Microwave ion thrusters that employ the gyromotion of the electrons around an external static magnetic field lines to obtain discharge are among the most selected devices for deep space missions that require longer life time. The phenomenon of electron cyclotron resonance (ECR) is utilized to a high degree to energize the electrons in magnetic tubes created by the lines of force and obtain discharge through impact ionization. A discussion for the trade-offs between microwave systems to other electric propulsion systems is presented along with numerical simulations on some designs from literature. COMSOL Multiphysics, a finite element software, is used to conduct magnetic field simulation, electromagnetic simulation and plasma simulation. Results show that the increase in number of magnets and orienting them to form cusps affect the density and temperature distribution by increasing the magnetic tubes in which the electrons are trapped and energized. The R-mode and X-mode resonance regions are plotted, and for 20mN-class ion thruster it is verified that X-mode resonance is the dominant energy transfer mechanism.
We experimentally and numerically study the dynamics of a liquid jet issued from a rotating orifice, whose breakup is regulated by a vibrating piezo element. The helical trajectory of the spiralling jet yields fictitious forces varying along the jet whose longitudinal projections stretch and thin the jet, affecting the growth of perturbations. We show that by quantifying these fictitious forces, one can estimate the jet intact length and size distribution of drops formed at jet breakup. The presence of the locally varying fictitious forces may render high-frequency perturbations, that would otherwise be stable in the abscence of stretching, unstable, as observed similarly in the case of straight jets stretching under gravity. The perturbation amplitude then dictates how strong the perturbation is coupled to the jet compared with random noise that is inherently present in any experimental set-up. In the present study we exploit the slenderness of the jet to separate the calculation of the base flow and the growth of perturbations. The fictitious forces calculated from the base flow trajectory are then used in a nonlinear slender-jet model, which treats the spiralling jet as a quasi-straight jet with locally varying body forces. We show both experimentally and numerically that jet breakup characteristics (e.g. intact length and drop size distribution) can be controlled by finite-amplitude perturbations created by mechanically induced pressure modulations. Finally, we revisit the integrated net gain approach developed for straight jets under gravity and we provide simple analogous relations for spiralling jets.
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