Coupling plasma plume of a low-power magnetically shielded Hall thruster with a hollow cathode

Gunghun ZHANG, Junxue REN, Wei LIANG, Ning OUYANG,Cho LU, Hiin TANG,d,e,*

a School of Space and Environment, Beihang University, Beijing 100083, China

b School of Astronautics, Beihang University, Beijing 100083, China

c Shanghai Institute of Space Propulsion, Shanghai 201112, China

d Key Laboratory of Spacecraft Design Optimization & Dynamic Simulation Technologies, Ministry of Education, Beijing 100083, China

e Laboratory of Space Environment Monitoring and Information Processing, Ministry of Industry and Information Technology, Beijing 100083, China

KEYWORDS Coupling region;Electron conduction;Hall thruster;Hollow cathode;Utilizations of propellant and current

Abstract The coupling region of a Hall thruster with a hollow cathode is the region between the cathode and the thruster plume. The characteristics of plasma in that region are complicated and strongly associated with the thruster working conditions and the cathode position. In this paper,a laboratory 100 W class magnetically shielded Hall thruster was coupled with a hollow cathode.Optical imaging and electrostatic probe were employed to monitor and scan the plasma plume.Plume characteristics in the coupling region in non-self-sustained mode and self-sustained mode were compared. Evolution of the coupling plume with the cathode position was studied. Experiments show that,when turning the thruster into self-sustained mode or moving the cathode further away axially, the discharge current can be reduced by 6.4-10.6% restraining the electron current and improving ionization.In particular,when the cathode is moved further,the electron conduction near the channel walls is suppressed. The electron current is reduced by 27.4% and the ion beam current is increased by 7%. Overall, this work shows that the working mode of the thruster and

1. Introduction

A Hall thruster is a type of electric propulsion device.An integrated Hall thruster contains a hollow cathode, which is the source of the initial electrons, and the thruster body, which constrains electrons ejected from the hollow cathode by orthogonal electric field and magnetic field(E×B field)inside the discharge channel, inducing collisions between the electrons and the propellant atoms.Ions are produced by collision ionization inside the discharge channel. The ions are then accelerated outside the channel by the axial electric field. In contrast to the energy limitations of chemical propulsion,Hall thruster uses external source of energy to accelerate the ions through an electric field generating higher specific impulse,nearly one magnitude higher than chemical rockets. As such,Hall thruster is a very efficient and competitive electric propulsion device for satellites and is widely used in commercial telecommunications and government spacecraft.1-3In recent years, electric propulsion has trend to be economical4and demands have emerged for low-power Hall thrusters, e.g. the Starlink mission of SpaceX. Ideally, low-power Hall thruster is attractive for a wide range of missions where propulsion is a challenge,5,6especially for thrusters ranging from a few tens to a few hundred watts where high specific impulse and long lifetime are required.

Being the connection between the thruster body and the hollow cathode, the coupling region is drawing increasing attention. This region, situated between the cathode and the thruster plume, has the particularity of having a complex plasma behavior and electron dynamics. The coupling process is closely related to the cathode position and the thruster working conditions. The behavior of plasma there affects the current oscillation,7plume extraction8,9and thruster performance.10

