Omega Beam Research Articles

Nonuniform Absorption and Scattered Light in Direct-Drive Implosions Driven by Polarization Smoothing.

Laser-direct-drive symmetric implosions on OMEGA illuminate a target with 60 laser beams and are designed to produce spherical implosions. Each beam is smoothed using orthogonal polarizations obtained by passing the laser beams through distributed polarization rotators (DPRs). Observations of light scattered from OMEGA implosions do not show the expected symmetry and have much larger variation than standard predictions. For the first time, we have quantified the scattered-light nonuniformity from individual beams and identified the DPRs as the source of the enhanced nonuniformity. An instrument was invented that isolated and measured the variation in the intensity and polarization of the light scattered from each OMEGA beam. The asymmetric intensity and polarization measurements are explained when the on-target offsets between the two orthogonal polarizations produced by the DPRs are modeled using a 3D cross-beam energy transfer (CBET) code that tracks the polarizations of each beam. The time-integrated nonuniformity in laser absorption and scattered light due to CBET and the DPR polarization offsets during high-performance OMEGA implosions is predicted to be significant and dominated by low spherical harmonic mode numbers. The nonuniformity is predicted to be greatly reduced by replacing the DPRs with new optics that create smaller offsets.

Development of bridge precast beams from high performance concrete in EUROVIA CS

The paper presents a summary of the development of bridge precast beams in EUROVIA CS company in the last years. Several types of beams are described. The first beam that has already been applied in real construction is an Omega beam which consists of shell outside envelope enabling construction of a bridge without intermediate supports in midspan. The second one is an inverted T beam for railway and road bridges. At present, the development of UHPC beams for footbridges is going on. The plan is to create a set of shapes and solutions usable for various configurations with spans up to 15 m, fully exploiting the UHPC advantages. Another element for bridge construction developed by Eurovia and its partners is the lost formwork panel used for bridge superstructures built from both concrete and steel beams.

Laser entrance window transmission and reflection measurements for preheating in magnetized liner inertial fusion

Laser-driven magnetized liner inertial fusion (MagLIF) is being developed on the OMEGA Laser System to study scaling. MagLIF targets require a preheat laser entrance window that can hold the gas in the target yet allow sufficient laser energy to enter the gas. For OMEGA MagLIF targets, 1.8-μm-thick polyimide foils were found to be sufficient to hold a fuel pressure of up to 14 atm. Transmission and reflection of an OMEGA beam incident on such foils were measured with a calorimeter and time-resolved spectrometers for 2.5-ns square-shaped pulses, with energies from 60 to 200 J, focused to intensities from 0.65 to 2.2 × 1014 W/cm2. The laser energy transmitted in every case exceeded that required to achieve the goal of preheating the gas to 100 eV. The time-resolved measurements showed an initial period with very low, decreasing transmission, the duration of which decreased with increasing intensity, followed by a rapid transition to full transmission, accompanied by brief sidescattering of the transmitted light with a significant red shift. Reflection was always negligible. Two-dimensional radiation–hydrodynamic simulations, using 3-D ray tracing with inverse bremsstrahlung energy deposition, did not capture the rapid transition to full transmission, showing instead a slow increase in transmission, without significant sidescatter or red shift. We propose that full transmission is achieved by self-focusing followed by ponderomotive blowout of the plasma.

FEM simulation of static loading test of the Omega beam

The paper deals with a FEM simulation of static loading test of the Omega beam. Omega beam is a precast prestressed high-performance concrete element with the shape of Greek letter omega. Omega beam was designed as a self-supporting permanent formwork member for construction of girder bridges. FEM program ATENA Science was exploited for simulation of load-bearing test of the beam. The numerical model was calibrated using the data from both static loading test and tests of material properties. Comparison of load-displacement diagrams obtained from the experiment and the model was conducted. Development of cracks and crack patterns were compared. Very good agreement of experimental data and the FEM model was reached. The calibrated model can be used for design of optimized Omega beams in the future without the need of expensive loading tests. The calibrated material model can be also exploited in other types of FEM analyses of bridges constructed with the use of Omega beams, such as limit state analysis, optimization of shear connectors, prediction of long-term deflections or prediction of crack development.

