Direct thrust measurements and modelling of a radio-frequency expanding plasma thruster

Abstract

It is shown analytically that the thrust from a simple plasma thruster (in the absence of a magnetic field) is given by the maximum upstream electron pressure, even if the plasma diverges downstream. Direct thrust measurements of a thruster are then performed using a pendulum thrust balance and a laser displacement sensor. A maximum thrust of about 2 mN is obtained at 700 W for a thruster length of 17.5 cm and a flow rate of 0.9 mg s À1 , while a larger thrust of 4 mN is obtained at a similar power for a length of 9.5 cm and a flow rate of 1.65 mg s À1 . The measured thrusts are in good agreement with the maximum upstream electron pressure found from measurements of the plasma parameters and in fair agreement with a simple global approach used to model the thruster. Expanding plasma and double layer (DL) thrusters [1][2][3][4] are receiving increasing interest due to their potential advantages over conventional systems such as ion or hall thrusters. Such advantages include the lack of an explicit neutralizer, high power and thrust densities, and the absence of biased electrodes.1 These systems typically produce plasma within an insulating source tube through radio-frequency (rf) means and make use of a magnetic field to help confine the plasma and produce expansion at the thruster exit.1,2,5 The plasma is then accelerated by ambipolar or double layer fields that form internal to the plasma. 1,2,4,6 While prolific studies of the fundamental physics in such systems have been done, detailed experimental measurements of the thrust of such devices are only now beginning to be performed. 7,8 These thrust measurements are vital, as they allow almost all other electric propulsion figures of merit to be determined, 9 and thus, the performance of such systems to be assessed.Fruchtman has provided much of the theoretical groundwork for such plasma thrusters and has performed a number of detailed analyses in collisionless 10 and collisional regimes, 11 as well as investigating the effects of neutral depletion 10,11 and magnetic expansion. 12 In particular, in the absence of a magnetic field, the thrust has been predicted to be strongly linked to the maximum upstream electron pressure. 10Recent direct thrust measurements 7 of a permanent magnet helicon double layer thruster (PM-HDLT) have agreed favourably with this result; however, it is unclear how the presence of both the DL and the magnetic field affects the measured thrust. In order to test these theoretical results conclusively, the thrust measurements in a system without a magnetic field or DL are needed. With an improved understanding of the physics involved in the thrust delivery mechanism, better optimization studies can be performed.In the present letter, we experimentally investigate the thrust of a simple rf plasma thruster in the absence of a magnetic field and under conditions where a DL is not present. We also extend the theoretical results of Fruchtman 10 by assessing the effect that plasma divergence has on the expected system thrust.We begin by addressing ...

Axial force imparted by a conical radiofrequency magneto-plasma thruster

“…2 Recent theoretical and simulation studies of the axial force imparted by a magnetised plasma have highlighted the complex role of the plasma dynamics 3-6 (charged particles velocity distributions, collisional processes…) and its relevance to the field of space plasma physics and electric propulsion. Lately, this axial force has been directly measured in rf plasmas, confirming the role of the maximum electron pressure in the plasma source cavity [7][8][9] and identifying an additional axial force produced by an electron diamagnetic drift in the expanding magnetic field. 10 These results were obtained using a cylindrical radiofrequency helicon source or "thruster" of various length and diameter where the magnetic field (if applied) would be generated using one or two axial solenoids or arrays of permanent magnets.…”

Direct Thrust Measurement of an Electron Cyclotron Resonance Plasma Thruster

“…The electron temperature is increased up to a few tens of eV [18], and the electrons ionize the propellant gas by inelastic collisions. The energetic electrons are expelled at the open side of the cavity (exhaust) by two main processes: plasma expansion (driven by the electron pressure) and diamagnetic effects in the divergent magnetic field [25,[27][28][29][30][31]. The difference of mobility between the ions and the electrons creates an ambipolar electric field that maintains the quasi-neutrality of the current-free plasma beam and accelerates the ions.…”

“…Takahashi and Lafleur have used a double pendulum balance with a laser displacement sensor [24,25]. They were able to measure thrusts up to 4 mN for powers up to 700 W. Nakamura used a torsional thrust balance with a laser displacement sensor, reaching 650 µN at 400 W [26].…”

Direct Measurement of Axial Momentum Imparted by an Electrothermal Radiofrequency Plasma Micro-Thruster

“…Both the rf cable and gas lines inside vacuum are anchored at one position at the top of the thrust balance mounting to minimize mechanical resistance and maintain adequate sensitivity of the thrust balance. This challenging configuration is very distinct from previous thrust measurements using higher-power lower-pressure inductive rf thrusters (in which axial momentum is primarily imparted by accelerated ions rather than neutrals) which did not require mechanical contact between the gas injector and the plasma cavity or between the rf antenna and the plasma cavity [13,14].…”

“…The small diameter Pocket Rocket plasma thruster called (MiniPR), is attached to a thrust balance comprising a grounded open cube frame (0.3 × 0.3 × 0.3 m 3 ) suspended from four "chimneys" (Figure 1). A laser-displacement sensor system [13,14] positioned near the top of the thrust balance mounting is used to measure the displacement of the thrust balance and hence the axial force imparted by the plasma heated gas flow. The base pressure is measured to be about 7 × 10 −7 Torr using an ion gauge and a baratron gauge located 2 m downstream of the thrust balance.…”