Near-field plume properties of an ion beam formed by alternating extraction and acceleration of oppositely charged ions

Abstract

This paper is devoted to study the expansion of a beam composed of packets of positively and negatively charged ions generated by alternating extraction and acceleration. This beam is extracted from an ion-ion plasma, i.e. the electron density is negligible compared to the negative ion density. The alternating acceleration of ions is ensured by two grids placed in the ion-ion plasma region. The screen grid in contact with the plasma is biased with a square voltage waveform while the acceleration grid is grounded. A two-dimensional particle-in-cell (2D-PIC) code and an analytical model are used to study the properties of the near-field plume downstream of the acceleration grid. It is shown that the possible operating bias frequency is delimited by an upper limit and a lower one that are in the low MHz range. The simulations show that alternating acceleration with bias frequencies close to the upper frequency limit for the system can achieve high ion exhaust velocities, similar to traditional gridded ion thrusters, and with lower beam divergence than in classical systems. Indeed, ion-ion beam envelope might be reduced to 15° with 70% of ion flux contained within an angle of 3°. Thus, this alternating acceleration method is promising for electric space propulsion.

Expansion of alternatively extracted ion–ion beam in a low pressure collisional medium

“…In contrast, the alternate extraction, subject of our interest in this work, relies on one ion source that extracts and accelerates alternatively positive and negative ions [17]. To insure the overlap of the extracted oppositely charged ion packets and avoid beam stagnation, it has been demonstrated that the frequency of oppositely charged ion packets extraction should be in the low MHz range, typically 1.25 MHz [18]. Indeed, the collisionless expansion of alternatively extracted ion-ion beam in vacuum has been studied numerically via particle-incell (PIC) [18] and fluid [19] simulations.…”

“…To insure the overlap of the extracted oppositely charged ion packets and avoid beam stagnation, it has been demonstrated that the frequency of oppositely charged ion packets extraction should be in the low MHz range, typically 1.25 MHz [18]. Indeed, the collisionless expansion of alternatively extracted ion-ion beam in vacuum has been studied numerically via particle-incell (PIC) [18] and fluid [19] simulations. These works [18,19] show that, a few ion packets away from grid system, the generated ion beam is neutralized and has a free expansion.…”

“…Indeed, the collisionless expansion of alternatively extracted ion-ion beam in vacuum has been studied numerically via particle-incell (PIC) [18] and fluid [19] simulations. These works [18,19] show that, a few ion packets away from grid system, the generated ion beam is neutralized and has a free expansion. It has been demonstrated that the alternative extraction and acceleration of oppositely charged ion enhances electrostatic waves propagating with beam velocity [18].…”

“…These works [18,19] show that, a few ion packets away from grid system, the generated ion beam is neutralized and has a free expansion. It has been demonstrated that the alternative extraction and acceleration of oppositely charged ion enhances electrostatic waves propagating with beam velocity [18]. This is due to the propagation of ion packets having an uncompensated charge space in the close vicinity of the source exhaust plane.…”

“…In the absence of collision, the amplitude of these waves decreases monotonically with the distance from the ion source, since the oppositely charged ion packets overlap and compensate mutually their space charge. These electrostatic waves interact [18] with ions, through Landau damping and its inverse mechanism, and affect significantly the ion velocity distribution function (IVDF). Furthermore, the alternatively extracted ion-ion beam has also been subject of experimental studies [20][21][22][23] where the observed beam properties and the measured IVDF are quite similar to what have been predicted in numerical investigations [18,19].…”