Fundamentals of ICF Hohlraums

Abstract

On the Nova Laser at LLNL, we demonstrated many of the key elements required for assuring that the next laser, the National Ignition Facility (NIF) will drive an Inertial Confinement Fusion (ICF) target to ignition. The indirect drive (sometimes referred to as ''radiation drive'') approach converts laser light to x-rays inside a gold cylinder, which then acts as an x-ray ''oven'' (called a hohlraum) to drive the fusion capsule in its center. On Nova we've demonstrated good understanding of the temperatures reached in hohlraums and of the ways to control the uniformity with which the x-rays drive the spherical fusion capsules. In these lectures we will be reviewing the physics of these laser heated hohlraums, recent attempts at optimizing their performance, and then return to the ICF problem in particular to discuss scaling of ICF gain with scale size, and to compare indirect vs. direct drive gains. In ICF, spherical capsules containing Deuterium and Tritium (DT)--the heavy isotopes of hydrogen--are imploded, creating conditions of high temperature and density similar to those in the cores of stars required for initiating the fusion reaction. When DT fuses an alpha particle (the nucleus of a helium atom) and a neutron are created releasingmore » large amount amounts of energy. If the surrounding fuel is sufficiently dense, the alpha particles are stopped and can heat it, allowing a self-sustaining fusion burn to propagate radially outward and a high gain fusion micro-explosion ensues. To create those conditions the outer surface of the capsule is heated (either directly by a laser or indirectly by laser produced x-rays) to cause rapid ablation and outward expansion of the capsule material. A rocket-like reaction to that outward flowing heated material leads to an inward implosion of the remaining part of the capsule shell. The pressure generated on the outside of the capsule can reach nearly 100 megabar (100 million times atmospheric pressure [1b = 10{sup 6} cgs]), generating an acceleration of the shell of about 10 trillion gees, and causing that shell to reach, over the course of a few nanoseconds, an implosion velocities of 300 km/sec. When the shell and its contained fuel stagnates upon itself at the culmination of the implosion, most of the fuel is in a compressed shell which is at 1000 times solid density. That shell surrounds a hot spot of fuel with sufficient temperature (roughly 10 keV or 100 million degrees) to ignite a fusion reaction. The capsule must not only be driven hard, but also uniformly over its entire surface to cause uniform compression of the fuel to the center. With direct drive, this uniform heating of the capsule is caused by simultaneously illuminating the capsule from all sides with many laser beams and taking great care (via beam conditioning to avoid speckle etc.) to assure that 2 points close to one another on the capsule surface are driven with the same illumination. With indirect drive, the capsule is positioned in the center of a cylindrically symmetric container called a hohlraum. Laser beams enter the hohlraum through holes in the end caps, heat the walls of the cylinder, which then radiate soft x-rays, filling the hohlraum with a bath of radiant energy. This energy causes the fuel capsule to implode. Typically, 70-80% of the laser energy can be converted to x-rays. The hohlraum concept leads to a natural, geometric uniformity of x-ray flux on the capsule surface, since two points close to one another on the capsule surface ''look out'' at the heated hohlraum walls and (for a wall to capsule radius ratio of order 4) see nearly identical large sections of the walls (thus making it irrelevant just how non-uniform those sections actually are) and hence a nearly identical heat environment and are thus driven nearly identically. We now proceed to study these hohlraums.« less