Inertial Fusion Scheme Research Articles

Magnetic field transport in propagating thermonuclear burn

High energy gain in inertial fusion schemes requires the propagation of a thermonuclear burn wave from hot to cold fuel. We consider the problem of burn propagation when a magnetic field is orthogonal to the burn wave. Using an extended-MHD model with a magnetized α energy transport equation, we find that the magnetic field can reduce the rate of burn propagation by suppressing electron thermal conduction and α particle flux. Magnetic field transport during burn propagation is subject to competing effects: the field can be advected from cold to hot regions by ablation of cold fuel, while the Nernst and α particle flux effects transport the field from hot to cold fuel. These effects, combined with the temperature increase due to burn, can cause the electron Hall parameter to grow rapidly at the burn front. This results in the formation of a self-insulating layer between hot and cold fuel, which reduces electron thermal conductivity and α transport, increases the temperature gradient, and reduces the rate of burn propagation.

Inertial fusion without compression does not work either with or without nanoplasmonics

AbstractA recently published scheme for inertial fusion based on instantaneous heating of an uncompressed fuel is criticized. It is shown that efficient fusion and “time-like” fusion burn propagation cannot be realized due to the low nuclear reaction cross-sections. The suggested use of nanospheres inside the volume of the target to support the fast heating of the fuel is also questioned.

Evolution of Gas Cell Targets for Magnetized Liner Inertial Fusion Experiments at the Sandia National Laboratories PECOS Test Facility

Z-beamlet experiments conducted at the PECOS test facility at Sandia National Laboratories (SNL) investigated the nonlinear processes in laser plasma interaction (or laser-plasma instabilities) that complicate the deposition of laser energy by enhanced absorption, backscatter, filamentation, and beam-spray that can occur in large-scale laser-heated gas cell targets. These targets and experiments were designed to provide better insight into the physics of the laser preheat stage of the Magnetized Liner Inertial Fusion scheme being tested on the SNL Z-machine. The experiments aim to understand the trade-offs between laser spot size, laser pulse shape, laser entrance hole window thickness, and fuel density for laser preheat. Gas cell target design evolution and fabrication adaptations to accommodate the evolving experiment and scientific requirements are described in this paper.

Influence of low-temperature resistivity on fast electron transport in solids: scaling to fast ignition electron beam parameters

The role of low-temperature electrical resistivity in defining the transport properties of mega-Ampere currents of fast (MeV) electrons in solids is investigated using 3D hybrid particle-in-cell (PIC) simulations. By considering resistivity profiles intermediate to the ordered (lattice) and disordered forms of two example materials, lithium and silicon, it is shown that both the magnitude of the resistivity and the shape of the resistivity-temperature profile at low temperatures strongly affect the self-generated resistive magnetic fields and the onset of resistive instabilities, and thus the overall fast electron beam transport pattern. The scaling of these effects to the giga-Ampere electron currents required for the fast ignition scheme for inertial fusion is also explored.

Effect of nonthermal electrons on the shock formation in a laser driven plasma

In the laser-driven inertial fusion schemes and specifically in the shock ignition concept, non thermal electrons may be generated. By depositing their energy far from the origin, they can significantly modify the target hydrodynamics. It is shown in this paper that these electrons may affect the laser-driven shock formation and its propagation through the target. These changes are induced by the target heating and depend on the electron energy spectrum. Furthermore, results of some passive diagnostic may be misinterpreted, indicating an apparent different pressure.

Controlling the fast electron divergence in a solid target with multiple laser pulses.

Controlling the divergence of laser-driven fast electrons is compulsory to meet the ignition requirements in the fast ignition inertial fusion scheme. It was shown recently that using two consecutive laser pulses one can improve the electron-beam collimation. In this paper we propose an extension of this method by using a sequence of several laser pulses with a gradually increasing intensity. Profiling the laser-pulse intensity opens a possibility to transfer to the electron beam a larger energy while keeping its divergence under control. We present numerical simulations performed with a radiation hydrodynamic code coupled to a reduced kinetic module. Simulation with a sequence of three laser pulses shows that the proposed method allows one to improve the efficiency of the double pulse scheme at least by a factor of 2. This promises to provide an efficient energy transport in a dense matter by a collimated beam of fast electrons, which is relevant for many applications such as ion-beam sources and could present also an interest for fast ignition inertial fusion.

Imprint reduction in rotating heavy ions beam energy deposition

The compression of a cylindrical target by a rotating heavy ions beam is contemplated in certain inertial fusion schemes or in heavy density matter experiments. Because the beam has its proper temporal profile, the energy deposition is asymmetric and leaves an imprint which can have important consequences for the rest of the process. In this paper, the Fourier components of the deposited ion density are computed exactly in terms of the beam temporal profile and its rotation frequency Ω. We show that for any beam profile of duration T, there exist an infinite number of values of ΩT canceling exactly any given harmonic. For the particular case of a parabolic profile, we find possible to cancel exactly the first harmonic and nearly cancel every other odd harmonics. In such case, the imprint amplitude is divided by 4 without any increase of Ω.

Features of a point design for Fast Ignition

Fast Ignition is an inertial fusion scheme in which fuel is first assembled and then heated to the ignition temperature with an external heating source. In this note we consider cone and shell implosions where the energy supplied by short pulse lasers is transported to the fuel by electrons. We describe possible failure modes for this scheme and how to overcome them. In particular, we describe two sources of cone tip failure, an axis jet driven from the compressed fuel mass and hard photon preheat leaking through the implosion shell, and laser prepulse that can change the position of laser absorption and the angular distribution of the emitted electrons.

