57 research outputs found

    Undamped electrostatic plasma waves

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    Electrostatic waves in a collision-free unmagnetized plasma of electrons with fixed ions are investigated for electron equilibrium velocity distribution functions that deviate slightly from Maxwellian. Of interest are undamped waves that are the small amplitude limit of nonlinear excitations, such as electron acoustic waves (EAWs). A deviation consisting of a small plateau, a region with zero velocity derivative over a width that is a very small fraction of the electron thermal speed, is shown to give rise to new undamped modes, which here are named {\it corner modes}. The presence of the plateau turns off Landau damping and allows oscillations with phase speeds within the plateau. These undamped waves are obtained in a wide region of the (k,ωR)(k,\omega_{_R}) plane (ωR\omega_{_R} being the real part of the wave frequency and kk the wavenumber), away from the well-known `thumb curve' for Langmuir waves and EAWs based on the Maxwellian. Results of nonlinear Vlasov-Poisson simulations that corroborate the existence of these modes are described. It is also shown that deviations caused by fattening the tail of the distribution shift roots off of the thumb curve toward lower kk-values and chopping the tail shifts them toward higher kk-values. In addition, a rule of thumb is obtained for assessing how the existence of a plateau shifts roots off of the thumb curve. Suggestions are made for interpreting experimental observations of electrostatic waves, such as recent ones in nonneutral plasmas.Comment: 11 pages, 10 figure

    Instability of ion kinetic waves in a weakly ionized plasma

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    The fundamental higher-order Landau plasma modes are known to be generally heavily damped. We show that these modes for the ion component in a weakly ionized plasma can be substantially modified by ion-neutral collisions and a dc electric field driving ion flow so that some of them can become unstable. This instability is expected to naturally occur in presheaths of gas discharges at sufficiently small pressures and thus affect sheaths and discharge structures.Comment: Published in Phys. Rev. E, see http://link.aps.org/doi/10.1103/PhysRevE.85.02641

    Lawson Criterion for Ignition Exceeded in an Inertial Fusion Experiment

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    For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin burn propagation into surrounding cold fuel, enabling the possibility of high energy gain. While scientific breakeven (i.e., unity target gain) has not yet been achieved (here target gain is 0.72, 1.37 MJ of fusion for 1.92 MJ of laser energy), this Letter reports the first controlled fusion experiment, using laser indirect drive, on the National Ignition Facility to produce capsule gain (here 5.8) and reach ignition by nine different formulations of the Lawson criterion

    Mediator Condensates Localize Signaling Factors to Key Cell Identity Genes

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    The gene expression programs that define the identity of each cell are controlled by master transcription factors (TFs) that bind cell-type-specific enhancers, as well as signaling factors, which bring extracellular stimuli to these enhancers. Recent studies have revealed that master TFs form phase-separated condensates with the Mediator coactivator at super-enhancers. Here, we present evidence that signaling factors for the WNT, TGF-β, and JAK/STAT pathways use their intrinsically disordered regions (IDRs) to enter and concentrate in Mediator condensates at super-enhancers. We show that the WNT coactivator β-catenin interacts both with components of condensates and DNA-binding factors to selectively occupy super-enhancer-associated genes. We propose that the cell-type specificity of the response to signaling is mediated in part by the IDRs of the signaling factors, which cause these factors to partition into condensates established by the master TFs and Mediator at genes with prominent roles in cell identity

    Lawson Criterion for Ignition Exceeded in an Inertial Fusion Experiment

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