The Z-Pinch dense plasma focus method is a Magneto-Inertial Fusion (MIF) approach that may potentially lead to a small, low cost fusion reactor/engine assembly1. Recent advancements in experimental and theoretical understanding of this concept suggest favorable scaling of fusion power output yield 2. The magnetic field resulting from the large current compresses the plasma to fusion conditions, and this process can be pulsed over short timescales (micro-second). This type of plasma formation is widely used in the field of Nuclear Weapons Effects testing in the defense industry, as well as in fusion energy research. A Decade Module 2 (DM2), ~500 KJ pulsed-power is coming to the RSA Aerophysics Lab managed by UAHuntsville in January, 2012.
The analysis of the Z-Pinch MIF propulsion system concludes that a 40-fold increase of Isp over chemical propulsion is predicted. An Isp of 19,436 sec and thrust of 3812 N-sec/pulse, along with nearly doubling the predicted payload mass fraction, warrants further development of enabling technologies.
A magnetic nozzle is essential for fusion engines to contain and direct the nuclear by-products created in pulsed fusion propulsion. The nozzle must be robust to withstand the extreme stress, heat, and radiation. The configuration of fuel injection directs the D-T and Li6 within the magnetic nozzle to create the Z-pinch reaction as well as complete an electrical circuit to allow some of the energy of the nuclear pulses to rapidly recharge the capacitors for the next power pulse. Li6 also serves as a neutron shield with the reaction between neutrons and Li6 producing additional Tritium and energy, adding fuel to the fusion reaction and boosting the energy output.
Fluorine-Lithium-Beryllium (FLiBe) is proposed as the main thermal coolant and would flow through channels inside the ring assemblies, as well as through all the Carbon-Carbon (C-C) structure supporting the coils and comprising the nozzle and thrust struts. This fluid is suggested for the dual purpose of heat removal and capturing gamma rays and neutrons. Eight ring assemblies, spaced at equal radial angles from the focal point of fusion, are supported within the C-C parabolic nozzle. The shape and configuration of a ring assembly, shown in cross-section in Figure 7, would be angled toward the focus of the fusion pulses to allow the FLiBe to protect the magnetic conductor coils from neutrons. This radiation protection would be in addition to the Li6 “liner” which is expected to absorb high energy neutrons and will slow down many more.
The total burn time is 5 days for a roundtrip Mars mission, equating to 27,500 m/s of ΔV and using 86.3 mT of propellant. The trajectory for a 30-day trip to Mars requires an 8.7 day Earth departure burn. For a roundtrip, this trajectory requires a total burned propellant load of 350.4 mT and has an equivalent ΔV of 93,200 m/s. While these numbers are significantly larger than the 90-day trajectories, this does show the feasibility of a 30-day trip to Mars. Trip time, propellant, and Delta V are compared in Table 2 for a vehicle with a 552 mT burn-out mass, which is in the range of the study’s “Best Estimate” mass shown in Table3.
A comparison of the payload mass fractions shows that only about 33% of the mass in the traditional, high-thrust chemical propulsion Mars cargo mission could be payload. The Z-Pinch propulsion system can deliver a higher payload mass fraction, estimated at 35-55% in half the time, (90 days vs. 180 days). Z-Pinch propulsion may also enable fast round-trip trajectories for human Mars missions with comparable payload mass fractions to current chemical propulsion vehicle estimates.
The technology development required for this propulsion system is achievable on a reasonable timescale given sufficient resources. The first stage of a development program would involve sub-scale experiments to establish the foundational aspects of the system, such as Z-Pinch formation utilizing annular nozzles. Furthermore, the experiments would yield quantitative information enabling more sophisticated configurations for test and evaluation