70 research outputs found
Response to Comment on `Undamped electrostatic plasma waves' [Phys. Plasmas 19, 092103 (2012)]
Numerical and experimental evidence is given for the occurrence of the
plateau states and concomitant corner modes proposed in \cite{valentini12}. It
is argued that these states provide a better description of reality for small
amplitude off-dispersion disturbances than the conventional
Bernstein-Greene-Kruskal or cnoidal states such as those proposed in
\cite{comment
Instability of ion kinetic waves in a weakly ionized plasma
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
Undamped electrostatic plasma waves
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 plane
( being the real part of the wave frequency and 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 -values and chopping the tail shifts them
toward higher -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
Recommended from our members
Observation of resonant energy transfer between identical-frequency laser beams
Enhanced transmission of a low intensity laser beam is observed when crossed with an identical-frequency beam in a plasma with a flow velocity near the ion sound speed. The time history of the enhancement and the dependence on the flow velocity strongly suggest that this is due to energy transfer between the beams via a resonant ion wave with zero frequency in the laboratory frame. The maximum energy transfer has been observed when the beams cross in a region with Mach 1 flow. The addition of frequency modulation on the crossing beams is seen to reduce the energy transfer by a factor of two. Implications for indirect-drive fusion schemes are discussed
Recommended from our members
Plasma diagnostic reflectometry
Theoretical and experimental studies of plasma diagnostic reflectometry have been undertaken as a collaborative research project between the Lawrence Livermore National Laboratory (LLNL) and the University of California Department of Applied Science Plasma Diagnostics Group under the auspices of the Laboratory Directed Research and Development Program at LLNL. Theoretical analyses have explored the basic principles of reflectometry to understand its limitations, to address specific gaps in the understanding of reflectometry measurements in laboratory experiments, and to explore extensions of reflectometry such as ultra-short-pulse reflectometry. The theory has supported basic laboratory reflectometry experiments where reflectometry measurements can be corroborated by independent diagnostic measurements
Lawson Criterion for Ignition Exceeded in an Inertial Fusion Experiment
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
Recommended from our members
Pinhole closure measurements
Spatial-filter pinholes and knife-edge samples were irradiated in vacuum by 1053-nm, 5-20 ns pulses at intensities to 500 GW/cm. The knife-edge samples were fabricated of plastic, carbon, ahnuinum, stainless steel, molybdenum, tantalum, gold and an absorbing glass. Time-resolved two-beam interferometry with a 40-ns probe pulse was used to observe phase shifts in the expanding laser-induced plasma. For all of these materials, at any time during square-pulse irradiation, the phase shift fell exponentially with distance from the edge of the sample. The expansion was characterized by the propagation velocity V2x of the contour for a 2(pi) phase shift. To within experimental error, V2x, was constant during irradiation at a particular intensity, and it increased linearly with intensity for intensities 2. For metal samples, V, exhibited an approximate M-0.5 dependence where M is the atomic mass. Plasmas of plastic, carbon and absorbing glass produced larger phase shifts, and expanded more rapidly, than plasmas of the heavy metals. The probe beam and interferometer were also used to observe the closing of pinholes. With planar pinholes, accumulation of on-axis plasma was observed along with the advance of plasma away from the edge of the hole. On-axis closure was not observed in square, 4-leaf pinholes
Recommended from our members
Strongly-driven laser plasma coupling
An improved understanding of strongly-driven laser plasma coupling is important for optimal use of the National Ignition Facility (NIF) for both inertial fusion and for a variety of advanced applications. Such applications range from high energy x- ray sources and high temperature hohlraums to fast ignition and laser radiography. We discuss a novel model for the scaling of strongly-driven stimulated Brillouin and Raman scattering. This model postulates an intensity dependent correlation length associated with spatial incoherence due to filamentation and stimulated forward scattering. We first motivate the model and then relate it to a variety of experiments. Particular attention is paid to high temperature hohlraum experiments, which exhibited low to modest stimulated Brillouin scattering even though this instability was strongly driven. We also briefly discuss the strongly nonlinear interaction physics for efficient generation of high energy electrons either _ by irradiating a large plasma with near quarter-critical density or by irradiating overdense targets with ultra intense lase
- …