Pulsed Liner Fusion basics
Pulsed Liner Fusion is an approach to Inertial Fusion Energy based on the compression of a cylindrical fusion fuel cell with high power electrical energy
The Liner
The liner is a metal cylinder that serves as a means to both contain the fuel initially and to compress and confine it at fusion conditions. This is made from a metal to allow electrical current to pass material, and it typically beryllium or aluminum. The liner is 6mm in diameter and 10mm tall for present research with a wall thickness of 0.3mm. The wall thickness is designed to withstand magnetohydrodynamic instabilities that form during compression, and the outer surface is diamond polished to a roughness of <40nm to limit the seeding of such instabilities under very high accelerations.
The Fuel
The fuel is deuterium and tritium (DT) mix and the energy from fusion is released in the form of a large pulsed neutron yield. This is the same methods used in most inertial and magnetic confinement approaches as it most energetically favorable reaction pathway.
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The liner is filled with either DT gas under high pressure which is then sealed with a plastic foil, or the liner is cooled to cryogenic temperatures (< -250 C or < 15 K) to allow a dense DT ice layer to form on the inside wall.
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Deuterium can be harvested from seawater and therefore supplies are very large. Tritium is radioactive and must be bred during fusion reactions to create a sustainable supply. This is true for all fusion approaches using DT fuel
Pulsed Power Drive
Pulsed power is the generation of very high electrical pulses and applying them to a load in an electrical circuit. The most common way of generating these pulses is by charging many high energy capacitors to high voltage and then releasing all their energy at the same time. Modern pulsed power systems charge capacitor up to 100kV and generate load voltages of several million volts (megavolts, MV) and currents up to 25 million amperes (mega-amps, MA).
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The fact that the load is physically connected to the pulsed power driver electrodes (purple cones) guarantees excellent and efficient energy coupling to the liner. Also, pre-vacuumed electrode-target cassettes mean the whole fusion target can be inserted into a vacuum chamber that has just fired and is robust to a wide range of conditions
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This physical connection is also a challenge for a power plant that needs to operate every few seconds - this is the 'stand-off problem'. Everything close to the liner will be destroyed and needs replacing for every shot, and how well we can do that determines how quickly our energy can be produced for conversion to electricity
Fuel Preheat and Magnetization
The rapid compression by the very large magnetic field provides much of the heating to the fusion fuel (i.e. PdV work), but additional physics systems are needed to reach fusion conditions.
Firstly, the fuel must be magnetized to limit energy losses as the fuel heats up. This is achieved using a pair of Helmholtz coils to apply a large (up to 20T) axial B-field. Secondly, the fuel must be heated from its initial temperature at the start of the compression. This presently uses a high energy laser delivered directly into the fuel from the top of the liner. Both these needs can be provided by a variety of methods, including novel liner designs and direct use of the main current drive, which may eliminate the need for the present external systems.
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Experiments have demonstrated both the requirement to control the plasma parameters with magnetization, fuel-preheat, and compression, and our ability to successfully do so.
Compression & Burn
​​Driving a very large current through the liner generates a very large magnetic field around the outside of it. This accelerates the liner towards its own central axis, compressing the fuel. The mass of the heavy imploding liner provides inertial confinement of the fuel, maintaining peak fusion burn conditions for 1 - 2 ns. The DT fuel temperature is >30 million degrees (> 3keV) and rapidly occurring fusion reactions produce large numbers of neutrons at 14.1 MeV.
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The yield, i.e. the energy we can extract from the reaction, is primarily in these high energy neutrons. There are captured on the structures surrounding the vacuum chamber, as is typical for mot fusion schemes. The heat from neutron capture is then used to drive steam turbines for electricity production.
Performance and Projections
The Magnetized Liner Inertial Fusion (MagLIF) program at Sandia National Laboratories has produced DD neutron yields of 1.1 x 10 in single shot experiments. This is equivalent to a fusion triple product of 1.1 x 10 keV m s
This is behind only to the ignition results of the indirect laser drive NIF at LLNL and the laser direct drive work at OMEGA for inertial fusion and surpasses all magnetic fusion triple product results to date.
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These results were achieved with peak currents of 20MA on the Z machine at Sandia. For fusion gain, this is a sub-ignition facility and was never designed to demonstrate fusion gain. The fact that MagLIF has been highly successful with very limited funding compared to many other fusion approaches is remarkable.
It is predicted that drive currents of up to 60MA are needed to demonstrate facility gain in single shot experiments. Building such a pulsed power driver is both possible with present day technology, and much more cost effective and energy efficient than any laser drive system. In fact, Pacific Fusion are building one right now.......
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Leading Institutions
Simon Bott-Suzuki