Plasma-Jet driven Magneto-Inertial Fusion (PJMIF)
by Milos Stanic
Feasible, practical and relatively cheap nuclear fusion concept is one of science’s holy grails and the search has been on for more than five decades now. It represents one of the ultimate sources of energy for both terrestrial and space practices. There are dozens of research projects, involving more than twenty different approaches for nuclear fusion, but no fusion with gain has been achieved yet. I will hereby try to go through some of the main fusion ideas and introduce you to a fusion concept named Plasma-Jet driven Magneto-Inertial Fusion (PJMIF). The two main categories of nuclear fusion are Magnetic Confinement Fusion (MCF) and Inertial Confinement Fusion (ICF). As the name reveals, MCF involves utilizing strong magnetic fields to confine a low density plasma over a large spatial scale, for long enough periods to establish fusion reactions, based on the Lawson Criteria. MCF takes advantage of the fact that magnetic fields keep the ions (which carry a significant portion of the energy) within the “reaction domain” and therefore reduces thermal losses. Another very important property of MCF is that it is a steady state process. Alternatively, ICF follows a radically different approach. It uses high-energy laser pulses to compress a solid-state target. It does so by using a large array of lasers, symmetrically distributed across a spherical chamber. One places the target in the center of the chamber and fires the lasers which then compress the target to an extremely high density state at which fusion occurs. There are numerous ways to accomplish this (direct, indirect, fast ignition etc…), but, in essence, they all involve high-energy lasers and a target. This approach relies on the fact that a huge amount of energy is delivered within a very short amount of time and on a small space scale, taking the plasma to a much “higher than necessary” state, so although there is no magnetic field to trap the ions within the reaction domain, the conditions are sufficient to achieve fusion. Current state-of-the-art technology allows us to do both of these experiments. The two most famous fusion facilities are ITER (International Thermonuclear Experimental Reactor) in France, which represents the MCF approach and NIF (National Ignition Facility) in California, USA, which is the ICF representative. Both of these facilities are enormous in size and cost. Each of the approaches has its own advantages and disadvantages, but for the sake of length of this article, I will only discuss those through a comparison with PJMIF. PJMIF can be called a hybrid approach, trying to take the best of both MCF and ICF. A magnetized plasma target is confined inertially (like ICF) by imploding plasma jets, and electron thermal conduction is suppressed by internal, usually closed field, lines. The suppression of thermal conduction is important because this way we prevent the precious energy from flowing away from the reaction domain. There is no material shell close to the plasma, so PJMIF provides an elegant solution for the stand-off problem (stand-off problem is referring to the issues of surrounding equipment being damaged during the fusion process). The time-scale of the whole process is supposed to be on the order of several microseconds, where the confinement time is proportional to the jet velocity over jet length ratio. The parameter space, which illustrates a range of operational parameters (primarily density), is intermediate, between ICF and MCF. The higher densities of fuel compared with MCF mean that reacting volumes are smaller by orders of magnitude (because of the Lawson criteria), and the magnetized target permits lower power drivers for implosion and confinement compared with ICF, resulting in a potentially low cost development path to fusion. Target magnetization can be achieved in several ways, of which probably the easiest one is so called “field-reversed configuration” (FRC) and this is a well-known topology, with many techniques for forming such plasmas, for all those which are in the field of pulsed plasma thrusters or nuclear fusion. As mentioned previously, target magnetization plays a key role in reducing thermal losses, by confining the hot ions within the magnetic field. So, how does it work? The idea is to use the converging plasma jets as a “spherical piston” for compression (in PJMIF common term is liner, rather than piston, see Figures below) and as the magnetized target compresses, its magnetic field strength should increase rapidly, since its strength is inversely proportional to the radius of the target. Therefore, PJMIF intends to apply high-compression, pulsed operation, by imposing a high-energy plasma liner on the target while preserving a strong magnetic field around the target that would hopefully significantly reduce thermal losses and therefore allow easier access to nuclear fusion.
Preliminary analytical and numerical studies (Thio, Lindemuth, Siemon, Cassibry et. al.) show that the parameters at which PJMIF should operate are almost literally halfway between MCF and ICF, which is a parameter space no fusion concept has yet explored. By introducing a magnetic field in the target, the requirements on the initial jet energy can be significantly lowered, because thermal losses are cut during the compression and so better overall efficiency is achieved. Lower initial jet energy requires less power capacitors and this results in a more compact system. If proven, PJMIF would require vastly lighter and smaller facilities than those needed for MCF and ICF. It is a matter of fairly complex physics, but in order for MCF to work, one would need a large volume of plasma, on the order of hundreds of cubic meters, while for PJMIF, the whole reaction is taking place at scales of only a few cubic centimeter. The ICF has incredibly complex optics that split the largest laser in the World in 192 beams and direct it towards the center of the chamber, so all of that results in a facility that is the size of a football field. Another interesting aspect is the price… Lindemuth & Siemon (2009) in their paper formed a fairly simple, but working (at least for cases of ITER and NIF), economic model for fusion concepts. According to that study, while ITER and NIF cost billions of dollars, PJMIF facility should only cost around fifty million dollars. This is primarily due to the lower input energy and the fact that the rail-guns are far simpler than lasers or tokamaks. Now, you might ask why hasn’t this already been done since the low cost estimates are clearly appealing. The key words lie at the beginning of the paragraph before the previous one – all of these studies are preliminary so far. The very idea of PJMIF, as it exists now, has been formed merely ten years ago and it wasn’t until last year that the funding for some mid-scale experiment was actually granted. The project is called PLX (Plasma Liner Experiment) and it is a collaborative project among several universities and companies, with Los Alamos lab at its top. The experiment is primarily concentrating on fundamental physics of high-energy-density plasmas, but is also representing a good test-field for PJMIF, since it utilizes the above-described machinery. Unfortunately, PLX energy is below scale, so no fusion can be achieved, but it will serve as an excellent facility to confirm (or deny) the feasibility of nuclear fusion via PJMIF. PJMIF, with all of it’s potential advantages over ICF and MCF, would probably be a “show stealer” when it comes to nuclear fusion. For the ICARUS project and all other potential missions that will utilize fusion power as primary source of power, probably the most important of all advantages is the low mass. As mentioned, this comes from the fact that less initial energy is required due to the fact that the entire plasma rail-gun machinery is of rather simple construction. To sum up, it is important to say that there is a very long way indeed for those who pursue PJMIF. PLX has been started and a lot of work is being done on a daily basis. Jet merging (liner forming) experiments are to be undertaken at the beginning of next year, many simulations and theoretical analysis need to be done to try and assess the plasma behavior as these complex geometries are being formed, propagated, compressed and potentially driven to fusion conditions. If proven to be feasible it could represent an exciting option for ICARUS to reach the stars.