Abstract
Continuing work in the design of shock ignition targets is described. Because of reduced implosion velocity requirements, low target adiabats, and efficient drive by short wavelength lasers, these targets produce high gain at laser energies well below 1 MJ. Effects of hydrodynamic instabilities such as Rayleigh–Taylor or Richtmyer–Meshkov are greatly reduced in these lowaspect ratio targets. Of particular interest is the optimum ratio of ignitor to compression pulse energy. A simple pellet model and simulationderived coupling coefficients are used to analyze optimal fuel assembly, and determine that shock ignition allows enough control to create theoretically optimum assemblies. The effects on target design due to constraints on the compression and ignitor pulse intensities are also considered and addressed. Significant sensitivity is observed from lowmode perturbations because of large convergence ratios, but a more powerful ignitor can mitigate this.
This work was supported by the U.S. Office of Naval Research and the U.S. Department of Energy. We wish to thank Dr. R. Betti and Dr. J. Perkins for many useful discussions, and K. Obenschain for his support of the massively parallel computing facilities used for the simulations.
I. INTRODUCTION
II. 1D STUDIES
A. Optimization of compression power
B. Coupling
C. Laser intensity
1. Ignitor pulse flexibility
2. Compression intensity
III. 2D STUDIES
A. Convergence ratio
B. Aspect ratio
IV. CONCLUSIONS
Key Topics
 Hydrodynamics
 12.0
 Plasma waves
 4.0
 Glass lasers
 3.0
 High pressure
 3.0
 Hot carriers
 3.0
Figures
The 1D gain curve constructed from the optimum gain cases at each scale factor for the shock ignition targets simulated in this study.
The 1D gain curve constructed from the optimum gain cases at each scale factor for the shock ignition targets simulated in this study.
(a) Optimum gain, (b) peak laser intensities, and (c) laser pulse energies as a function of compression power, for a 250 ps FWHM ignitor and the scale2 target.
(a) Optimum gain, (b) peak laser intensities, and (c) laser pulse energies as a function of compression power, for a 250 ps FWHM ignitor and the scale2 target.
Comparison of simulation and predicted hotspot (open diamonds) and coldfuel (solid squares) energies. The predicted energies use the formula from Table II in Appendix A, and the assembled fuel masses, stagnation adiabats, and hottocold pressure ratios measured in the simulations.
Comparison of simulation and predicted hotspot (open diamonds) and coldfuel (solid squares) energies. The predicted energies use the formula from Table II in Appendix A, and the assembled fuel masses, stagnation adiabats, and hottocold pressure ratios measured in the simulations.
Absorption and coupling efficiencies for the laser pulse to the pellet ignition assembly. The coupling efficiencies have the absorption factored out, i.e., they denote coupling of absorbed energies. The diamond symbols denote the laser compression pulse, and the square symbols denote ignitor pulse coupling.
Absorption and coupling efficiencies for the laser pulse to the pellet ignition assembly. The coupling efficiencies have the absorption factored out, i.e., they denote coupling of absorbed energies. The diamond symbols denote the laser compression pulse, and the square symbols denote ignitor pulse coupling.
The ignitor shock pressures as a function of time, as the ignitor pulse width changes. The ignitor peak power is 450 TW. The solid lines are the peak shock pressures, while the dotted lines are the pressure at the laser critical surface. Inset: The pellet yield (diamond) and gain (triangle) vary with the ignitor pulse width.
The ignitor shock pressures as a function of time, as the ignitor pulse width changes. The ignitor peak power is 450 TW. The solid lines are the peak shock pressures, while the dotted lines are the pressure at the laser critical surface. Inset: The pellet yield (diamond) and gain (triangle) vary with the ignitor pulse width.
The gain vs ignitor timing and power (and pulse length) holding the ignitor energy fixed at 90 kJ.
The gain vs ignitor timing and power (and pulse length) holding the ignitor energy fixed at 90 kJ.
The maximum laser intensities and gains for scale2 pellets rescaled to different initial aspect ratios. All targets have the same laser energy and implosion velocity (without ignitor).
The maximum laser intensities and gains for scale2 pellets rescaled to different initial aspect ratios. All targets have the same laser energy and implosion velocity (without ignitor).
Relative rms perturbation of the areal mass in the pellet for the scale2 target with a 300 kJ, 110 TW compression pulse. The solid black line shows the growth of outer surface perturbations (initially rms), the dashdotted line is the inner surface perturbation (initially rms), the dashed line is for 300 overlapped 1 THz bandwidth ISI beams, all for a 150 kJ ignitor. The light solid line is for all three perturbation sources simultaneously and with a 200 kJ ignitor. The vertical gray lines denote the times (a) as the compression shock breaks out of the rear of the fuel shell; (b) as the ignitor shock turns on; and (c) when the pellet stagnates and ignition occurs.
