Implosion Mode Research Articles

Direct fast heating efficiency of a counter-imploded core plasma employing a laser for fast ignition experiments (LFEX)

Fast heating efficiency when a pre-imploded core is directly heated with an ultraintense laser (heating laser) was investigated. ‘Direct heating’ means that a heating laser hits a pre-imploded core without applying either a laser guiding cone or an external field. The efficiency, η, is defined as the increase in the internal core energy divided by the energy of the heating laser. Six beams (output of 1.6 kJ) from the GEKKO XII (GXII) green laser system at the Institute of Laser Engineering (ILE), Osaka University were applied to implode a spherical deuterated polystyrene (CD) shell target to form a dense core. The DD-reacted protons and the core x-ray emissions showed a core density of 2.8 ± 0.7 g cm−3, or 2.6 times the solid density. Furthermore, DD-reacted thermal neutrons were utilized to estimate the core temperature between 600 and 750 eV. Thereafter, the core was directly heated by a laser for fast-ignition experiments (LFEX, an extremely energetic ultrashort pulse laser) at ILE with its axis lying along or perpendicular to the GXII bundle axis, respectively. The former and latter laser configurations were termed ‘axial’ and ‘transverse modes’, respectively. The η was estimated from three independent methods: (1) the core x-ray emission, (2) the thermal neutron yield, and (3) the runaway hot electron spectra. For the axial mode, 0.8% 2.1% at low power (low LFEX energy) and 0.4% 2.5% at high power (high LFEX energy). For the transverse mode, 2.6% 7% at low power and 1.5% 7.7% at high power. Their efficiencies were compared with that in the uniform implosion mode using 12 GXII beams, 6% 12%, which appeared near to the η for the transverse mode, except that the error bar is very large.

Investigation of rocket effect in bright-rimmed clouds using Gaia EDR3

ABSTRACT Bright-rimmed clouds (BRCs) are excellent laboratories to explore the radiation-driven implosion mode of star formation because they show evidence of triggered star formation. In our previous study, BRC 18 has been found to accelerate away from the direction of the ionizing H ii region because of the well-known ‘Rocket Effect’. Based on the assumption that both BRC 18 and the candidate young stellar objects (YSOs) are kinematically coupled and using the latest Gaia EDR3 measurements, we found that the relative proper motions of the candidate YSOs exhibit a tendency of moving away from the ionizing source. Using BRC 18 as a prototype, we made our further analysis for 21 more BRCs, a majority of which showed a similar trend. For most of the BRCs, the median angle of the relative proper motion of the candidate YSOs is similar to the angle of on-sky direction from the ionizing source to the central IRAS source of the BRC. Based on the Pearson’s and Spearman’s correlation coefficients, we found a strong correlation between these two angles, which is further supported by the Kolmogorov–Smirnov (K−S) test on them. The strong correlation between these two angles supports the ‘Rocket Effect’ in the BRCs on the sky plane.

Magnetic fields and young stellar objects in cometary cloud L1616

ABSTRACT The Lynds’ Dark Nebula (LDN) 1615/1616 and CB 28 (hereafter L1616) together form a cometary globule located at an angular distance of about 8° west of the Orion OB1 association, aligned roughly along the east–west direction, and showing a distinct head–tail structure. The presence of massive stars in the Orion belt has been considered to be responsible for the radiation-driven implosion mode of star formation in L1616. Based on the latest Gaia Early Data Release 3 (EDR3) measurements of the previously known young stellar objects (YSOs) associated with L1616, we find the distance to this cloud to be 384 ± 5 pc. We present optical polarimetry towards L1616 that maps the plane-of-sky component of the ambient magnetic field (BPOS) geometry. Based on the proper motion of the YSOs associated with L1616, we investigate their plane-of-sky motion relative to the exciting star ϵ Ori. Using the Gaia EDR3 measurements of the distances and proper motions of the YSOs, we find two additional sources comoving with the known YSOs. One comoving source is HD 33056, a B9 star, and the other might be a young pre-main-sequence star not reported in previous studies. The mean direction of BPOS is found to follow the cloud structure. This could be the effect of dragging of the magnetic field lines by the impact of the ionizing radiation from ϵ Ori. Based on the pressure exerted on L1616, and the ages of the associated YSOs, we show that it could possibly be the main source of ionization in L1616, and thus the star formation in it.

