Experimental study of the domain of existence of intensive non-self-sustained discharge in crossed fields

A set of experiments was performed on the breadboard model of the Hall thruster with the anode layer. Use of the thermoemission cathodes as the cathode-compensators made it possible to determine the domain of existence of the non-self-sustained type of discharge in the crossed electric and magnetic fields at different values of the anode voltage, the magnetic field, and the working body mass flow rate. According to the electron current transfer mechanism and the current oscillation character, two modes of the discharge are distinguished burning strictly differing, even visually: the “accelerative” and the “stochastic” ones.     

Particularities of ignition of the non-self-sustained discharge with a thermoemission cathode in crossed fields

The experimental results of use of a thermoemission cathode as the cathode-compensator when working with the Hall electroreactive thruster are presented. The operation particularities of the thruster with the thermoemission cathode are investigated, and the technical requirements for the cathodes in use are adjusted. Data making it possible to specify the technical requirements on the power supplies for the Hall reactive thrusters (HRTs) are obtained. The study of the discharge ignition process in the crossed fields inside the Hall electroreactive thruster shows that, at the moment of ignition, the total discharge current may exceed by 10–20 times that of the stationary regime; here, the value and the duration of the current “spike” depend directly on the emissive ability of the cathode-compensator. The influence of the magnetic field value in the accelerator channel on the ignition potential is discovered.     

Performance characteristics of very low power cylindrical Hall thrusters for the nano-satellite “PROITERES-3”

Development of Hall thrusters for nano, small and low power satellites below 100 W is expected. In lowering Hall thruster power, the cylindrical type Hall thruster is more advantage than conventional coaxial-type Hall thrusters. In this study, a very low power cylindrical Hall thruster for nano-satellite “PROITERES-3” under development in Osaka Institute of Technology was designed, and the thruster performance was measured. As a result, a stable operation was achieved even with 10 W. The anode specific impulse and the anode thrust efficiency were 1570 s and 18.1%, respectively, with 66 W. Also, the discharge current oscillation was lower compared with that of SPT-type Hall thruster.     

Investigation of thrust characteristics of a dummy hollow-anode hall thruster

To measure the thrust of a dummy hollow-anode Hall Effect thruster, an IT-20 thrust meter was used whose mode of functioning is based on automatic compensation for the engine’s thrust by the force generated by a magnetoelectric compensator in the negative feedback loop. The dependences of the thrust on the discharge voltage and the magnitude of the magnetic field were obtained. The existence of oscillations in the discharge current was shown. The frequency of these oscillations matches the transit-time oscillation frequency of neutral particles, which may have an effect on the divergence between the calculated thrust values with those measured by the thrust meter.     

阳极磁屏蔽霍尔离子源磁场的数值模拟

为了提高离子束电流和离子束能量,进一步增加霍尔离子源的效率,对霍尔离子源阳极进行磁屏蔽,达到优化磁场位形,增强磁场对放电等离子体的约束。由磁场的数值模拟结果可知:阳极磁屏蔽后,有明显凸向阳极表面的弯曲磁场线,并且提高了磁镜磁场的磁镜比;大的正梯度磁场的存在,增强了磁场的径向分量,相对减弱了放电等离子体的震荡;不仅保留原有鞍形场的作用,而且还充分发挥了另外两个鞍形场的优势。最后通过实验结果和数值模拟结果的比较,验证了此数值模拟结果的正确性。     

Effects of channel wall material on thrust performance and plasma characteristics of Hall-effect thrusters

The effects of channel wall material on Hall thruster performance and on plasma characteristics were investigated. A laboratory-model Hall thruster THT-III was operated with three channel wall materials of BN, BNSiN and BNAlN. Both the discharge current and the thrust were affected by the nature of the channel wall materials. The measured axial distributions of wall and plasma potentials, radial and axial electron temperatures, and electron number density near the channel walls showed that the wall material affected ionization region and ion wall loss in the channel, resulting from secondary electron emission, although ion acceleration region was determined by the axial distribution of radial magnetic field. The difference in discharge current between channel wall materials was attributed to the difference in axial current density near the inner channel wall, depending on secondary electron emission.     

A Study of the “Needle” and “Bell” Discharge Burning Modes in a 1.5-kW Hall Thruster

Technical Physics Letters - Two discharge burning modes (“needle” and “bell”) have been experimentally studied for a Hall thruster with a nominal power of 1.5 kW. The...     

Investigation of Nonself-Maintained Discharge in Crossed Electric and Magnetic Fields with Closed Hall Current

A nonself-maintained discharge in crossed fields with a closed Hall current is experimentally investigated. Argon and xenon are used as the working medium. The discharge gap is characterized by a magnetic field uniform along the channel, by the absence of an insulating coating on the channel walls, and by the delivery of the working medium uniform over the channel section through the porous end diaphragm of the anode. Measurements of current distributions over the structural elements are performed. It is shown that, in the presence of a self-incandescent compensator cathode supporting a nonself-maintained discharge, an intense flux of accelerated ions is created with a high recovery factor of the working medium with about 30 eV spent for ion formation and the energy efficiency exceeding 0.5.     

