On collisionless ion and electron populations in the magnetic nozzle experiment (MNX)

Abstract

The Magnetic Nozzle Experiment (MNX) is a linear magnetized helicon-heated plasma device, with applications to advanced spacecraft-propulsion methods and solar-corona physics. This paper reviews ion and electron energy distributions measured in MNX with laser-induced fluorescence (LIF) and probes, respectively. Ions, cold and highly collisional in the main MNX region, are accelerated along a uniform magnetic field to sonic then supersonic speeds as they exit the main region through either mechanical or magnetic apertures. A sharp decrease in density downstream of the aperture(s) helps effect a transition from collisional to collisionless plasma. The electrons in the downstream region have an average energy somewhat higher than that in the main region. From LIF ion-velocity measurements, we find upstream of the aperture a presheath of strength , where is the electron temperature in the main region, and length 3 cm, comparable to the ion-neutral mean-free-path; immediately downstream of the aperture is an electrostatic double layer of strength and length 0.3-0.6 cm, 30-. The existence of a small, ca. 0.1%, superthermal electron population with average energy is inferred from considerations of spectroscopic line ratios, floating potentials, and Langmuir probe data. The superthermal electrons are suggested to be the source for the large .

Particle-in-cell modeling of magnetized argon plasma flow through small mechanical apertures

“…When secondary emission models are included, the overall dynamics again remain approximately the same and so are not reported here. As noted earlier, this is just one of many possible magnetic nozzle and aperture arrangements; others (and a similar configuration) experimentally reported can be found in [9].…”

“…The cooler bulk value is bounded by T e,1 ∼ 4.8 eV ±5% and the high-energy tail value is bounded by T e,2 ∼ 21.5 eV ±5%; the relative population is bounded by f e,2 /f e,1 ∼ 2.5% ± 0.5%. High-energy tails in the EEDF have been measured in the MNX device [9] and are in quantitative agreement. The model excludes the consideration of excited Ar + * states, which may influence the EEDF; excited ions may be included in future work, although their relative population in the MNX is estimated to be low [21].…”

“…As shown, apertures have been placed in the center of the Helmholtz coil pair, position a, where the field is uniform, and on either side of the nozzle coil, where the field is either diverging or converging. All positions have shown, under similar conditions of field, helicon power, and gas pressure, ion acceleration to supersonic speed in short distances (2−5 mm) [9]. Because of the similarity of these experimental results, this paper concentrates on studies of only one configuration, with the aperture placed at position c, corresponding to a region in which the field first converges then diverges.…”

“…Particle-in-cell (PIC) simulations [10] using the commercial LSP code [11,12] are employed to selfconsistently model the plasma dynamics and formation of the electrostatic layer measured in the MNX [9] near the mechanical aperture. Argon plasma is formed upstream, in the MNX main chamber (MC), by absorption of helicon waves.…”

“…The argon plasma about 30 cm downstream from the antenna has a density of n p ∼ 10 13 cm −3 , a bulk electron temperature near T e ∼ 5 eV and a tail temperature near 30 eV. The tail contains about 1% of the density [9]. This plasma flows towards mechanical apertures of varying sizes, from 1−10 mm diameter, and varying axial positions, marked a, b, c and d in Fig.…”