Real experiments are irreplaceable in providing new insights into subtle physics issues and in stirring the creative imagination of scientists.

“Magnetohydrodynamic scaling: From astrophysics to the laboratory”, Ryutov, Remington, Robey and Drake in Physics of Plasmas (2001)

PUFFIN is design to be highly versatile, and will drive a range of loads in different geometries to produce shocks and jets, magnetized turbulence, magnetic reconnection, magnetized heat flow. Research areas of interest include magnetohydrodynamic turbulence, magnetic reconnection and magnetized target fusion.

Magnetohydrodynamic Turbulence

Laser shadowgraphy image of a turbulent column of magnetized plasma, driven by pulsed power.

Turbulence is a fundamental process in all fluids, including plasmas. It dissipates large scale fluid motions through a cascade to ever smaller scales, until viscosity dominates and dissipates the kinetic energy as heat. In a plasma, magnetic fields couple to the charged particles, breaking the symmetry and creating anisotropic, fully three-dimensional turbulence. In MHD, the basic building blocks of turbulence are magnetic flux tubes, which interact, creating current sheets which in turn drive flux-tube merging through magnetic reconnection.

In a plasma, the familiar Reynolds number Re=uL/\nu\propto n T^{-5/2}), has a magnetic counterpart Re_M=uL/\eta\propto T^{3/2}, where \eta is the magnetic diffusivity. Both must be large to sustain MHD turbulence, requiring a high-energy-density (HED) state with high density and high temperature. Such plasmas can be created by high-powered lasers or pulsed-power driven currents. Pulsed-power creates long lasting, inherently magnetised plasmas, with an astrophysically-relevant equipartition between the magnetic, thermal and kinetic energies. This is in contrast to long-lived magnetic confinement plasmas, dominated by magnetic pressure, or short-lived laser-driven plasmas, dominated by kinetic and thermal pressures.

The PUFFIN current pulse is around ten times longer than existing pulsed-power facilities, allowing it to drive the plasma over many Alfvénic timescales. Combined with the astrophysically relevant equipartition between the energy components, PUFFIN provides a unique and ideal driver for MHD turbulence experiments.

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Magnetic Reconnection

Ultra-high speed imaging of the self-emission from a magnetic reconnection experiment, showing the development of plasmoids within the reconnection layer.

Magnetic reconnection is a ubiquitous and important process throughout the Universe. It explosively reconfigures the topology of magnetic field lines, and enables the rapid dissipation of magnetic energy, heating and accelerating the plasma. This process has been studied in both space and laboratory experiments, including the collisionless regime relevant to the solar wind. In recent years, high energy density experiments driven by lasers or pulsed-power have further expanded the plasmas in which we can study reconnection.

In particular, recent pulsed-power experiments have demonstrated the development of the plasmoid instability, a tearing of the reconnecting current sheet, which leads to very rapid reconnection and dissipation of the magnetic energy. Although in the purely MHD regime plasmoids can only develop at high Lundquist numbers (a dimensionless measure of plasma conductivity), recent theoretical work has demonstrated the existence of a semi-collisional regime, in which two fluid effects are important, which enables plasmoids to form at much more modest conditions. As such, these pulsed-power experiments offer a qualitative window into the plasmoid instability, which is believe to be important in reconnection throughout the Universe.

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Magnetized Target Fusion

Two approaches to controlled thermonuclear fusion have dominated the research landscape for decades – magnetic confinement fusion, which uses large, externally generated magnetic fields to confine a large, sparse plasma over long timescales, and inertial confinement fusion, which uses intense radiation fields (lasers or X-rays) to implode capsules of fuel, transiently creating powerful fusion explosions.

A third approach, magnetized target fusion (MTF, also called magneto-inertial fusion) utilises aspects of both of these techniques. A magnetised plasma target is formed and then compressed inside a “liner”, which can be made from imploding, heavy plasma, or a solid metal wall. The magnetic fields insulate the target from the cold liner, keeping the fuel hot as it sub-sonically compresses to a hot, dense state where fusion occurs.

An outstanding challenge in MTF is understanding how heat is transported in these hot, dense, magnetized plasmas. Exotic effects outside of standard magnetohydrodynamics, such as the Nernst term, can lead to rapid destruction of the insulating magnetic fields. Tangled magnetic field lines, generated by turbulence can provide very long connection lengths for plasma, effectively lower the bulk conductivity. Using PUFFIN we will create and sustain MTF-relevant plasmas and study heat transport effects within them to assess the viability of various MTF approaches.

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