Nova (laser)

Nova was a high-power laser built at the Lawrence Livermore National Laboratory (LLNL) in California, United States, in 1984 which conducted advanced inertial confinement fusion (ICF) experiments until its dismantling in 1999.

Nova was the first ICF experiment built with the intention of reaching "ignition", the condition where self heating of the fusion plasma exceeds all losses.

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.

Inertial confinement fusion (ICF) devices use drivers to rapidly heat the outer layers of a target in order to compress it.

The remaining portion of the target is driven inwards due to Newton's Third Law, eventually collapsing into a small point of very high density.

Given the right overall conditions of the compressed fuel—high enough temperature and high enough product of density and time during which the plasma is confined by its own inertia—this heating process can result in ignition, initiating a burn wave outward from the central hot spot.

Calculations show that the energy must be delivered quickly in order to compress the core before it disassembles, as well as creating a suitable shock wave.

Although other "drivers" have been suggested, notably heavy ions driven in particle accelerators, lasers are currently the only devices with the right combination of features.

Building large Nd:glass lasers had not been attempted before, and LLNL's early research focused primarily on how to make these devices.

Even minor variations in intensity of the beams would result in "self-focusing" in the air and glass optics in a process known as Kerr lensing.

The infrared light generated by the Nd:glass lasers was found to interact very strongly with the electrons in the plasma created during the initial heating through the process of stimulated Raman scattering.

This process, referred to as "hot electron pre-heating", carried away a great amount of the laser's energy, and also caused the core of the target to heat before it reached maximum compression.

During the initial construction phase, Nuckolls found an error in his calculations, and an October 1979 review chaired by John Foster Jr. of TRW confirmed that there was no way Nova would reach ignition.

[11] The foil shells, or hohlraums, are generally formed as small open-ended cylinders, with the laser arranged to shine into 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.

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.

Based on the LASNEX computer models, it was estimated that LMF would require a driver of about 10 MJ,[10] in spite of nuclear tests that suggested a higher energy.

The report concluded that "considering the extrapolations required in target physics and driver performance, as well as the likely $1 billion cost, the committee believes that an LMF [i.e. a Laser Microfusion Facility with yields to one gigajoule] is too large a step to take directly from the present program."

[18] The report was also critical of the gas laser experiments being carried out at LANL, and suggested they, and similar projects at other labs, be dropped.

Starting in 1992, LLNL staff modified one of Nova's existing arms to build an experimental CPA laser that produced up to 1.25 PW.

[22][23] The basic amplification system used in Nova and other high-power lasers of its era was limited in terms of power density and pulse length.

In order to avoid filamentation or damage to the optical elements, the entire end of the beamline is placed in a large vacuum chamber.

Although Petawatt was instrumental in advancing the practical basis for the concept of fast ignition fusion, by the time it was operational as a proof-of-concept device, the decision to move ahead with NIF had already been taken.

This loan was controversial, as the only other operational laser at LLNL at the time, Beamlet (a single experimental beamline for NIF), had recently been sent to Sandia National Laboratory in New Mexico.