Feasibility of transition radiation as a diagnostic of hot electrons generated in indirect-drive experiment
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In the experiment of indirect-drive inertial confinement fusion, hot electrons in hohlraum are usually inferred from the bremsstrahlung, measured with filter-fluoresce X-ray spectrometer. Here, we study the feasibility of measuring hot electrons by detecting the transition radiation, emitted from energetic electrons passing through the outer surface of hohlraum. With the aid of Monte Carlo simulations, it was found that the intensity of black-body radiation in optical range, due to the energy deposition of electrons, is at least one order of magnitude larger than that of optical transition radiation, but two orders of magnitude smaller than the intensity in THz range. Hence, it would be plausibly feasible to detect the transition radiation in the far infrared and THz range. Furthermore, the sensitivity of intensity versus thickness and temperature are discussed with two temperature components of hot electrons. Finally, a proposal of diagnostic for hot electrons is put forward by adopting the wedge or stepped plate.Keywords:
Hohlraum
Transition radiation
Currently, laboratory created energy density of laser-driven inertial confinement fusion (ICF) is extremely close to that for ignition, while the divergence between experiment and simulation is increasing. One of the key issues is the lack of advanced knowledge of laser-hohlraum coupling process, which has shown the complexity of hohlraum environment. Optical Thomson scattering (OTS) becomes the standard technique for diagnosing the ICF hohlraum plasma parameters, due to its capability of providing unperturbed, local and precise measurement. The development of OTS in China is closely related with the Shenguang series laser facilities, on which most of the ICF experiments are carried out. In recent years, 4ω(263 nm) Thomson scattering technique has been set up on Shenguang-III prototype and 100 kJ-level laser facility, the corresponding results help the understanding of ICF physics. In the near future, several novel methods will be developed, for high-precision diagnostics of ICF ignition hohlraum plasmas and the research of new physical phenomena.
Hohlraum
National Ignition Facility
Thomson scattering
Nova (rocket)
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A highly uniform thermal x-radiation field for indirect-drive inertial confinement fusion implosions may be obtained by irradiating a four-hole, tetrahedral geometry, spherical hohlraum with all 60 Omega laser beams. Implosion studies and calculations [J. M. Wallace et al., Phys. Rev. Lett. 82, 3807 (1999)] indicate a drive uniformity comparable to that expected for the National Ignition Facility [J. A. Painser et al., Laser Focus World 30, 75 (1994)]. With 60 beams distributed over the cavity wall, tetrahedral hohlraums have a natural insensitivity to power balance and pointing errors. Standard, smooth Nova capsules imploded with this drive indicate that moderate convergence-ratio implosions, Cr∼18, have measured-neutron yield to calculated-clean-one-dimensional-neutronyield ratios similar to those previously investigated using the comparatively poor drive uniformity of Nova cylindrical hohlraums. This may indicate that a nonsymmetry-related neutron yield degradation mechanism, e.g., hydrodynamic mixing of cold, dense ablator material with the hot-spot region or some combination of nonsymmetry effects, is dominating in this Cr regime.
Hohlraum
Implosion
National Ignition Facility
Nova (rocket)
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A novel rugby-ball shaped hohlraum is designed in the context of the indirect-drive scheme of inertial-confinement fusion (ICF). Experiments were performed on the OMEGA laser and are the first use of rugby hohlraums for ICF studies. Analysis of experimental data shows that the hohlraum energetics is well understood. We show that the rugby-ball shape exhibits advantages over cylinder, in terms of temperature and of symmetry control of the capsule implosion. Simulations indicate that rugby hohlraum driven targets may be candidates for ignition in a context of early Laser MegaJoule experiments with reduced laser energy.
