Numerical Studies of Ponderomotive Acceleration and Ion Cyclotron Resonance: Application to Next Generation Electric Thrusters

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We have examined ponderomotive acceleration/ion cyclotron resonance (PA/ICR) of argon ions by performing test particle simulations. The PA gives rise to the pure parallel acceleration of ions, while the ICR causes the perpendicular ion heating followed by the energy conversion from the perpendicular to the parallel direction in the presence of a divergent background magnetic field. The energy gain by the PA/ICR is classified in terms of the adiabatic parameter, Λ = LBΩ0/v0||, where LB is the axial divergent scale length of the background magnetic field, Ω0 is the ion gyrofrequency at the resonance, and v0|| is the initial ion drift velocity along the axial magnetic field. For Λ < 100, the energy gain, Δε, due to the PA/ICR increases as Λ increases. For Λ > 100, Δε saturates since the increased axial velocity of the ion via the PA reduces the transit time to cross the acceleration region. When the externally applied rf electric field intensity is increased to 1000 V/m, we find a maximal 60% increase in the energy gain for the PA/ICR scheme compared with the energy gain by the ICR only. We have applied the PA/ICR scheme to the next-generation electric thruster, and have estimated the thrust including ion wall-loss and ion-neutral collisions.