posted on October 11, 2013 |
| 4656 views
On August 13th, Lawrence Livermore's National Ignition Facility (NIF) exploded a capsule containing deuterium and tritium releasing 8000 J (3x1015 neutrons) in a nanosecond. This broke the previous NIF record by a factor of 3.
Figure courtesy of NIF.
One of NIF’s goals has long been to produce energy from a fusion rather than fission reaction because the radioactive waste has a half-life
significantly shorter than fission waste. Until now, break-even fusion reactions (ie reactions that produce as much energy as required to ignite them) have eluded scientists.
Early calculations indicate that the reaction of the hot plasma started to self-heat the core. This enhanced the yield by nearly 50%.
NIF’s principle associate director Ed Moses stated that "The yield was significantly greater than the energy deposited in the hot spot by the implosion. … This represents an important advance in establishing a self-sustaining burning target, the next critical step on the path to fusion ignition on NIF."
The NIF system is designed to produce temperatures of tens of millions of degrees and pressures of many billion atm. These conditions are only traditionally seen in stars, nuclear weapon explosions, and planet cores. Amazingly, this all happens within a dime sized gold cavity called a hohlraum.
Dime sized hohlraum. Figure courtesy of NIF.
Early Fusion Research
Fusion within a star is perpetuated by the high pressures caused by gravity. But, in the 1940’s magnetic fields were used to confine the reaction of ions and electron plasma. The temperature was kept at 100 to 300 million Kelvin. When this occurred the deuterium (consisting of a single neutron and proton) and tritium (consisting of a single proton and two neutrons) overaome the electrostatic force repelling them and fused into helium (containing 2 neutrons and two protons). The atomic weight of the Helium (or alpha particle) is lighter than the starting materials. This difference in mass was converted into kinetic energy as famously described by Einstein’s formula E=mc2. Heat is produced when the alpha particles and extra neutrons interact via this kinetic energy.
Lasers were used to heat the system in the start of the 70’s in a process called inertial confinement fusion (ICF). In this process, the lasers are focused on a small spherical pellet of fuel causing the outer layer to explode and the inner layer to implode as per Newton’s Third Law. This implosion is followed by a heating shockwave that results in a self-sustaining ignition that propagates outwards. In this system the inertia of the plasma’s mass serves to confine the system as opposed to the magnetic fields that were used in the earlier systems.
The Indirect Drive
NIF on the other hand were the first to use an “indirect drive.” This method employs lasers to heat the inner surface of the hohlraum, which heats the containing pellet into plasma and produces a uniform X-ray “bath”. This high speed heating process creates a blow off of the surface imploding the fuel capsule similar to the direct laser method. This allows the fuel to burn before it can break up. Estimates are that this method, when optimized, will produce 10 to 100 times the energy needed to initiate the reaction.
Figure courtesy of NIF.
Previous experiments have suffered from the collapse of the high velocity imploding target capsule shell. This hindered the compression of the target. To alleviate this issue, the 192 lasers used to kick start the process were turned down during the start of the pulse.
"In this particular experiment, we intentionally lowered the goal in order to gain control and learn more about what Mother Nature is doing. The results were remarkably close to simulations and have provided an important tool for understanding and improving performance." said lead scientist Omar Hurricane.
After the experiment Director Moses expressed his excitement for his team "… who seamlessly integrated their capabilities in order to field this experimental campaign."
Diagram courtesy of NIF.