Hohlraum
Implosion
National Ignition Facility
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Summary form only given. Results from the first NIF indirect-drive ICF experiments and Omega experiments testing new hohlraum and capsule designs are described. In the area of laser-hohlraum coupling, the effects of laser beam smoothing by spectral dispersion (SSD) and polarization smoothing (PS) on the beam propagation in long scale gas-filled targets has been studied at NIF at plasma scales relevant to indirect drive low Z filled ignition hohlraum designs. At Omega, a NIF-relevant high electron temperature gas-filled hohlraum platform has been developed for laser-plasma coupling studies. In the area of hohlraum energetics, isolated and integrated measurements of albedo and conversion efficiency have been performed on well characterized multi-element ("cocktail") hohlraum wall materials predicted to improve coupling efficiency. In parallel, a joint energetics and symmetry campaign testing new foam-filled ignition hohlraum designs and laser entrance hole shields in a NIF-like multicone illumination geometry has begun. We are also testing the hydrodynamic stability of more efficient Cu-doped Be capsule ablators and the hydrodynamic effects of fuel fill-tubes using X-ray absorption and emission imaging on hohlraum-driven surrogate capsules. Finally, the first set of high convergence implosions in NIF-relevant >250 eV hohlraums with NIF-like hohlraum-to-capsule coupling ratios have been performed, demonstrating good coupling and symmetry
Hohlraum
National Ignition Facility
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A novel ignition hohlraum named three-axis cylindrical hohlraum (TACH) is designed for indirect-drive inertial confinement fusion. TACH is a kind of 6 laser entrance holes (LEHs) hohlraum, which is orthogonally jointed of three cylindrical hohlraums. Laser beams are injected through every entrance hole with the same incident angle.
Hohlraum
National Ignition Facility
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Hohlraum
National Ignition Facility
Nova (rocket)
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The first indirect drive implosion experiments using Beryllium (Be) capsules at the National Ignition Facility confirm the superior ablation properties and elucidate possible Be-ablator issues such as hohlraum filling by ablator material. Since the 1990s, Be has been the preferred Inertial Confinement Fusion (ICF) ablator because of its higher mass ablation rate compared to that of carbon-based ablators. This enables ICF target designs with higher implosion velocities at lower radiation temperatures and improved hydrodynamic stability through greater ablative stabilization. Recent experiments to demonstrate the viability of Be ablator target designs measured the backscattered laser energy, capsule implosion velocity, core implosion shape from self-emission, and in-flight capsule shape from backlit imaging. The laser backscatter is similar to that from comparable plastic (CH) targets under the same hohlraum conditions. Implosion velocity measurements from backlit streaked radiography show that laser energy coupling to the hohlraum wall is comparable to plastic ablators. The measured implosion shape indicates no significant reduction of laser energy from the inner laser cone beams reaching the hohlraum wall as compared with plastic and high-density carbon ablators. These results indicate that the high mass ablation rate for beryllium capsules does not significantly alter hohlraum energetics. In addition, these data, together with data for low fill-density hohlraum performance, indicate that laser power multipliers, required to reconcile simulations with experimental observations, are likely due to our limited understanding of the hohlraum rather than the capsule physics since similar multipliers are needed for both Be and CH capsules as seen in experiments.
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Hohlraum
National Ignition Facility
Nova (rocket)
Thermonuclear Fusion
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Hohlraum
National Ignition Facility
Thermonuclear Fusion
Nova (rocket)
Fusion power
Implosion
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We demonstrate the hohlraum radiation temperature and symmetry required for ignition-scale inertial confinement fusion capsule implosions. Cryogenic gas-filled hohlraums with 2.2 mm-diameter capsules are heated with unprecedented laser energies of 1.2 MJ delivered by 192 ultraviolet laser beams on the National Ignition Facility. Laser backscatter measurements show that these hohlraums absorb 87% to 91% of the incident laser power resulting in peak radiation temperatures of ${T}_{\mathrm{RAD}}=300\text{ }\text{ }\mathrm{eV}$ and a symmetric implosion to a $100\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ diameter hot core.
Hohlraum
Implosion
National Ignition Facility
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