Indirect-Drive ICF
In indirect-drive ICF, researchers use x rays to ablate a capsule of fusion fuel, driving the capsule inward and creating an implosion. The ‑rays come from a high-Z (high atomic number) metal canister called a hohlraum. Lasers or z‑pinches can be used with hohlraums to generate the bath of x rays, which surround the implosion and cause the surface of the capsule to ionize and expand (ablate), generating pressures reaching hundreds of millions of atmospheres.
Hohlraums provide a spatial and temporal smoothing effect on the energy deposition to the capsule, which can have benefits for the hydrodynamic stability of the capsule and symmetry control of the implosion. However, the majority of the laser energy delivered to the fusion target is diffused into the hohlraum wall instead of the capsule.
Energy management in the design of indirect-drive ICF targets is therefore critical, as there is an interplay between laser energy entering the hohlraum through laser entrance holes and x-ray energy escaping via those holes. In addition, laser–plasma interactions (LPIs) inside the hohlraum can redirect laser energy in undesirable or helpful ways. The distribution of radiant energy inside the hohlraum determines the symmetry of the implosion because the distribution of x-ray flux experienced by the capsule is generally nonuniform and evolves over time. The implosion is particularly sensitive to asymmetry in x-ray flux during times of peak acceleration. Thus, asymmetry management is energy management, since asymmetries in an implosion at the time of peak compression are a signature of wasted kinetic energy.
The principal techniques that researchers use to manage implosion asymmetry in a hohlraum include adjustments to laser-beam pointing; laser power rebalancing between sets of beams, altering the helium gas-fill inside the hohlraum; and utilizing the LPI process of crossed-beam energy transfer to force energy from one set of beams to another. Researchers test these techniques at NIF, along with a variety of ablator materials and target designs, bringing them closer to achieving new ICF advances.
Contact
- Omar Hurricane
- hurricane1 [at] llnl.gov (hurricane1[at]llnl[dot]gov)
Direct-Drive ICF
For a fixed laser, direct-drive ICF involves greater energy coupling to the capsule by avoiding the intermediate hohlraum laser-to-x-ray energy conversion step of indirect drive. This approach can offer a significant energy advantage. However, some of this energy advantage may be offset by crossed-beam energy transfer, which redirects incoming laser energy outward before transferring energy to the ablator.
While the ablation pressures for direct- and indirect-drive are similar, the mass ablation-rate for indirect drive is larger due to the deeper penetration of x rays into the ablator. This leads to a higher hydrodynamic efficiency (ratio of implosion kinetic energy to energy absorbed) in the case of indirect drive, but the net laser energy efficiency of direct drive is generally higher than that of indirect drive. Due to the energy advantage of direct drive, the stagnation pressure requirement for ignition of a direct-drive implosion is approximately half of the requirement for indirect drive. In principle, this leads to lower implosion convergence requirements for ignition.
The energy advantage of direct drive leads to larger capsules, which can provide larger fusion yields for a given implosion velocity. However, the direct-drive advantage in energy coupling is offset by the higher adiabat (lower fuel compression) requirements of direct-drive that are needed for hydrodynamic stability control. This increased sensitivity of direct-drive implosions is essentially due to the steeper ablation density profile associated with the short range of electron conduction as opposed to a less steep, and more forgiving, profile in x-ray–driven ablation.
Direct-drive implosions have an additional source for high-mode (>30) hydrodynamic instability due to laser imprinting, which the indirect-drive approach avoids by using a hohlraum. Because the laser directly impinges on a direct-drive capsule and because this approach uses relatively thin ablators, electron preheating of direct-drive capsules is more challenging than the approach used in indirect-drive ICF.
The primary research facility for direct-drive ICF is the OMEGA Laser Facility. The spherical arrangement of the lasers at this facility is optimized for direct drive. However, at NIF, researchers have been exploring polar-direct-drive for many years. Since the NIF laser geometry consists of a non-spherical beam configuration, it is well adapted to polar-direct-drive. Direct-drive is a complementary approach to indirect-drive ICF, offering different strengths and weaknesses that make it a rich environment to study ICF physics.
