What the Recent Progress in Laser Fusion Means

On Aug. 8, 2021, Lawrence Livermore National Laboratory’s (LLNL’s) National Ignition Facility (NIF) made a significant step toward ignition, achieving a yield of more than 1.3 megajoules (MJ).

This is still twelve times less than the 20 megajoules that simulations indicate can be achieved. Everything would need to be redesigned to try to get to net power generation for the whole system. 422 megajoules of power are needed to charge the capacitors for each shot. It would probably need to have triple the 422 megajoules to get to replacement breakeven of the energy generation needed to get the power to the outlets to charge the capacitors.

The performance needs further improvement by over 1000 times, the system needs to be recreated with rapid pellet loading and handling in order to create anything approaching a viable energy generation system. This system is not the path to new energy generation. It is a decades-long research project for nuclear weapons.

A one-gigawatt nuclear fission reactor can generate about 8 terawatt-hours of power in a year. This is about 29 billion megajoules in a year. The LLNL nuclear fusion shot produced 22 billion times less energy.

Initial analysis shows an 8X improvement over experiments conducted in spring 2021 and a 25X increase over NIF’s 2018 record yield.

They focused many lasers target the size of a BB that produces a hot-spot the diameter of a human hair, generating more than 10 quadrillion watts of fusion power for 100 trillionths of a second.

The experiment built on several advances gained from insights developed over the last several years by the NIF team including new diagnostics; target fabrication improvements in the hohlraum, capsule shell and fill tube; improved laser precision; and design changes to increase the energy coupled to the implosion and the compression of the implosion.

250 kilo-joules of energy was deposited on the target (roughly 2/3 of the energy from the beams). Lawrence Livermore National Laboratory has its 1.9 MJ laser system running at full power.

Sankey diagram of the laser energy to hohlraum x-ray to target capsule energy coupling efficiency. Note the “laser energy” is after conversion to UV, which loses about 50% of the original IR power. The conversion of x-ray heat to energy in the fuel loses another 90% – of the 1.9 MJ of laser light, only about 10 kJ ends up in the fuel itself.

The name National Ignition Facility refers to the goal of igniting the fusion fuel, a long-sought threshold in fusion research. In existing (non-weapon) fusion experiments the heat produced by the fusion reactions rapidly escapes from the plasma, meaning that external heating must be applied continually in order to keep the reactions going. Ignition refers to the point at which the energy given off in the fusion reactions currently underway is high enough to sustain the temperature of the fuel against those losses. This causes a chain-reaction that allows the majority of the fuel to undergo a nuclear burn. Ignition is considered a key requirement if fusion power is to ever become practical.

NIF is designed primarily to use the indirect drive method of operation, in which the laser heats a small metal cylinder instead of the capsule inside it. The heat causes the cylinder, known as a hohlraum (hollow cavity), to re-emit the energy as intense X-rays, which are more evenly distributed and symmetrical than the original laser beams. Experimental systems, including the OMEGA and Nova lasers, validated this approach through the late 1980s. The baseline NIF pellet design is about 2 mm in diameter, chilled to about 18 kelvins (−255 °C) and lined with a layer of frozen DT fuel. The hollow interior also contains a small amount of DT gas.

In a typical experiment, the laser will generate 3 MJ of infrared laser energy of a possible 4. About 1.5 MJ of this is left after conversion to UV, and about 15 percent of this is lost in the x-ray conversion in the hohlraum. About 15 percent of the resulting x-rays, about 150 kJ, will be absorbed by the outer layers of the target. The coupling between the capsule and the x-rays is lossy, and ultimately only about 10 to 14 kJ of energy is deposited in the fuel itself.

The resulting inward directed compression is expected to compress the fuel in the center of the target to a density of about 1,000 g/cm3 (or 1,000,000 kg/m3);for comparison, lead has a normal density of about 11 g/cm3 (11,340 kg/m3). The pressure is the equivalent of 300 billion atmospheres.

Based on simulations, it was expected this would produce about 20 MJ of fusion energy to be released, resulting in a net fusion energy gain, denoted Q, of about 15 (fusion energy out/UV laser energy in). Improvements in both the laser system and hohlraum design are expected to improve the energy absorbed by the capsule to about 420 kJ (and thus perhaps 40 to 50 in the fuel itself), which, in turn, could generate up to 100-150 MJ of fusion energy. However, the baseline design allows for a maximum of about 45 MJ of fusion energy release, due to the design of the target chamber. This is the equivalent of about 11 kg of TNT exploding.

These output energies are still less than the 422 MJ of input energy required to charge the system’s capacitors that power the laser amplifiers. The net wall-plug efficiency of NIF (UV laser energy out divided by the energy required to pump the lasers from an external source) would be less than one percent, and the total wall-to-fusion efficiency is under 10% at its maximum performance. An economical fusion reactor would require that the fusion output be at least an order of magnitude more than this input. Commercial laser fusion systems would use the much more efficient diode-pumped solid state lasers, where wall-plug efficiencies of 10 percent have been demonstrated, and efficiencies 16-18 percent are expected with advanced concepts under development.

Written By Brian Wang, Nextbigfuture.com