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Now showing items 49 - 64 of 8240

  • An ultra-short and TeV quasi-monoenergetic ion beam generation by laser wakefield accelerator in the snowplow regime

    Zheng, F. L.   Wu, S. Z.   Zhou, C. T.   Wang, H. Y.   Yan, X. Q.   He, X. T.  

    A new laser-plasma ion acceleration mechanism, snowplow ion acceleration, is proposed using an ultra-relativistically intense laser pulse irradiating on a combination target. When the thickness of the foil D is less than the length where the double-layer consisting of electron and ion layers is formed, the relativistic ion beam pre-accelerated by radiation pressure acceleration can be trapped and accelerated to the TeV level by the laser plasma wakefield over a long distance in the snowplow regime. Based on the classic wakefield theory, the snowplow structure can control the beam quality in terms of pulse duration, leading to the fact that the heavy-ion beam is theoretically shorter than half of the total wakefield structure, the size of the acceleration region. An analytical model is established, suggesting that ultra-short (70 mu m) and ultra-highly energetic (3.2 TeV) carbon ion bunches generated by a centimeter-scale laser wakefield acceleration are expected, driven by a circularly polarized (CP) laser pulse with intensity of 10(23) W/cm(2) and duration of 66 fs. The particle-in-cell simulations agree well with theoretical results. Copyright (C) EPLA, 2011
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  • Characterization of Chemically Fixed Liquid Anhydrous Ammonia in an Illinois Drummer Soil

    He, X. T.   Mulvaney, R. L.   Stevenson, F. J.   Vanden Heuvel, R. M.  

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  • The interplay between the kinetic nonlinear frequency shift and the flowing gradient in stimulated Brillouin scattering

    Wang, Q.   Zheng, C. Y.   Liu, Z. J.   Xiao, C. Z.   Feng, Q. S.   Zhang, H. C.   He, X. T.  

    The effect of the kinetic nonlinear frequency shift (KNFS) on backward stimulated Brillouin scattering (SBS) in homogeneous plasmas and inhomogeneous flowing plasmas is investigated by three-wave coupled-mode equations. When the positive contribution to the KNFS from electrons as well as the negative contribution from ions is included, the net KNFS can become positive at a large electron-ion temperature ratio ZT(e)/T-i. In homogeneous plasmas, KNFS can greatly reduce the SBS reflectivity at low or large ZT(e)/T-i but has a weak effect on SBS ZT(e)/T-i where the positive frequency shifts from electrons almost cancels out the negative shifts from ions. In inhomogeneous plasmas, the net negative frequency shift can enhance the backward SBS reflectivity for the negative gradient of the plasma flowing, and can suppress the reflectivity for the positive case. On the contrary, the net positive frequency can suppress the reflectivity for the negative case of the flowing gradient and enhance the reflectivity for the positive case. This indicates that the SBS in inhomogeneous flowing plasmas can be controlled by changing the sign of the nonlinear frequency shift.
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  • Study of strong enhancement of synchrotron radiation via surface plasma waves excitation by particle-in-cell simulations

    Pan, K. Q.   Zheng, C. Y.   Wu, Dong   Cao, L. H.   Liu, Z. J.   He, X. T.  

    Synchrotron radiation is strongly enhanced by the resonant excitation of surface plasma waves (SPWs). Two-dimensional particle-in-cell simulations show that energy conversion efficiency from laser to radiation in the case of SPWs excitation is about 18.7%, which is improved by more than 2 orders of magnitude compared with that of no SPWs excitation. Besides the high energy conversion efficiency, the frequency spectrum and the angular distribution of the radiation are also improved in the case of SPWs excitation because of the quasi-static magnet field induced by surface plasma waves excitation. (C) 2015 AIP Publishing LLC.
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  • Generation of high-energy mono-energetic heavy ion beams by radiation pressure acceleration of ultra-intense laser pulses

    Wu, D.   Qiao, B.   McGuffey, C.   He, X. T.   Beg, F. N.  

