Absolute number densities and the spatial distribution of atomic hydrogen in a large-scale microwave plasma reactor were measured by means of two-photon absorption laser-induced fluorescence (LIF) with laser radiation at 205 nm. The microwave discharge was operated at 2.45 GHz with a maximum power of 2 kW. Absolute number densities were obtained by calibrating the LIF detection system via NO 2 titration in a flow tube reactor and in the range from 0.5 × 10 15 to 2.5 × 10 15 cm −3 . Compared to small-scale reactors with a volume-to-surface ratio less than 3 cm, this reactor has a relatively homogeneous spatial distribution of atomic hydrogen. The mole fraction of atomic hydrogen in hydrogen plasmas ranges from 10% to 20%. The gas temperature-volume effect plays an important role in the spatial distribution of atomic hydrogen in this reactor.

In recent years the two-photon absorption laser-induced fluorescence technique has widely been applied to measure the density of atomic hydrogen. In most studies the influence of laser property fluctuations over multipulses in the determination of the H atom density has not been considered. In this work we derive the effect of such fluctuations and give a method to experimentally determine the influence of the fluctuations in the laser’s properties over multipulses on the measurement of H atoms.

Non-intrusive characterization of the singly ionized xenon velocity in Hall thruster plume using laser induced fluorescence (LIF) is critical for constructing a complete picture of plume plasma, deeply understanding the ion dynamics in the plume, and providing validation data for numerical simulation. This work presents LIF measurements of singly ionized xenon axial velocity on a grid ranging from 100 to 300 mm in axial direction and from 0 to 50 mm in radial direction for a 600 W Hall thruster operating at the nominal condition of discharge voltage 300 V and discharge current 2 A, the influence of discharge voltage is investigated as well. The ion velocity distribution function (IVDF) results in the far-field plume demonstrate a profile of bimodal IVDFs, especially prominent at radial distances greater than channel inner radius of 22 mm at axial position of 100 mm, which is quite different from that of the near-field plume where bimodal IVDFs occur in the central core region for the same power Hall thruster when compared to previous LIF measurements of BHT-600 by Hargus (2010 J. Propulsion Power 26 135). Beyond 100 mm, only single-peak IVDFs are measured. The two-dimensional ion velocity vector field indicates the bimodal axial IVDF is merely a geometry effect for the annular discharge channel in the far-field plume. Results about the IVDF, the most probable velocity and the accelerating potential profile along the centerline all indicate that ions are still accelerating at axial distances greater than 100 mm, and the maximum most probable velocity measured at 300 mm downstream of the exit plane is about 19 km s −1 . In addition, the most probable velocity of ions along radial direction changes a little except the lower velocity ion populations in the bimodal IVDF cases. The ion temperature at axial distances of 10 and 300 mm oscillates along the radial direction, while the ion temperature first increases, and then decreases for the 200 mm case. Finally, the axial position for the ion peak axial velocity on the thruster centerline is shifted upstream for higher discharge voltages, and the velocity curve is becoming steeper with the discharge voltage before reaching the maximum. This observation can be used as a criterion to optimize the thruster performance.

Non-intrusive characterization of the ion velocity distribution functions at several points of interest in 600 W magnetically shielded MSHT-600 and unshielded USHT-600 Hall thrusters plume by laser-induced fluorescence was conducted. Several visual clues supporting the magnetic shielding effect of MSHT-600 were presented. The measured ion axial velocity distribution functions show that the ion axial velocity is higher and observed to reach the maximum at nearer location to the exit plane on the centerline in the magnetically shielded Hall thruster than that in the unshielded one under the same operating conditions but uniformly distributed in the radial direction when ignoring the slower population for both thrusters. These results are in accordance with the higher specific impulse measured in several other shielded Hall thrusters. Moreover, the ion axial velocity on the centerline is higher than the average axial velocity at other radial locations for the same axial distance downstream of the exit plane in USHT-600, while the higher plume divergence or the inner pole erosion causes the velocity on the centerline to decrease in MSHT-600. The downstream shift of the accelerating potential and electric field distribution are also the typical features in magnetically shielded Hall thrusters as reported in other studies. Finally, the xenon ion velocity vector fields show that the ion population starts to meet and diverge further downstream of the exit plane in MSHT-600 than that in USHT-600, which may be associated with the difference of the plume divergence.

The ion velocities within the discharge channel of the 600 W magnetically shielded and unshielded Hall thrusters are measured by the laser-induced fluorescence technique to quantitatively evaluate the erosion of the channel walls. Visual inspection of the magnetically shielded Hall thruster has qualitatively indicated that the discharge channel has been successfully shielded from the bombarding of the plasma, while the erosion occurs in the unshielded counterpart. In addition, the ion velocities along the thruster centerline indicate that the voltage utilization efficiency of MSHT-600 is higher than that of USHT-600 and comparable to the state-of-the-art BHT-600 thruster. Furthermore, measurements performed inside the discharge channel show that the ions deep inside the channel impact the channel walls backward in the shielded thruster, while forward in the unshielded one. Moreover, the plasma potential along the channel walls remains as high as the anode potential in the shielded thruster, while it dramatically decreases in the unshielded one. When accounting for the sheath potential acceleration, the maximum erosion rate of the inner and outer walls of USHT-600 is 1.3 and 3.1 μm/h, respectively, based on the plasma parameters of SPT-100, while the maximum value of the weighted average erosion rate of MSHT-600 is 0.08 and 0.04 μm/h, respectively, for two cases of the ion number density. Compared to the erosion rate of SPT-100, it is qualitatively and quantitatively confirmed that strong shielding of the discharge channel is realized and high performance is maintained in the designed thruster with a magnetically shielded configuration.