PhD opportunity

Inertial confinement fusion using laser driven and wire array Z-pinch driven hohlraums

Supervisor Prof. Jeremy Chittenden

One of the main approaches to inertial confinement fusion (ICF) is ‘indirect drive’ where lasers or X-ray sources are used to heat a cylindrical cavity called a ‘holhraum’ which surrounds a spherical capsule containing the fusion fuel. This hohlraum is typically heated to several million degrees Centigrade at which point the inner surface acts as a black body radiation source. The X-rays emitted by this black body then bombard the capsule causing the surface to expand rapidly. The reaction force due to this surface expansion causes the interior of the capsule to implode, compressing the fusion to very high densities and heating it to fusion temperatures.
Much of the recent work investigating the use of hohlraums in indirect drive ICF has taken place on the National Ignition Facility laser at Lawrence Livermore National Laboratory. Recent experiments have concentrated on trying to achieve the process of ‘ignition’, where alpha particles heat the plasma and enhance the energy yield. Plasma pressures inside the capsule reaching half the value required for ignition have been demonstrated along with significant levels of alpha particle heating. One of the factors currently thought to be inhibiting further increases in fusion yield is a lack of symmetry in the radiation from the hohlraum reaching the capsule. There are a number of potential causes for this, such as the non-uniform expansion of the inside wall of the hohlraum that occurs as it is heated by the laser. In addition, the extreme plasma density and temperature gradients that exist within the hohlraum are thought to be a source of spontaneously generated magnetic fields which can strongly affect the uniformity of the black body temperature distribution that is obtained.
An alternative to the use of lasers to heat the hohlraum is to use X-ray sources from magnetically driven implosions. One such scheme uses a cylindrical target formed from an array of fine metallic wires which is imploded using the electro-mechanical force from a pulsed electrical driver with several mega-amperes of current. This ‘wire array Z-pinch’ plasma provides a highly efficient X-ray source which can in turn be used to heat a larger scale hohlraum and capsule than is used with laser driven indirect drive. This approach provides a potential means of realising plasma ignition on a larger scale plasma with substantially higher fusion yields.
This PhD project will involve large scale high performance computing simulations of laser driven and wire array driven hohlraums using the 3D radiation magneto-hydrodynamics code ‘Chimera’ developed at Imperial College. One of the principle objectives is to undertake the first comprehensive three-dimensional treatment of the effects of magnetic fields on radiation symmetry in laser driven hohlraums. Further simulations will investigate potential future designs for wire array Z-pinch driven hohlraums. This work will also involve modelling in-house experiments on the Magpie pulsed electrical generator designed to study the ablation of materials using X-ray pulses from imploding wire array Z-pinches.
The project will be based within the Centre for Inertial Fusion Studies at Imperial College and will involve close collaborative work with experimental groups at Lawrence Livermore National Laboratory and Sandia National Laboratory as well as the Magpie group at Imperial.
Background reading
O. A. Hurricane, et. al. Nature 506, 343 (2014).
J.P. Chittenden et. al. Physics of Plasmas 23, 052708 (2016).
C. Walsh et. al. Phys. Rev. Lett. 118, 155001 (2017).
J.H. Hammer, et. al. Physics of Plasmas 6, 2129 (1999).

Magnetised ignition and burn in inertial confinement fusion plasmas
Supervisor Prof. J. Chittenden
 
One of the principle components of inertial confinement fusion (ICF) is the process of ‘ignition’, where energetic alpha particles released by the fusion process become the dominant heat source, driving further fusion reactions. The robust ignition of a central fusion ‘hotspot’ leads to strong heat flow into the surrounding fuel, resulting in a ‘burn wave’ propagating outwards, that leads to large amplification of the energy yield.
Recent experiments on the National Ignition Facility (NIF) laser at Lawrence Livermore National Laboratory have reached the regime where alpha particle heating is dominant, but have yet to achieve sufficiently robust ignition to trigger a self-sustaining burn wave. Fusion performance on NIF is currently limited by inherent asymmetries in the radiation source and capsule structure which give rise to inhomogeneous implosions and effectively reduce the heat coupled to the hotspot. While the majority of current research efforts at NIF are directed at understanding and controlling the growth of these asymmetries, an alternate route accepts that perturbations are ubiquitous in ICF experiments and instead reduces hot-spot cooling through the application of external magnetic fields. Magnetisation of the electrons within the hotspot plasma suppresses thermal conduction losses and sustains the high temperatures for longer. Preliminary simulations suggest that applying a 50 Tesla initial field to the best performing the capsules, may be sufficient to push the hotspot over an ‘ignition cliff’ into a regime where self-sustaining burn begins, with an accompanying ten-fold increase in overall energy yield.
 
