| 411. |
- Westerberg, Lars-Göran, et al.
(författare)
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Effects of varying magnetosheath flow and dissipation on the two-dimensional reconnection structure at the magnetopause
- 2009
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 16:5, s. 052902-1
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Tidskriftsartikel (refereegranskat)abstract
- The stationary Riemann problem including dissipation in form of resistivity and viscosity for the reconnection structure at the dayside magnetopause is considered. Including tangential velocity shear and gradients across the complete reconnection structure from the magnetosheath side into the magnetosphere, it is shown how dissipation affects the thickness of the intermediate shock wave in the vicinity of a reconnection site. We present how plasma flow structure undergoes a significant change for a nonzero uniform plasma velocity and velocity gradient. It is also shown how the reconnection rate changes and leads to a cutoff when the velocity becomes Alfvénic.
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| 412. |
- Westerberg, Lars-Göran, et al.
(författare)
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Hall effects on the steady structure of the rotational layer at the dayside magnetopause
- 2007
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 14
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Tidskriftsartikel (refereegranskat)abstract
- The influence of the Hall term in the generalized Ohm's law on the large-scale plasma flow during conditions of ongoing magnetic reconnection at the dayside magnetopause (MP) is investigated. Of special interest is the plasma flow behavior during the transition of the MP transition layer as the Hall effect grows in proportion to the viscous-resistive effects. The governing equations are solved approximately by an ordinary perturbation expansion in orders of large Reynolds and Lundqvist numbers. It is shown that the flow pattern is strongly dependent on the magnitude of the Hall parameter; as it approaches zero, the viscous-resistive solution is obtained, while for an ordering of the same magnitude as the resistivity/viscosity, the Hall effect begins to affect the flow structure severely. For an increasing value on the Hall parameter, oscillations are brought into the system, an effect that is enhanced with the magnitude of the Hall parameter. Furthermore, it is shown that as the Hall effect begins to dominate, the transition layer thickens.
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| 413. |
- Wilkie, George, 1983, et al.
(författare)
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Fundamental form of the electrostatic δf-PIC algorithm and discovery of a converged numerical instability
- 2016
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 23:5, s. 052111-
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Tidskriftsartikel (refereegranskat)abstract
- The δf particle-in-cell algorithm has been a useful tool in studying the physics of plasmas, particularly turbulent magnetized plasmas in the context of gyrokinetics. The reduction in noise due to not having to resolve the full distribution function indicates an efficiency advantage over the standard (“full-f”) particle-in-cell. Despite its successes, the algorithm behaves strangely in some circumstances. In this work, we document a fully resolved numerical instability that occurs in the simplest of multiple-species test cases: the electrostatic ΩH mode. There is also a poorly understood numerical instability that occurs when one is under-resolved in particle number, which may require a prohibitively large number of particles to stabilize. Both of these are independent of the time-stepping scheme, and we conclude that they exist if the time advancement were exact. The exact analytic form of the algorithm is presented, and several schemes for mitigating these instabilities are also presented.
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| 414. |
- Wilkie, George, 1983, et al.
(författare)
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Transport and deceleration of fusion products in microturbulence
- 2016
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 23:6, s. 060703-
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Tidskriftsartikel (refereegranskat)abstract
- The velocity-space distribution of alpha particles born in fusion devices is subject to modification at moderateenergies due to turbulent transport. Therefore, one must calculate the evolution of an equilibrium distributionwhose functional form is not known a priori. Using a novel technique, applicable to any trace impurity, wehave made this calculation for fully nonlinear gyrokinetic simulations not only possible, but particularlyefficient. We demonstrate a microturbulence-induced departure from the local slowing-down distribution, aninversion of the energy distribution, and associated modifications to the alpha heating and pressure profilesin an ITER-like scenario.
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| 415. |
- Wu, D.J., et al.
(författare)
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An analytical solution of finite‐amplitude solitary kinetic Alfvén waves
- 1995
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 2, s. 4476-4481
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Tidskriftsartikel (refereegranskat)abstract
- An analytical solution of finite-amplitude solitary kinetic Alfven waves (SKAWs) in a low-beta (beta much less than m(e)/m(i) much less than 1) plasma is presented. This solution has been compared with the solution of the Korteweg-de Vries (KdV) equation in the small-amplitude limit. It is found that the KdV soliton solution is valid only for the maximum relative density perturbation N-m<0.1. For the larger N-m, the exact analytical solution shows that the SKAWs have a much wider structure and much stronger perturbed fields than the KdV solitons with the same N-m. Moreover, the relations between the width and the amplitude of SKAWs are also considerably different from that of the KdV solitons. In addition the possibility for applying these results to some events observed from the Freja scientific satellite is discussed. (The Freja is a Swedish-German scientific project for the investigation of ionospheric and magnetospheric plasmas, and the Freja satellite was launched on a Long-March II rocket of China on October 6, 1992.) (C) 1995 American Institute of Physics.
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| 416. |
- Wu, D J, et al.
