze a regenerative-amplifier free-electron laser (FEL) to produce fully coherent, hard x-ray pulses. The method makes use of narrow-bandwidth Bragg crystals to form an x-ray feedback loop around a relatively short undulator. Self-amplified spontaneous emission (SASE) from the leading electron bunch in a bunch train is spectrally filtered by the Bragg reflectors and is brought back to the beginning of the undulator to interact repeatedly with subsequent bunches in the bunch train. The FEL interaction with these short bunches regeneratively amplifies the radiation intensity and broadens its spectrum, allowing for effective transmission of the x rays outside the crystal bandwidth. The spectral brightness of these x-ray pulses is about 2 to 3 orders of magnitude higher than that from a single-pass SASE FEL.     

Effect of the ponderomotive scattering and injection position on electron-bunch injection into a laser wakefield

For the purpose of laser wakefield acceleration, it turns out that the injection of electron bunches longer than the plasma wavelength can also generate accelerated femtosecond bunches with a relatively low energy spread. This is of great interest because such injecting bunches can be provided, e.g., by photo cathode rf linacs. Here we show that when an e-bunch is injected into the wakefield, it is important to take into account the interaction of the injected bunch with the laser pulse in the vacuum region located in front of the plasma. We show that at low energies of the injected bunch, this leads to ponderomotive scattering of the bunch and results in a significant drop of the collection efficiency. For certain injection energies the ponderomotive scattering may result in a smaller energy spread in the accelerated bunch. It is found that the injection position in the laser wakefield plays an important role. Higher collection efficiency can be obtained for certain injection energies, when the bunch is injected in plasma at some distance from the laser pulse; the energy spread, however, is typically larger in this case. We also estimate the minimum trapping energy for the injected electrons and the length of the trapped bunch. PACS 52.38.Kd; 41.75.Jv; 41.85.Ar     

Experimental verification of laser photocathode RF gun as aninjector for a laser plasma accelerator

The feasibility of the laser photocathode RF gun, BNL/GUN-IV, as an injector for a laser plasma accelerator was investigated at the subpicosecond S-band twin linac system of the Nuclear Engineering Research Laboratory, University of Tokyo. Electron beam energy of 16 MeV, emittance of 6π mm mrad, bunch length of 240 fs (FWHM), and charge per bunch of 350 pC were confirmed at 10 Hz. As for diagnosis of the femtosecond electron bunch, the quantitative comparison of performance of the femtosecond streak camera, the coherent transition radiation (CTR) Michelson interferometer, and the far-infrared polychromator was carried out. We concluded that the streak camera is the most reliable up to 200 fs and that the polychromator is the best for the shorter electron bunch. The 3.5-ps (rms) resolved synchronization between the YLF laser driver for the gun and the electron bunch was achieved. Based on the above experiences, we have designed and installed a much better laser-electron synchronization system using the Kerr-lens mode-locked Ti:Sapphire laser with the min harmonics synchrolocker and the stable 15-MW klystron. The timing jitter is expected to be suppressed down to 320 fs (rms)     

Equivalent charge of photons and neutrinos in a plasma

It is suggested that it is possible to define an equivalent electric charge for an intense laser pulse (which can be described as a photon bunch) propagating in a plasma. It is also shown that this equivalent charge can be a source of new radiation processes in an inhomogeneous plasma. The results are extended to the case of a neutrino bunch, which is coupled to the plasma by weak nuclear forces.     

Coaxial injector of a plasma opening switch operating in the nonsteady arc-discharge mode

Results are presented from interferometric studies of a coaxial injector operating in the nonsteady arc-discharge mode. The discharge was initiated and stabilized by pulsed gas puffing into the interelectrode gap of the injector. The dynamics of the plasma bunch formation and the spatial characteristics of the bunch are investigated, and the density of the generated plasma is determined.     

Near-GeV-energy laser-wakefield acceleration of self-injected electrons in a centimeter-scale plasma channel

The first three-dimensional, particle-in-cell (PIC) simulations of laser-wakefield acceleration of self-injected electrons in a 0.84 cm long plasma channel are reported. The frequency evolution of the initially 50 fs (FWHM) long laser pulse by photon interaction with the wake followed by plasma dispersion enhances the wake which eventually leads to self-injection of electrons from the channel wall. This first bunch of electrons remains spatially highly localized. Its phase space rotation due to slippage with respect to the wake leads to a monoenergetic bunch of electrons with a central energy of 0.26 GeV after 0.55 cm propagation. At later times, spatial bunching of the laser enhances the acceleration of a second bunch of electrons to energies up to 0.84 GeV before the laser pulse intensity is significantly reduced.     

Passive harmonic mode locking of soliton bunches in a fiber ring laser

We report on the experimental observation of passive harmonic mode locking of bunches of single-pulse solitons or twin-pulse solitons in an Erbium-doped fiber ring laser. Experimental investigations on the phenomenon revealed that, although the soliton interaction between the adjacent single-/twin-pulse solitons in a bunch is weaker than that of the pulse interaction in the twin-pulse solitons, a soliton bunch could also function as a unit and form the state of passively harmonic mode-locking. Harmonic mode-locking is one of the intrinsic characteristics of soliton emission in passively mode-locked fiber ring lasers. It can be formed based on the single-pulse soliton, twin-pulse soliton, or bunch of solitons.