Efficiency and symmetry of heavy ion driven inertial fusion hohlraum targets

High conversion efficiency, a high degree of radiation symmetry and a large transfer efficiency onto the capsule surface are among the most important issues associated with an indirect drive inertial fusion scheme. These problems are interrelated and it is possible that when we try to improve one of these factors, the others might be affected in an adverse manner. For example, by using a sufficiently large hohlraum-to-capsule radii ratio (of the order of 4 - 5), or by keeping this ratio reasonably small (of the order of 3) and introducing an appropriate number of radiation shields at suitable locations in the hohlraum, a very high degree of symmetry (of the order of 99%) may be achieved, but the transfer efficiency may become as low as 15% or even less. It is therefore extremely important to optimize these effects simultaneously to design an efficient indirect drive target. In this paper we study the above problems in a self-consistent manner using two-dimensional numerical simulation. The problem considered is three-dimensional with an axial symmetry. The hohlraum is an ellipsoid made of solid gold with a spherical capsule at the centre. Two identical cylindrical converters made of low-density solid gold are placed symmetrically around the capsule. In addition, a number of radiation shields have been placed at appropriate places inside the hohlraum. It has been found that this design leads to an 80 - 90% conversion efficiency, 15 - 20% transfer efficiency and symmetry of the radiation field.

Antimatter-driven fusion propulsion scheme for solar system exploration

The potential use of the proton-antiproton annihilation reaction as a driver for an inertially confined, magnetically insulated fusion plasma with application to advanced space propulsion is examined. The fusion scheme utilized is the magnetically insulated inertial confinement fusion (MICF) concept which combines the favorable aspects of both inertial and magnetic fusions into one. Using an appropriate set of governing equations for the fusion plasma, along with those that detail the annihilation reactions and the energy deposition by the annihilation products including contributions from muon catalysis, we calculate the energy gain for the system as well as the amount of antihydrogen needed to ignite the plasma. We find about 13 ng of antihydrogen are needed to supply a megajoule of energy to the plasma, and about 10 g will be needed for a 220-mT space vehicle to make a one-way trip to Mars in about 2 months. I. Introduction I N several previous publications1-3 we examined the potential use of the magnetically insulated inertial confinement fusion (MICF) concept as a propulsion device that could be utilized in solar explorations and/or interplanetary travel. The concept in question combines the favorable aspects of both magnetic and inertial fusions in that physical containment of the hot plasma is provided by a metallic shell while its thermal energy is insulated from the material wall by a strong, self-generated magnetic field as illustrated in Fig. 1. Unlike the conventional implosion-type inertial fusion schemes, energy production in this approach does not require compression of the fusion fuel to many times solid-state densities and simultaneous delivery of energy to the core to initiate the burn. Instead the fusion plasma is created through wall ablation by an incident laser beam that enters the target through a hole. The same laser gives rise to the strong magnetic field through a process known as the thermoelectric effect, whereby it can be shown that such a field can be generated in a hot plasma when its density gradient is perpendicular to its temperature gradient. We have seen that MICF is capable of producing specific impulses of several thousand seconds and thrusts of tens of kilonewtons that would allow round trips to Mars, for example, to be made in relatively short periods (a few months) even when the massive power supply system for the laser driver is carried on board. It is clear that such travel times can be substantially reduced if the power supply component is eliminated from the dry weight of the vehicle. Several recent studies4 have proposed beaming the needed energy from an Earth-orbiting space station, but it is evident that a more reliable performance can be assured if a compact energy source is taken along. Clearly no source can match that of matter-antima tter annihilation reactions since such a fuel (e.g., anti-hydrogen) possesses the largest specific energy, i.e., energy per unit mass, provided, of course, that the technology for producing, storing, and manipulating such

Compact Fusion Advanced Rankine (CFAR II) Power Cycle

The Compact Fusion Advanced Rankine (CFARII) power cycle is a direct plasma energy conversion scheme for inertial fusion (ICF) and magnetically-insulated, inertially confined fusion (MICF) reactors...

Electron- and ion-beam transportation channel formation by laser ionization based on resonance saturation—LIBORS

We have been able to show that ’’laser ionization based on resonance saturation’’, LIBORS, represents a powerful new form of laser interaction that can be used to efficiently couple laser energy into either a cold gas or a plasma. In essence, the dense population of resonance-state atoms resulting from laser resonance saturation represents both a reservoir of energy that rapidly provides translation energy to the free electrons through superelastic collisions and a large pool of atoms having an ionization energy reduced by the laser photon energy. We show that over a wide range of conditions LIBORS can be superior to both multiphoton ionization and inverse bremsstrahlung by many orders of magnitude. Indeed, LIBORS appears to be particularly well suited to create long plasma channels needed for electron- or ion-beam transportation in future inertial fusion schemes. We have estimated that a suitably tuned laser pulse of about 0.7 J and 400-nsec duration should be capable of creating a plasma channel with an electron density of close to 1015 cm−3 over an area of 0.2 cm2 for a length of 5 m in sodium vapor of about 0.1 Torr. A similar result should be possible in lithium vapor.