Yanlin HU (扈延林), Zhongxi NING (宁中喜), Xiaoyu LIU (刘晓宇),Yanfeng CHU (初彦峰), Wei MAO (毛威), Yan SHEN (沈岩) and Ximing ZHU (朱悉铭)

1 Beijing Institute of Control Engineering, Beijing 100080, People's Republic of China

2 Plasma Propulsion Lab, Institute of Advanced Power, Harbin Institute of Technology, Harbin 150001,People's Republic of China

Abstract Hollow cathodes are widely used as electron sources and neutralizers in ion and Hall electric propulsion.Special applications such as commercial aerospace and gravitational wave detection require hollow cathodes with a very wide discharge current range.In this paper,a heater is used to compensate for the temperature drop of the emitter at low current. The self-sustained current can be extended from 0.6 to 0.1 A with a small discharge oscillation and ion energy when the flow rate is constant.This is also beneficial for long-life operation.However,when the discharge current is high(＞1 A),heating can cause discharge oscillation,discharge voltage and ion energy to increase.Further,combined with a rapid decline of pressure inside the cathode and an increase in the temperature in the cathode orifice plate, electron emission in the orifice and outside the orifice increases and the plasma density in the orifice decreases. This leads to a change in the cathode discharge mode.

Keywords:electric propulsion hollow cathode,discharge oscillation, heater, extended operation range(Some figures may appear in colour only in the online journal)

1. Introduction

With the development of space science and technology, new space missions require increasingly stable and static platforms. In space applications such as gravitational wave detection,gravity field measurement and high-precision space telescopes,the attitude orbit control system needs to be highly accurate, thus increasing the demand for μN- to mN-class small thrust units [1-3]. The high-efficiency multistage plasma thruster (HEMP) is a preferred device for the miniaturization of thrusters, owing to its low system complexity and long life [4, 5]. Hollow cathodes are widely used as electron sources in various space missions [6-8], where they are included in electric thrusters,space station active potential control [9], etc. They can also be applied to other plasma discharge-related fields[10-13].A cathode with a heater uses a heating wire to heat the emitter to the thermal emission temperature. This kind of cathode, which has the bestdeveloped theoretical research, is the most widely used in orbit[14].In a μN-class thruster such as HEMP the cathode is required to work at a low current and low flow rate,but there is no strict limit on the power of the cathode, which ensures the possibility of providing heating power in this case.In low operating conditions it is often difficult for the cathode to maintain a self-sustained discharge. At this point, turning on the heater maintains normal operation of the cathode to meet the demand for μN propulsion. The HEMP requires a large power supply when raising the orbit and a small power supply for station keeping, which requires a wide range of working capabilities. However, the traditional low-power cathode is designed for low working conditions using a hollow cathode dedicated to a discharge current below 1 A.

With the start of various space missions, the production of low-power thrusters has driven the development of low-power cathodes.Domonkos[15]studied the operation of a barium-tungsten cathode with a cathode tube diameter of 3.2 mm at low current and a low flow rate (a minimum power consumption of 10 W and minimum flow rate of 0.08 mg s−1). It was found that a hollow cathode with a closed keeper has low power consumption and requires less working substance in spot mode for the same discharge current. The temperature of the orifice mainly depends on the discharge current. The heat loss is mainly caused by heat radiation and heat conduction. The NASA Glenn Research Center [16] conducted an experimental evaluation of a miniature ion thruster based on a small cathode.The cathode had an inner diameter of 3.5 mm. The cathode orifice plate was convex to reduce ion recombination on the surface.A swaged heater was welded around the gas supply pipe. The cathode needed to be pre-heated for 2 min before normal operation.The current emission range was 0.5-2.5 A, the Xe flow rate was 0.5-3.0 sccm and the anode discharge voltage was 14-44 V. To overcome the shortcomings of conventional coating technology,NASA[17]used a small diamond-coated barium-tungsten hollow cathode that had no keeper housing or heater and had a stable discharge current of 0.75-3 A.The flow rate of argon gas was 8-12 sccm, the plane temperature of the orifice was 875 °C-1100 °C (which increased with an increase in the discharge current) and the discharge voltage was below 45 V. At the University of Southampton in 2007,the T5 hollow cathode-based miniature thruster was evaluated[18].The cathode emitter had a normal operating temperature of approximately 1000 °C, a maximum discharge current of 3.2 A, a flow rate of 0.04 mg s−1and a power of less than 90 W. Pedrini [19] from Sitael conducted separate cathode tests and thruster-coupling tests with two kinds of cathodes: HC1 (0.3-1 A, 0.08-0.5 mg s−1) and HC3 (1-3 A,0.08-0.5 mg s−1) for HT100 and HT400, respectively. The theoretical model showed good consistency with the experimental results. Lev [20, 21] of Rafael Advanced Defense Systems of Israel validated the characteristics of a low-power heaterless cathode with a discharge current of 0.2-0.5 A and a flow rate of 0.2 and 0.25 mg s−1, indicating self-sustaining operation at a minimum discharge current of of 0.35 A. A keeper current was provided below 0.35 A to maintain the discharge. It can be seen that research on small cathodes is a developing field.The small cathode size greatly increases the difficulty of manufacturing various component parts, and the thermal design of the cathode is also a challenge.At the same time, from the current research results, small cathodes also have the lowest power limit (approximately 10 W).