Two types of hollow cathode setup related to the thruster body were published in previous studies: internal and external cathodes. For medium- or high-power thruster, an internal cathode is a better choice to increase the performance.10-12However, an internal cathode would cause overheating of the magnetic components and the demagnetization of the magnetic circuit,which would destruct the thruster.Thermal loads are serious challenge for kilowatt thrusters and they become even more rigorous for low-power thrusters. Therefore, an external cathode is considered the most appropriate scheme for this type of systems. The position of an external cathode influences directly the coupling process and thus thruster performance. Far cathode makes thruster ignition difficult,whereas near cathode is eroded by high-energy ions ejected from the thruster channel.13The position with minimum coupling voltage is regarded as the slowest erosion point.14High electron density near the cathode would cause a plume deflection angle in radial dimension toward the cathode, demonstrated by the test of the 3D plume distribution of a Hall thruster.9Effects of the separatrix on cathode-anode coupling were emphatically studied on different facilities.15-18Separatrix is the boundary that divides the magnetic field lines into two directions. The first part of the magnetic line originates from the outer magnetic pole,taking an inward sharp curve,and terminates at the inner pole. The second part takes an outward direction and draws a big magnetic circuit, entering the outer pole from the back of the thruster. The magnetic separatrix serves as a reference position for the cathode.15-17Electron conduction routes and effects on a multi-cusped field thruster were described both employing simulation code and experimental facilities. Reduction in leaked electrons decreases the peak electron temperature and density,and moves the position of the maximum electron density away from the exit.19The formation of the two types of electron conduction routes is attributed to the magnetic mirror effect in the multi-cusped field.With the cathode moving towards downstream,the electron current in the inner route increases but the electron current in the outer route decreases, promoting the ionization in the exit region and restraining ionization in the upstream region.20

Regarding the influence of the coupling process on the thruster working conditions, researches about the effects of enhanced cathode electron emission on Hall thruster performance were carried out. Electron emission affects the electron cross-field transport in the thruster discharge. As the electron emission increases with wire heating, the discharge current increases,the plasma plume angle decreases and the ion energy distribution function shifts toward higher energies, accompanied with a suppression of large amplitude,low frequency discharge current oscillations usually related to an ionization instability. Optimal placement of the cathode with respect to the magnetic field could further enhance the thruster discharge parameters.These effects correlate strongly with the reduction of the voltage drop in the coupling region with the fringe magnetic field.21,22Inadequate electron-emission, induced by decreasing the emitter temperature, could be compensated by the self-adjustment of the space potential on the plasma bridge.However,it usually results in a decrease in the effective Hall thruster discharge voltage, and consequently in a degradation of the overall performance of the thruster.23

Besides the cathode position and the thruster operating conditions, there are effects on coupling process derived from the experimental facilities. These effects include discharge oscillations,performance and plume properties of the thruster.The vacuum chamber walls act as an artificial electrical boundary condition that keeps the Hall thruster plume plasma potential within certain boundaries. Cathode position alters the recombination pathways taken by the cathode electrons in the Hall thruster circuit. Cathode electrons preferentially travel to the thruster body, the plume, and facility surfaces in the near field, midfield, and far field, respectively.7,24

However,most of the above studies concentrated on thruster with more than kilowatt consumption or with traditional orthometric E×B field. As for low-power magnetically shielded Hall thruster, the coupling of the thruster with a hollow cathode makes a more dramatic impact on the thruster performance.21,25A laboratory Low-power Magnetically Shielded Hall Thruster (LMSHT) with permanent magnets is developed in our lab.26Compared with the thrusters mentioned above, LMSHT has three main differences. First, a strong magnetic field with a maximum value around 500 G(1 G=10-4T) on the centerline of the channel near the exit achieves a better constraint of electrons. Second,nearly paralleled magnetic lines around the ceramic channel walls form conduction routes of electrons along the channel walls. Third,the compact volume makes the effect of electric field between the cathode and the thruster plume on the coupling process more considerable.9In this study, different thruster working modes and cathode positions are tested, the plume morphology is monitored, the plume plasma characteristics are scanned, and the electron conduction routes variation and the effects on the current and propellant utilizations are analyzed.

The remainder of the paper is organized as follows: Section 2 describes the facilities and experimental setup,Section 3 discusses the experimental results, and conclusions and future work are presented in Section 4.

2. Facilities and experimental setup

2.1. Low-power magnetically shielded Hall thruster: LMSHT

LMSHT shown in Fig.1 is a 100 W class magnetically shielded Hall thruster.LMSHT could work stably from 13 W to 263 W,generating a thrust ranging from 2.9 mN to 10.8 mN. The anode specific impulse varies between 729 s and 1576 s. The anode efficiency higher than 30% could be achieved for mass flow rate of 0.5-0.7 mg/s with the highest efficiency reaching 37.8%.

