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Papers by Karnig Mikaelian

Physics of Plasmas, 2006

Radial profiles of nuclear burn in directly driven, inertial-confinement-fusion implosions have b... more Radial profiles of nuclear burn in directly driven, inertial-confinement-fusion implosions have been systematically studied for the first time using a proton emission imaging system sensitive to energetic 14.7MeV protons from the fusion of deuterium (D) and 3-helium (He3) at the OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Experimental parameters that were varied include capsule size, shell composition and thickness, gas fill pressure, and laser energy. Clear relationships have been identified between changes in a number of these parameters and changes in the size of the burn region, which we characterize here by the median “burn radius” Rburn containing half of the total DHe3 reactions. Different laser and capsule parameters resulted in burn radii varying from 20to80μm. For example, reducing the DHe3 fill pressure from 18to3.6atm in capsules with 20μm thick CH shells resulted in Rburn changing from 31to25μm; this reduction is attributed to increased fue...

We consider the photon-photon channel, gamma + gamma yields e/sup +/ + e/sup -/ + e/s up +/ + e/s... more We consider the photon-photon channel, gamma + gamma yields e/sup +/ + e/sup -/ + e/s up +/ + e/sup -/, with respect to the possible relevant experimental roles it plays in (i) the photoproduction process gamma + Z yields Z + e/sup +/ + e/sup -/ + e/sup +/ + e/sup -/, (ii) the colliding beam process e/sup plus or minus / + e/sup -/ yields e/sup plus or minus / + e/sup +/ + e/sup -/ + e/sup +/ + e/sup -/, and (iii) the absorption of high-energy cosmic photons. Discussion of the first role includes calculation of rates at different energies, enor estimates, and effects of screening and threshold cuts. In the second role, emphasis is placed on scale changes due to energy and angle kinematic cuts. Competition with the single-pair absorption process (i.e., gamma + gamma yields e/sup +/ + e/sup -/) at the higher cosmic gamma -ray e nergies ole. (auth)

APS Division of Plasma Physics Meeting Abstracts, Oct 1, 2001

ABSTRACT

Bulletin of the American Physical Society, 2019

Bulletin of the American Physical Society, 2016

Ablative Richtmyer-Meshkov (ARM) instability develops while a strong radiation pulse, rapidly ris... more Ablative Richtmyer-Meshkov (ARM) instability develops while a strong radiation pulse, rapidly rising to its constant peak intensity, drives a constant-strength shock wave from the rippled irradiated surface of a solid target into its volume. For the direct laser irradiation, the theory, experiment, and simulations have demonstrated that the development of the ARM results in decaying oscillations of the areal mass/optical thickness modulation amplitude. Much less is known about the ARM with the indirect drive. This effect causing oscillations of the ablation front is the physical basis of the recently proposed [D. S. Clark et al., Phys. Plasmas 21, 112705 (2014)] and successfully demonstrated [H. F. Robey et al., Phys. Plasmas 23, 056303 (2016)] adiabat-shaping approach to improving the NIF target performance. We report a theoretical and numerical stability analysis of the indirectly-driven shock-piston flow performed to investigate the physical mechanism of the ablation-front oscillations detected in the simulations and the NIF experiments on adiabat shaping.

We apply the Canuto-Goldman (CG) analytical model to turbulence generated by the class of instabi... more We apply the Canuto-Goldman (CG) analytical model to turbulence generated by the class of instabilities which have a power-law growth rate. For the case of Rayleigh-Taylor (RT) instability we compare the results of the model with the experiments of Read and Youngs and find reasonable agreement on the scale size of large eddies: they are predicted to be a0% of the mixing length and are at least an order of magnitude larger than the smallest eddies. We interpret this as chunk mix. Based on the experiments of Read and Yotmgs we develop a simple model for turbulence generated by Richtmyer-Meshkov (RIM) instabilities. The CG method applied to our model predicts that the relative sizes of large eddies and small eddies do not vary by more than 60%. We interpret this as atomic mix. Independently of the CG model we find that the ratio of turbulent to directed energy is about 2~ and 9% for RT and RM respectively, and varies as the square of the Atwood number in both cases.