For the cathodes needed for a wide range of propulsion applications an additional method is needed to ensure stable operation of the cathode at low current levels, and the influence of high temperatures on the emitter also needs to be studied.In this paper,based on existing cathodes,the range of operation under low working conditions is expanded by heating during operation to ensure that the existing size of the cathode can meet the demands of general propulsion applications and μN-class propulsion while avoiding the disadvantages of a dedicated small cathode.Research on the low operating conditions of existing cathodes also helps to reduce the minimum power limit of the current small cathodes to meet the needs of multiple missions. Generally, a cathode with a heater turns the heater off after successful ignition,and the emitter maintains a self-sustained electron emission capability. In this paper, it is proposed that when the cathode is operating, the heater is turned on to continuously heat the cathode.The results show that under low operation conditions(low flow rate and low discharge current), the discharge voltage of the cathode can be reduced and the operating range of the cathode is obviously widened, while the influence of heating on the oscillation of the cathode is small. At a low flow rate and large discharge current, heating may cause a synchronous rise in the oscillation and voltage:it is then easy to overheat the cathode due to excessive heating power,resulting in a sharp increase in low-frequency oscillation[22].

2. Experimental equipment and method

The HEP-100MF model is an electric propulsion hollow cathode which has a standard flow rate of 3 sccm and a discharge current of 4 A. The material of the emitter is LaB6and the structure of the cathode is shown in figure 1. The structure of the heater is described in detail in[23].A CE0030200T power supply and a voltage and current control precision of 0.1 V were adopted. The thermal emission performance of the cathode is shown in figure 2. In the case of a heating current of 7 A, the emitter of the cathode reaches its thermal emission state,and the temperature at this time is approximately 1320 °C.

This experiment was carried out on the ground vacuum equipment at the Institute of Advanced Power,Harbin Institue of Technology, China. The chamber was 0.3 m in diameter and 0.5 m long with a pumping speed of 2000 l s−1and an ultimate vacuum of 2 × 10−5Pa [24]. The heater was operated in constant-current heating mode. The cathode was successfully ignited after about 5-8 min at a flow rate of 3 sccm,anode voltage of 50 V,ignition voltage of 220 V and heating current of 7.5 A. The flow rate and anode discharge current were adjusted, and the cathode was self-sustained when the heater was turned off. Then the heating power was turned on, and different heating currents (5, 6 and 7 A) were fixed to operate the cathode. A schematic diagram of the experimental device and the measuring circuit is shown in figure 3. Gas supply lines include Xe storage tanks, a mass flow meter (MFM) and a mass flow controller (MFC). A capacitive vacuum gauge [25] was installed between the cathode and the mass flow controller to measure the pressure at 10 cm upstream of the cathode emitter in order to characterize the internal pressure of the cathode tube. The results of the internal pressure were showed by a real-time digital display. A two-channel oscilloscope [26] was used to collect the mean and oscillation of the cathode discharge voltage and discharge current. The temperature of a certain point of the cathode orifice plate was monitored by a perforated anode plate using a pyrometer. The temperature measurement point is shown in figure 3. The ion energy distribution of the cathode plume was collected by a homemade retarding potential analyzer(RPA)[26]which was placed on the side of the cathode plume. The RPA used in the experiment was a four-grid structure. The first grid is suspended, the second grid is loaded with a voltage of −24 V for repelling electrons,the third grid is loaded with a linearly varying voltage of 0-140 V at 1 Hz and the fourth grid is loaded with a voltage of −24 V near the collector to suppress secondary electron emission. A high-speed camera [27] was used to shoot the plume area discharge oscillation.The effects of heating on the discharge characteristics of the hollow cathode were analyzed in combination with the various diagnostic means above.

