预脉冲对超短激光与薄膜靶相互作用加速产生质子的影响

采用波长为744 nm、聚焦功率密度为6×10¹⁶W/cm²的超短激光分别与两种不同厚度的铝薄膜靶相互作用,根据鞘层加速机制在靶后法线方向测量质子束角分布和能谱随靶厚度的变化,研究了预脉冲对质子加速的影响。随着薄膜靶厚度的降低,质子计数迅速增加,但当薄膜靶厚度太薄时,激光预脉冲形成的预等离子体影响了薄膜靶的面型,导致质子横向发散角迅速增加,而薄膜靶面型的破坏减少了激光与等离子体相互作用过程中的电子回流,从而降低了超热电子的产生和鞘层加速电场的维持,影响了质子的加速能谱。因此,超短脉冲激光与薄膜靶相互作用加速产生质子束,应尽量降低预脉冲,不能采用太薄的薄膜靶,以避免预等离子体影响薄膜靶的面型,导致质子的能量降低、发散角增大。     

紫外超短激光驱动铜薄膜靶产生质子的实验研究

在中国原子能科学研究院的放电泵浦的紫外KrF超短脉冲激光放大装置上,开展了紫外超短脉冲激光与铜薄膜靶相互作用加速产生质子束的实验研究。紫外超短脉冲激光输出能量为30 mJ、波长为248 nm、脉冲宽度为500 fs,采用离轴抛物面镜聚焦获得激光聚焦功率密度为1.2×10¹⁷ W/cm²。激光以45°入射5 μm厚的铜薄膜靶,质子最大能量超过300 keV。紫外超短脉冲激光的高对比度和高吸收效率是紫外激光加速的优点。     

电子磁谱仪法测量超热电子温度

利用超热电子磁谱仪测量了紫外超短脉冲激光与固体等离子体相互作用产生超热电子的能谱,在无预脉冲、激光强度为1017 W/cm2 条件下,紫外超短脉冲激光与固体(Cu)等离子体相互作用产生超热电子的能谱呈双温麦克斯韦分布,超热电子温度为81 keV,激光吸收的主导机制为真空吸收。     

高强度紫外激光驱动的质子加速

正超强脉冲激光驱动等离子体加速产生强流脉冲质子束,在高能量密度物理和惯性约束聚变等领域有着重要的研究意义。本文研究了超短脉冲激光加速质子的物理过程,研究了激光强度、激光波长、激光对比度、薄膜靶厚度等对超短脉冲激光驱动薄膜靶加速质子束的影响。研究了紫外超短脉冲激光在质子加速过程中的优势,高对比度的紫外激光有效抑制等离子体对质子加速的影响,波长短,具有高临界密度和更好的激光吸收效率,可产生具有超高密度梯度的高密度等离子体,有利于提高超热电子密度,提高质子加速的束流强度和能量转换效率。P极化激光以45°入射角入     

超短激光与薄膜靶作用加速产生质子初步研究

物理学家利用高能粒子加速器进行了多方面的研究，但高能粒子加速器庞大且耗资巨大。随着超短超强激光的发展，现在的激光的功率密度可达到10^22W／cm^2。许多实验室利用不同功率密度的激光与固体靶、薄膜靶及气体等相互作用，进行加速产生高能粒子的研究。其中，利用超短超强激光与薄膜薄相互作用加速产生质子是一重要的研究课题，利用超热电子加速产生超热电子，     

激光等离子体X射线光谱分峰方法研究

分析了激光等离子体发射的X射线光谱的线型轮廓,构造出三种线型拟合函数,采用最小二乘算法,提出了一种多峰值数据处理方法,并应用于超短超强激光脉冲加热产生的镁铝等离子体的类H类He X光光谱验证其有效性,比较三种拟合线型的拟合精度。     

固体径迹探测器CR39的标定

固体径迹探测器广泛应用于科学和技术方面，CR39是其中使用很频繁的一种塑料探测器。由于电子和伽马光子在CR39中的碰撞截面很小，远小于中子、质子或其他离子的碰撞截面，因此可认为固体径迹探测器CR39对电子和光子不响应，而仅对中子、质子或其他离子响应，这给CR39在实验中的应用带来很大优点。在超短超强脉冲激光与等离子体相互作用的实验中，会产生大量的强伽马射线、热电子或超热电子，而在有些实验如超短超强脉冲激光加速产生高能质子束的研究中，需单独对质子束的通量、角分布、能谱等参数进行详尽的测量。     

15 TW激光与氮气作用产生稳定电子束的实验研究

利用15 TW激光脉冲，系统研究了基于电离化注入的激光尾波场加速。实验中，研究了等离子体密度、相互作用位置、激光脉宽以及激光能量对电子束的电荷量、发散角、指向性、能量以及产生概率的影响。将约400 mJ、25 fs的激光脉冲聚焦在喷嘴前沿，等离子体密度约9×10¹⁸ cm^-3时，电子的产生概率高达100%，获得了水平（竖直）发散角（6.5±0.5） mrad（（5.3±0.3） mrad）、水平（竖直）指向稳定性±1.2 mrad （±0.7 mrad）、峰值能量（135±8） MeV和电荷量（13.5±2.0） pC（＞50 MeV）的稳定电子束，为其应用奠定了基础。     

