Specially engineered layered laser targets could enable the ‘fast ignition’ of inertial confinement fusion by guiding and focusing electron beams with magnetic fields.

Fast Ignition ICF is an advanced variant of inertial confinement fusion in which hot spot generation is done separately from fuel compression.  The most studied fast ignition candidate scheme uses an ultra-high current beam of relativistic electrons generated by a petawatt laser pulse to generate the hot spot.  It is typically envisaged that a gold cone will be inserted into the spherical deuterium-tritium target to funnel the petawatt laser to the high density region of the target.

Fast Ignition is an attractive alternative to conventional central hot spot ignition, as it can achieve much higher gains (> 100) for a total laser energy of around 300kJ, which compares very favourably with the > 1MJ needed for central hot spot schemes.  Fast Ignition is thus thought to be a way to realize inertial fusion energy for less capital.

The fast electron beam must create a hot spot of not dissimilar radius to the laser spot after transversing a distance of up to 100µm.  This means that the fast electron beam must be well collimated in order to make the process sufficiently energetically efficient.  Unfortunately it is now clear that fast electrons are naturally produced with a significant angular distribution.  The possibility that beams might undergo magnetic collimation without any interference has been studied, but it is likely that this will not be sufficient.

Alex Robinson from the UK’s Central Laser Facility, together with Mike Key and Max Tabak of Lawrence Livermore National Laboratory in the US, has recently proposed a new scheme for tackling this problem.  The idea builds on previous theoretical and experimental work done by the CLF and user groups such as the Queen’s University Belfast group which has established that strong magnetic field growth can occur at resistivity gradients (such as the interface between two materials of different Z) thus providing a way to engineer the guiding of fast electron beams.

What Robinson and his co-workers suggest is that an array of several shells embedded in less resistive material can effectively guide and focus a fast electron beam by guiding different sub-populations of the fast electron beam along different paths.  This is achieved by the growth of very strong magnetic fields along the material interfaces.  The array, or “Magnetic Switchyard”, would enter as an insert that would sit inside the cone tip and thus not intrude on the fuel assembly.  

The results of fully 3D fast electron transport simulations that are close to the full fast ignition scale indicate that coupling efficiencies of >25% can be achieved and that energies around 10kJ can be coupled into the hot spot even when the fast electron beam is generated with a divergence half-angle in excess of 50 degrees.

This work outlines a route to overcoming the fast electron divergence problem with target engineering and suggests that Fast Ignition remains a bright prospect for advanced inertial fusion.

The work has been published in the March 2012 edition of Physical Review Letters (link opens in a new window).