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A self-consistent model is presented for performing steady-state fullykinetic Particle-in-Cell simulations of magnetised plasma plumes. Anenergy-based electron reflection prevents the numerical pump instabilityassociated with a typical open-outflow boundary, and is shown to besufficiently general that both the plume kinetics and plasma potentialdemonstrate domain independence (within 4%). This is upheld by non-stationaryRobin-type boundary conditions on the Poisson's equation, coupled to acapacitive circuit that allows physical evolution of the downstream potentialdrop in the transient. The method has been validated against experiments,providing results that fall within the uncertainty of measurements. Simulationsare then carried out to study collisional xenon discharges into axisymmetricdiverging magnetic nozzles. Particular discussion is given to theidentification of a potential well arising from charge separation at the edgeof the plume, the role of ion-neutral charge exchange, and a three-regionpiecewise polytropic cooling regime for electrons. The polytropic index isshown to depend on the degree of magnetisation. Specifically, in the regionnear the thruster outlet, the plume is weakly-magnetised due to the cross-fielddiffusion of electron-heavy particle collisions. Downstream, astrongly-magnetised region of near-isothermal expansion occurs. Finally, in thedetached region, the polytropic index tends to that of a more adiabaticunmagnetised case. With an increasing magnetic nozzle field strength, aninferior limit is found to the average polytropic index of$\bar{\gamma}_e\sim1.16$.

Fully kinetic model of plasma expansion in a magnetic nozzle