紫外超短脉冲激光与固体靶相互作用产生超热电子能谱测量

利用电子磁谱仪测量紫外超短脉冲激光与固体等离子体相互作用产生超热电子的能谱,在无预脉冲、激光强度为1017 W/cm2的条件下,紫外(248 nm)超短(440 fs)脉冲激光与固体(Cu)等离子体相互作用产生超热电子的能谱呈双温麦克斯韦分布, 超热电子温度为81 keV,激光吸收的主导机制为真     

高功率激光驱动核反应研究进展与展望

近年来，随着高功率激光技术的持续进步，目前在实验室已可获得超过10²² W/cm²的激光聚焦强度，且还在不断提升，这为超高强度激光脉冲触发核过程和核应用开辟了可能性。这样的超强脉冲激光与物质发生相互作用时，会出现很多新的物理现象，并且其产生的高温高压高密度的等离子体极端环境以及诱发的核反应次级粒子束等，也为其他基础和应用研究提供了独特的平台。强激光产生的等离子体环境可用来模拟天体核反应的环境，研究电子屏蔽效应等因素带来的低能核反应截面测量中的不确定性因素，这是目前实验室条件下直接研究天体环境中核反应的唯一技术手段。同时，激光驱动等离子体中核反应的研究也与惯性约束聚变中的燃料设计息息相关。与常规环境温度和压力下的物质相比，等离子体环境中的核反应动力学要复杂得多，对于研究天体核反应和惯性约束聚变也至关重要。因此，除加速器和反应堆外，高功率激光器正成为研究核物理的新平台。激光与核物理的结合不仅有利于具有新颖思想和方法的基础科学研究，也有利于广泛的物理应用领域。激光核物理已成为国际上一门新的交叉学科，也是物理学的重要前沿之一。与传统核物理装置相比，超...     

Equilibrium evolution on the resistive time scale in a tokamak reactor

In the Engineering Test Facility (ETF), the plasma pulse duration is expected to be hundreds of seconds, which is comparable to the resistive time scale that governs the resistive diffusion of the equilibrium. The resistive evolution of the safety factorq profile may, for MHD stability reasons, limit the duration of the plasma burn in a tokamak reactor. It may be possible to control this evolution and extend the plasma burn time through proper profile tailoring. We study the evolution of theq profile on the resistive time scale numerically using a one- and-one-half-dimensional (1 1/2-D) single fluid transport code. Two high beta (T 7–16%) cases are considered: (a) a beam-driven hydrogen plasma with no nuclear alpha heating for which the beam energy is used as a device to control the temperature profile, and (b) an ignited D-T plasma in which the neutral injection has been turned off. For the beam-driven plasma, it is shown that low beam energy heating profiles lead to resistive steady states having broad temperature profiles and flatq profiles, while high beam energy heating profiles lead to resistive steady states having peaked temperature profiles and deepq profiles. The centralized nuclear heating in an ignited D-T plasma causes the evolution of theq profile for this case to behave much like that in the high energy, beam-driven case: namely, theq values near the plasma center decrease on the resistive time scale until a deep, resistive, steady-stateq profile is reached.Research sponsored by the Office of Fusion Energy, U.S. Department of Energy, under contract W-7405-eng-26 with the Union Carbide Corporation.     

Simulation study on electron heating characteristics in magnetic enhancement capacitively coupled plasmas with a longitudinal magnetic field

The electron heating characteristics of magnetic enhancement capacitively coupled argon plasmas in presence of both longitudinal and transverse uniform magnetic field have been explored through both theoretical and numerical calculations. It is found that the longitudinal magnetic field can affect the heating by changing the level of the pressure heating along the longitudinal direction and that of the Ohmic heating along the direction which is perpendicular to both driving electric field and the applied transverse magnetic field, and a continuously increased longitudinal magnetic field can induce pressure heating to become dominant. Moreover, the electron temperature as well as proportion of some low energy electrons will increase if a small longitudinal magnetic field is introduced, which is attributed to the increased average electron energy. We believe that the research will provide guidance for optimizing the magnetic field configuration of some discharge systems having both transverse and longitudinal magnetic field.     