Research on the quasi-static collapse and instantaneous implosion of the deep-sea spherical pressure hull

Manned submersibles are an important deep-sea survey and operation tool. As the most important component of manned submersible, the spherical pressure hull provides a safe living space for pilots and scientists under high pressure. Therefore, research on the collapse load, collapse mode and impact of implosion on nearby structures is essential. In this paper, a tensile test of a titanium alloy specimen is conducted to determine the value of fracture strain in finite element modelling. Then, transient dynamic and quasi-static simulations are performed based on the scale model experimental process of a deep-sea pressure hull with a designed service depth of 4500 m. The first-order buckling mode is introduced as the structural geometric imperfection during calculation, and quasi-static step loading is applied to the surface of the spherical pressure hull. The results show that the collapse load and mode by the simulation and experimental results are consistent with each other, and the instantaneous impact is studied with the collapse pressure set as the initial pressure. The study of the implosion shock wave will provide a reference for protective measures during the experimental process in a pressure chamber and for underwater operation safety assessments of manned submersibles.

Investigation of Rocket Effect in BRC 18 using Gaia EDR3

ABSTRACT Bright-rimmed clouds (BRCs) are ideal candidates to study radiation-driven implosion mode of star formation as they are potential sites of triggered star formation, located at the edges of Hii regions, showing evidence of ongoing star formation processes. BRC 18 is located towards the eastern edge of relatively closer (∼400 pc) H ii region excited by λ Ori. We made R-band polarimetric observations of 17 candidate young stellar objects (YSOs) located towards BRC 18 to investigate any preferred orientation of the discs with respect to the ambient magnetic field and the direction of energetic photons from λ Ori. We found that the discs are oriented randomly with respect to the projected magnetic field. Using distances and proper motions from the Gaia EDR3 of the candidate YSOs, we investigated the possible acceleration of BRC 18, away from λ Ori due to the well-known ‘Rocket Effect’, by assuming that both the candidate YSOs and BRC 18 are kinematically coupled. The relative proper motions of the candidate YSOs are found to show a trend of moving away from λ Ori. We computed the offset between the angle of the direction of the ionization front and the relative proper motion of the candidate YSOs and found it to lie close to being parallel to each other. Additionally, we found 12 sources that are co-moving with the known candidate YSOs towards BRC 18. These co-moving sources are most likely to be young and are missed in previous surveys conducted to identify potential YSOs of the region.

Study of Implosion of Combined Nested Arrays

New experimental data on the implosion of plasma of nested kapron−tungsten arrays are obtained at the Angara-5-1 facility. The mode of plasma implosion is implemented in which a shock wave region forms in the space between the inner and outer arrays where a transition from the super-Alfvenic (V r > V A ) to sub-Alfvenic (V r < V A ) plasma flow takes place. Specific features of the formation and decay of the shock region are studied using laser shadow imaging and X-ray frame photography. The plasma density in the transition region is estimated. By comparing the experimental data with the results of simulations of quasi-steady implosion of a nested array with allowance for extended plasma production, the physical conditions are determined at which the implosion mode with the formation the shock region takes place. Stable compression of the plasma of the inner array was observed during the implosion of combined nested arrays with a fiber outer array and tungsten inner array. Suppression of magnetic Rayleigh-Taylor instability during the compression of the inner array plasma results in the formation of a compact radiating Z-pinch and generation of a soft X-ray pulse with a peak power of 4 TW and duration of about 5 ns.

Plasma Radiation Source on the Basis of the Gas Puff with Outer Plasma Shell in the Circuit of a Mega-Ampere Load Current Doubler

Characteristics of Z-pinch plasma radiation in the form of a double shell neon gas puff with outer plasma shell are investigated in the microsecond implosion mode. Experiments are performed using a GIT-12 mega-joule generator with load current doubler having a ferromagnetic core at implosion currents up to 5 MA. Conditions for matching of the nonlinear load with the mega-ampere current multiplier circuit are determined. The load parameters (plasma shell characteristics and mass and geometry of gas puff shells) are optimized on the energy supplied to the gas puff and n energy characteristics of radiation. It is established that the best modes of K-shell radiation in neon are realized for such radial distribution of the gas-puff material at which the compression velocity of the shell is close to a constant and amounts to 27–30 cm/μs. In these modes, up to 40% of energy supplied to the gas puff is converted into K-shell radiation. The reasons limiting the efficiency of the radiation source with increasing implosion current are analyzed. A modernized version of the energy supply from the current doubler to the Z-pinch is proposed.

