High energy electron beam generation during interaction of a laser accelerated proton beam with a gas-discharge plasma

The study of the interaction between ion beam and plasma is very important to the areas of inertial fusion energy and high energy density physics. With detailed one-dimensional electromagnetic particle-in-cell simulations, we investigate here the interaction of a laser-accelerated proton beam assuming an ideal monoenergetic beam with a gas-discharge plasma. After the saturation stage of the two-stream instability excited by the proton beam, significant high energy electrons are observed, with maximum energy approaching 2 MeV, and a new two-stream instability occurs between the high energy electrons and background electrons. The trajectories of plasma electrons are studied, showing the process of electron trapping and de-trapping from the wakefield.     

Enhancement of proton collimation and acceleration by an ultra-intense laser interacting with a cone target followed by a beam collimator

A special method is proposed of a laser-induced cavity pressure acceleration scheme for collimating, accelerating and guiding protons, using a single-cone target with a beam collimator through a target normal sheath acceleration mechanism. In addition, the problems involved are studied by using two-dimensional particle-in-cell simulations. The results show that the proton beam can be collimated, accelerated and guided effectively through this type of target. Theoretically, a formula is derived for the combined electric field of accelerating protons. Compared with a proton beam without a beam collimator, the proton beam density and cut-off energy of protons in the type II are increased by 3.3 times and 10% respectively. Detailed analysis shows that the enhancement is mainly due to the compact and strong sheath electrostatic field, and that the beam collimator plays a role in focusing energy. In addition, the simulation results show that the divergence angle of the proton beam in type II is less than 1.67 times that of type I. The more prominent point is that the proton number of type II is 2.2 times higher than that of type I. This kind of target has important applications in many fields, such as fast ion ignition in inertial fusion, high energy physics and proton therapy.     

用离子束技术研究稀磁半导体和纳米磁性材料

综述用离子束技术研究稀磁半导体和纳米磁性材料的构建、调控和改性的进展和现状。离子束注入是优选的构建和调控方法,测试结果表明在GaN和ZnO等新半导体光电材料中注入Mn、Ni和Co等过渡金属离子可得到低温和室温下的铁磁性。离子束分析技术给出这些新材料薄膜的微结构信息,透射电镜下观察到过饱和剂量Mn+注入GaN形成的纳米磁性颗粒。     

pA量级质子束流测量系统

研制的质子束流测量系统,本底电流仅为１０＾－１４Ａ量级,可测最大电流达ｍＡ量级。在质子单粒子效应实验中,测得了ｐＡ量级的质子束及其随时间变化的关系,为半导体体器件粒子翻转截面的计算提供了必需的数据。     

Experimental determination of the absorbed dose to water in a scanned proton beam using a water calorimeter and an ionization chamber

The absorbed dose to water is the reference physical quantity for the energy absorbed in tissue when exposed to beams of ionizing radiation in radiotherapy. The SI unit of absorbed dose to water is the gray (Gy = 1 J/kg). Ionization chambers are used as the dosimeters of choice in the clinical environment because they show a high reproducibility and are easy to use. However, ionization chambers have to be calibrated in order to convert the measured electrical charge into absorbed dose to water. In addition, protocols require these conversion factors to be SI traceable to a primary standard of absorbed dose to water. We present experimental results where the ionization chamber used for the dosimetry for the scanned proton beam facility at PSI is compared with the direct determination of absorbed dose to water from the METAS primary standard water calorimeter. The agreement of 3.2% of the dose values measured by the two techniques are within their respective statistical uncertainties.     

Investigation of the energy characteristics of the external proton beam in the six-meter synchrocyclotron

The energy spectrum and the average energy of the external proton beam of the six-meter synchrocyclotron of the Joint Institute for Nuclear Studies have been investigated under various operating conditions. The spectrum is described by a Gaussian curve with a dispersion of (2.8±0.3) Mev. The mean energy has been determined to within an accuracy of 0.1%.In conclusion we wish to thank T'ang Hsiao-wei and A. A. Tyapkin for discussion of the results of the present work.     