LMSHT adopts a Boron Nitride (BN) ceramic discharge channel suiting the magnetic field. The discharge channel of LMSHT has a mean diameter of 34 mm and a width to mean diameter ratio of 0.3.In addition,the anode has been designed to optimally distribute the propellant in the discharge channel.An assembled gas distributor with multiple venting grooves is employed to improve the symmetry of neutral propellant atom distribution in azimuthal and radial direction inside the ceramic channel.

Fig. 1 LMSHT with partially shielded discharge channel.

Fig. 2 Partially magnetic shielding inside discharge channel of LMSHT.

A group of coaxial coppery heat sink is assembled inside the thruster,as shown in Fig.2.When the thruster is working,the heat sink could transfer the heat around the inner magnetic pole and the channel walls to the backside of LMSHT. Monitoring of the temperature during the tests shows that the temperature near the inner magnet ring levels off just taking several minutes and stays below 160°C even under the working point of 172 W, much lower than the permanent magnet limit of 550°C ensuring the safe operation of LMSHT.

LMSHT replaces the electromagnetic coils of traditional thruster with permanent magnets, generating a stronger magnetically shielded field27,28inside the channel, as shown in Fig. 2. Partially grazing lines carry cold electrons near the anode to the channel walls,which can reduce the sheath potential drop accelerating ions toward the walls. Meanwhile, high anode potential introduced from the anode to the walls by partially grazing lines decreases the potential gradient parallel to the walls, which reduces the energy of ions accelerated along the walls. Furthermore, High potential near the walls refrains ions from colliding the walls and repels them away from the walls. Partial magnetic shielding could be proved by comparing the shielded and unshielded walls after test for tens of minutes, which could reduce interaction between the plasma and the discharge channel walls, slowing down the erosion rate of the channel.The partial shielding of LMSHT corresponded to the configuration of the magnetic field lines near the walls.Comparing the boundary of the magnetically shielded walls with the junctures of the separatrices with the channel walls,region covered by the separatrices agrees with the shielded section of the channel walls.26For better performance,a magnetic field higher than 500 G is employed inside the channel, which induces the problem of ignition. An additional anode was adopted to overcome the difficulty successfully.

2.2. Vacuum chamber

All the experiments were carried out in DIWENBENG-2500(DWB-2500) vacuum chamber in Joint Laboratory of Plasma and Propulsion (JLPP).DWB-2500 is built on a stainless-steel tank with a diameter of 2.5 m and length of 5.5 m.DWB-2500 is equipped with two 57000 s-1cryopumps and two Xenon cryopumps evacuating Xenon 60000 s-1and 30000 s-1respectively. To protect these pumps from the impact of ion beam,every cryopump is covered by stainless-steel grids. DWB-2500 could provide vacuum environment better than 4×10-5Pa background pressure.

Fig. 3 Experimental facilities.

2.3. Experimental setup.

During the experiments, 0.48 mg/s and 0.4 mg/s Xenon was fed to the anode and the cathode respectively. The dynamic pressure for Xenon under this working condition was kept at 3×10-3Pa. An IT6723G auto range DC stabilized power supply was used for anode. A DH1724A-5 and a DH1716A-6 DC stabilized power supply was assigned to the cathode keeper and emitter heater. The heater power was shut down after starting the cathode. The anode worked at 250 V constant voltage, whether the keeper power was on or off. All these powers shared a common negative, floating to the ground.The cathode and the anode were insulated from each other and both insulated from the ground connected vacuum chamber.To study the effect of cathode position on thruster plume,the cathode was installed on a step motor controlling platform on the vertical plane, as shown in Fig. 3.

A planar probe was employed to diagnose the ion current density and the voltage-current characteristics of the nearfield plasma.9,29A 2 mm tungsten bar was flush coated by an Al2O3tube with outer diameter of 3 mm. Probes scan the vertical plane coinciding with the axis of the cathode.Spatial resolution of all the probes scanning was 3 mm,the same with the outer diameter of probes. Probe signals were recorded by a GEN 3i data recorder with a frequency of 4 kHz.

