Browsing by Subject "pulses"
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Item All-optical suppression of relativistic self-focusing of laser beams in plasmas(2008-11) Kalmykov, S. Yu; Yi, S. Austin; Shvets, G.; Kalmykov, S. Yu; Yi, S. Austin; Shvets, G.It is demonstrated that a catastrophic relativistic self-focusing (RSF) of a high-power laser pulse can be prevented all-optically by a second, much weaker, copropagating pulse. RSF suppression occurs when the difference frequency of the pulses slightly exceeds the electron plasma frequency. The mutual defocusing is caused by the three-dimensional electron density perturbation driven by the laser beat wave slightly above the plasma resonance. A bienvelope model describing the early stage of the mutual defocusing is derived and analyzed. Later stages, characterized by the presence of a strong electromagnetic cascade, are investigated numerically. Stable propagation of the laser pulse with weakly varying spot size and peak amplitude over several Rayleigh lengths is predicted.Item Characterization of two distinct, simultaneous hot electron beams in intense laser-solid interactions(2009-11) Cho, B. I.; Osterholz, J.; Bernstein, A. C.; Dyer, G. M.; Karmakar, A.; Pukhov, A.; Ditmire, T.; Cho, B. I.; Osterholz, J.; Bernstein, A. C.; Dyer, G. M.; Ditmire, T.The transport of energetic electron beams generated from aluminum foils irradiated by ultraintense laser pulses has been studied by imaging coherent transition radiation from the rear side of the target. Two distinct beams of MeV electrons are emitted from the target rear side at the same time. This measurement indicates that two different mechanisms, namely resonance absorption and jxB heating, accelerate the electrons at the targets front side and drive them to different directions, with different temperatures. This interpretation is consistent with 3D-particle-in-cell simulations.Item Fast-electron-relaxation measurement for laser-solid interaction at relativistic laser intensities(2007-11) Chen, H.; Shepherd, R.; Chung, H. K.; Kemp, A.; Hansen, S. B.; Wilks, S. C.; Ping, Y.; Widmann, K.; Fournier, K. B.; Dyer, G.; Faenov, A.; Pikuz, T.; Beiersdorfer, P.; Dyer, G.We present measurements of the fast-electron-relaxation time in short-pulse (0.5 ps) laser-solid interactions for laser intensities of 10(17), 10(18), and 10(19) W/cm(2), using a picosecond time-resolved x-ray spectrometer and a time-integrated electron spectrometer. We find that the laser coupling to hot electrons increases as the laser intensity becomes relativistic, and that the thermalization of fast electrons occurs over time scales on the order of 10 ps at all laser intensities. The experimental data are analyzed using a combination of models that include K alpha generation, collisional coupling, and plasma expansion.Item Measurement of the equation of state of solid-density copper heated with laser-accelerated protons(2017-03) Feldman, S.; Dyer, G.; Kuk, D.; Ditmire, T.; Feldman, S.; Dyer, G.; Kuk, D.; Ditmire, T.We present equation of state (EOS) measurements of solid-density copper heated to 5-10 eV. A copper sample was heated isochorically by hydrogen ions accelerated from an adjacent foil by a high intensity pulsed laser, and probed optically. The measured temperature and expansion are compared against simulations using the most up-to-date wide range EOS tables available.Item Optimization Of Laser Wakefield Acceleration(2001) Rundquist, A. R.; LeBlanc, S. P.; Gaul, E. W.; Cheshkov, S.; Grigsby, F. B.; Tajima, T. T.; Downer, M. C.; Rundquist, A. R.; LeBlanc, S. P.; Gaul, E. W.; Cheshkov, S.; Grigsby, F. B.; Tajima, T. T.; Downer, M. C.Using an evolutionary strategy algorithm, we optimize the generalized transformer ration of a laser wakefield accelerator. The algorithm tests several realistic pulse shapes by integrating the fluid wakefield differential equation and it converges to the shape that most efficiently produces a strong accelerating gradient while experiencing minimal distortion.Item Optimization of the neutron yield in fusion plasmas produced by Coulomb explosions of deuterium clusters irradiated by a petawatt laser(2013-02) Bang, W.; Dyer, G.; Quevedo, H. J.; Bernstein, A. C.; Gaul, E.; Donovan, M.; Ditmire, T.; Bang, W.; Dyer, G.; Quevedo, H. J.; Bernstein, A. C.; Gaul, E.; Donovan, M.; Ditmire, T.The kinetic energy of hot (multi-keV) ions from the laser-driven Coulomb explosion of deuterium clusters and the resulting fusion yield in plasmas formed from these exploding clusters has been investigated under a variety of conditions using the Texas Petawatt laser. An optimum laser intensity was found for producing neutrons in these cluster fusion plasmas with corresponding average ion energies of 14 keV. The substantial volume (1-10 mm(3)) of the laser-cluster interaction produced by the petawatt peak power laser pulse led to a fusion yield of 1.6x10(7) neutrons in a single shot with a 120 J, 170 fs laser pulse. Possible effects of prepulses are discussed. DOI: 10.1103/PhysRevE.87.023106Item Parametric Amplification Of Laser-Driven Acceleration In A Plasma Channel(2012-06) Arefiev, A. V.; Schollmeier, M.; Khudik, V. N.; Arefiev, Alexey V.; Khudik, Vladimir N.Two-dimensional particle-in-cell simulations are presented for a laser-irradiated solid-density target with and without an underdense preplasma. It is shown that an underdense preplasma can generate an energetic electron tail in addition to the warm electrons generated at