Nova (laser) - Fusion in Nova

Fusion in Nova

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 era published in general science magazines and similar material either gloss over the issue, or imply that Nova was using the direct drive approach, lacking the hohlraum. It was only during the design of NIF that the topic become public, so Nova was old news by that point.

As had happened with the earlier Shiva, Nova failed to meet expectations in terms of fusion output. In this case the problem was tracked to instabilities that "mixed" the fuel during collapse and upset the formation and transmission of the shock wave. The maximum fusion yield on NOVA was about 1013 neutrons per shot. 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.