Direct-drive Inertial Confinement Fusion Target Research Articles

Effect of fast electrons on the gain of a direct-drive laser fusion target

The results of numerical and theoretical studies of the gain of a direct-drive inertial confinement fusion target, which includes a kinetic description of energy transfer by laser-accelerated fast electrons, are presented. The range of the initial temperature of the fast electrons and the fraction of laser energy contained in these particles were chosen based on the results of recent experiments at the National Ignition Facility. The effect of ‘wandering’ of fast electrons is taken into account which is due to the remoteness of the region of fast electron generation from the ablation surface of the imploded target. As a result a significant fraction of particles do not fall into the compressed part of target. The ‘wandering’ effect leads to a decrease in the negative effect of fast electron generation on the gain of the target.

Effect of ‘wandering’ and other features of energy transfer by fast electrons in a direct-drive inertial confinement fusion target

The heating of an inertial confinement fusion (ICF) target by fast electrons, which are generated as a result of laser interaction with the expanding plasma (corona) of a target, is investigated theoretically. It is shown that due to remoteness of the peripheral region, where electrons are accelerated, a significant portion of these particles, moving in corona and repeatedly crossing it due to reflection in a self-consistent electric field, will not hit into the compressed part of target. Using the modern models of fast electron generation, it is shown that in a typical target designed for spark ignition, the fraction of fast electrons that can pass their energy to the compressed part of the target is small. Only 12% of the total number of fast electrons can do it. Such an effect of ‘wandering’ of fast electrons in corona leads to a significant decrease in the negative effect of fast electrons on target compression. Taking into account the wandering effect, the distribution of energy transmitted by fast electrons to different parts of target and the resulting reduction of deuterium–tritium (DT) fuel compression are established.

Mitigation of Laser Imprinting with Diamond Ablator for Direct-Drive Inertial Confinement Fusion Targets

We propose a novel scheme to mitigate the initial perturbation imprinting due to irradiation non-uniformity. Diamond was potential candidate for the ablator material for ICF targets due to its stiffness. The stiffness is very important parameter for imprint mitigation because the laser imprint is primary as a function of pressure perturbation due to lase irradiation non-uniformity. We measured the imprint amplitude of diamond foils and plastic (CH) foils. The experimental data suggest the initial imprinting is drastically mitigated for the diamond foils.

Linear stability analysis of double ablation fronts in direct-drive inertial confinement fusion

A linear stability theory of double ablation fronts is developed for direct-drive inertial confinement fusion targets. The so-called electron radiative ablation front [S. Fujioka et al., Phys. Rev. Lett. 92, 195001 (2004)] is studied with a self-consistent model. Numerical results are presented as well as an analytical approach for the radiation dominated regime of very steep double ablation front structure. Dispersion relation formula is tackled by means of a sharp boundary model.

Radiation hydrodynamic theory of double ablation fronts in direct-drive inertial confinement fusion

The one-dimensional theory of double ablation fronts is developed for direct-drive inertial confinement fusion targets. The theory is based on the subsonic ablation front approximation and includes the effects of both radiation and electron heat fluxes. It is found that the structure of the ablation front is determined by two dimensionless parameters: the Boltzmann number and the effective mean free path. The Boltzmann number represents the ratio of the convective thermal and radiation energy fluxes, while the effective mean free path is the ratio between the characteristic plasma temperature gradient conduction scale length and the radiation mean free path. The development of a double ablation front is determined based on the range of the above dimensionless parameters. Temperature and density profiles in double ablation fronts are derived from a simplified analytic model and compared with the results of numerical simulations.

Laser absorption, mass ablation rate, and shock heating in direct-drive inertial confinement fusion

Direct-drive laser absorption, mass ablation rate, and shock heating are experimentally studied on the OMEGA Laser System [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] to validate hydrodynamics simulations. High-gain, direct-drive inertial confinement fusion target implosions require accurate predictions of the shell adiabat α (entropy), defined as the pressure in the main fuel layer to the Fermi-degenerate pressure, and the implosion velocity of the shell. The laser pulse shape determines the shell adiabat and the hydrodynamic efficiency determines the implosion velocity. A comprehensive set of measurements tracking the flow of energy from the laser to the target was conducted. Time-resolved measurements of laser absorption in the corona are performed on spherical implosion experiments. The mass ablation rate is inferred from time-resolved Ti K-shell spectroscopic measurements of nonaccelerating, solid CH spherical targets with a buried tracer layer of Ti. Shock heating is diagnosed in planar-CH-foil targets using time-resolved x-ray absorption spectroscopy and noncollective spectrally resolved x-ray scattering. The highly reproducible experimental results achieved with a high level of laser drive uniformity [S. P. Regan et al., J. Opt. Soc. Am. B 22, 998 (2005)] constrain the modeling of direct-drive energy coupling. A detailed comparison of the experimental results and the simulations reveals that a single-value flux limiter in the thermal transport model cannot explain all of the experimental observables. Simulations of laser absorption measurements need a time-dependent flux limiter to match the data. Modeling of both resonance absorption and nonlocal effects in the electron thermal conduction from the critical density to the ablation front are underway to resolve the observed discrepancies.

Modeling of stimulated Brillouin scattering near the critical-density surface in the plasmas of direct-drive inertial confinement fusion targets

The nonlinear propagation of laser beams, smoothed by spatial and temporal bandwidth, near the critical density surface of direct-drive inertial confinement fusion targets has been modeled. The interplay between filamentation and forward and backward stimulated Brillouin scattering (SBS) is described in the presence of light reflected from the critical density surface and high absorption of light near the critical density. The spectrum of backscattered light develops a red shift due to SBS, which can be seeded by the reflection of light from the critical surface. The intensity of backscattered light decreases moderately as the bandwidth of smoothing by spectral dispersion is increased.