On the stability of small-scale ballooning modes in axisymmetric mirror traps

It is shown that a steepening of the radial plasma pressure profile leads to a decrease in the critical value of beta, above which, small-scale balloon-type perturbations in a mirror trap become unstable. This may mean that small-scale ballooning instability leads to a smoothing of the radial plasma profile. The critical beta values for the real magnetic field of the gas-dynamic trap and various plasma pressure radial profiles was also calculated. For a plasma with a parabolic profile critical beta is evaluated at the level of 0.72. A previous theoretical prediction for this trap was almost two times lower than maximal beta 0.6 achieved experimentally.     

Control of first-wall surface conditions in the 2XIIB magnetic mirror plasma confinement experiment

The control of first-wall surface conditions in the 2XIIB Magnetic Mirror Plasma Confinement experiment is described. Before each plasma shot, the first wall is covered with a freshly gettered titanium surface. Up to 5 MW of neutral beam power has been injected into 2XIIB, resulting in first-wall bombardment fluxes of 10¹⁷ atoms · cm^?2 · s^?1 of 13-keV mean energy deuterium atoms for several ms. The background gas flux is measured with a calibrated, 11-channel, fast-atom detector. Background gas levels are found to depend on surface conditions, injected beam current, and beam pulse duration. For our best operating conditions, an efective reflex coefficient of 0.3 can be inferred from the measurements. Experiments with long-duration and high-current beam injection are limited by charge exchange; however, experiments with shorter beam duration are not limited by first-wall surface conditions. We conclude that surface effects will be reduced further with smoother walls.     

环形电子束在周期永磁场中传输的理论研究

通过对环形电子束中电子的受力分析，得出了在周期永磁场(PPM)中电子束传输的条件和电子束的形状。研究认为可以使用PPM对环形电子束进行传输。     

Numerical simulation of inducing characteristics of high energy electron beam plasma for aerodynamics applications

The problem of flow active control by low temperature plasma is considered to be one of the most flourishing fields of aerodynamics due to its practical advantages.Compared with other means,the electron beam plasma is a potential flow control method for large scale flow.In this paper,a computational fluid dynamics model coupled with a multi-fluid plasma model is established to investigate the aerodynamic characteristics induced by electron beam plasma.The results demonstrate that the electron beam strongly influences the flow properties,not only in the boundary layers,but also in the main flow.A weak shockwave is induced at the electron beam injection position and develops to the other side of the wind tunnel behind the beam.It brings additional energy into air,and the inducing characteristics are closely related to the beam power and increase nonlinearly with it.The injection angles also influence the flow properties to some extent.Based on this research,we demonstrate that the high energy electron beam air plasma has three attractive advantages in aerodynamic applications,i.e.the high energy density,wide action range and excellent action effect.Due to the rapid development of near space hypersonic vehicles and atmospheric fighters,by optimizing the parameters,the electron beam can be used as an alternative means in aerodynamic steering in these applications.     

Investigation of the characteristics of a beam of protons accelerated in a 10 Bev proton synchrotron

The article gives the results of the investigations of the characteristics of a beam accelerated to the extreme energy in a 10 Bev proton synchrotron. The investigation methods are described; the article describes the filling of the phase stability region, the distribution of particles by amplitude of free oscillation, and the value of the energy dispersion of particles in the accelerated beam.The authors sincerely thank N. B. Rubin for his comments on the work.     

Evaluation of NBI absorption and fast ion stored energy to improve thermal energy confinement time calculation in EAST

The absorption of neutral beam power and the fast ion stored energy in EAST plasmas with neutral beam injection(NBI)is analyzed to improve the calculation of thermal energy confinement time.The neutral beam power absorption and fast ion stored energy are systematically calculated using the TRANSP code,through the investigation of global parameters including plasma current,line averaged density and beam energy.Results have shown that scaling laws for the NBI absorption coefficient and fast ion energy rate are obtained through statistical analysis.A comparison of the confinement improvement factor H98y2 with these new scaling laws against those assuming fixed coefficients is given.     