Lightweight HPC beam OMEGA

In the design and construction of precast bridge structures, a general goal is to achieve the maximum possible span length. Often, the weight of individual beams makes them difficult to handle, which may be a limiting factor in achieving the desired span. The design of the OMEGA beam aims to solve a part of these problems. It is a thin-walled shell made of prestressed high-performance concrete (HPC) in the shape of inverted Ω character. The concrete shell with prestressed strands is fitted with a non-stressed tendon already in the casting yard and is more easily transported and installed on the site. The shells are subsequently completed with mild steel reinforcement and cores are cast in situ together with the deck. The OMEGA beams can also be used as an alternative to steel - concrete composite bridges. Due to the higher production complexity, OMEGA beam can hardly substitute conventional prestressed beams like T or PETRA completely, but it can be a useful alternative for specific construction needs.

Laser-driven magnetized liner inertial fusion

A laser-driven, magnetized liner inertial fusion (MagLIF) experiment is designed for the OMEGA Laser System by scaling down the Z point design to provide the first experimental data on MagLIF scaling. OMEGA delivers roughly 1000× less energy than Z, so target linear dimensions are reduced by factors of ∼10. Magneto-inertial fusion electrical discharge system could provide an axial magnetic field of 10 T. Two-dimensional hydrocode modeling indicates that a single OMEGA beam can preheat the fuel to a mean temperature of ∼200 eV, limited by mix caused by heat flow into the wall. One-dimensional magnetohydrodynamic (MHD) modeling is used to determine the pulse duration and fuel density that optimize neutron yield at a fuel convergence ratio of roughly 25 or less, matching the Z point design, for a range of shell thicknesses. A relatively thinner shell, giving a higher implosion velocity, is required to give adequate fuel heating on OMEGA compared to Z because of the increase in thermal losses in smaller targets. Two-dimensional MHD modeling of the point design gives roughly a 50% reduction in compressed density, temperature, and magnetic field from 1-D because of end losses. Scaling up the OMEGA point design to the MJ laser energy available on the National Ignition Facility gives a 500-fold increase in neutron yield in 1-D modeling.

Laser-driven magnetized liner inertial fusion on OMEGA

Magneto-inertial fusion (MIF) combines the compression of fusion fuel, a hallmark of inertial confinement fusion (ICF), with strongly magnetized plasmas that suppress electron heat losses, a hallmark of magnetic fusion. It can reduce the traditional velocity, pressure, and convergence ratio requirements of ICF. The magnetized liner inertial fusion (MagLIF) concept being studied at the Z Pulsed-Power Facility is a key target concept in the U.S. ICF Program. Laser-driven MagLIF is being developed on OMEGA to test the scaling of MagLIF over a range of absorbed energy of the order of 1 kJ on OMEGA to 500 kJ on Z. It is also valuable as a platform for studying the key physics of MIF. An energy-scaled point design has been developed for OMEGA that is roughly 10 × smaller in linear dimensions than Z MagLIF targets. A 0.6-mm-outer-diameter plastic cylinder filled with 2.4 mg/cm3 of D2 is placed in a ∼10-T axial magnetic field, generated by a Magneto-inertial fusion electrical discharge system, the cylinder is compressed by 40 OMEGA beams, and the gas fill is preheated by a single OMEGA beam propagating along the axis. Preheating to >100 eV and axially uniform compression over 0.7 mm have been demonstrated, separately, in a series of preparatory experiments that meet our initial expectations. The preliminary results from the first integrated experiments combining magnetization, compression, and preheat demonstrating a roughly 2 x increase in the neutron yield will be reported here for the first time.

