Physics of Ignition: Collaboration with the National Ignition Facility: Diagnosing Hot-Spot Mix via X-Ray Spectroscopy

The fusion of light nuclei is the energy source that powers the sun. If harnessed on earth, it could provide limitless low-carbon energy. The basic fuel - the Deuterium and Tritium (D&T) forms of heavy hydrogen, are either readily available in sea-water, or can be 'bred' from the abundant element Lithium (the element in a mobile phone battery). The primary nuclear waste products are harmless - the main being helium (an alpha particle), an inert gas found in party balloons. This all sounds too good to be true - and in a sense it is - because getting the reaction to occur is incredibly difficult - because pushing the D and T close together such that the strong force causes them to bind takes a lot of energy (they repel as they are positively charged nuclei). Getting them to move fast enough so that when by chance they have a head-on collision and get close enough to fuse corresponds to heating them to 100 million K. Confining such a hot plasma for long enough for the collisions to occur is no mean feat. There are two approaches: the first uses a magnetic bottle to keep a low density gas away from the walls of a container. As the density is low, collisions take several seconds - this is the magnetic fusion approach. The second idea uses lasers irradiating a small spherical balloon containing the heavy hydrogen. The laser heats the outside of the balloon from different directions, creating a hot plasma that expands into the vacuum, and then, like a spherical rocket, the shell moves towards the centre, compressing the heavy hydrogen to high temperatures and densities 100s of times denser than ordinary liquid. No magnetic fields are needed, because owing to the high density, the collisions are very rapid, and although the compressed miniature sun will expand again (and blow up more quickly if fusion takes place), the reaction occurs faster than the explosion itself - the material is confined by its own inertia. This is called inertial confinement fusion. In current studies at the National Ignition Facility in California, this goal is close to being realised. However, at present there are still problems to be overcome. One of the major ones is that the shell does not compress uniformly, and it is known that if the implosion is not close to being perfectly spherical, then any ripples will grow, breaking up the wall of the shell before the peak of the implosion. The shell of the balloon then mixes into the fuel, and starts to 'glow' due to the high temperatures, and cools the system, preventing fusion. Therefore, two interlinked problems need to be tackled - firstly, we need to find out how much of the shell is mixing into the heavy hydrogen core - and secondly we need to work out how to prevent this happening (either by making better targets, or illuminating the sphere more uniformly). This research grant addresses the first measurement problem. For various physics reasons the shell of the balloon contains some heavy elements (particularly Germanium) which, if they mix into the hot core, 'light-up' and emit characteristic X-ray lines. From a study of the absolute and relative brightness of these lines, it is possible to gain information on the temperature of the material, and of the density, and also, of the amount of the shell that has mixed into the core. Some of this work has already been performed by our US colleagues. However, at present the technique is not quite accurate enough to say if the amount that has mixed in is really enough to extinguish the reaction. The Oxford and York groups in the UK here put forward several new ideas to improve the theory and experimental technique to a point where we believe we will be able to say if the mix level is acceptable. These ideas are based on a new high resolution x-ray instrument, novel spectroscopic theory looking at the brightness of X-rays from different elements, and by performing sophisticated full 3 dimensional simulations of the emission process.