Relative rms perturbation of the areal mass in the pellet for the scale2 target with a 300 kJ, 110 TW compression pulse. The solid black line shows the growth of outer surface perturbations (initially rms), the dashdotted line is the inner surface perturbation (initially rms), the dashed line is for 300 overlapped 1 THz bandwidth ISI beams, all for a 150 kJ ignitor. The light solid line is for all three perturbation sources simultaneously and with a 200 kJ ignitor. The vertical gray lines denote the times (a) as the compression shock breaks out of the rear of the fuel shell; (b) as the ignitor shock turns on; and (c) when the pellet stagnates and ignition occurs.
The areal mass perturbation amplitude spectrum, normalized to the average areal mass at four times during the implosion, for the different perturbations sources included here. The different times shown are at (a) ; (b) 12.8 ns when the shocks break out from the rear of the target; (c) 16 ns, when the ignitor shock turns on; and (d) 16.9 ns, when the pellet ignites. The sources included are perturbation on the outer surface (black), inner surface (dashdotted), ISI laser imprint (dashed), and all three sources combined at once (gray). The modes greater than have been locally averaged in wavenumber from to to reduce statistical fluctuations that obscure the plot.
The areal mass perturbation amplitude spectrum, normalized to the average areal mass at four times during the implosion, for the different perturbations sources included here. The different times shown are at (a) ; (b) 12.8 ns when the shocks break out from the rear of the target; (c) 16 ns, when the ignitor shock turns on; and (d) 16.9 ns, when the pellet ignites. The sources included are perturbation on the outer surface (black), inner surface (dashdotted), ISI laser imprint (dashed), and all three sources combined at once (gray). The modes greater than have been locally averaged in wavenumber from to to reduce statistical fluctuations that obscure the plot.
Images of an imploding pellet at times (a) 100 ps after the ignitor starts; (b) as the ignitor shock is halfway through the shell; and (c) at stagnation as the burn begins. The pellet is initially perturbed on the outer and inner surfaces with “NIFspec” spectra (Ref. 4) with nominal amplitudes of 0.48 and , respectively, and was subject to laser imprinting from 300 overlapped mutually incoherent 1 THz ISI beams. The resulting gain is 102.
Images of an imploding pellet at times (a) 100 ps after the ignitor starts; (b) as the ignitor shock is halfway through the shell; and (c) at stagnation as the burn begins. The pellet is initially perturbed on the outer and inner surfaces with “NIFspec” spectra (Ref. 4) with nominal amplitudes of 0.48 and , respectively, and was subject to laser imprinting from 300 overlapped mutually incoherent 1 THz ISI beams. The resulting gain is 102.
Lowmode simulations for target with two different convergence ratios. The perturbation amplitude is measured as a ratio to the nominal perturbation level ( rms in the wetted foam). Solid lines are the laser energy used at the optimal gain (left axis), while the dotted lines give the optimal gain (right axis).
Lowmode simulations for target with two different convergence ratios. The perturbation amplitude is measured as a ratio to the nominal perturbation level ( rms in the wetted foam). Solid lines are the laser energy used at the optimal gain (left axis), while the dotted lines give the optimal gain (right axis).
Growth of outer surface modes from highresolution simulations comparing the original lower AR target with a target that is designed for about 40% lower drive pressure . The cross marks on the curves denote the time of ignitor turnon and ignition.
Growth of outer surface modes from highresolution simulations comparing the original lower AR target with a target that is designed for about 40% lower drive pressure . The cross marks on the curves denote the time of ignitor turnon and ignition.
Images of the imploded density just as the pellet begins to burn (gain is at these times) for (a) lower AR target and (b) a higher AR target .
Images of the imploded density just as the pellet begins to burn (gain is at these times) for (a) lower AR target and (b) a higher AR target .
For the isobaric pellet assembly , the total and component energies of the pellet assembly as a function of assembly pressure. The fuel assembly gain (gray curve, with axis at right) is also shown. Note that the peak gain occurs at a pressure that is greater than that at the point of minimum total energy.
For the isobaric pellet assembly , the total and component energies of the pellet assembly as a function of assembly pressure. The fuel assembly gain (gray curve, with axis at right) is also shown. Note that the peak gain occurs at a pressure that is greater than that at the point of minimum total energy.
The function describes the sensitivity of the core radius to initial mass perturbations, whereas the function (in gray) describes the sensitivity of the final target mass to pressure perturbations.
The function describes the sensitivity of the core radius to initial mass perturbations, whereas the function (in gray) describes the sensitivity of the final target mass to pressure perturbations.
Tables
Target specifications listed by relative mass. In all cases, the ablator is made of CH foam wicked with solid DT and the fuel layer is DT ice. The target linear dimensions are scaled by the cube root of the scale factor, which denotes the relative target mass. Simulation parameters listed are those found at highest gain.
Target specifications listed by relative mass. In all cases, the ablator is made of CH foam wicked with solid DT and the fuel layer is DT ice. The target linear dimensions are scaled by the cube root of the scale factor, which denotes the relative target mass. Simulation parameters listed are those found at highest gain.
Fuel assembly parameters at peak gain, expressed as a function of mass .
Fuel assembly parameters at peak gain, expressed as a function of mass .
Fuel assembly parameters at peak gain in terms of total energy.
Fuel assembly parameters at peak gain in terms of total energy.
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