Human hopes of reaching stars other than the Sun are currently limited by the maturity of advanced propulsion technologies. One of the few candidate propulsion systems for providing interstellar flight capabilities is nuclear fusion. In the past many fusion propulsion concepts have been proposed and some of them even explored in high detail (Project Daedalus), however, as scientific progress in this field has advanced, new fusion concepts have emerged that merit evaluation as potential drivers for interstellar missions. Plasma jet driven Magneto-Inertial Fusion (PJMIF) is one of those concepts. PJMIF involves a salvo of converging plasma jets that form a uniform liner, which compresses a magnetized target to fusion conditions. It is an Inertial Confinement Fusion (ICF)-Magnetic Confinement Fusion (MCF) hybrid approach that has the potential for many benefits over both ICF and MCF, such as lower system mass and significantly lower cost.
PJMIF involves converging plasma jets that are launched from symmetrically distributed plasma rail-guns (or plasma guns), so as jets come in, they merge and form a plasma liner that compresses the plasmoid target (spheromak or FRC), which reaches fusion conditions at peak compression.
The main advantage of PJMIF over classical ICF and MCF is that it does well in combining the best of both concepts, namely inertial compression and strong magnetic fields. Magnetic field is embedded in the target plasmoid, so when the target compresses, the magnetic flux increases inversely proportional to the radius of the target, taking magnetic field strength to MG levels.
The benefit of this mechanism is more efficiency, allowing PJMIF to operate at an intermediate parameter space, theoretically allowing fusion with gain at low input energies of 50 to 75 MJ.
For terrestrial purposes PJMIF the rail-guns would be distributed across a full sphere, however for propulsion purposes a parabolic nozzle has to be used in order to allow the exhaust jet to exit. The principle of such a fusion propulsion magnetic nozzle concept has been well described by several previous studies, so here we will only present the basic concepts. The nozzle consists several superconducting coils which create a specific magnetic field configuration inside the nozzle chamber.
Very high specific impulses and specific jet powers can be achieved with fairly low jet velocities of a few hundred km/s. These regimes of PJMIF propulsion represent a very minor technological extrapolation, while the performance would be sufficient for manned Solar system or precursor interstellar missions. However, if one needs extreme performance for interstellar flight, two options are available: increase in jet velocity to over 1000 km/s or significant improvement of fusion gain.
Conservative calculations prove not to be sufficient for interstellar distances, but the numbers do show potential for Solar system missions. The medium extrapolated scenario with significantly increased jet velocity is already stepping into the interstellar mission parameter range. With specific impulse of roughly 455000 s, thrust of 1.34 MN and total jet power of close to 3 TW, such a propulsion system would be more than enough for a precursor mission to Oort cloud or even a full interstellar mission with reduced ΔV. Certainly the most interesting of the three calculations is the optimistic extrapolation. It is assumed that by the time Icarus mission would mature, we would be able to launch total plasma mass of 1 g with speeds up to 1500 km/s. Although small amounts of plasma have been accelerated to speeds of 2500 km/s, it is not unreasonable to think that same can be done with more significant amounts of plasma. In addition, even if one would argue that such high velocities are impossible for plasma masses on the order of 0.01 g, it is good to keep in mind that plasma guns are significantly less complex than i.e. lasers and that we can keep the total mass of the fuel simply by increasing the number of the guns. Liner kinetic energy scales with square of the velocity and so the optimistic scenario easily reaches and surpasses the required performance demands.
There are several very important highlights that need to be pointed out.
1. The overall estimated fuel mass for the optimistic scenario is only 25000 t, which is more than 40% less fuel than what was estimated for Daedalus mission first stage.
2. PJMIF propulsion system would completely overcome some of the most complex engineering problems that ICF has embedded involving fuel storage, complex pellet production and pellet injection.
3. Vast amounts of heat will be generated in the coils during operation and in order to avoid excessive radiator mass, more efficient heat exchangers need to be developed with specific heat output larger than 50 kW/kg.
4. Presented optimistic scenario still involves gain of only 50 and if this number would increase even slightly by further advances in driver technology, hybridization with ICF or utilization of outside magnetic fields, it would provide an additional boost to the whole concept, potentially driving initial mass even lower or lowering demands for initial jet velocity.
Successful 2012 Kickstarter Fundraising for PJMIF – Early HyperV Technology to develop PJMIF for space propulsion
Plasma electric thruster technology will be good for propelling spacecraft around the solar system. For example, we can use solar panels for electric power out to at least the asteroid belt beyond Mars without requiring a large unwieldy array of solar panels. For beyond the asteroid belt, in the darker outer solar system, a compact fission reactor capable of providing electrical power for years is the most practical energy source.
Four jets experimental test firing
If you assume an efficiency of 50%, a thruster producing 1 kW of output at an Isp of 2000 sec yields about 0.1 N of thrust and requires 2 kW of input from the solar cells.
Thrust for any electric thruster, including ours, is given ideally by the formula T = (2*P)/(g*Isp), where T is thrust in Newtons, P is power in Watts (into the exhaust stream), g is the acceleration constant 9.8 m/sec^2, and Isp is in sec.