The electron current Ieis calculated by subtracting the ion beam current from the discharge current, as the following formula:

The efficiency of the discharge current ηcand the propellant Xenon gas ηpare defined in Eq. (4) and Eq. (5) respectively:

and

where MXe+is the absolute mass of Xenon ion,and ˙mXeis the anode mass flow rate.

Moreover, a commercial digital camera and two filters of 480 nm and 560 nm with half-band width of 10 nm, were used to capture the features and plasma parameters evolution,when the cathode position was changed.

3. Experimental results and discussion

3.1. Two types of morphologies of coupling region

During the experiments, LMSHT worked at two different discharge modes controlled by the cathode keeper. When the cathode keeper was turned on, the thruster worked in nonself-sustained mode,whereas when it was turned off,the thruster worked in self-sustained mode. In the non-self-sustained discharge mode,the thruster anode voltage was remained constant at 250 V, and the anode discharge current oscillated around 0.47 A, with the keeper power working at 15 V/1 A under constant current mode. However, in the self-sustained mode,the anode worked at 250 V/0.44 A under constant voltage mode, with the keeper floating. The discharge current shows periodic oscillation as shown in Fig. 4. Plumes of the thruster under these two different modes were captured by digital camera equipped with filters.

Fig. 5(a) and (b) show the photographs of the thruster plumes with a 560 nm filter. The thruster plume features are significantly different when the thruster works in different modes.Although both are asymmetric,the asymmetry is more pronounced in the self-sustained mode. In the non-selfsustained mode, the plume is absorbed by the cathode and expands slightly in the direction of the cathode. There is an apparent black-area between the thruster plume and the cathode, where nothing could be caught optically. The transition between these two modes can be observed when turning off the keeper. The black-area between the thruster beam and cathode orifice fades away as the thruster and cathode plumes merge into a single plume. This previous black-area becomes filled with plasma that remains stable in the self-sustained mode.

Fig. 4 Discharge current oscillation of LMSHT in different modes.

Fig. 5 Plume features of LMSHT plume in different modes.

For a better comparison,information about the light intensity was extracted from the plume morphology with a 480 nm filter. As shown in Fig. 5(c) and (d), differences in these two modes are notable. If it is assumed that the thruster body and its plume work as a virtual plasma anode, the differences between the two modes shown in Fig. 5 can be reasoned. For the non-self-sustained mode, the cathode works in spot mode.However, the cathode switches to plume mode when the keeper is turned off. There are remarkable distinctions in the plasma characteristics for these two types of cathode modes.For a typical plume mode, the plasma between the cathode and the anode shapes as a plume overall,needing higher anode discharge voltage,and generating electrons with higher energy.On the contrary,in the spot mode,the plasma forms a hot-spot near the keeper orifice,and then a black-area downstream.The anode discharge voltage and the electron temperature are both lower in the spot mode than in the plume mode.30,31In this simplified cathode and virtual plasma anode system, the plasma works in the same way.

Fig. 6 shows the plasma potential variation of the thruster plume measured with a 2 mm planar Langmuir probe, fixed 40 mm away from the thruster exit plane. The probe scanned along the radius in vertical plane from the cathode side to the opposite side. The cathode radial position is marked with the dashed line in Fig. 6.

Fig. 6 Plasma potential of LMSHT plume in different modes.

Fig. 7 Ion beam current density of LMSHT plume in different modes.

For the cathode and virtual plasma anode system, the plasma potential here acts as the virtual anode discharge voltage. It is clear that the plasma potential is much higher in the self-sustained mode when the keeper is turned off, consistent with the plume mode of the hollow cathode.With the increase of the plasma potential in the plume,especially in the coupling region, electrons ejected from the cathode gain more power from the electric field between the cathode and the virtual plasma anode. High-energy electrons intensify the collisions with Xenon atoms there, enhancing the ionization in the coupling region even in the main plume of the thruster body. Ion current density(D Iion)in Fig.7 is measured by the 2 mm planar probe placed at an axial distance of 20 mm. An obvious growth of the ion current density on the half side of the cathode certifies the contribution of the high-energy electrons to the collision ionization in the coupling region. Hence, the black-area in Fig. 5 is replaced with a distinct plasma bridge.

