Takuma Endo

Pressure History at the Thrust Wall of a Simplified Pulse Detonation Engine

Gas dynamics in a simplified pulse detonation engine (PDE) was theoretically analyzed. A PDE was simplified as a straight tube with a fixed cross section. One end of the tube was closed, namely, this end was the thrust wall, and the other end was open. A detonation wave was initiated at the closed end and simultaneously started to propagate toward the open end. When the detonation wave broke out from the open end, a rarefaction wave started to propagate from the open end toward the closed end. This rarefaction wave was reflected by the closed end. By considering this rarefaction wave to be self-similar in the analysis of the interference between this rarefaction wave and its reflection from the closed end, we analytically formulated the decay portion of the pressure history at the closed end (thrust wall) without any empirical parameters. By integrating the obtained pressure history at the thrust wall with respect to time, important performance parameters of a PDE were also formulated. The obtained formulas were compared with numerical and experimental results and agreed with them very well.

Numerical Studies on Specific Impulse of Partially Filled Pulse Detonation Rocket Engines

Performance analyses of pulse detonation rocket engines (PDREs) were numerically studied, focusing on partialfill effects at ground tests. The initial detonable mixture, inert gas, fuel-fill fraction, equivalence ratio, and initial temperature of inert gas were changed as governing parameters. The simulation results were compared against those of previous studies and agreed well with them. The simulation results indicated that the initial mass fraction of the detonable mixture to total mass of the gas was the predominant factor for the specific impulse of partially filled PDREs. Based on the numerical results, a new, simple empirical formula is proposed to predict the specific impulse of partially filled PDREs.

Direct-drive hydrodynamic instability experiments on the GEKKO XII laser

Hydrodynamic instabilities, such as the Rayleigh–Taylor (R–T) instability, play a critical role in inertial confinement fusion as they finally cause fuel-pusher mixing that potentially quenches thermonuclear ignition. Good understanding of the instabilities is necessary to limit the mixing within a tolerable level. A series of experiments has been conducted on the GEKKO XII laser facility [C. Yamanaka et al., IEEE J. Quantum Electron. QE-17, 1639 (1981)] to measure hydrodynamic instabilities in planar foils directly irradiated by 0.53 μm laser light. It has been found that (1) the imprint is reasonably explained by an imprint model based on the equation of motion with the pressure perturbation smoothed by the cloudy-day effect, and (2) the experimental R–T growth rate is significantly reduced from the classical growth rate due probably to ablative stabilization enhanced by nonlocal heat transport.

Homogeneous-dilution model of partially fueled simplified pulse detonation engines

A model for estimating the propulsive performance of a partially fueled simplified pulse detonation engine is proposed. The model has two significant advantages: no empirical parameter is required, and the model enables estimation of both the impulse and the duration during which the pressure at the thrust wall remains higher than its initial value. In the model, an objective partially fueled pulse detonation engine is replaced with the equivalent fully fueled pulse detonation engine. The equivalent pulse detonation engine is fully filled with a homogeneous mixture of the detonable and inert gases that separately fill the objective partially fueled pulse detonation engine. The performances of the equivalent fully fueled pulse detonation engine are instead estimated by a previously developed performance-estimation model for a fully fueled pulse detonation engine. The applicability of the model is examined by comparing the results of the model with numerical and experimental results in the cases where hydrogen or ethylene was used as fuel. Further, the applicability limits, which arose from replacement of the objective partially fueled pulse detonation engine with the equivalent fully fueled pulse detonation engine, are described.

High-Mach number collisionless shock and photo-ionized non-LTE plasma for laboratory astrophysics with intense lasers

We propose that most of the collisionless shocks in the Universe, for example, supernova remnant shocks, are produced because of the magnetic field generated by Weibel instability and its nonlinear process. In order to verify and validate the computational result confirming this theory, we are carrying out model experiments with intense lasers. We are going to make a collisionless counter-streaming plasma with intense laser ablation based on the scaling law to laser plasma with the particle-in-cell simulation resulting in Weibel-mediated shock formation. Preliminary experimental data are shown. The photo-ionization and resultant non-LTE plasma physics are also very important subjects in astrophysics related to mainly compact objects, for example, black hole, neutron star and white dwarf. Planckian radiation with its temperature 80–100 eV has been produced in gold cavity with irradiation of intense lasers inside the cavity. The sample materials are irradiated by the radiation inside the cavity and absorption and self-emission spectra are observed and analyzed theoretically. It is demonstrated how the effect of non-LTE is essential to reproduce the experimental spectra with the use of a precision computational code.