부분매입형 신형상 합성보의 단면성능에 관한 연구

The demand for the structural system of reduction of story height increases because buildings are getting higher. The existing structural systems are not efficiency. Thus, it is hard to reduce the story height and existing methods cannot secure economics as expected. This study aims at developing the partially concrete-filled new type composite beam, which can efficiently resist against the end negative moment and central positive moment, also reduce deflection of beams. Through case studies on loading of concentrated load and uniformly distributed load to fixed beam, we could find the most efficient ratio of moment of inertia and the ratio <TEX>${\alpha}$</TEX>(end beam length to span). The gap space between middle and end beam can be used as facilities installation, consequently the suggested Omega beam system is expected to get the effect of reduction in story height as well as reduction of quantity.

Multi-keV x-ray sources from HYBRID targets on GEKKO and OMEGA facilities

The feasibility of efficient X-ray sources for radiography on the LMJ (Laser MegaJoule) in the multi-kJ/ns range was demonstrated on the OMEGA laser facility (Univ. Rochester) from 2002 to 2004 [1,2]. We significantly enhanced the conversion efficiency of titanium (4–6 keV), copper (8–10 keV) and germanium (9–13 keV) foils by using an optimized pre-pulse/pulse combination. Since higher X-ray energy and therefore electronic temperature need hydroconfinement, plastic cylindrical hohlraums internally coated with titanium, copper and germanium with various OMEGA beam configurations were successfully tested from 2005 to 2009 [3–5]. In addition, many shots with metal-doped aerogel (Ti, Fe, Ge) were tested on OMEGA [6].Recently we tested a new concept of “HYBRID sources” based on the combination of a thin titanium foil at the exit hole of a plastic cylinder filled with very low density SiO 2 aerogel (2 and 5 mg/cc). The benefit of the underdense medium is, first, to transport the laser energy to the titanium foil after its conversion into a supersonic ionization front and, second, to prevent foil expansion and excessive kinetic energy losses by longitudinal hydroconfinement.

Recent progress in metal-lined cylindrical as efficient x-ray sources

Feasability of efficient X-ray sources for LMJ (LaserMégaJoule) targets radiography in the multi-keV/ns regime was demonstrated on OMEGA laser facility (University of Rochester) from 2002 to 2004 [1][2][3]. We significantly enhanced the conversion efficiency of titanium, copper and germanium foils using an optimized prepulse/pulse combination [4]. Since higher X-ray energy and therefore high electronic temperature require more confinement, we built and successfully tested in 2005, plastic cylindrical hohlraums internally coated with titanium in various OMEGA beam configurations, pulse types (with and without prepulse) and target designs. The conversion efficiency (CE), depends on hohlraum length and diameter and the highest CE was measured above 17%, which is better than any other x-ray sources in this photon energy range (i.e. 4.7 keV). The best experimental setup was a 2-cone irradiation scheme without prepulse i.e. the simplest and the most economic configuration in view of radiographic purposes. These studies were carried on in february 2007 with Ge-lined hohlraums and Ti-lined halfraums. We describe and show comparisons between experimental results (time integrated and resolved x-ray imaging, pinholes and x-ray diode) with 2D hydrorad simulations.

Experimental measurements of bifurcations, chaos and three cycle behaviour on a neutralized electron beam

An experiment involving the natural oscillations on an electron beam propagating parallel to a magnetic field in a low pressure gas is described. For conditions such that omega pe2<< omega ge2 and beam currents above a few mu A, the beam system behaves as if it were approximately neutralized by cold ions. Increasing the beam current, Ib, increases the frequency and amplitude of the oscillation until period doubling occurs. With a further small increase, many more period doublings occur in rapid succession until the spectrum of the oscillations is essentially flat out to the original frequency f0 above which it rapidly decreases. This state remains as Ib is increased until, suddenly, a perfectly coherent three cycle oscillation appears with a periodicity of three times that of f0; with a small further increase of Ib, the three cycle becomes a 6 cycle and then the chaotic regime returns. The frequency f0 agrees well with the ion plasma frequency of the neutralized electron beam and is probably driven by some form of shear instability. By considering only the amplitude of the oscillations the evolution of the system has been successfully modelled with a simple one dimensional difference equation which takes account of the competition between exponential growth of the instability and quadratic saturation leading to the final steady state.