3. Results and discussion

3.1.Influence of heating on the V-I discharge characteristics of cathode

In order to study the effects of heating on the performance of the HEP-100MF cathode under a low flow rate and low discharge current, the cathode was operated below the standard gas supply flow rate (0.6, 0.7 and 0.8 sccm) and below the standard discharge current(≤2 A).Comparative discharge tests with no heating and with heating were performed.At the same time, different heating currents were used to study the effect of the heater on the cathode discharge under different heating conditions. The experimental results are shown in figure 4. It can be seen that the discharge voltage at the low flow rate is high under the same discharge current, which is due to the presence of fewer current carriers in the cathode and anode regions at a low flow rate, resulting in an increase in plasma resistivity [20]. The anode voltage is high in the near-anode region where the gas content is low [28]. In the case of a low current higher than 1 A,the performance of heating conditions was consistent with the above. At lower discharge currents (less than 1 A), the effect of the flow rate on the discharge voltage became small with and without heating, and the discharge voltage tended to be uniform at different flow rates and the same discharge current.When the discharge current was less than 1 A, as the discharge current decreased, the potential rose under the same flow rate; this is the result of plasma bombardment heating of the emitter[29].As the discharge current decreases,the cathode sheath voltage must increase in order to increase the velocity of the ion bombardment,thereby providing the emitter with the required ion flux [20].

With regard to the effects of heating on the cathode discharge, heating at a low supply flow rate (0.6-0.8 sccm)relative to unheated conditions causes different changes in the discharge voltage of the cathode at different discharge currents. These changes roughly occur below and above a discharge current of 1 A.At about less than 1 A,heating causes a significant decrease in the cathode discharge voltage, and as the heating current increases the reduction in the discharge voltage becomes more pronounced.This is due to the fact that the extra heat input at low operating conditions increases the thermal electron emission capability of the cathode, producing more plasma and thus reducing the discharge voltage.This result is similar to the phenomenon in the previous experiment where the spatial potential increased due to the decrease in heating power to ensure the emission current density [21]. When the discharge current is higher than 1 A,the discharge voltage increases rapidly as the discharge current increases. Heating promotes a rapid increase in the discharge voltage,and may even exceed the value for the case of no heating. Overheating (7 A heating) causes a certain reduction in the discharge voltage. The occurrence of this phenomenon is closely related to the change in the cathode discharge position when the discharge is greater than 1 A. A detailed discussed is given in section 3.4.

It can be seen from figure 4 that in addition to the reduction in the discharge voltage at low current, the heating also significantly broadens the operating range of the cathode,extending it to low flow and low current. This contributes to ensuring stable operation of the cathode under low operating conditions. Taking 0.6 sccm as an example, the minimum discharge current of the cathode can only reach 0.6 A without heating. At this time, the lower limit of the self-sustained discharge (a state of continuous electron thermal emission relying only on the anode voltage without external heating)has been reached. Continued reduction of the discharge current will cause the cathode to be extinguished immediately or after a short period of operation. Turning on the heater can effectively suppress the occurrence of cathode extinction and can continue to maintain extended operation of the cathode at a lower discharge current.This is mainly due to the additional heat input after the heater is turned on. This part of the heat can be used to heat the emitter to achieve an electron emission temperature under low operating conditions,and results in an electron emission capability that is sufficient to maintain plasma discharge.

3.2. Influence of heating on discharge oscillations of the cathode

Discharge oscillation is an important phenomenon that cannot be ignored in an electric propulsion plasma. When studying the influence of heating on cathode discharge characteristics it is necessary to consider changes in the discharge oscillation.The oscillations of the cathode discharge circuit were collected by a two-channel oscilloscope,and the obtained results for oscillation of the voltage under different conditions are shown in figure 5. At 0.8 sccm with a 0.5 A discharge,heating resulted in a significant decrease in the discharge voltage. However, the oscillation was obviously small at this time and the discharge was relatively stable, indicating that heating under this condition can significantly reduce the discharge voltage while maintaining low oscillations. This is advantageous for improving the life of the cathode. At 0.8 sccm with a 2 A discharge,even in the case of no heating,the discharge oscillation was still large.After 6 A heating,the discharge oscillation significantly increased, while the magnitude of the discharge voltage also significantly increased.