Study of indirectly driven implosion by x‐ray spectroscopic measurements

Fusion pellet implosion by laser‐generated x rays was investigated by means of time‐integrated spectroscopic measurements. Deuterium fuel was seeded with a small amount of Ar in order to determine the electron temperature and the density of the compressed fuel from, respectively, the emission intensity ratio and the broadening of the Lyβ (Ar17+1s‐3p) and Heβ (Ar16+1s2‐1s3p) lines. Comparison of the observed results with volume‐averaged temperatures and densities obtained from one‐dimensional (1‐D) fluid‐dynamic simulations showed large discrepancies at maximum compression. One possible explanation is that the fuel is stably compressed until the beginning of pusher deceleration by collision with a reflected shock wave from the pellet center, and that further compression during the deceleration phase is terminated in particular for heavy stagnation cases. Similar results were obtained for fusion output. Experimentally obtained neutron yields were close to those from the 1‐D simulations at the beginning of t...

Chapman-Jouguet Oblique Detonation Structure Around Hypersonic Projectiles

How to generate a steady-state detonation around a hypersonic projectile in stoichiometric hydrogen-oxygen premixed gases is studied. The speed of the hypersonic projectiles was beyond the Chapman ‐Jouguet(C‐J) detonation speed. The e owe eld around the projectile was visualized by using a gate intensie ed charge-coupled device camera(single-frame schlieren picturesand OHradical self-emissionimages ). Threeparameters are varied:1 ) the projectile e ight length from diaphragm rupture location, 2 )the initial pressure of mixture below/above the critical pressureforsteady detonation initiation around a hypersonicprojectile, and3 )theprojectilespeed.Atthepressure condition below the criteria, the detonation structure is composed of three different shock and detonation waves, which appear just after a diaphragm rupture and evolve in time: an overdriven bow detonation wave, a strong detonation wave, and a diffracted shock wave. Their propagation after diaphragm rupture is investigated, and it is found that the C ‐J detonation wave moved away from the projectile and only a reactive bow shock wave remained around projectile far away from the diaphragm. At the pressure condition above the criteria, a steady oblique detonation wave was generated around the projectile as soon as the projectile broke the diaphragm. In this steady-state detonation-wave case, with respect to the e ow Mach number behind the wave front, the whole detonation wavewas divided into fourparts: 1 )strong overdriven detonation wave, 2 ) weak overdriven detonation wave, 3) quasi-C‐J detonation wave, and 4 ) C‐J detonation wave. It has been found that a rarefaction wave is generated at the projectile shoulder and that curvature of the wave has a signie cant effect on the structure of the detonation wave.

Shock Hugoniot and temperature data for polystyrene obtained with quartz standard

Equation-of-state data, not only pressure and density but also temperature, for polystyrene (CH) are obtained up to 510 GPa. The region investigated in this work corresponds to an intermediate region, bridging a large gap between available gas-gun data below 60 GPa and laser shock data above 500 GPa. The Hugoniot parameters and shock temperature were simultaneously determined by using optical velocimeters and pyrometers as the diagnostic tools and the α-quartz as a new standard material. The CH Hugoniot obtained tends to become stiffer than a semiempirical chemical theoretical model predictions at ultrahigh pressures but is consistent with other models and available experimental data.

X‐ray emission from high‐Z mixture plasmas generated with intense blue laser light

A mixture plasma composed of high‐Z elements is proposed as an efficient converter of laser light to x rays. It is shown theoretically that an energy band of small opacity for one element, corresponding to a partial cutout on its emission spectrum, can be enhanced with a large opacity band of another element so that the mean opacity for the mixture with an optimum mixing ratio becomes larger than the opacities of the individual elements. As a result, higher re‐emission and efficient conversion to x rays can be expected for the mixture. Absolute x‐ray emission from Au‐Sm and Au‐Tb mixture plasmas irradiated with a blue‐laser pulse was measured for the first time. The results show a trend that the mixtures at the optimum mixing ratio give higher x‐ray conversion efficiencies than their constituent materials.

X-ray source studies for radiography of dense matter

Studies of short-pulse laser-generated hard x-ray (18–60 keV) sources, suitable for radiographs of large samples of dense matter, are presented. The spatial and dynamic resolutions for different target types and laser parameters have been investigated. A high quality radiograph with good spatial resolution in two dimensions was demonstrated by irradiating freestanding thin W wires. The influence of the geometry for the quality of the radiograph, which is crucial for the design of experiments probing laser-compressed matter, is reported.