Experimental investigations of a breadboard model of Hall thruster

Various breadboard models of Hall thrusters are analyzed with a view to optimizing the output parameters which characterize the degree of processing of the working medium and the energy efficiency of the device. It is demonstrated that the channel geometry with the magnetic field uniform along the channel affects only slightly the efficiency of acceleration of ion flow. In a number of working modes, the formation of accelerated ion flow is accompanied by the emergence of pulsations of discharge current, this leading to destructive processes of acceleration and formation of ion flow. The development of a breadboard model with a nonuniform magnetic field along the channel significantly decreasing in the direction of anode with cylindrical space makes possible the elimination of discharge current pulsations in a wide range of parameters. The increase in the density of the flow of working medium by a factor of 1.7 results in a 30% increase in the energy efficiency and provides for the working medium utilization factor of the order of 1.2.     

The effect of magnetic field configuration and propellant supply rate on insulator erosion in a hall thruster

Dependence of the rate of insulator erosion on the walls of discharge chamber of a Hall thruster on the propellant supply rate and magnetic field configuration at a constant discharge voltage has been studied. The regime of thruster operation was controlled by varying the propellant mass flow rate. The magnetic field configuration was optimized by monitoring the intensity of the spectral line of sputtered boron atoms. It is established that the rate of insulator erosion in the discharge chamber significantly decreases upon optimization of the magnetic field configuration, while the integral characteristics of the thruster change rather insignificantly. With the optimum magnetic field configuration, the rate of erosion of discharge chamber walls depends rather insignificantly on the propellant supply rate.     

圆柱形阳极层霍尔推进器的工作特性与离子束流研究

主要对圆柱形阳极层霍尔推进器进行实验研究,得到推进器的工作特性、离子束分布、束流中电子的含量等结果。并且给出圆柱形阳极层霍尔推进器的最佳工作参数:工作电压在700~1000 V之间,电流在0.15~0.65 A,气体流量低于8 ml/min。从离子束流的径向分布可知:高电压、低电流、小气量时离子束的发散角较小。     

A theoretical study of the impact of magnetic field of Hall current on the parameters of a high-current vacuum-arc discharge

It is demonstrated that the magnetic field of Hall current in a short high-current vacuum-arc discharge significantly distorts the external axial magnetic field and affects the distribution of current density in the discharge gap. This effect decreases with increasing external magnetic field, with decreasing arc current, and with decreasing ratio of the length of discharge gap to its transverse dimension. A 2D magnetohydrodynamic mathematical model is used to calculate the discharge parameters for different values of induction of external magnetic field. The calculation results are compared with similar results obtained using a 1.5D model in which the impact made by magnetic field of Hall current is ignored. It is inferred that the simpler 1.5D model may be employed in calculations of parameters of vacuum arc in a wide range of variation of arc current and of induction of external magnetic field.     

Use of emission spectroscopy for real-time assessment of relative wall erosion rate of BHT-200 hall thruster for various regimes of operation

Radiation emission due to Boron atoms sputtered from the Boron-Nitride ceramic walls of a BHT-200 Hall thruster was measured as a diagnostic for real time assessment of thruster wall erosion and to determine the effects of various operation conditions on thruster lifetime. Boron neutral 249.677 and 249.773 nm lines were measured using a high-resolution spectrometer. Spectral measurement results and the accompanying analysis and discussion are presented in this study. From the spectral measurements it was observed that the Boron emission intensity significantly increases for increased discharge voltage pointing to a large increase in the thruster wall erosion rate. Additionally, the measurements show that for the nominal discharge voltage and the applied magnetic field intensity, there is an optimum propellant flow rate for minimum Boron emission, thus minimum wall erosion rate. The variation in the current to the magnet coils showed that the Boron emission intensity increases for increased magnetic field and the Boron emission intensity shows similar behavior to that of the Xenon single ion emission line intensity at 248.911 nm. The findings of the study show that emission spectroscopy can be used in determining the optimum operational parameters for minimum wall erosion for SPT type Hall thrusters.     

Numerical prediction of wall erosion on a Hall thruster

Hall thrusters are promising space propulsion options for spacecraft. Current major interest in Hall thrusters is their life time evaluation for long time operation. Sputtering erosion of the insulation wall determines the life time of the thrusters. In this study, we present two-dimensional Hybrid-PIC modeling of Hall thrusters, which predicts the wall erosion effect. The principle challenges of our modeling is (1) solving both the plasma generation and acceleration in the channel and the exhaust plume in the near field of the exit and (2) predicting the channel erosion by ion sputtering with deforming the calculation grid. As a result, it was shown that the modeling simulated successive operations and wall erosions by ion sputtering during 2000 h operation of a Hall thruster.     