Fig. 8 Utilizations of LMSHT in different modes.

The increase of the ion beam current in Fig.7 coming from the plasma bridge in coupling region and the fringe area of the thruster plume deteriorates the symmetry of the plume. Ions produced in coupling region are usually not accelerated by strong electric field.These ions play a limited role in the interaction between plume and downstream facilities. However,enhancing the ionization in the coupling region could increase conductivity there, making electrons more easily and effectively enter the discharge channel. Rise of ion beam current density in the center of the plume in Fig.7 reveals that the ionization inside the channel is promoted.In view of this,it is useful to enhance the ionization in coupling region.

As can be seen in Fig. 8, the discharge currents, ion beam current, electron current, current utilization and propellant utilization all show clear variation. Compared to non-selfsustained mode,the ion beam current presents a slight increase when it discharges self-sustainably.However,the electron current decreases sharply. This reduction in electron current submerges the rise of ion beam current, reducing the discharge current from 0.47 A to 0.44 A. As the electron current decreases and ion beam current lifts, both improve the utilization of the discharge current.A 9.9%current utilization rise is shown in Fig. 8. Because of the slight increment of ion beam current, the propellant utilization acquires a slight promotion in self-sustained mode.

3.2. Coupling plume evolution with different cathode positions

Two typical cathode positions were selected during the coupling experiments according to the cases where the features had a more pronounced variation. The distance between the keeper orifice and the edge of the thruster exit plane was regarded as the cathode position. These two positions were designated at 9 mm and 21 mm respectively in axial direction,and 6 mm in radial direction as highlighted in Figs. 9(a) and(b).In both positions,the cathode was kept at a 45°angle from the centerline of the thruster all the time. Figs. 9(a)-(d) show the plume evolution of the thruster in non-self-sustained mode and Figs. 9(e)-(h) show that of self-sustained mode.

Results are clear for the self-sustained mode, as shown in Figs. 9(e) and (f). As the cathode moves further, the visible plasma bridge fades away. The plume changes into gourd shape,shrinking firstly outside the channel and then expanding near the cathode position. This second expansion arises not only on the cathode side,but also on the opposite side.To capture more features of the coupling plumes,light intensity information was extracted and processed.Regarding the line where the light intensity is maximum as the axial centerline of the plume, the difference of light intensity between the side of the plume near the cathode and the opposite side could be calculated. This difference highlights the characteristics of the coupling plumes as shown in Figs. 9(g) and (h). For position-01, the junction of the coupling plume and the thruster plume starts from the outer pole and stretches downstream of the thruster plume. However, for position-02, the junction just exists downstream of the plume.

Fig. 9 Plume features of LMSHT plume at different cathode positions.

Fig. 10 Sketch map of electron conduction routes in different cathode positions.

For the non-self-sustained mode, evolution of the plume feature is not drastic but still obvious.As previous description of the two types of coupling region phenomena, the first cathode position presents an asymmetric plasma plume, deflecting to the cathode.When the cathode was moved from position-01 to position-02,further in the axial direction,the plume shrinks towards the discharge channel and evolves into an approximately axisymmetric plasma cone, as shown in Fig. 9(a) and(b)Figs.9(a)and(b).Moreover,the light intensity near the exit of the outer channel walls is highest for position-01, as shown in Fig. 9(c), whereas this maximum disappears when the cathode was moved to position-02 in Fig. 9(d).

Whether the thruster works in the non-self-sustained mode or in the self-sustained mode, there is a clear relation between the coupling plume and the cathode location.These differences mentioned above in the coupling plume reveal the variation of the electron conduction routes entering the thruster.