The physics of ignition in magnetised plasmas becomes fundamentally different to that in conventional ICF implosions. Magnetising the electrons requires fields of several tens of thousands of Tesla, which relies on effective compression of the seed field by the implosion itself. A complete treatment of the magneto-hydrodynamic (MHD) models for the plasma reveals additional terms which redistribute the magnetic flux and modify the heat flow. The field results in an inherent directional bias meaning that the heat flow, the ignition process and the burn propagation all become intrinsically anisotropic. For very large magnetic fields the alpha particles become trapped within hotspot such that burn propagation becomes driven by radiation transport rather through electrons and alphas. As part of this work, we will study the physics of magnetised burn in ICF, investigating modifications to the heat flow, alpha heating and burn propagation and the inherent asymmetries that result. Magnetisation and alpha heating also affect the burning plasma on a microscopic level, changing the electron distribution function and hence all of the collisional processes within the plasma.
 
This PhD project will involve large scale high performance computing simulations of magnetised ignition and burn in ICF plasmas using the 3D radiation magneto-hydrodynamics code ‘Chimera’ developed at Imperial College. One of the principle objectives is to undertake the first comprehensive three-dimensional treatment of the effects of magnetic fields on ignition in high yield NIF capsule implosions. The physical processes of interest are, however, common to all approaches to magnetised ICF. The project will therefore also explore the benefits of fuel magnetisation in direct drive designs as well as understanding the process of ignition within magnetised liner inertial fusion where fuel magnetisation is an intrinsic component of the design. The project will also explore how magnetisation affects the extrapolation of fusion performance to next generation facilities in indirect, drive and magnetically driven scenarios. The work will involve a combination of skills including the development of theoretical models for magnetised microphysics, their integration into large scale computer models and design simulations for experiments on NIF, the Omega laser and the ‘Z’ pulsed power facility.
 
The project will be based within the Centre for Inertial Fusion Studies at Imperial College and will involve close collaborative work with experimental groups at Lawrence Livermore National Laboratory, the Laboratory for Laser Energetics at the University of Rochester and Sandia National Laboratory.
 
Background reading
L. J. Perkins, et. al. Physics of Plasmas 24, 062708 (2017).
C. Walsh et. al. Phys. Rev. Lett. 118, 155001 (2017).
J.P. Chittenden et. al. Physics of Plasmas 23, 052708 (2016).
O. A. Hurricane, et. al. Nature 506, 343 (2014).
Modelling the effects of radiation and magnetic fields on shocks and turbulent flows in laboratory astrophysics experiments
 
Laboratory astrophysics provides a mechanism to test our understanding of the behaviour of astrophysical bodies by using dimensional scaling to design laboratory experiments which behave in a similar manner. Such experiments must be carefully designed to replicate the physical processes which make the non-linear evolution of supersonic flows in dense astrophysical plasmas very different to that found in conventional fluid dynamics. The intense X-ray radiation generated within shock fronts allows the plasma to cool, making it more compressible and collapsing the shock until it becomes too thin to remain hydrodynamically stable. This process is believed to contribute to the break-up of expanding blast waves which form supernovae remnants. Transport and reabsorption of some of this X-ray radiation produces a ‘precursor’ propagating ahead of the shock discontinuity which fundamentally changes the nature of the shock front. Strong density and temperature gradients at the shock front or elsewhere in the plasma can also be the source of spontaneously generated magnetic fields which are thought to be a possible candidate for the first ‘primordial’ magnetic fields generated within the universe. Compression of these fields can lead to strongly magnetised flows which again modify the nature of the shock as is the case in the Earth’s magneto-spheric bow-shock. Magnetic fields can also be responsible for the development of instabilities within supersonic flows, leading eventually to turbulent magnetised flows which again have very different properties to turbulence in conventional fluid dynamics.
Experiments designed to replicate these processes in the laboratory, typically require the use of large scale lasers or pulsed power generators in order to achieve the high energy density plasma states required. Recent work has included the use of the Orion laser to produce radiative blast waves and the use of the 1.5MA Magpie generator at Imperial College to investigate magnetic reconnection in supersonic flows. The principle computational tool which enables the design of new experiments on Magpie and other pulsed drivers is the Gorgon 3D magneto-hydrodynamics tool which was developed within the Plasma Physics group at Imperial College.
This PhD studentship will involve the adaptation of the Gorgon code to new laboratory astrophysics configurations to enable the study of the effects of radiation and magnetic fields on the stability of blast waves, particle acceleration in processes in magnetised shocks, studies of magnetically decelerated supersonic flows and the compression of turbulent magnetised plasmas. The work will involve the continued development of elements of the core MHD model as well as post-processing models which generate ‘synthetic diagnostics’ to facilitate comparison to experimental data. The student will be expected to collaborate with experimental groups at Imperial College, Cornell University, UCSD and Sandia National Laboratories as well as computational and theoretical groups at the Universities of Rochester and Paris VI.
Background reading
B.Remington, Plasma Phys. Control. Fusion 47 A191 (2005) doi:10.1088/0741-3335/47/5A/014
F. Suzuki-Vidal, et. al. Phys. Rev. Lett. 119, 055001 (2017) doi: 10.1103/PhysRevLett.119.055001
M. Bocchi, B. Ummels, J.P. Chittenden, et. al. The Astrophysical Journal 84, p767 2013 (doi:
10.1088/0004-637X/767/1/84)
J. Hare Phys. Rev. Lett 118, 085001 (2017) doi: 10.1103/PhysRevLett.118.085001

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