Evaluation of the Hall parameter of electrolyte solutions in thermosyphonic MHD flow

The objective of this work is to determine experimentally the Hall parameter of electrolyte solutions using a closed loop thermosyphonic magnetohydrodynamic flow. The upper and lower parts of the loop, which represent the heat sink and heat source of the system respectively, are constructed from copper pipe coated with varnish on the inside surface. The middle region, connecting the upper and lower parts of the loop, is made from plastic vertical pipes, with segmented copper electrodes placed vertically opposite to each other on each side of the loop plastic walls and connected as a Hall generator to measure the open circuit voltage. A transverse magnetic field is imposed in the middle non-conducting plastic-wall region by a set of permanent magnets. The magnets provide a magnetic field strength of up to 0.225 T, whereas the driving temperature difference between the hot and cold portion of the loop ranges from 10 to 80 °C. Measurements of the induced flow rate and induced open circuit voltage are reported as a function of driving temperature difference and magnetic field strength.The analytical one-dimensional model of Ghaddar [Int. J. Heat Mass Transfer 41 (8-9) (1998) 1075] is extended to account for the electrode design and the Hall effect pertinent to electrolyte solutions. The open circuit voltage is related to the driving temperature difference, flow characteristic, magnetic field strength, electrolyte electric properties and electrode design. The developed 1-D model and the measured open circuit voltage are used to evaluate the Hall parameter (ωτ), which is a property of the fluid of the electrolyte liquid. It is found that ωτ can be as large as 100 for electrolytes and causes a significant loss in power output at the electrodes due to electron drift in the fluid leading to generation of current in an axial direction at the expense of the current flowing in the transverse direction between the electrodes.     

Nondestructive Measurement of Hall Coefficient for Materials Characterization

The Hall effect is widely exploited in NDE for measuring unknown weak magnetic fields using a small piece of conducting material of known high Hall coefficient. The Hall effect could be also exploited in NDE for measuring the unknown weak Hall coefficient of conducting materials using a strong applied magnetic field, but such measurements are fraught with difficulties because of the need to cut the specimen into a small piece similar to a Hall sensor, which of course is inherently destructive. This paper tries to answer the question how the need for destructive cutting in order to produce a measurable Hall voltage could be avoided. The underlying problem is that the Hall effect produces a Hall current that is normal to the conduction current but does not directly perturb the electric potential distribution unless the Hall current is intercepted by the boundaries of the specimen. This study investigated the feasibility of using alternating current potential drop techniques for nondestructive Hall coefficient measurement in plates. Specifically, the directional four-point square-electrode configuration is investigated with superimposed external magnetic field. Two methods are suggested to make Hall coefficient measurements in large plates without destructive machining. At low frequencies constraining the bias magnetic field can replace destructively constraining the dimensions of the specimen. At sufficiently high inspection frequencies the magnetic field of the Hall current induces a strong enough Hall electric field that produces measurable potential differences between points lying on the path followed by the Hall current even when it is not intercepted by either the edge of the specimen or the edge of the magnetic field. Both techniques are investigated first analytically to illuminate the underlying physics and then by numerical simulations to make useful quantitative predictions.     

Particle simulation of discharge current oscillation in Hall thrusters

Twenty kHz-range discharge current oscillation with a large amplitude is observed in Hall thrusters, causing unstable operation. In our previous studies, the effects of the anode orifice configuration on the current oscillation have been investigated. Using the results of neutral particle simulations, the relationship between the neutral distribution and the propellant inlet condition was analyzed. In the present study, to confirm the effect of the neutral distribution and investigate the ionization process in detail, axisymmetric analysis in the acceleration channel was carried out using a hybrid-PIC code. As a result, the periodic ionization and current oscillation were obtained. The oscillation amplitude for a smaller orifice is greater due to the higher ionization rate. The results agree with the effect of the neutral distribution predicted by the simulations.     

Distribution of local plasma parameters in a channel of the Hall thruster at two different types of discharge burning

According to experiments, a jumplike repartition of the discharge burning takes place at some parameters of Hall-thruster operating. This paper presents results of probing measurements demonstrating the difference in the distribution of floating and plasma potentials, electron temperature, and electric-field strength in a channel of the Hall thruster at two different types of discharge burning. The measurements were carried out along the discharge-channel axis using Langmuir probes.     

Hall effect in bismuth in quantizing magnetic fields

The Hall effect has been investigated in the very low-temperature limit in bismuth, when the lattice scattering is dominant in solids. The energy band structure of bismuth carriers is assumed to follow the modified nonellipsoidal, nonparabolic (MNENP) model. Results show that the Hall coefficient and Hall angle oscillate with the dc magnetic field, and the period and amplitude of the oscillations increase with the dc magnetic field. We also compare our numerical results with those found using other types of energy bands.     

The effect of axial magnetic filed on the process of anode spot formation in a vacuum-arc discharge with CuCr50 electrodes

An experimental study is made of the effect of an external axial magnetic field on the process of anode spot formation in a pulsed vacuum-arc discharge in the range of currents from 5 to 12 kA in a discharge gap with CuCr50 electrodes. The times and currents, at which an anode spot is formed, are determined for each amplitude value of current depending on the magnitude of magnetic field. The minimal value of magnetic field preventing the anode spot formation is determined for each current. The measured values of diameters of the current channel are used to calculate the anode temperature. It is demonstrated that, under experimental conditions, the heating of anode is insufficient for marked evaporation, and the anode spot formation is associated with the critical flow of fast cathode ions.