Radiation implosion

The term radiation implosion describes the process behind a class of devices which use high levels of electromagnetic radiation to compress a target. The major use for this technology is in fusion bombs and inertial confinement fusion research.

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Fission bomb radiation source

Most of the energy released by a fission bomb is in the form of x-rays. The spectrum is approximately that of a black body at a temperature of 50,000,000 kelvins. The amplitude can be modeled as a trapezoidal pulse with a one microsecond rise time, one microsecond plateau, and one microsecond fall time. For a 30 kiloton fission bomb, the total x-ray output would be 100 terajoules.

Radiation transport

In a Teller-Ulam bomb, the object to be imploded is called the "secondary". It contains fusion material, such as lithium deuteride, and its outer layers are a material which is opaque to x-rays, such as lead or uranium-238.

In order to get the x-rays from the surface of the primary, the fission bomb, to the surface of the secondary, a system of "x-ray reflectors" is used.

The reflector is typically a cylinder made of a material such as uranium. The primary is located at one end of the cylinder and the secondary is located at the other end. The interior of the cylinder is commonly filled with a foam which is mostly transparent to x-rays, such as polystyrene.

The term reflector is misleading, since it gives the reader an idea that the device works like a mirror. Some of the x-rays are diffused or scattered, but the majority of the energy transport happens by a two-step process: the x-ray reflector is heated to a high temperature by the flux from the primary, and then it emits x-rays which travel to the secondary. Various classified methods are used to improve the performance of the reflection process^{[citation needed]}.

The implosion process

The term "radiation implosion" suggests that the secondary is crushed by radiation pressure, and calculations show that this pressure is very large. In fact, what happens is that the outer layers of the secondary become so hot that they vaporize and fly off the surface at high speeds. The recoil from this surface layer ejection produces pressures which are an order of magnitude stronger than the simple radiation pressure. The so-called radiation implosion is therefore really a radiation-powered ablation-drive implosion.

Laser radiation implosions

There has been much interest in the use of large lasers to ignite small amounts of fusion material. This process is known as inertial confinement fusion (ICF). As part of that research, much information on radiation implosion technology has been declassified.

When using optical lasers, there is a distinction made between "direct drive" and "indirect drive" systems. In a direct drive system, the laser beam(s) are directed onto the target, and the rise time of the laser system determines what kind of compression profile will be achieved.

In an indirect drive system, the target is surrounded by a shell (called a Hohlraum) of some intermediate-Z material, such as selenium. The laser heats this shell to a temperature such that it emits x-rays, and these x-rays are then transported onto the fusion target. Indirect drive has various advantages, including better control over the spectrum of the radiation, smaller system size (the secondary radiation typically has a wavelength 100 times smaller than the driver laser), and more precise control over the compression profile.