Physical Review A, 1984

The growth of Rayleigh-Taylor instabilities in inertial-confinement-fusion targets can be reduced... more The growth of Rayleigh-Taylor instabilities in inertial-confinement-fusion targets can be reduced if smooth density gradients are introduced at the interfaces. Keeping the total mass fixed, we consider spreading a heavy shell either continuously or in a discrete number of subshells. We calculate the rate for the fastest growing mode as a function of the fraction of mass spread and find the minima for the cases of 4, 6, and 8 subshells. The rates are reduced by 1.4-1.6. If all the mass is spread continuously into an exponential profile, we find that the rates are reduced by approximately V a~=2.5.

Le Journal de Physique Colloques, 1974

CONTRIBUTIONS (where X is any C = + state) or amplitudes ; subtractions have then been fixed impo... more CONTRIBUTIONS (where X is any C = + state) or amplitudes ; subtractions have then been fixed imposp + p + X 1 + y * + X 2 + y *-+ X 1 + ing off-mass-shell current algebra limits both for one + x2 + y + + p-and two massless external mesons. Predictions on the S wave amplitudes have also been derived from (where XI, ' 2 are hadron states) is reviewed in Some the recent analysis of meson-meson scattering by detail. Both the theoretical aspects and the experi-Protopopescu ol mental feasibility of various processes are discussed especially for experimentalists' convenience. The equivalent polarised-photon approximation Absorption of high-energy cosmic photons through double pair production in photon-photon collisions

Physical Review E, 2016

We consider viscous fluids in spherical geometry, a lighter fluid supporting a heavier one. Chand... more We consider viscous fluids in spherical geometry, a lighter fluid supporting a heavier one. Chandrasekhar (Quart. J. Mech. Appl. Math. 8, 1 (1955)) analyzed this unstable configuration providing the equations needed to find, numerically, the exact growth rates for the ensuing Rayleigh-Taylor instability. He also derived an analytic, but approximate solution. We point out a weakness in his approximate dispersion relation (DR) and offer a somewhat improved one. A third DR, based on transforming a planar DR into a spherical one, suffers no unphysical predictions and compares reasonably well with the exact work of Chandrasekhar and a more recent numerical analysis of the problem (G.

Physical Review Fluids, 2016

In a typical Richtmyer-Meshkov experiment a fast moving flat shock strikes a stationary perturbed... more In a typical Richtmyer-Meshkov experiment a fast moving flat shock strikes a stationary perturbed interface between fluids A and B creating a transmitted and a reflected shock, both of which are perturbed. We propose shock tube experiments in which the reflected shock is stationary in the laboratory. Such a standing perturbed shock undergoes well known damped oscillations. We present the conditions required for producing such a standing shock wave which greatly facilitates the measurement of the oscillations and their rate of damping. We define a critical density ratio R critical in terms of the adiabatic indices of the two fluids, and a critical Mach number M s critical of the incident shock wave which produces a standing reflected wave. If the initial density ratio R of the two fluids is less than critical R then a standing shock wave is possible at M s =M s critical. Otherwise a standing shock is not possible and the reflected wave always moves in the direction opposite the incident shock. Examples are given for present-day operating shock tubes with sinusoidal or inclined interfaces. We consider the effect of viscosity which affects the damping rate of the oscillations. We point out that nonlinear bubble and spike amplitudes depend relatively weakly on the viscosity of the fluids, and that the interface area is a better diagnostic.

Physikalisch Chemische Trenn Und Messmethoden, Nov 1, 1988

We have carried out a number of 2D numerical simulations on an ALE code for shock-tube experiment... more We have carried out a number of 2D numerical simulations on an ALE code for shock-tube experiments in which a shock crosses one or more contact discontinuities and, after traveling through a homogeneous medium, reflects off a rigid wall at the end of the shock-tube and re-crosses the contact discontinuity. We have considered two-fluid and three-fluid experiments: the first fluid, which carries the original shock, is air; the other fluids are helium, freon, SF/sub 6/, or air again. Helium is lighter than air, while freon and SF/sub 6/ are heavier than air. The interface(s) between the fluids serve as contact discontinuities and are subjected to the original shock, the re-shock, and subsequent rarefactions/compressions. 9 refs., 6 figs.