Two-dimensional magnetohydrodynamic studies of implosion modes of nested wire array z-pinches

Implosion dynamics of nested wire arrays in (r, θ) geometry was studied with two-dimensional magnetohydrodynamic (2D MHD) simulations. Three different implosion modes are obtained by just changing the wire number of the outer array, when the other conditions, such as the initial radius, length, mass of each array, the wire number of the inner array, and the discharge voltage waveform, are fixed. Simulation results show that the effect of discrete wires, which cannot be described by the thin shell inductive model, will influence the distribution of current between the outer and inner arrays at the early stage, and the discrepancy between results from MHD and thin shell model increases with the interwire gap of the outer array.

Experimental study on current division of nested wire array Z pinches

The current division between the inner and the outer arrays is one of the key factors that impact the implosion dynamics and modes of nested wire array. This paper presents the first quantitative measurements of current division for nested wire array Z pinch on the Qiang Guang-I pulsed power facility. In experiments, the nested wire arrays, made of 3.8 μm-diameter tungsten wires, were 20 mm in height and 8 mm/16 mm in diameter for inner/outer array. Measurements for loads consisting of 42/21 wires in inner/outer array were compared with that of 21/42 in inner/outer array. Data of current versus time at various radial positions were obtained using magnetic probes until 20 ns before the current peak. Results show that the currents in inner and outer arrays increase during the rise of the driving current, the proportion of current in outer array decreases and that in inner array increases, while the current in outer array, for loads with 21/42 wires in inner/outer array, was larger than those with 42/21 wires in inner/outer array.

Specific features of the structure of the Z-pinch emitting region formed during the implosion of a foam-wire load at the ANGARA-5-1 facility

Results are presented from experimental studies of the structure of the compressed plasma of a Z-pinch produced during the implosion of a foam-wire load at the current of up to 3 MA. The foam-wire load consisted of two nested cylindrical cascades, one of which was a solid or hollow cylinder made of low-density agar-agar foam, while the other was a wire array. The wall thickness of a hollow foam cylinder was 100–200 μm. The images of the pinch and its spectrum obtained with the help of multiframe X-ray cameras and a grazing incidence spectrograph with a spatial resolution were analyzed. Data on the spatial structure of the emitting regions and the soft X-ray (SXR) spectrum of the Z-pinch in the final stage of compression of a foam-wire load were obtained. The implosion modes characterized by the formation of hot regions during implosion of such loads were revealed. The characteristic scale lengths of the hot regions were determined. It is shown that the energy distribution of SXR photons in the energy range from 80 eV to 1 keV forms the spatial structure of Z-pinch images recorded during the implosion of foam-wire loads. It is revealed that the spectral density of SXR emission in the photon energy range of 300–600 eV from hot Z-pinch regions exceeds the spectral density of radiation from the neighboring Z-pinch regions by more than one order of magnitude. Groups of lines related to the absorption and emission of radiation by atoms and multicharged ions of carbon and oxygen in the outer foam cascade of a foam-wire load were recorded for the first time by analyzing the spatial distribution of the SXR spectra of multicharged ions of the Z-pinch. The groups of absorption lines of ions (C III, O III, O IV, and O VI) corresponding to absorption of SXR photons in the Z-pinch of a tungsten wire array, which served as the inner cascade of a foam-wire load, were identified. The plasma electron temperature measured from the charge composition of carbon and oxygen ions in the outer agar-agar foam cascade was 10–40 eV. During the implosion of foam-wire loads at currents of up to 3 MA, SXR pulses (hν > 100 eV) with a duration of 10 ns and peak power of 3 TW were detected. It is shown that the temporal profile of single-peak and double-peak SXR pulses can be controlled by varying the parameters of the outer and inner cascades of the foam-wire load.

