查看更多>>摘要：Previous studies have shown that the use of laser bandwidth may mitigate the growth of stimulated Raman scattering (SRS) in laser plasma interaction experiments, in particular, when the spectrum of the driving (or pump) laser is composed of uniformly distributed frequency components with a well-chosen bandwidth [for example, Luo et al., Phys. Plasmas 29, 032102 (2022); Wen et al., ibid. 28, 042109 (2021); and Follett et al., ibid. 26, 062111 (2019)]. Here, we investigate the effects of frequency chirp in the pump laser on backward SRS in inhomogeneous plasmas, taking into account kinetic effects associated with the nonlinear detuning of the parametric resonance due to high-amplitude electron plasma waves (EPW). Through theoretical considerations and numerical simulations, using a multi-dimensional particle-in-cell (PIC) code, it is shown that positive frequency chirp rates lead to a displacement of the resonance in the plasma profile. For a sufficiently strong positive chirp rate, such that the resonance displacement is faster than the EPW group velocity, the EPWs prove to remain limited in amplitude such that SRS is suppressed. The required frequency chirp rate corresponds to a laser bandwidth of about 1%-2%. Published under an exclusive license by AIP Publishing.
Blackman, D. R.Nuter, R.Korneev, Ph.Arefiev, A....
11页
查看更多>>摘要：An accurate description of plasma waves is fundamental for the understanding of many plasma phenomena. It is possible to twist plasma waves such that, in addition to having longitudinal motion, they can possess a quantized orbital angular momentum. One such type of plasma wave is the Laguerre-Gaussian mode. Three-dimensional numerical particle-in-cell simulations demonstrate the existence of stable long-lived plasma waves with orbital angular momentum. These waves can be shown to create large amplitude static magnetic fields with unique twisted longitudinal structures. In this paper, we review the recent progress in studies of helical plasma waves and present a new analytical description of a standing Laguerre-Gaussian plasma wave mode along with 3D particle-in-cell simulation results. The Landau damping of twisted plasma waves shows important differences compared to standard longitudinal plasma wave Landau damping. These effects include an increased damping rate, which is affected by both the focal width and the orbital number of the plasma wave. This increase in the damping rate is of the same order as the thermal correction. Moreover, the direction of momentum picked up by resonant particles from the twisted plasma wave can be significantly altered. By contrast, the radial electric field has a subtle effect on the trajectories of resonant electrons. Published under an exclusive license by AIP Publishing.
查看更多>>摘要：The rapid electrical explosion of thin metal wires in a vacuum can completely vaporize the wire and form a fast-expanding gas cylinder of metal atoms. Thin metal wires 12.7 mu m in diameter made of W, Mo, Pt, Pd, Ni, Cu, Au, and Al completely evaporated during the first similar to 10 ns under the action of a fast-rising current of similar to 1 kA/ns. Laser interferometry and the integrated phase technique were used to reconstruct the dynamic atomic polarizability for fully vaporized thin metal wires at 532 and 1064 nm wavelength, calculate the static atomic polarizability, the first dipole-allowed electronic transition energy, and transition wavelengths, and compare our reconstructions with available tabulated values. Published under an exclusive license by AIP Publishing.
查看更多>>摘要：In this paper, we present the effects of a radiative shock (RS) on the morphology of jet-like objects subjected to hydrodynamic instabilities. To this end, we used an experimental platform developed to create RSs on high energy laser facilities such as LULI2000 and GEKKO XII. Here, we employed modulated targets to initiate Richtmyer-Meshkov and Rayleigh-Taylor instability (RTI) growth in the presence of an RS. The RS is obtained by generating a strong shock in a dense pusher that expands into a low-density xenon gas. With our design, only a limited RTI growth occurs in the absence of radiative effects. A strongly radiative shock has opposite effects on RTI growth. While its deceleration enhances the instability growth, the produced radiations tend to stabilize the interfaces. Our indirect experimental observations suggest a lower instability growth despite the interface deceleration. In addition, the jets, produced during the experiment, are relevant to astrophysical structures such as Herbig-Haro objects or other radiatively cooling jets. Published under an exclusive license by AIP Publishing.
