It is generally accepted that magnetic fields generated in the nonlinear development of the transverse Weibel instability provide effective collisionality in unmagnetized collisionless shocks. ...Recently, extensive two- and three-dimensional simulations improved our understanding of the growth and saturation of the instability in colliding plasma shells. However, the steady state structure of the shock wave transition layers remains poorly understood. We use basic physical considerations and order-of-magnitude arguments to study the steady state structure in relativistic unmagnetized collisionless shocks in pair plasmas. The shock contains an electrostatic layer resulting from the formation of stationary, magnetically focused current filaments. The filaments form where the cold upstream plasma and the counterstreaming thermal plasma interpenetrate. The filaments are not entirely neutral, and strong electrostatic fields are present. Most of the downstream particles cannot cross this layer into the upstream medium because they are trapped by the electrostatic field. We identify the critical location in the shock transition layer where the electromagnetic field ceases to be static. At this location, the degree of charge separation in the filaments reaches a maximum value and the current inside the filaments comes close to the Alfven limit. We argue that the radius of the current filaments upstream of the critical location is about equal to the upstream plasma skin depth. Finally, we show that some downstream particles cross the electrostatic layer and run ahead of the shock into the preshock medium without causing instability. These particles may play an important role in diffusive particle acceleration.
Simulations of the interaction region in a photon-photon collider Chen, P; Ohgaki, T; Spitkovsky, A ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
10/1997, Letnik:
397, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The status and initial performance of a simulation program CAIN for interaction region of linear colliders is described. The program is developed to be applicable for e
+e
−, e
−e
−, e
−γ and γγ ...linear colliders. As an example of an application, simulation of a γγ collider option of NLC is reported.
We discuss the preliminary estimates to create Neutron Star atmospheric conditions in the laboratory and the possibility of generating photon bubbles. The minimal requirements for photon-bubble ...instability could potentially be met with a properly configured 10 ps petawatt laser experiment. The high energy (multi-MeV) electrons generated by an ultra-intense laser interacting with a foil are coupled to the electrons in the solid to heat the entire solid generating high thermal temperatures. Small amounts of matter could potentially be heated to ~1 keV temperatures with large radiation temperature. Additionally, 2-D PIC simulations show large B-fields on both the front and back of these targets with B fields consistent with experiments using the petawatt at Rutherford Appleton Laboratory (Tatarakis, M. et al.: 2002c, Nature415, 280).
Collisions of high Mach number flows occur frequently in astrophysics, and the resulting shock waves are responsible for the properties of many astrophysical phenomena, such as supernova remnants, ...Gamma Ray Bursts and jets from Active Galactic Nuclei. Because of the low density of astrophysical plasmas, the mean free path due to Coulomb collisions is typically very large. Therefore, most shock waves in astrophysics are “collisionless”, since they form due to plasma instabilities and self-generated magnetic fields. Laboratory experiments at the laser facilities can achieve the conditions necessary for the formation of collisionless shocks, and will provide a unique avenue for studying the nonlinear physics of collisionless shock waves. We are performing a series of experiments at the Omega and Omega-EP lasers, in Rochester, NY, with the goal of generating collisionless shock conditions by the collision of two high-speed plasma flows resulting from laser ablation of solid targets using ∼10
16 W/cm
2 laser irradiation. The experiments will aim to answer several questions of relevance to collisionless shock physics: the importance of the electromagnetic filamentation (Weibel) instabilities in shock formation, the self-generation of magnetic fields in shocks, the influence of external magnetic fields on shock formation, and the signatures of particle acceleration in shocks. Our first experiments using Thomson scattering diagnostics studied the plasma state from a single foil and from double foils whose flows collide “head-on”. Our data showed that the flow velocity and electron density were 10
8 cm/s and 10
19 cm
−3, respectively, where the Coulomb mean free path is much larger than the size of the interaction region. Simulations of our experimental conditions show that weak Weibel mediated current filamentation and magnetic field generation were likely starting to occur. This paper presents the results from these first Omega experiments.
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