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Contact
- Omar Hurricane
- hurricane1 [at] llnl.gov (hurricane1[at]llnl[dot]gov)
Magnetic-Drive ICF
In magnetically driven ICF, scientists use the z‑pinch effect to generate high-energy-density states by directly compressing and heating fusion fuel using magnetic pressure, or by generating x rays for indirect-drive ICF. The z‑pinch effect takes advantage of the fact that for fixed electric current, the magnetic pressure increases as the inverse square of radius. For electric currents in the many tens of mega-amps regime, the magnetic pressure generated can be comparable to the ablation pressure generated from the laser-based indirect-drive or direct-drive ICF techniques for similar spatial-scale targets.
LLNL scientist help design targets for the Z‑machine at Sandia, developing improvements to the electrical engineering of pulsed-power devices and diagnostics. In addition, LLNL researchers perform experiments on the Z‑machine, along with related experiments at NIF.
An important distinction between laser-driven ICF and direct magnetic ICF is that the equivalent of the capsule (called a “liner” in z-pinches) doesn’t ablate, and as a result, it doesn’t shed mass. Instead, the liner is conducting, carrying the electric current on the outside while holding the fusion fuel inside. Thus, the implosion velocity of direct magnetic ICF approaches is generally slower than laser-drive approaches. This slower velocity results in lower central temperatures at the time of stagnation, temperatures that are typically too low to access ignition. Thus, most direct magnetic ICF approaches require additional heating of the fusion fuel and additional magnetic fields (e.g., in the fuel) to have a chance of accessing ignition conditions.
Despite having a tiny physical volume, magnetic ICF targets have strong feedback on the facility driving them, since the target has a significant time-varying inductance and impedance in the inductance–capacitance–resistance circuit that defines the facility’s operating parameters. Magnetic implosions cause a spike in inductance and voltage that will cause the current driving the magnetic pressure on the target to drop and potentially short-circuit if not carefully managed as part of an integrated target and power-flow design. Some modern designs for pulsed-power facilities are “impedance matched” to deliver a specific and rapid pulse-shaped current to magnetic ICF targets.
Contact
- Omar Hurricane
- hurricane1 [at] llnl.gov (hurricane1[at]llnl[dot]gov)
ICF Simulations
Simulations are centrally important to ICF, both for guiding experimental programs and for building physical understanding of the target behavior. Before an experiment begins, LLNL researchers use simulations to design targets, specify laser power curves, and determine settings for experimental diagnostics. After an experiment takes place, scientists use simulations to generate synthetic diagnostics that can be directly compared with measurements to interpret the results. Scientists also use simulations in parametric studies and ensembles to explore sensitivities and improve designs. More than 100 design parameters are specified for each ignition target, based upon simulation results.
At LLNL, ICF research focuses on indirectly driven targets, where laser energy is converted to x rays inside an enclosure called a hohlraum. Simulations of these systems can model the full hohlraum in three dimensions, including the walls, the fusion capsule at the center, and the windows that hold in the hohlraum gas. These integrated simulations are complemented by higher resolution calculations, which focus on the capsule alone, allowing more detailed modeling of small-scale features. Capsule-only simulations can include all currently known sources that degrade implosion symmetry. These sources include:
- The thin tent that supports the capsule
- The narrow fill tube used to introduce fusion fuel
- Surface roughness on the capsule
- Grain boundaries and voids in the diamond ablator
- Grooves and other asymmetries in the frozen fuel layer
ICF codes must model a very wide range of challenging physics relevant to implosions, including:
- Transport and absorption of laser light, including crossbeam energy transfer
- Electron heat transport
- Radiation transport
- Compressible hydrodynamics and hydrodynamic instabilities
- Thermonuclear burn
- Transport and energy deposition of fusion products
- Effects of applied and self-generated magnetic fields
These simulations rely on detailed microphysics, such as equations of state and opacities that include non-local thermodynamic equilibrium effects. The growth of high performance computing over recent decades has made it possible to include much more of this physics at higher resolution, which in turn has substantially improved the completeness and predictive capability of the simulations.
Learn More
Cracking the fusion codes: Advancing ICF modeling and simulation with supercomputers
Contact
- Marty Marinak
- marinak1 [at] llnl.gov (marinak1[at]llnl[dot]gov)
