Characterization of impulse and plasma expansion dynamics in a magnetic nozzle for laser fusion propulsion applications

A laser fusion rocket generates thrust by exhausting high-energy, high-density plasma using a magnetic nozzle. We investigated the relationship between the plasma expansion radius in the early phase and the resultant impulse bit, and evaluated the energy-scaling law and momentum efficiency based on the impulse bit and emission measurements. The absolute values of the changes in the impulse bit and expansion radius appear to correlate as functions of the magnetic field energy. The impulse bit follows a power-law dependence on laser energy with an exponent of 1.2. In contrast, the initial momentum scales with laser energy to the power of 0.85. This difference in the exponents is attributed to the reduced impulse bit in the low laser energy region. Therefore, the momentum efficiency did not follow simple power-law scaling with an exponent of 0.35, instead, it exhibited a significant decrease at low laser energies. This suggests that the impulse bit varies not only with the magnetic field energy and laser energies but also with the plasma density distribution. • Impulse bit and plasma emission are measured in a magnetic nozzle for fusion rocket. • Impulse bit and expansion radius are correlated with magnetic field energy. • Linear dependence of impulse on laser energy shown. • Plasma distribution may affect impulse bit.