LASER FUSION
UK Inertial Fusion research is principally focussed on Laser Fusion. The goal of laser fusion is to assemble, then ignite, a tiny star on earth using lasers. If this can be achieved, some of the mass of this 'star' is converted into energy (E=mc^2) liberating clean, green, energy. As the fuel source is found in sea water, this offers an essentially limitless source of clean energy for humankind - this is our goal.
Laser fusion, also called laser inertial confinement fusion (ICF), is initiated by heating the surface of a small spherical capsule containing fusion fuel (a mixture of Deuterium and Tritium). The heating turns the surface into a high-pressure plasma. The high-pressure plasma then accelerates outwards from the capsule surface, in a process called ablation. This creates an equal and opposite inward force, accelerating the remaining capsule and fuel inwards, initiating a spherical implosion.
A sufficiently fast and spherical implosion can compress this fuel to 4000 times solid density (1,000,000 kg/m^3), and heats the centre to 60 million degrees Kelvin. This creates a tiny star, in fact the conditions are more extreme than those in the Sun.
These ignition conditions trigger a chain reaction; a thermo-nuclear burn-wave then spreads into the rest of the fuel. Only once this occurs is more energy is liberated from the implosion than that input by the lasers. To-date, attempts to achieve ignition have not succeeded, yet they are very close. The challenge is to get these implosions to very high velocity which keeping them spherical.
There are two principle methods being used to perform Laser Fusion; direct drive and indirect drive.
LASER DIRECT DRIVE
In the Laser Direct Drive concept many lasers impinge directly on a ~millimetre sized capsule containing the DT fuel. In a few billionths of second, this heats the outer surfaces of the capsule driving an implosion. Within 10 billionths of a second the fuel is compressed to ignition densities and temperatures. As the fuel collapses onto a central region a thermonuclear burn lasting one-tenth of a billionth of second occurs generating many more times the energy in the laser. This net gain is essential and required for the economic viability of any future power plant. Direct drive is efficient as it couples the laser directly to the target, it uses relatively simple, albeit near-perfect, targets and requires the use of only low Atomic Number, and cheap, raw materials.
A schematic of the direct drive Laser Fusion concept.
Internationally, direct drive research is led by the US Laboratory for Laser Energetics (LLE), using the Omega laser facility. Ignition is not possible on Omega as the laser energy is insufficient, yet it is possible to demonstrate the creation of ignition-equivalent implosion performance at reduced scale. This is called 'hydrodynamically-equivalent ignition'. Recent progress on Omega seen a generalised Lawson criteria of 0.74, this is just shy of ignition. If scaled to the National Ignition Facility this would yield approximately 600 kJ.
LASER INDIRECT DRIVE
Indirect drive uses a hollow metal cavity ('hohlraum') to convert the laser light into x-rays, which then ablate the implosion capsule. This has the advantage that the x-ray radiation ablating the capsule is very smooth which helps limit the growth of damaging hydrodynamic instabilities, but is disadvantaged by the fact that the process is less efficient at coupling energy to the implosion. This means a larger, and more expensive, laser is required for ignition than if the implosion process was more efficient. The world's largest laser, the National Ignition Facility (NIF) in the USA principally investigates the indirect drive approach to Laser Inertial Fusion, as will the French Laser MegaJoule (when complete).
A schematic of the indirect drive Laser Fusion concept.
Fusion energy-gain via Indirect Drive Laser Fusion is yet to be achieved, however experiments at the US National Ignition Facility (NIF) suggest this approach is closing in on ignition. Progress continues, with fusion-energy output tripling since October 2020, while the pressure and confinement time exceed 80% of those required for fusion energy-gain (ignition). As NIF makes progress towards entering the burning plasma regime – a key milestone on the path to ignition – an influential US report has called for the US to move aggressively towards the development of Inertial Fusion Energy.
DIRECT DRIVE SHOCK IGNITION
The Direct Drive approach to laser fusion illuminates the implosion capsule directly with the lasers. This is a lot more energetically efficient than other 'indirect' methods.
In conventional Direct Drive, as the implosion converges towards it's centre, the imploding capsule is decelerated by the pressure building within it's core. This converts the momentum of the implosion into pressure, compressing and heating the fuel within the core, initiating ignition. However this requires high implosion velocities, which can cause the growth of fluid instabilities (e.g. the Rayleigh-Taylor instability), these can prevent ignition.
In the Shock Ignition approach, the implosion velocity is deliberately reduced so the fluid instabilities are no longer a problem. However now the momentum of the implosion is insufficient to initiate ignition. Instead a strong shock is launched towards the end of the implosion, this further heats and compresses the core, creating the required ignition conditions.
Based on simulations, the Shock Ignition approach should make energy gain possible with a significantly smaller, and therefore cheaper, laser system than current multi-billion dollar laser fusion systems.