So for an Isp of 2000 sec, T = 0.1 N per kilowatt of power into the exhaust. It should be noted, of course, that an efficiency has not yet been included in this calculation. If you assume an efficiency of 50%, then for every kilowatt that goes into your exhaust, the solar panel needs to deliver 2 kW of power.
Based on previous work and some modeling, we should be able to achieve an efficiency around 50%, but only if we use a well-matched pulse forming network and an energy recovery circuit to recycle the remaining magnetic field energy at the end of each pulse. We have plans to do that if we receive sufficient funds.
Most of HyperV Technologies work to date has been single shot testing at effective Isp’s of 3000-5000 sec (some tests to even 8000 sec). We chose 2000 sec for our KickStarter prototype demonstration for two reasons. First, there are a large number of missions for which an Isp of 2000 sec is actually preferred over values in the 3000-4000 sec range, so this matches an Isp of real world interest. The reason for this is because thrust is inversely proportional to Isp for a constant input power. For missions requiring extremely high deltaV, you still need to operate at higher Isp to keep your fuel requirements manageable, but for the lower deltaV missions, reducing trip time is often equally important to minimizing fuel consumption. In other words, a value of 2000 sec is a nice compromise – it gives you most of the advantages of lower fuel consumption, but with higher thrust. Secondly, that value of Isp allows for a somewhat simpler test in our laboratory using lower peak currents while we are
learning how to establish repetitive operation.
In 2012, HyperV was firing six plasma jets into a large vacuum tank and recording the density, shape and speed of the plasma as it merges together in the tank.
Hyperv ran the E3P-1 thruster for 92.5 seconds at 5 Hz with an input power of 2.3 kW and an Isp of 2,000-2,800 seconds exceeding all stated Kickstarter firing event goals!
Stated Kickstarter Firing Event Goals:
Operate at an average continuous input power level of about 1.0 kW
Achieve a specific impulse (Isp) of 2000 sec (which means an average exhaust velocity of about 20,000 m/s)
Operate at 5 pulses per second (5 Hz) for a minimum duration of one minute
Achieved firing event performance:
Input power 2.3 kW
5 Hz for a total run time of 92.5 seconds
This is just the beginning for developing a spaceflight qualified system, since we must be able to run the thruster continuously for years.
The electric pulsed plasma propulsion (E3P) technology beyond the successful conclusion of our first Kickstarter project.
We have identified a unique new approach to using a non-gaseous propellant, which maintains the same high Isp and thrust of argon or xenon gas propellants in our thruster. This new propellant approach provides the E3P thruster technology with the following advantages:
1) The non-gaseous propellant is completely inert and non-toxic, and requires no high-pressure propellant tanks which can explode, or valves which can fail. Given that the propellant is inert and non-toxic, it is an ideal candidate technology for safe use with, and storage aboard, manned platforms and on cubesats.
2) This thruster technology could enable interplanetary missions on cubesat spacecraft as small as 6U (briefcase size spacecraft) and with similarly low cost.
3) It is scalable up to large spacecraft in the kilowatt power range.
4) It can be throttled for fine maneuvering of the spacecraft.
5) Both the thruster technology and propellant are mechanically robust.
6) It has high thrust per unit area, taking up less space on the rear of the spacecraft.
Los Alamos Computational Work on getting PJMIF for power generation
Arxiv – Possible Energy Gain for a Plasma Liner-Driven Magneto-Inertial Fusion Concept by Los Alamos National Laboratory
A one dimensional (1-D) parameter study of a Magneto-Inertial Fusion (MIF) concept indicates that significant gain may be achievable. This concept uses a dynamically formed gaseous shell with inwardly directed momentum to drive a magnetized fuel to ignition, which in turn partially burns an intermediate layer of unmagnetized fuel. The concept is referred to as the Plasma Jet MIF or PJMIF. The results of an Adaptive Mesh Refinement (AMR) Eulerian code are compared to those of a Lagrangian code.
Cases with and without electron heat conduction (e-HC) have been run in an attempt to bracket similar cases with and without MHD. The magnetic field in an MHD calculation should significantly retard the escape of thermal energy. Indeed, it appears that below 90 km/sec the gain is greatly diminished for the AMR calculations with e-HC on. Lasnex agrees. For the AMR calculations, as the liner velocity increases to 100 km/sec and above, the gains for the cases with e-HC on suddenly jump to near those with e-HC off. This does not agree with the Lasnex calculations. However, for Lasnex at the lowest liner velocity (60 km/sec, with or without e-HC or a magnetic field) only low gain or no gain was achieved. Gains up to 60 were obtained in with reduced density of the target DT (DT2), but these are in question and have not been verified with other codes yet.
In conclusion, the main result is that for the xenon liner velocities above 60 km/sec a magnetic field reduces the energy loss by electron thermal heat conduction sufficiently to allow fusion ignition to be obtained with ensuing gains that are thought to be sufficient for a PJMIF-based energy producing reactor.
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
Known for identifying cutting edge technologies, he is currently a Co-Founder of a startup and fundraiser for high potential early-stage companies. He is the Head of Research for Allocations for deep technology investments and an Angel Investor at Space Angels.
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