365. Inertial confinement – using lasers for compression

I hope you are not getting bored too much by my discussion of thermonuclear fusion in inertial confinement reactors, because today I’m going to continue and finally start explaining why do they want to use lasers in HiPER to compress plasma.

Basically, the main bonus of using lasers is their ability to concentrate huge energy in a very small volume () during tiny amounts of time (), so that the very phenomenon of inertial confinement becomes possible. Actually, nowadays laser technology is the only technology available that allows us to build up energy densities as high as , matter densities of the order and temperatures about 10 keV at which fusion reactions start.

The time scale during which plasma is confined in inertial confinement devices is of the order , so devices can clearly only work in impulse regime.

Last time I presented a photo of fuel capsule for inertial confinement reactor (naturally, it should be a sphere since sphere compresses the best). Actually, such fuel cell may have a rather complicated structure. The outer layer of the capsule is called ablator (one can show that it is energetically favourable to make ablator from the material with large ). Its main purpose is to hold the form of the capsule and evaporate as fast as possible after the laser starts to heat the microcapsule. In the simplest case, below the ablator lays fuel – d+t ice or gas (at normal or high pressure). In more complicated cases additional insulating layers may be present that, say, protect d+t ice from melting.

In indirect drive thermonuclear fusion reactors, capsule is placed inside this NIF hohlraum…

… and then the laser is turned on. The beam heats up the material of hohlraum, and the latter emits X-rays heating the capsule in turn.

Laser emission is focused on the capsule in a spherically symmetric way (and least, we try to focus it this way – why? see discussion in the previous post). If the power of the beam is about , ablator is evaporated and ionized at time scales . Its material becomes plasma with characteristic temperature of the order and density around . This plasma blows off, with typical velocities being around 300-1000 km/s, while laser emission continues to heat the lower layers of the capsule up – laser emission strongly interacts with plasma.

To be continued.