A frequency spectrum analysis was performed on the discharge oscillation waveform at a discharge current of 2 A,and the results are shown in figure 6. It can be seen that the maximum amplitude at a large discharge current (＞1 A) is concentrated around 30 kHz,and heating causes a significant increase in the amplitude here,i.e.further enhancement of the cathode oscillation.However,as the heating current increases from 5 to 7 A it can be seen that the amplitude has a certain degree of decline with high oscillations.

The oscillation data measured by the oscilloscope under different operating conditions were processed,and the effects of heating on the magnitude of oscillation of the cathode discharge voltage and current are shown in figure 7.It can be seen that under a low discharge current the influence of heating on the oscillation is relatively small, and there is almost no influence on the voltage oscillation. The heating causes a slight increase in the current oscillation. At a higher discharge current, heating causes a significant increase in discharge oscillations. From the small oscillation of the low current to the large oscillation of the high current there is a transition in the discharge oscillation. The inflection point of this transition is approximately the 1 A discharge current,and cathode oscillation significantly increases after the inflection point. At low flow rates (＜1 sccm) and higher discharge currents(＞1 A),heating causes a sharp increase in discharge oscillations and discharge voltage. This is clearly disadvantageous for cathode operation. Therefore, heating is only suitable for operation under low current, and it is not advisable to turn the heater on when operating at high current.

3.3. Influence of heating on discharge in a plume

A camera was used to take pictures of the plume area.With the 1 A discharge as the inflection point, plumes with heating and no heating at the two discharge currents are shown in figures 8 and 9, respectively. It can be seen that the area of the keeper plate becomes obviously bright owing to the significantly increase in local temperature after heating. When discharged below 1 A, heating causes a slight contraction of the plume divergence angle, showing a better spot mode of operation.Above a 1 A discharge, heating causes a significant change in the plume color and the entire plume is more divergent, at which point the cathode oscillation is further increased.

To further illustrate the different discharge oscillation modes in the plume region, the plume region was photographed at two different discharge currents using a high-speed camera.The results are shown in figures 10 and 11.It can be seen that at 0.8 A, the overall plume region has a weak luminous intensity and a small area, and the oscillation is weak. Thus, it is difficult to see the obvious change in the plume. At a 2 A discharge, the overall plume region has a strong luminous intensity, and the area increases. The oscillation is enhanced, and a periodic change in the obvious oscillation can be observed. In addition, the plume region changes to a more diffuse plume operation mode.

To further analyze the plasma energy distribution in the plume, the ion energy distribution of the cathode plume is analyzed by RPA,and the results are shown in figure 12.At a low discharge current, heating causes the ion energy distribution in the plume to concentrate toward low ion energy.The higher the heating current, the more obvious this phenomenon of low energy concentration. At a higher discharge current (1.8 A), the ion energy in the plume region is always high.This is because,with heating,the additional heat input causes a decrease in the discharge voltage under lowcurrent conditions, resulting in a relative decrease in the ion energy.It is beneficial to reduce the plume ion energy,as this reduces the sputter corrosion effect of the plume on the thrust system and helps to improve the lifetime reliability of the cathode and even the entire thrust system.

3.4. Influence of heating on the temperature and pressure of the cathode

Research on existing cathode thermal characteristics has shown that the high-temperature region of the cathode is mainly concentrated in the emitter region. The downstream outlet region has the highest temperature, which can be approximately characterized by the highest tungsten temperature [26, 30]. In this paper, the temperature of the cathode orifice plate is monitored by a perforated anode plate using a pyrometer.The cathode temperature under different operating conditions is shown in figure 13.This temperature monitoring device passes through the plasma in the plume region when measuring the temperature of the orifice plate. Therefore, the temperature measurement result cannot completely characterize the actual temperature of the orifice plate,and there is interference with the plasma. The temperature is measured when the cathode reaches the emission state and no plasma is generated. The temperature difference is approximately 20 °C-30 °C compared with the temperature when the cathode is successfully ignited. In this paper, the temperature of the pyrometer is used directly to characterize the operating temperature of the cathode.