the critical surface. Preplasma electrons are accelerated in a quasi-static positively charged channel formed by the laser. At ultra-relativistic laser intensities (a(0) = 10), the acceleration mechanism is not sensitive to the laser polarization. An energetic tail with energies significantly exceeding the energy expected for a single electron in a vacuum is present in simulations with s and p-polarized beams. This suggests that the mechanism of parametric amplification of laser-driven electron acceleration is a likely explanation for the observed phenomenon.Item Plasma acceleration and cooling by strong laser field due to the action of radiation reaction force(2008-12) Berezhiani, V. I.; Mahajan, S. M.; Yoshida, Z.; Mahajan, S. MIt is shown that for super intense laser pulses propagating in a hot plasma, the action of the radiation reaction force (appropriately incorporated into the equations of motion) causes strong bulk plasma motion with the kinetic energy raised even to relativistic values; the increase in bulk energy is accompanied by a corresponding cooling (intense cooling) of the plasma. The effects are demonstrated through explicit analytical calculations.Item Surveying The Dynamic Radio Sky With The Long Wavelength Demonstrator Array(2010-12) Lazio, T. Joseph W.; Clarke, Tracy E.; Lane, W. M.; Gross, C.; Kassim, N. E.; Ray, P. S.; Wood, D.; York, J. A.; Kerkhoff, A.; Hicks, B.; Polisensky, E.; Stewart, K.; Dalal, N. P.; Cohen, A. S.; Erickson, W. C.; York, J. A.; Kerkhoff, A.This paper presents a search for radio transients at a frequency of 73.8 MHz (4 m wavelength) using the all-sky imaging capabilities of the Long Wavelength Demonstrator Array (LWDA). The LWDA was a 16-dipole phased array telescope, located on the site of the Very Large Array in New Mexico. The field of view of the individual dipoles was essentially the entire sky, and the number of dipoles was sufficiently small that a simple software correlator could be used to make all-sky images. From 2006 October to 2007 February, we conducted an all-sky transient search program, acquiring a total of 106 hr of data; the time sampling varied, being 5 minutes at the start of the program and improving to 2 minutes by the end of the program. We were able to detect solar flares, and in a special-purpose mode, radio reflections from ionized meteor trails during the 2006 Leonid meteor shower. We detected no transients originating outside of the solar system above a flux density limit of 500 Jy, equivalent to a limit of no more than about 10(-2) events yr(-1) deg(-2), having a pulse energy density greater than or similar to 1.5 x 10(-20) J m(-2) Hz(-1) at 73.8 MHz for pulse widths of about 300 s. This event rate is comparable to that determined from previous all-sky transient searches, but at a lower frequency than most previous all-sky searches. We believe that the LWDA illustrates how an all-sky imaging mode could be a useful operational model for low-frequency instruments such as the Low Frequency Array, the Long Wavelength Array station, the low-frequency component of the Square Kilometre Array, and potentially the Lunar Radio Array.Item Time dependence of fast electron beam divergence in ultraintense laser-plasma interactions(2012-08) Akli, K. U.; Storm, M. J.; McMahon, M.; Jiang, S.; Ovchinnikov, V.; Schumacher, D. W.; Freeman, R. R.; Dyer, G.; Ditmire, T.; Dyer, G.; Ditmire, T.We report on the measurement and computer simulation of the divergence of fast electrons generated in an ultraintense laser-plasma interaction (LPI) and the subsequent propagation in a nonrefluxing target. We show that, at I lambda(2) of 10(20) Wcm(-2) mu m(2), the time-integrated electron beam full divergence angle is (60 +/- 5)degrees. However, our time-resolved 2D particle-in-cell simulations show the initial beam divergence to be much smaller (<= 30 degrees). Our simulations show the divergence to monotonically increase with time, reaching a final value of (68 +/- 7)degrees after the passage of the laser pulse, consistent with the experimental time-integrated measurements. By revealing the time-dependent nature of the LPI, we find that a substantial fraction of the laser energy (similar to 7%) is transported up to 100 mu m with a divergence of 32 degrees.Item Ultrafast Radial Transport In A Micron-Scale Aluminum Plasma Excited At Relativistic Intensity(2004-06) Bowes, B. T.; Downer, M. C.; Langhoff, H.; Wilcox, M.; Hou, B.; Nees, J.; Mourou, G.; Bowes, B. T.; Downer, M. C.; Langhoff, H.; Wilcox, M.; Hou, B.; Nees, J.; Mourou, G.Using femtosecond microscopy, we observe a thermal/ionization front expand radially at similar to10(8)cm/s from a lambda(2)-size spot of an aluminum target excited at >10(18)W/cm(2). Numerical modeling shows transport is predominantly radiative and may be initially nonlocal.Item Vortex bubble formation in pair plasmas(2013-07) Berezhiani, V. I.; Shatashvili, N. L.; Mahajan, S. M.; Aleksic, B. N.; Mahajan, S. M.It is shown that delocalized vortex solitons in relativistic pair plasmas with small temperature asymmetries can be unstable for intermediate intensities of the background electromagnetic field. Instability leads to the generation of ever-expanding cavitating bubbles in which the electromagnetic fields are zero. The existence of such electromagnetic bubbles is demonstrated by qualitative arguments based on a hydrodynamic analogy, and by numerical solutions of the appropriate nonlinear Schrodinger equation with a saturating nonlinearity.