For position-01, electrons move toward the outer walls chamfer of the channel and are constrained by strong magnetic field near the outer SmCo pole, as the arrow inside the discharge channel shows in Fig. 10. Collisions and conductions happen there. A magnetically shielded Hall thruster, because of the partially grazing magnetic lines which are parallel to the channel walls, has analogical magnetic lines with a multicusped thruster near the outer channel walls.20Lots of electrons get trapped by the parallel magnetic lines near the channel walls and unconstrained towards the anode, resulting in a considerable number of electrons being caught by the anode. When the cathode is moved further, it is more easily for electrons to enter the plume for the further cathode location.7,24The electron conduction routes near the channel walls are suppressed. Electrons enter the thruster plume first, and then move towards the channel at a larger angle with the radial magnetic field lines.On account of the strong magnetic field of 500 G, electrons in this high incidence angle trajectory are trapped and collide with the propellant atoms more effectively,increasing the ionization inside channel. At the same time,electron current is lower,improving the utilization of discharge current.

Fig.11 Ion beam current density of LMSHT plume in different cathode positions.

Fig. 12 Utilizations of LMSHT in different cathode positions.

To describe the plume quantitatively and verify the analysis of electron motions made above, a planar probe was used to scan the plume radially in non-self-sustained mode, 50 mm away from the thruster exit plane.In Fig.11,the ion beam current density shows an increment in the center of the thruster plume of position-02 in comparison with position-01. A 7%rise of total ion beam current rises the propellant utilization from 65.59%to 70.11%as shown in Fig.12.The electron current decreases by 27.4%profiting from the suppression of electrons moving to the anode near the outer walls. The evident decrease of the discharge current, from 0.47 A to 0.42 A, the increase of ion beam current and the reduction of electron current,all contribute to the notable increment of the current utilization from 48.75% to 58.31%.

4. Conclusions

In this paper, a laboratory low-power magnetically shielded Hall thruster was coupled with a hollow cathode. The Hall thruster employs a strong magnetically shielded magnetic field with parallel lines near channel walls.The experiment includes two parts. The first part is that the plume characteristics was compared when changing the thruster working mode between non-self-sustained mode and self-sustained mode. In the second part, the evolution of the coupling plume was studied for different cathode positions,while keeping the cathode conditions the same.

(1) When the cathode keeper is turned off, the discharge current decreases from 0.47 A to 0.44 A,and the thruster turns into self-sustained mode from non-self-sustained mode. The thruster plume is taken as a virtual plasma anode. The self-sustained and non-self-sustained mode correspond to the plume mode and the spot mode of the hollow cathode respectively. In the plume mode,the plasma potential increases and hence the electron temperature in coupling region increases,improving ionization in that region.The plasma in the plume expands and connects with the cathode orifice. The thruster plume becomes more asymmetric, and a visible plasma bridge appears in the coupling region between the cathode and the thruster. Better ionization in the selfsustained mode increases the ion beam current and reduces the electron current, indirectly improving the utilizations of current by 9.9%.

(2) When the cathode is moved away from the thruster in the axial direction, the discharge current decreases from 0.47 A to 0.42 A,and the plume shrinks towards the discharge channel and evolves into an approximately axisymmetric plasma cone.Grazing lines of the magnetically shielded field facilitate the electron conduction near the outer channel walls. When the cathode is moved away, the electron conduction routes near the channel walls are suppressed. Electrons enter the plume first and then move towards the channel under a larger angle with respect to the radial magnetic field lines.Due to the strong magnetic field inside the channel,electrons collide with propellant atoms more effectively,increasing the ionization inside channel. A 27.4% sharp decrease in the electron current and a 7% slight growth in the ion beam current improve notably the efficiency of current and propellant.

In addition to the effects on the coupling plasma plume,the position and the working conditions of the hollow cathode greatly affect the discharge oscillation and the stability of the thruster.As a future work,the oscillation of the discharge current of the thruster will be monitored when the conditions of the cathode are changed. The analysis of the oscillation can differentiate which mode the thruster works in and offer information about ionization and acceleration of ions inside the discharge channel. The ion energy distribution in the thruster plume and the slight change of the thrust will be studied to make verification against the results of this manuscript and the oscillation in the future work. Furthermore, new material and technology are desired to be employed to improve the performance of the thruster.32

Acknowledgement

This work was supported by the National Natural Science Foundation of China (No. 11872093).