Low Fuel Convergence Path to Direct-Drive Fusion Ignition

Abstract

A new class of inertial fusion capsules is presented that combines multishell targets with laser direct drive at low intensity (2.8 × 10 14 W=cm 2 ) to achieve robust ignition. The targets consist of three concentric, heavy, metal shells, enclosing a volume of tens of μg of liquid deuterium-tritium fuel. Ignition is designed to occur well "upstream" from stagnation, with minimal pusher deceleration to mitigate interface Rayleigh-Taylor growth. Laser intensities below thresholds for laser plasma instability and cross beam energy transfer facilitate high hydrodynamic efficiency (∼10%). DOI: 10.1103/PhysRevLett.116.255003 A large convergence ratio and high implosion velocity have been the hallmarks of inertial fusion from its inception [1,2]. Target design has evolved over the past forty-plus years to culminate in the National Ignition Facility (NIF) ignition targets fired during the Ignition Campaign of 2009-2012. These targets sought to ignite a central hot spot formed by a small fraction of fuel that could be heated to ignition temperatures. Burn was then projected to propagate to the main fuel and produce high gain. A specific example (Fig. 107 of Ref.[2]) composed entirely of light materials is driven by 1.35 MJ of laser light. It required an implosion velocity u I ¼ 41 cm=μs and convergence ratio (initial capsule radius, R i divided by final radius R f ) of C ¼ 36 from a plastic and deuterium-tritium (DT) ice shell with an in-flight aspect ratio of 40. The predicted yield of 15 MJ from 180 μg of DT fuel corresponds to a burn fraction of 25%. Nearly perfect spherical symmetry was essential. The experimental results achieved to date on the NIF have underperformed predictions [3,4].A fundamentally different path to ignition is described in this Letter. A new class of targets capable of producing multi-megajoule yields from DT fuel masses of tens of μg, absorbed drive energies less than 2 MJ, and burn fractions exceeding 50% is defined. High gain is abandoned as a goal. Instead, we seek a mechanically robust implosion and large margin for ignition. The Revolver targets described here consist of three nested, spherical metal shells with buffer gas between the shells and a central volume filled with cryogenic liquid DT fuel. The baseline target is depicted in Fig. 1, which also shows the implosion diagram from a HYDRA [5] 1D simulation. Energy is absorbed by an ablator shell from a short laser pulse that leaves 70% of the ablator mass as payload to implode the target. The implosion is entirely mechanical, dominated by the metal shells. The metal multishell system is intended to be more robust hydrodynamically than a single plastic-DT ice shell. Ignition well upstream of stagnation is a key feature of the Revolver targets. This is controlled with a design parameter that allows for adjustment of the ignition margin.All the physics pieces we will assemble have long been known and studied. Metal pushers were first discussed in the literature by Kirkpatrick and co-workers [6,7], who showed the benefits of radiation t...

First-principles equation-of-state table of beryllium based on density-functional theory calculations

“…In inertial confinement applications (ICF), [5][6][7] beryllium has been used as an ablator for indirect-drive fuel capsule designs [8][9][10][11] because of its advantages of high density, low opacity, high thermal conductivity, 12 and low instability growth. 13,14 Since these target designs are based mainly on the radiation-hydrodynamic simulations where the materials will experience many different pressures (up to 10 5 Mbar) and temperature (up to 10 8 K) conditions, 15 accurate properties of beryllium under such extreme conditions are essential for ICF applications. Experimental data are limited to 20 Mbar, however, because of the difficulty in generating shocks and measuring them accurately.…”

The role of incidence angle in the laser ablation of a planar target

“…Radiation hydrodynamic simulations are used to indicate the duration of validity of the model as the coronal plasma expands and to evaluate the ablative properties for large spherical targets. It is found that the planar model can be appropriately applied to a 3.1 mm radius capsule, similar to the one encountered in the Revolver target baseline design 3 , early in the laser pulse. The recovery of planar ablation scaling early in time is attributed to the length scale of the rarefied coronal plasma being small compared to the target radius.…”

“…Polar direct drive (PDD) is the only near-term viable method for driving a variety of proposed direct drive ignition target designs (see Refs. [1][2][3][4] on the National Ignition Facility (NIF). In its current indirect drive configuration, NIF's 192 laser beams emanate from laser ports at angles of 23.5, 30, 44.5, and 50 degrees from the poles of its spherical target chamber.…”

Effect of Impurity Atoms on Thermonuclear Yield

“…Directly-driven large capsules tested their applicability for the Revolver tripleshell ignition capsule [116] and for plasma kinetic studies. The Revolver concept uses a very large outer shell, 6-mm diameter, to initiate the implosion.…”

“…Recent experiments investigating the Revolver concept of triple-shell implosions [116,128] tested the concept of launching very large, low-velocity Be shells at Omega [119].These experiments showed that a 1.2-mm-diameter, 15-µm-thick, Be shell could be accelerated to a velocity of 7.5 cm/µsec as opposed to the usual single-shell ignition design velocity of 35 cm/µsec [119]. These experiments did not use a gas fill.…”