The electron velocity distribution in Hall thrusters is anisotropic, which not only makes the sheath oscillate in time, but also causes the sheath to oscillate in space under the condition of low electron temperatures. The spatial oscillation sheath has a significant effect on near-wall transport current. In this Letter, the method of particle-in-cell (2D + 3 V) was adopted to simulate the effect of anisotropic electron temperatures on near-wall conductivity in a Hall thruster. Results show that the electron-wall collision frequency is within the same order in magnitude for both anisotropic and isotropic electron temperatures. The near-wall transport current produced by collisions between the electrons and the walls is much smaller than experimental measurements. However, under the condition of anisotropic electron temperatures, the non-collision transport current produced by slow electrons which reflected by the spatial oscillation sheath is much larger and closes to measurements.

Benign paroxysmal positional vertigo (BPPV) is a clinical condition. The existing diagnostic methods cannot determine the specific location of otolith on the short or long brachial sides. Thus, visual and quantitative evaluation of the existing clinical standard diagnostic modality Dix–Hallpike test is needed to improve medical efficiency. Our goal was to develop a real-time virtual simulation system to assess a BPPV treatment manipulation. In this study, we used the proposed simulation system to observe otolith movement during a posterior semicircular canal BPPV diagnostic test, and to analyze the diagnostic mechanisms and strategies. Through visual cluster analysis of otolith position and analysis of otolith movement time in the standard Dix–Hallpike test, we can find that the positions of otoliths are relatively scattered, especially on the z-axis (z 1 = 10.67 ± 3.98), and the fall time of otoliths at different positions has relatively large changes (t 1 = 22.21 ± 1.40). But in the modified experiment z 2 = 4.93 ± 0.32 and t 2 = 26.21 ± 0.28. The experimental results show that the simulation system could track the state and the movement of otolith in real-time, which is of great significance for understanding the diagnostic mechanisms of BPPV evaluations and improving the diagnostic method.

Despite its affordability, the krypton Hall-effect thruster in applications always had problems in regard to performance. The reason for this degradation is studied from the perspective of the nearwall conductivity of electrons. Using the particle-in-cell method, the sheath oscillation characteristics and its effect on near-wall conduction are compared in the krypton and xenon Hall-effect thrusters both with wall material composed of BNSiO 2 . Comparing these two thrusters, the sheath in the krypton-plasma thruster will oscillate at low electron temperatures. The near-wall conduction current is only produced by collisions between electrons and wall, thereby causing a deficiency in the channel current. The sheath displays spatial oscillations only at high electron temperature; electrons are then reflected to produce the non-oscillation conduction current needed for the krypton-plasma thruster. However, it is accompanied with intensified oscillations. V C 2014 AIP Publishing LLC. INTRODUCTIONAt present, using Hall-effect thrusters in interplanetary missions and geostationary satellites with affordable propellants can reduce costs and improve economics. Optional propellants include gases of argon, krypton, nitrogen, and nitrogen dioxide. Although the atomic masses of these gases are smaller and the degrees of first ionization are higher than xenon, thereby yielding lower propulsive efficiency, these propellants are more affordable. 1 The Hall-effect thruster is universally applicable in many countries involved in aeronautics and in which giving the krypton-plasma thruster greater research attention. [2][3][4] Although previous studies show that compared with xenon, the thruster efficiency is low with krypton, some studies show that improvements in efficiency can be made in propellant utilization rate, along with other means, which approach the efficiency of xenon. [5][6][7][8]12 Other research shows that thruster performance based on krypton is significantly lower than that on xenon. 6,[9][10][11] With a low utilization rate of krypton, krypton (with a decrease of 37% in atomic mass) relative to xenon under normal conditions, the thrust decreases 20%-30% and the specific impulse decreases 37%. 13 Previous studies have concluded that the main factor of krypton's low efficiency is its low propellant utilization rate. To study the reason for this low efficiency, it is necessary to study the plasma in the thruster's discharge channel. 14 Estimates of classical conduction produced by collisions between electrons and atoms is far less than experimental results; that is, other unusual conduction occurs allowing electrons to cross magnetic field lines. 15 Some researchers believe that this unusual conduction is due to collective oscillation in the plasma of the thruster, which is called Bohm conduction. Bohm conduction is used to explain the reason why the current is greater than that produced solely by collision between electrons and atoms in discharge devices using plasma with a high degree of ionization. The Bohm conduct...

More and more physics engines are used to simulate the dynamics between two rigid bodies. However, it is still unknown whether physics engine is suitable for computing the physical information of objects with fluid effect, such as the trajectory and collision dynamics of otolith in semicircular canal in benign paroxysmal postural vertigo (BPPV). The purpose of this study is to determine whether the fluid effect in Bullet physics engine has an impact on the movement data of otoliths in semicircular canal. Based on the experimental data obtained previously, we discuss the position and trajectory of the otolith when it falls in the Dix-Hallpike test. Root Mean Square Error (RMSE) is used to evaluate the static position of otolith under two conditions, and the RMSE is 0.716. We found that the initial position of otolith is affected by fluid effects, but the final static position and trajectory of otolith is similar (x=-5.838±0.294, y=19.348±0.143, z=-9.540±0.635). The experimental results show that the fluid effect does not affect the experimental results of the Dix-Hallpike test, and it has sufficient applicability for the evaluation of BPPV diagnostic methods.