托卡马克等离子体中快粒子触发的高频鱼骨模不稳定性研究

本文考虑托卡马克等离子体中快粒子的空间密度分布剖面,在中性束及电子回旋共振加热的条件下,建立了研究鱼骨模的色散关系并对鱼骨模作了数值研究。结果表明:快粒子对内扭曲模有致稳作用,且在近轴加热条件下可激发高频鱼骨模,其频率与快粒子的环向进动频率一致。鱼骨模的增长率与快粒子的密度梯度有关,一般随密度梯度的增加而增大。在高比压区间,勉强通行快粒子可驱动鱼骨模进入第二稳定区,在该区域鱼骨模是稳定的。     

Divertor detachment operation in helium plasmas with ITER-like tungsten divertor in EAST

Detachment in helium (He) discharges has been achieved in the EAST superconducting tokamak equipped with an ITER-like tungsten divertor. This paper presents the experimental observations of divertor detachment achieved by increasing the plasma density in He discharges. During density ramp-up, the particle flux shows a clear rollover, while the electron temperature around the outer strike point is decreasing simultaneously. The divertor detachment also exhibits a significant difference from that observed in comparable deuterium (D) discharges. The density threshold of detachment in the He plasma is higher than that in the D plasma for the same heating power, and increases with the heating power. Moreover, detachment assisted with neon (Ne) seeding was also performed in L- and H-mode plasmas, pointing to the direction for reducing the density threshold of detachment in He operation. However, excessive Ne seeding causes confinement degradation during the divertor detachment phase. The precise feedback control of impurity seeding will be performed in EAST to improve the compatibility of core plasma performance with divertor detachment for future high heating power operations.     

Effects of the Hot Electron Interchange Instability on Plasma Confined in a Dipolar Magnetic Field

The Levitated Dipole Experiment (LDX) explores confinement and stability of plasma created within the dipole field of a strong superconducting magnet. During initial experiments, long-pulse, quasi-steady state discharges that last more than 10 s and have peak beta of more than 20% are studied. The plasma is created by multi-frequency electron cyclotron resonance heating (ECRH) at 2.45 and 6.4 GHz. A population of energetic electrons, with mean energies above 50 keV, dominates the plasma pressure. Creation of high pressure, high beta plasma is possible only when intense hot electron interchange (HEI) instabilities are stabilized by sufficient neutral gas fueling. The instabilities resonate with the magnetic drift motion of the energetic electrons and can cause rapid radial transport. Measurements of the electrostatic and magnetic fluctuations of the HEI instability are described along with observations of the instability’s spectral characteristics. Fluctuations of the outer poloidal field induced by the HEI show a rapid evolution of the perturbed pressure profile.      

Threshold of parametric instability in the ionospheric heating experiments

Many observations in the ionospheric heating experiment, by a powerful high frequency electromagnetic wave with ordinary polarization launched from a ground-based facility, is attributed to parametric instability (PI). In this paper, the general dispersion relation and the threshold of the PI excitation in the heating experiment are derived by considering the inhomogeneous spatial distribution of pump wave field. It is shown that the threshold of PI is influenced by the effective electron and ion collision frequencies and the pump wave frequency. Both collision and Landau damping should be considered in the PI calculation. The derived threshold expression has been used to calculate the required threshold for excitation of PI for several ionospheric conditions during heating experiments conducted employing EISCAT high frequency transmitter in Troms?, Norway, on 2nd October 1998, 8th November 2001, 19th October 2012 and 7th July 2014. The results indicate that the calculated threshold is in good agreement with the experimental observations.     

Ignition Approach by Neutral Beam Injection Heating in Impurity Contaminated Tokamak Reactors

Available heating power by neutral beam injection in a tokamak reactor is evaluated semi-empirically. Using this estimated value, device and plasma parameters to ignite the plasma in impurity contaminated tokamak reactors are investigated. By lowering the plasma density and concurrently by enlarging the plasma minor radius or aspect ratio, the difficulty of NBI heating can be avoided, and the ignition is almost always possible both for trapped ion mode and Alcator scaling laws.     

Semiempirical transport in tokamaks

A class of semiempirical transport models is proposed for testing against confinement data from tokamaks and for use in operations planning and machine design. A reference model is proposed to be compatible with published confinement data. Theoretical considerations are used to express the anomalous transport coefficients in terms of appropriate dimensionless parameters. The reference model is applied to heating of an advanced tokamak experiment.     

Plasma Confinement Characteristics in Heliotron J--Spontaneous Change of Plasma Confinement State

Spontaneous transition of the plasma confinement mode was observed in the helicalaxis heliotron device “Heliotron J“ for three different plasma heating schemes, i.e. ECHonly, NBI-only and the combination of ECH and NBI. The transition seems to occur above a certain critical density. In addition to the confinement transition, a spontaneous shift of the hitting position of the divertor plasma flux on the wall was observed. This shift could be related with the change of the edge field topology caused by non-inductive toroidal currents.     

Results and Future Plan of JT-60U towards Steady-State Tokamak Reactor

Recent results of JT-60U towards establishment of physics basis for ITER and advanced tokamak operation are presented.Progress in high integrated performance is achieved with improvement of N-NB and ECRF heating systems.In the next experimental campaign 2003-2004,discharge duration with 17 MW heating will be extended up to 30s for sustaining high-beta plasma longer than the current diffusion time.Superconducting modification of JT-60 is planned to demonstrate high-beta plasma sustainment exceeding ideal MHD instability limit without wall stabilization.