    Generation of high-energy mono-energetic heavy ion beams by radiation pressure acceleration (RPA) of intense laser pulses is investigated. Different from previously studied RPA of protons or light ions, the dynamic ionization of high-Z atoms can stabilize the heavy ion acceleration. A self-organized, stable RPA scheme specifically for heavy ion beams is proposed, where the laser peak intensity is required to match with the large ionization energy gap when the successive ionization state passes the noble gas configurations [ such as removing an electron from the helium-like charge state (Z - 2)(+) to (Z - 1)(+)]. Two-dimensional particle-in-cell simulations show that a mono-energetic Al13+ beam with peak energy 1.0 GeV and energy spread of only 5% can be obtained at intensity of 7 x 10(20)W/cm(2) through the proposed scheme. A heavier, mono-energetic, ion beam (Fe26+) can attain a peak energy of 17 GeV by increasing the intensity to 10(22)W/cm(2). (C) 2014 AIP Publishing LLC.
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  • Stimulated Raman scattering instability of a left-handed circularly polarized laser in strongly axially magnetized plasmas

    Pan, K. Q.   Guo, L.   Li, Z. C.   Yang, D.   Li, S. W.   Jiang, S. E.   Zhang, B. H.   Zheng, C. Y.   He, X. T.  

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  • Nonlinear transition from convective to absolute Raman instability with trapped electrons and inflationary growth of reflectivity

    Wang, Y. X.   Wang, Q.   Zheng, C. Y.   Liu, Z. J.   Liu, C. S.   He, X. T.  

    We propose a nonlinear mechanism for transition from convective to absolute in stimulated Raman backscattering instability due to the effect of trapped particles in the plasma wave. Convective instability saturates at the low level, yet it is sufficient to trap electrons near the plasma phase velocity. The trapped electrons tend to flatten the distribution function. With spatial averaging over the trapped region, we find that the flattened distribution function reduces the damping rate due to bounce resonance of the plasma wave and then decreases the threshold for absolute instability. So the transition from a weak, convective instability to a strong, absolute instability can occur, leading to exponential growth everywhere and inflation of reflectivity of several orders of magnitude as observed in the experiment, once the threshold is exceeded. Published by AIP Publishing.
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  • ‘Well, he would say that, wouldn\"t he?’

    Hastings   Gerard  

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  • Lin He: \"Junk\" DNA isn\"t

    Sedwick   C.  

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  • Qualit?t mit Praxisn?he ist gefragt!

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  • Rotational excitation of cyanogen ion, CN+ (X 1Σ+) by He collisions

    Anusuri, Bhargava  

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  • A Borel chain condition of T(X)

    Todorcevic, S.; Xiao, M.  

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  • Sample(x)=(a*x<=t) Is a Distinguisher with Probability 1/8

    Thorup, Mikkel  

    A random sampling function Sample : U -> {0, 1} for a key universe U is a distinguisher with probability alpha if for any given assignment of values v(x) to the keys x is an element of U, including at least one nonzero v(x) not equal 0, the sampled sum Sigma(x is an element of U) Sample(x)v(x) is nonzero with probability at least alpha. Here the key values may come from any commutative monoid (addition is commutative and associative and zero is neutral). Such distinguishers were introduced by Vazirani [Randomness, Adversaries, and Computation, Ph.D. thesis, University of California, Berkeley, CA, 1986], and Naor and Naor used them for their small-bias probability spaces [SIAM T. Comput., 22 (1993), pp. 838-856]. Constant probability distinguishers are used for testing in contexts where the individual key values are not computed directly, yet where the sum is easily computed. A simple example is when we get a stream of key value pairs (x(1), v(1)), (x(2), v(2)), ... ,(x(n), v(n)) where the same key may appear many times. The accumulated value of key x is v(x) =3D Sigma(i is an element of{1, ... ,n},xi=3Dx) v(i), and we want to check if there is any nonzero v(x) (a nonzero could indicate some accounting mistake). For space reasons, we may not be able to maintain v(x) for every key x, but the sampled sum is easily maintained as the single value Sigma(n)(i=3D)(1) Sample(x(i))v(i). Here we show that when dealing with w-bit integers, if a is a uniform odd w-bit integer and t is a uniform w-bit integer, then Sample(x) =3D [ax mod 2(w) <=3D t]is a distinguisher with probability 1/8. Working with standard units, that is, w =3D 8, 16, 32, 64, we exploit that w-bit multiplication works modulo 2(w), discarding overflow automatically, and then the sampling decision is implemented by the C-code a*x<=3Dt. Previous such samplers were much less computer friendly; e.g., the distinguisher of Naor and Naor [SIAM T. Comput., 22 (1993), pp. 838-856] was more complicated and involved a 7-independent hash function.
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  • t(X;11)(q28;p15) NUP98/HMGB3

    Gindina, Tatiana  

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  • Vet can\"t retake test because he didn\"t request that accommodation

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  • Enhancement of the surface emission at the fundamental frequency and the transmitted high-order harmonics by pre-structured targets

    Pan, K. Q.   Yang, D.   Guo, L.   Li, Z. C.   Li, S. W.   Zheng, C. Y.   Jiang, S. E.   Zhang, B. H.   He, X. T.  

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