Large-scale high-resolution simulations of high gain direct-drive inertial confinement fusion targets

Targets have been designed that produce moderate to high gain when directly driven by lasers. The intrinsic sensitivity of these targets to hydro instabilities is found using the FAST(2D) multidimensional radiation hydrocode [J. H. Gardner, A. J. Schmitt, J. P. Dahlburg et al., Phys. Plasmas 5, 1935 (1998)], which simulates the simultaneous behavior of a large bandwidth (e.g., l=2–256) of perturbations from compression to acceleration, and then to stagnation and burn. The development of the structure in these multimode simulations is benchmarked to theoretical analysis and single-mode calculations, which reveals the need to “renormalize” the simulation after compression. The simulations predict that a direct drive point design is expected to degrade significantly from its one-dimensional clean yield, yet still ignite and give appreciable gain. Simulations of high-gain pellets using a spike prepulse to inhibit Richtmyer–Meshkov growth show a considerable robustness, with high (>100) gains possible even with nominal surface finishes and laser imprint.

Improved performance of direct-drive inertial confinement fusion target designs with adiabat shaping using an intensity picket

Hydrodynamic instabilities seeded by laser imprint and surface roughness limit the compression ratio and neutron yield in the direct-drive inertial confinement fusion target designs. New improved-performance designs use adiabat shaping to increase the entropy of only the outer portion of the shell, reducing the instability growth. The inner portion of the shell is kept on a lower entropy to maximize shell compressibility. The adiabat shaping is implemented using a high-intensity picket in front of the main-drive pulse. The picket launches a strong shock that decays as it propagates through the shell. This increases the ablation velocity and reduces the Rayleigh–Taylor growth rates. In addition, as shown earlier [T.J.B. Collins and S. Skupsky, Phys. Plasmas 9, 275 (2002)], the picket reduces the instability seed due to the laser imprint. To test the results of calculations, a series of the picket pulse implosions of CH capsules were performed on the OMEGA laser system [T.R. Boehly, D.L. Brown, R.S. Craxton et al., Opt. Commun. 133, 495 (1997)]. The experiments demonstrated a significant improvement in target yields for the pulses with the picket compared to the pulses without the picket. Results of the theory and experiments with adiabat shaping are being extended to future OMEGA and the National Ignition Facility’s [J.A. Paisner, J.D. Boyes, S.A. Kumpan, W.H. Lowdermilk, and M.S. Sorem, Laser Focus World 30, 75 (1994)] cryogenic target designs.

Imprint reduction using an intensity spike in OMEGA cryogenic targets

A novel technique for reducing laser imprint in OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] cryogenic targets has been developed. A typical direct-drive inertial confinement fusion target consists of a shell of deuterium–tritium ice with a thin outer layer of polystyrene (CH). The presence of the CH layer gives rise to a brief period of early-time Rayleigh–Taylor instability. This effectively increases the amount of laser imprint by about a factor of 2 for typical target configurations, and especially for the nonuniformity modes considered most dangerous for target performance. Two-dimensional hydrodynamic simulations show that by introducing a short, high-intensity spike at the start of the implosion, this early-time growth can be significantly reduced with only a modest decrease in the calculated one-dimensional neutron yield.

Development of polystyrenemicroshells doped with ultrafine metal particles as radiation-preheatedICF targets using optimized multiple emulsion technique

Growth rates of the Rayleigh-Taylor instability, in direct driveinertial confinement fusion targets, can be reduced using aradiation-preheated polymer ablator. Polystyrene microshells doped withultrafine metal particles (200-300 Å) are fabricated using themultiple emulsion technique, which can serve as the outer layer forradiation-preheated target. Preliminary results show that microshells ofapproximately 200-1500 µm can be produced with various wallthicknesses. Further optimization of the process is in progress toproduce very thin (wall thickness ~1 µm) microshells withultrasmooth surface finish (~15 nm).

The effect of optical prepulse on direct-drive inertial confinement fusion target performance

The effect of optical prepulses on the performance of imploding inertial confinement fusion (ICF) targets has been measured. The neutron yields from nearly identical spherical targets imploded by the OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] laser were measured as a function of prepulse fluences in the drive laser. These experiments indicate that the cumulative prepulse fluence on target must be less than ∼0.2 J/cm2, which corresponds to a prepulse intensity contrast of ⩾107. This effect is the result of damage to the Al barrier layer used on OMEGA ICF targets. To verify this, the effect of low-intensity irradiation on these Al layers was observed on planar targets using the time-resolved reflectivity and the time-integrated transmission measurements. These experiments show that damage to the Al layers begins at ∼0.1 J/cm2, consistent with the implosion performance results.

Comparison of beam-smoothing methods for direct-drive inertial confinement fusion

Analysis and numerical simulations of the smoothing of the speckled illumination of a direct-drive inertial confinement fusion target are presented. In particular, the spatial spectrum of the integrated target fluence is compared across smoothing methods. Two categories of smoothing methods are considered. In one method spatially incoherent light is amplified and directed onto the target, whereas in the other the light is phase modulated and spectrally dispersed before being amplified and then focused through a random phase plate onto the target. The dependence of the smoothed spatial spectrum on the characteristics of phase modulation and dispersion is examined for both sinusoidal and more general phase modulation. It is shown that smoothing with nonsinusoidal phase modulation can result in spatial spectra that are substantially identical to that obtained with the incoherent light method in which random phase plates are present in both methods and identical beam divergence is assumed.