Sub-Scale Demonstration of an Axial Pulsed Magnetic Nozzle for Nuclear Propulsion Systems

With current technology, crewed and uncrewed missions to interplanetary destinations take a relatively long time. For example, a crewed mission to Mars takes 2-4 years, and a robotic mission an outer plant takes 5-20 years. During this time, astronauts are subject to cramped conditions and damage from cosmic rays; robotic missions face programmatic issues. Advanced propulsion systems, such as pulsed nuclear propulsion systems can solve these issues, reducing trip times by as much as 90%, due to their inherent high specific power (1-10 kW/kg). However, these systems face several technical challenges, namely high heat loads incident on the nozzle; metals melt at the required exhaust temperatures. Instead, it has been proposed to use a magnetic nozzle; a magnetic nozzle functions by directing the plasma exhaust with high-strength magnetic fields. Previous work investigated magnetic nozzles with the solenoidal topology, but recent work has found this to be ineffective. Instead, the axial topology is proposed, however, this configuration has never before been tested in the laboratory. In this work, the researchers undertake the first sub-scale test of an axial magnetic nozzle for a pulsed nuclear propulsion system. The nozzle tested here is cylindrical, 0.12 m in diameter and 0.12 m in length with 40 struts. A current of 1 kA is run through each strut to produce a 1T field. The plasma is generated using a 1064 nm Nd:YAG laser with a maximum energy of 0.65 J and a laser spot diameter of 0.5 mm. The researchers simulate the experiment with the computational code SPFMax and estimate thrust using measurements from a series of charge collectors. Using previous work, they find that, while the simulation predicts thrusts between 6.2-6.7 µNs for the nozzle on and nozzle off, respectively, the experiment only measured thrust between 1.0-2.2 µNs for the same conditions. This difference is most likely due to differences between the computational model setup and experimental setup. Future work includes performing simulations that more accurately model the experimental setup, and devising alternate ways to more accurately estimate thrust from the charge-collector data.