page_title: Nova (laser)
section_title: Fusion in Nova
hierarchical_section_title: Nova (laser) / Fusion in Nova
caption_attribution_description: Implosion of a fusion microcapsule on the NOVA laser system. This miniature “star” was created in the Nova laser target chamber as 300 trillion watts of power hit a 0.5-millimeter-diameter target capsule containing deuterium–tritium fuel.
context_page_description: Nova was a high-power laser built at the Lawrence Livermore National Laboratory in California, United States, in 1984 which conducted advanced inertial confinement fusion experiments until its dismantling in 1999. Nova was the first ICF experiment built with the intention of reaching "ignition", a chain reaction of nuclear fusion that releases a large amount of energy. Although Nova failed in this goal, the data it generated clearly defined the problem as being mostly a result of Rayleigh–Taylor instability, leading to the design of the National Ignition Facility, Nova's successor. Nova also generated considerable amounts of data on high-density matter physics, regardless of the lack of ignition, which is useful both in fusion power and nuclear weapons research.
context_section_description: Research on Nova was focussed on the indirect drive approach, where the laser shine on the inside surface of a thin metal foil, typically made of gold, lead, or another high-z metal. When heated by the laser, the metal re-radiates this energy as diffuse x-rays, which are more efficient than UV at compressing the fuel pellet. In order to emit x-rays, the metal must be heated to very high temperatures, which uses up a considerable amount of the laser energy. So while the compression is more efficient, the overall energy delivered to the target is nevertheless much smaller. The reason for the x-ray conversion is not to improve energy delivery, but to "smooth" the energy profile; since the metal foil spreads out the heat somewhat, the anisotropies in the original laser are greatly reduced.
  The foil shells, or hohlraums, are generally formed as small open-ended cylinders, with the laser arranged to shine in the open ends at an oblique angle in order to strike the inner surface. In order to support the indirect drive research at Nova, a second experimental area was built "past" the main one, opposite the laser bay. The system was arranged to focus all ten beams into two sets of five each, which passed into this second area and then into either end of the target chamber, and from there into the hohlraums.
  Confusingly, the indirect drive approach was not made widely public until 1993. Documents from the Nova era published in general science magazines and similar materials either gloss over the issue, or imply that Nova was using the direct drive approach, lacking the hohlraum.
  As had happened with the earlier Shiva, Nova failed to meet expectations in terms of fusion output. The maximum fusion yield on NOVA was about 10¹³ neutrons per shot. In this case the problem was tracked to instabilities that caused turbulent mixing of the fuel during collapse and upset the formation and transmission of the shock wave. The problem was caused by Nova's inability to closely match the output energy of each of the beamlines, which meant that different areas of the pellet received different amounts of heating across its surface. This led to hot spots on the pellet which were imprinted into the imploding plasma, seeding Rayleigh–Taylor instabilities and thereby mixing the plasma so the center did not collapse uniformly.
  Nevertheless, Nova remained a useful instrument even in its original form, and the main target chamber and beamlines were used for many years even after it was modified as outlined below. A number of different techniques for smoothing the beams were attempted over its lifetime, both to improve Nova as well as better understand NIF. These experiments added considerably not only to the understanding of ICF, but also to high-density physics in general, and even the evolution of the galaxy and supernovas.
caption: Fusion target implosion on Nova. The green coloring of the target holder is due to the leftover laser light that was upconverted only "half way" to UV, stopping at green. The optics are arranged to focus this light "short" of the target, and here it strikes the holder. A small amount of IR light is also leftover, but this cannot be seen in this visible-light photograph. An estimate of the size of the implosion can be made by comparing the size of the target holder here with the image above.
exif: {}