Modifying Wire-Array Z-Pinch Ablation Structure and Implosion Dynamics Using Coiled Wires

Coiled arrays, which are cylindrical arrays in which each wire is formed into a helix, suppress the modulation of ablation at the fundamental wavelength. Outside the vicinity of the wire cores, ablation flow from coiled arrays is modulated at the coil wavelength and has a two-stream structure in the r, thetas plane. Within the vicinity of the helical wires, ablation is concentrated at positions with the greatest azimuthal displacement, and plasma is axially transported from these positions such that the streams become aligned with the sections of the coil furthest from the array axis. The GORGON MHD code accurately reproduces this observed ablation structure, which can be understood in terms of J times B forces that result from the interaction of the global magnetic field with a helical current path as well as additional current paths suggested by the simulations. With this ability to control where the ablation streamers occur, large wavelength coils were constructed such that the breaks that form in the wires had sufficient axial separation to prevent perturbations in the implosion sheath from merging. This produced a new mode of implosion in which the global instability can be controlled, and perturbations correlated between all wires in an array. For large-wavelength eight-wire coiled arrays, this produced a dramatic increase in X-ray power, equaling that of a 32-wire straight array. These experiments were carried out on the mega ampere generator for plasma implosion experiments (1 MA, 240 ns) at Imperial College London, London, U.K.

Study on target structure for direct–indirect hybrid implosion mode in heavy ion inertial fusion

The key issues in heavy ion beam (HIB) inertial confinement fusion (ICF) include particle accelerator, physics of intense beam, beam final transport, target-plasma hydrodynamics, etc. In this paper, we focus on fuel implosion. In order to realize an effective implosion, beam illumination non-uniformity on a fuel target must be suppressed less than a few percent. In this study a direct–indirect hybrid implosion mode is discussed in heavy ion beam inertial confinement fusion (HIF) in order to release sufficient fusion energy in a robust manner. In the direct–indirect hybrid mode target, a low-density foam layer is inserted, and the radiation energy is confined in the foam layer. In the foam layer, the radiation transport is expected to smoothen the HIB illumination non-uniformity in the lateral direction. In this paper, we study the influences of the foam thickness and the inner radiation-shield Al density on implosion uniformity. Two-dimensional fluid simulations demonstrate that the hybrid target contributes to the HIB non-uniformity smoothing and releases a sufficient fusion energy output in HIF.

Observation of Cascade Implosions in “Star”-Like Wire-Array $Z$-Pinches on the Zebra Generator

The dynamics of implosions in ldquostarrdquo-like wire arrays were investigated in the 1-MA Zebra generator. The hydrodynamic mode of implosion was confirmed by several plasma diagnostics, including laser probing, an optical streak camera, and a time-gated charge-coupled-device camera. Implosion begins on the edge wires and cascades from wire to wire accelerating toward the center.

Direct-indirect hybrid mode implosion in heavy ion inertial fusion

A direct-indirect hybrid implosion mode is proposed and discussed in heavy ion beam (HIB) inertial confinement fusion (HIF) in order to release sufficient fusion energy in a robust manner. On the other hand, the HIB illumination non-uniformity depends strongly on a target displacement dz from the centre of a fusion reactor chamber. In a direct-driven implosion mode, dz of ~ 20 μm was tolerable, and in an indirect-implosion mode, dz of ~ 100 μm was allowable. In the direct-indirect mixture mode target, a low-density foam layer is inserted, and the radiation energy is confined in the foam layer. In the foam layer the radiation transport is expected to smooth the HIB illumination non-uniformity in the lateral direction. Two-dimensional implosion simulations are performed, and show that the HIB illumination non-uniformity is well smoothed in the direct-indirect hybrid-mode target. Our simulation results present that a large pellet displacement of ~ a few hundred μm is allowed in order to obtain a sufficient fusion energy output in HIF.

Direct–indirect mixed implosion mode in heavy ion inertial fusion

In order to realize an effective implosion, beam illumination non-uniformity on a fuel target must be suppressed less than a few percent. In this study, a direct–indirect mixture implosion mode is proposed and discussed in heavy ion beam (HIB) inertial confinement fusion (HIF) in order to release sufficient fusion energy in a robust manner. On the other hand, the HIB illumination non-uniformity depends strongly on a target displacement dz from the center of a fusion reactor chamber. In a direct-driven implosion mode, dz of ∼20 μm was tolerable, and in an indirect-implosion mode, dz of ∼100 μm was allowable. In the direct–indirect mixture mode target, a low-density foam layer is inserted, and the radiation energy is confined in the foam layer. In the foam layer, the radiation transport is expected to smooth the HIB illumination non-uniformity in the lateral direction. Two-dimensional implosion simulations are performed, and show that the HIB illumination non-uniformity is well smoothed in the direct–indirect mixture target. Our simulation results present that a large pellet displacement of approximately a few hundred microns is allowed in order to obtain a sufficient fusion energy output in HIF.