Neutronic performance of the MEGAPIE spallation target under high power proton beam

The MEGAPIE project, aiming at the construction and operation of a megawatt liquid lead-bismuth spallation target, constitutes the first step in demonstrating the feasibility of liquid heavy metal target technologies as spallation neutron sources. In particular, MEGAPIE is meant to assess the coupling of a high power proton beam with a window-concept heavy liquid metal target. The experiment has been set at the Paul Scherrer Institute (PSI) in Switzerland and, after a 4-month long irradiation, has provided unique data for a better understanding of the behavior of such a target under realistic irradiation conditions. A complex neutron detector has been developed to provide an on-line measurement of the neutron fluency inside the target and close to the proton beam. The detector is based on micrometric fission chambers and activation foils. These two complementary detection techniques have provided a characterization of the neutron flux inside the target for different positions along its axis. Measurements and simulation results presented in this paper aim to provide important recommendations for future accelerator driven systems (ADS) and neutron source developments.     

The influence of magnetic field on the beam quality of relativistic electron beam long-range propagation in near-Earth environment

In recent years,it has been proposed to use satellite-mounted radio-frequency(RF)accelerators to produce high-current relativistic electron beams to complete debris removal tasks.However,when simulating the long-range propagation(km-range)process of the electron beam,it is difficult to directly use the particle-in-cell method to simultaneously consider the space charge effect of beam and the influence of the geomagnetic field.Owing to these limitations,in this paper,we proposed a simplified method.The ps-range electronic micropulses emitted by the RF accelerator were transmitted and fused to form a ns-range electron beam;then,combined with the improved moving window technology,the model was constructed to simulate the long-range propagation process of the relativistic electron beam in near-Earth environment.Finally,by setting the direction of movement of the beam to be parallel,perpendicular and at an inclination of 3° to the magnetic field,we analyzed and compared the effects of the applied magnetic fields in different directions on the quality of the beam during long-range propagation.The simulation results showed that the parallel state of the beam motion and magnetic fields should be achieved as much as possible to ensure the feasibility of the space debris removal.     

Micro-fluidic target chamber machined by proton beam writing for the in situ analysis of gas absorption in synthetic crystals

Many synthetic crystals used for chemical and industrial applications have special internal structures, e.g. nano-pores, which allow separating different gases and fluids. Ion beam analytical methods can be used to study the gas diffusion and absorption in these materials in situ and to visualize their inner surfaces which affect these processes. For this purpose, a small target chamber was constructed in PMMA (polymethyl methacrylate) using proton beam writing (PBW). This micro-fluidic structure enables the establishment of a defined atmosphere around a crystal and allows the simultaneous ion beam analysis. In order to confine the gas from the high vacuum in the measurement chamber Si₃N₄-windows of 200 nm thickness were thermally bonded to the structured PMMA block yielding a closed target chamber with the possibility to accomplish PIXE and RBS measurements. In addition, two capillaries were connected to the chamber for gas inlet and evacuation. First tests showed that the construction is leak-proof and allows to establish a defined atmosphere. After that, the Argon gas diffusion into Zn(tbip)-crystals was studied. These measurements have shown unexpectedly high nickel concentrations in the host crystal which reduces the Argon density in these areas after absorption, because the Ni atoms decrease the pore size by replacing Zn-atoms in the Zn(tbip)-lattice. It could be demonstrated that gas diffusion and absorption in organic crystals can be studied in situ with high lateral resolution using ion beam analysis in a dedicated target chamber machined by PBW.     

Effects of resonant magnetic perturbations on the loss of energetic ions in tokamak pedestal

Resonant magnetic perturbations (RMPs) are extensively applied to mitigate or suppress the edge localized mode in tokamak plasmas, but will break the axisymmetric magnetic field configuration and increase the loss of energetic ions. The mechanism of RMPs induced energetic ion loss has been extensively studied, and is mainly attributed to resonant effects. In this paper, in the perturbed non-axisymmetric tokamak pedestal, we analytically derive the equations of guiding center motion for energetic ions including the bounce/transit averaged radial drift velocity and the toroidal precession frequency modified by strong radial electric field. The loss time of energetic ions is numerically solved and its parametric dependence is analyzed in detail. We find that passing energetic ions cannot escape from the plasma, while deeply trapped energetic ions can escape from the plasma. The strong radial electric field plays an important role in modifying the toroidal precession frequency and resulting in the drift loss of trapped energetic ions. The loss time of trapped energetic ions is much smaller than the corresponding slowdown time in DIII-D pedestal. This indicates that the loss of trapped energetic ions in the perturbed non-axisymmetric pedestal is important, especially for the trapped energetic ions generated by perpendicular neutral beam injection.