查看更多>>摘要：In direct-drive inertial confinement fusion, laser imprint can cause areal density perturbations on the target shell that seed the Rayleigh-Taylor instability and further degrade the implosion. To mitigate the effect of laser imprint, a foam overcoating layer outside the target shell has been suggested to increase the thermal smoothing of the conduction region (between the ablation front and the critical density surface) and mass ablation of the ablation front. In this paper, we use a two-dimensional radiation hydrodynamic code FLASH to investigate the laser imprint mitigation performance and find other physical mechanisms of foam overcoatings. First, radiation ablation dynamically modulates density distribution not only to increase the frequency of the perturbed ablation front oscillation but also to decrease the amplitude of the oscillation. Second, a larger length of the shocked compression region reduces the amplitude of the perturbed shock front oscillation. The areal density perturbations decrease with the decrease in the perturbations of the ablation front and shock front. Based on the abovementioned physical mechanisms, we propose the optimal ranges of foam parameters to mitigate laser imprint with the aid of dimensional analysis: the foam thickness is about two to three times that of the perturbation wavelength, and the foam density is about 1/2-3/2 times that of the critical density. (C)2022 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/)
查看更多>>摘要：Cost-effective vertical etching of plug holes and word lines is crucial in enhancing 3D NAND device manufacturability. Even though multiscale technology computer-aided design (TCAD) methodology is suitable for effectively predicting etching processes and optimizing recipes, it is highly time-consuming. This article demonstrates that our deep learning platform called TCAD-augmented Generative Adversarial Network can reduce the computational load by 2 600 000 times. In addition, because well-calibrated TCAD data based on physical and chemical mutual reactions are used to train the platform, the etching profile can be predicted with the same accuracy as TCAD-only even when the actual experimental data are scarce. This platform opens up new applications, such as hot spot detection and mask layout optimization, in a chip-level area of 3D NAND fabrication. Published under an exclusive license by AIP Publishing.
查看更多>>摘要：Since high density operation is advantageous for building an efficient fusion reactor, understanding the density limit in tokamaks has been seen as one of the most important issues. This paper reports a series of measurements around the last-closed flux surface (LCFS) in L-mode plasmas by using a thermal helium beam diagnostic. Fluctuation analysis has been employed to characterize the poloidal flow and the turbulence structure. A reversal of the poloidal flow in the scrape-off layer and concomitant cooling of the outer divertor plasma are observed as the density is raised. While, in the confined region, the change in the density barely affects the poloidal flow, a higher density shifts the fluctuation power spectral densities toward lower frequencies and wave numbers. The eddy tilting of this region is consistent with what is expected from the magnetic shear effect. A radially coherent low frequency mode appears in the case of the highest density investigated in this study ( (n) over bar(e)/n(e,GW) = 0.51), and higher frequencies near the LCFS are modulated by this mode. (C) 2022 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)
查看更多>>摘要：The presence and the position of an X-point, namely, if the ion diamagnetic drift points toward it or not, strongly impact the edge plasma rotation in tokamaks. In the absence of kinetic effects such as magnetic ripple or ion orbit loss, the shape of the velocity profile results from the balance between neoclassical predictions and turbulent flow generation. In this contribution, we derive a reduced model of turbulence plunged in a shear flow. This model is based on (1) a description of the impact of a sheared flow on the interchange turbulence and (2) a prediction of the poloidal momentum generated by the turbulence. It includes the effects of both the magnetic topology and the finite shear layer width. The model is verified against 2D non-linear flux-driven simulations. Finally, the model predictions of the edge rotation resulting from the equilibrium between the neoclassical prediction and the poloidal momentum generation by the turbulence are invoked to describe the observations from experiment managed in the WEST tokamak. It points out the important role of the magnetic shear in the turbulence tilting and in the flow generation. Published under an exclusive license by AIP Publishing.
Zheng, LinjinKotschenreuther, M. T.Waelbroeck, F. L.Todo, Y....
11页
查看更多>>摘要：A radially adaptive numerical scheme is developed to solve the Grad-Shafranov equation for axisymmetric magnetohydrodynamic equilibrium. A decomposition with independent solutions is employed in the radial direction, and Fourier decomposition is used in the poloidal direction. The independent solutions are then obtained using an adaptive shooting scheme together with the multi-region matching technique in the radial direction. Accordingly, the adaptive toroidal equilibrium (ATEQ) code is constructed for axisymmetric equilibrium studies. The adaptive numerical scheme in the radial direction improves considerably the accuracy of the equilibrium solution. The decomposition with independent solutions effectively reduces the matrix size in solving the magnetohydrodynamic equilibrium problem. The reduction of the matrix size is about an order of magnitude as compared with the conventional radially grid-based numerical schemes. Also, in this ATEQ numerical scheme, no matter how accuracy in the radial direction is imposed, the size of matrices basically does not change. The small matrix size scheme gives ATEQ more flexibility to address the requirement of the number of Fourier components in the poloidal direction in tough equilibrium problems. These two unique features, the adaptive shooting and small matrix size, make ATEQ useful to improve tokamak equilibrium solutions. Published under an exclusive license by AIP Publishing.
NSTL