Direct-indirect mixture implosion in heavy ion fusion

In order to realize an effective implosion, the beam illumination non-uniformity and implosion non-uniformity must be suppressed to less than a few percent. In this paper, a direct-indirect mixture implosion mode is proposed and discussed in heavy ion beam (HIB) inertial confinement fusion (HIF) in order to release sufficient fusion energy in a robust manner. On the other hand, the HIB illumination non-uniformity depends strongly on a target displacement (dz) in a reactor. In a direct-driven implosion mode dz of ∼20 μm was tolerance and in an indirect-implosion mode dz of ∼100 μm was allowable. In the direct-indirect mixture mode target, a low-density foam layer is inserted, and radiation is confined in the foam layer. In the foam layer the radiation transport is expected in the lateral direction for the HIB illumination non-uniformity smoothing. Two-dimensional implosion simulations are performed and show that the HIB illumination non-uniformity is well smoothed. The simulation results present that a large pellet displacement of ∼300 μm is tolerable in order to obtain sufficient fusion energy in HIF.

The different dynamical modes of nested wire array Z pinches

Magnetohydrodynamic simulations are used to predict which modes of implosion are in operation in nested wire array experiments. Separate r–θ plane simulations of the flux of plasma imploding towards the axis from the outer array and the bombardment of the inner array by this flux are presented. The different implosion modes are distinguished by the level of momentum transfer and magnetic flux compression during collision. For typical configurations, large interwire gaps (&amp;gt;1.5 mm) are required in the inner array in order to access the advantageous mode where the outer array material passes through.

Kinetic effects on the electron thermal transport in ignition target design

Preheating is one of the most critical issues in laser fusion, because of significant reduction of volume compression. The nonlocal heat transport in an ablative plasma is found to play an important role in the preheating under high intensity laser irradiation. Namely, the electron heat transport should be described by the Fokker–Planck (FP) equation in the fluid implosion code. The Spitzer–Härm (SH) thermal conduction model is not applicable because the electron mean free path is comparable to the temperature scale length. The numerical simulations of the implosion with the FP heat transport have been carried out for the fast (high entropy) implosion mode in which the implosion velocity reaches as high as 6×107 cm/s. In the fast implosion, the required laser energy for ignition can be reduced. It is found in the simulation that the isentrope in the FP simulation code is higher by two to four times than that in the flux limited SH simulation.

Effect of Nonuniform Implosion on High-Gain Intertial Confinement Fusion Targets

High-gain target design and gain sensitivity to nonuniform implosion dynamics have been studied numerically. First of all, sensitivity of the pellet gain to the tailored pulse shape has been studied using the laser implosin code ILESTA-1D, and the gain scaling law has been obtained through an optimization of target and laser parameters. Then, the two-dimensional code, ILESTA-2D, was used to study the sensitivity of the pellet gain to the nonuniformity in target implosion. It is found that the fire-polishing effect by alpha-particle heating drastically smooths the spark structure. The implosion mode optimized with the ILESTA-1D code is relatively sensitive to nonuniformity and, therefore, higher implosion velocity modes are recommended from stability.

Self-similar solutions for the supersonic implosion of a screw-pinch plasma

Self-similar solutions are obtained for the supersonic compression of a plasma containing an axial current and an entrained axial magnetic field (screw pinch). These solutions represent a complete generalization of previous works done on the Z-pinch and Θ-pinch schemes. The solutions presented herein represent a screw-pinch plasma with a diffuse current profile. Three separate plasma implosion modes are identified. These modes differ qualitatively from the Z-pinch and Θ-pinch schemes. They are comprised of the plasma annulus implosion, a converging transverse magnetohydrodynamic shock, and the propagation of fast magnetosonic waves toward the axis (weak discontinuity). These modes are chosen on the basis of their ability to achieve magnetic field cumulation during the course of the implosion. Analytic expressions describing the asymptotic behavior of the hydromagnetic profiles are obtained for each of the three implosion modes. Numerical examples for selected boundary conditions, representing realistic plasma parameters for the purpose of ultrahigh magnetic field generation, are presented.