(författare)
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Solitary kinetic Alfven waves in the two-fluid model
- 1996
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 3, s. 2879-2884
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Tidskriftsartikel (refereegranskat)abstract
- Employing the two-fluid model, a generalized Sagdeev equation governing solitary kinetic Alfven waves (SKAWs) and the criterion for the existence of SKAWs, which are valid for different ranges of plasma pressure parameter beta, are presented. In the limit cases of beta much greater than m(e)/m(i) and beta much less than m(e)/m(i), the present results correspond, respectively, with conclusions obtained by Hasegawa ct al. [Phys. Rev. Lett. 37, 690 (1976)] and by Shukla et al. [J. Plasma Phys. 28, 125 (1982)], that is, SKAWs accompanied by, respectively, hump and dip density solitons for beta much greater than m(e)/m(i) and beta much less than m(e)/m(i). However, for the case of beta similar to m(e)/m(i), the present results show that SKAWs accompanied by both hump and dip density solitons are possible, and lead to KdV solitons in the small amplitude limit. In addition, the possibility for applying these results to electromagnetic spikes observed by the Freja scientific satellite is discussed [detailed information about the Freja satellite experiments can be found in serial papers presented in Space Sci. Rev. 70, Nos. 3/4 (1994)]. (C) 1996 American Institute of Physics.
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| 417. |
- Xu, J., et al.
(författare)
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Dynamics of electron injection in a laser-wakefield accelerator
- 2017
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Ingår i: Physics of Plasmas. - : AMER INST PHYSICS. - 1070-664X .- 1089-7674. ; 24:8
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Tidskriftsartikel (refereegranskat)abstract
- The detailed temporal evolution of the laser-wakefield acceleration process with controlled injection, producing reproducible high-quality electron bunches, has been investigated. The localized injection of electrons into the wakefield has been realized in a simple way-called shock-front injection-utilizing a sharp drop in plasma density. Both experimental and numerical results reveal the electron injection and acceleration process as well as the electron bunch's temporal properties. The possibility to visualize the plasma wave gives invaluable spatially resolved information about the local background electron density, which in turn allows for an efficient suppression of electron self-injection before the controlled process of injection at the sharp density jump. Upper limits for the electron bunch duration of 6.6 fs FWHM, or 2.8 fs (r. m. s.) were found. These results indicate that shock-front injection not only provides stable and tunable, but also few-femtosecond short electron pulses for applications such as ultrashort radiation sources, time-resolved electron diffraction or for the seeding of further acceleration stages. Published by AIP Publishing.
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| 418. |
- Yadikin, Dimitry, et al.
(författare)
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Advanced feedback control methods in EXTRAP T2R
- 2006
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 13:7, s. 072109-
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Tidskriftsartikel (refereegranskat)abstract
- Previous experiments in the EXTRAP T2R reversed field pinch device have shown the possibility of suppression of multiple resistive wall modes (RWM). A feedback system has been installed in EXTRAP T2R having 100% coverage of the toroidal surface by the active coil array. Predictions based on theory and the previous experimental results show that the number of active coils should be sufficient for independent stabilization of all unstable RWMs in the EXTRAP T2R. Experiments using different feedback schemes are performed, comparing the intelligent shell, the fake rotating shell, and the mode control with complex feedback gains. Stabilization of all unstable RWMs throughout the discharge duration of t(d)approximate to 10 tau(w) is seen using the intelligent shell feedback scheme. Mode rotation and the control of selected Fourier harmonics is obtained simultaneously using the mode control scheme with complex gains. Different sensor signals are studied. A feedback system with toroidal magnetic field sensors could have an advantage of lower feedback gain needed for the RWM suppression compared to the system with radial magnetic field sensors. In this study, RWM suppression is demonstrated, using also the toroidal field component as a sensor signal in the feedback system.
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| 419. |
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| 420. |
- Yagi, Y., et al.
(författare)
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The magnitude of sawtooth crash events in multiple and quasi-single helicity states in a reversed-field-pinch plasma
- 2005
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 12:8, s. 082507-
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Tidskriftsartikel (refereegranskat)abstract
- Factors affecting the magnitude of sawtooth crashes in soft x-ray (SXR) signals (I-SXR) have been studied using data from high-Theta (pinch parameter) discharges in the toroidal pinch experiment TPE-1RM20 reversed-field pinch device [Y. Yagi , J. Plasma Fusion Research 69, 700 (1993)]. In TPE-1RM20, discharges with a high Theta that are free of large sawteeth (improved high-Theta mode discharges), spontaneously appeared [Y. Hirano , Nucl. Fusion 36, 721 (1996)] among typical conventional discharges exhibiting large sawtooth crashes. The reason for the spontaneous appearance of sawtooth-free discharges has been unclear. It is shown that the normalized magnitude of the SXR crashes, [Delta(I-SXR)/I-SXR], is well correlated with the m (poloidal mode number)=0 mode amplitude in the postcrash phase. It is also shown that Delta(I-SXR)/I-SXR is smaller when the quasi-single helicity state precedes the crash rather than when the multiple-helicity state precedes the crash, and Delta(I-SXR)/I-SXR is smaller when E-parallel to (the parallel electric field at the plasma surface) is approximately zero in the precrash phase rather than E-parallel to being negative.
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