The temperature of the cathode after heating is about 200 °C higher than that without heating, and the temperature shows an upward trend with an increase in the discharge current and heating current. Turning on the heater during cathode operation provides more heat to the cathode, which can be characterized by changes in the temperature of the cathode. Studies have shown that as the heating power decreases, it causes an increase in the ignition voltage due to the low electron emission caused by the low emitter surface temperature [19]. As shown in figure 2, during the cathode ignition process the cathode temperature gradually increases with an increase in the heating current,and finally reaches the thermal emission temperature of the cathode (approximately 1300 °C) when it is heated at 7 A. At this moment, after the cathode is successfully operated, cathode operation can be self-sustained without the heater.However,the cathode is not usually self-sustained under low operating conditions, so the cathode is continuously heated to ensure that the emitter maintains a sufficient electron emission capability.

Heating can increase the temperature of the entire emitter, thus ensuring a self-sustaining cathode discharge under low-flow and low-current conditions. As the heating current increases, the voltage and ion energy of the plume region decrease, and the discharge oscillation remains unchanged.This cathode can be used as a low-current source of electrons for electric propulsion and a neutralizer. However, at a low flow rate and high current (＞1 A), the cathode voltage will increase with an increase in the discharge current, and the amplitude of the discharge oscillation will also increase. The discharge mode tends to occur in the plume mode. At this time, heating the emitter with the heater causes a rapid increase in the discharge voltage and oscillation, and the ion energy in the plume region is always high. This is consistent with the sharp increase in the low-frequency oscillation of the Hall thruster owing to the high cathode heating power obtained in previous studies [22]. During the oscillation,the plasma luminescence fluctuates simultaneously in the entire plume region (figure 11). The oscillation frequency is∼30 kHz, which should be related to the amount of excited neutral gas during electron conduction to the anode.

The internal pressure of the cathode was also monitored while noting the changes in the cathode temperature, and the results are shown in figure 14. It can be seen that in most cases the heating causes an increase in the internal pressure of the cathode. It is worth noting that in the case of a large discharge current, the internal pressure of the cathode drops and heating causes the drop in the pressure to be more pronounced. This might be due to a change in the discharge position at this time. The discharge voltage and oscillation at a high heating power increase with an increase in the cathode current, which may also be related to the transfer of the position of cathode electron emission from the emitter region to the orifice region. Figures 13 and 14 show the changes in the surface temperature of the cathode orifice plate and the internal pressure of the cathode with discharge current under different heating currents. The temperature of the cathode orifice plate increases linearly with an increase in the discharge current, while the internal pressure of the cathode suddenly drops after 1.5 A. When the cathode structure is constant and the temperature rises, the drop in internal gas pressure is mainly caused by the rapid decrease of the plasma throttling effect. When the position of electron emission gradually transfers from the emitter to the throttling orifice region or the outer surface of the cathode orifice plate,the flux of electrons through the orifice decreases. At this time, the plasma density generated in the orifice declines drastically,causing a rapid decrease in the internal pressure of the cathode. This conversion is not stable because the thermal electron emission capability of tungsten at the same temperature is much smaller than that of the LaB6emitter, resulting in an increase in oscillation.

4. Conclusion

In view of the requirements for a the wide range of operation of the HEMP, based on the existing cathode, research on widening the operating ability of the cathode under low operation conditions without changing the cathode structure was carried out. It was proposed that the heater could be turned on when the cathode is not in self-sustained operation under low operation conditions (low flow rate and low discharge current). The results showed that the self-sustaining current can be extended from 0.6 to 0.1 A when the flow rate is constant.Heating can significantly reduce the discharge voltage under low-current conditions with low discharge oscillations, which reduces the energy of the plasma in the cathode plume and reduces the corrosion of the cathode structure and the entire thrust system. The feasibility of applying the cathode to the expansion of low operation conditions was proved. The main reason why a low-current cathode cannot be self-sustained is that the temperature of the emitter is lowered, resulting in insufficient capability for thermal electron emission. Therefore, by optimizing the structure size, the self-sustaining cathode current can also be reduced, and increasing the heating efficiency of the emitter or reducing heat dissipation might be considered.

It is worth noting that heating at low flow rates(＜1 sccm)and higher currents(＞1 A)causes a synchronous increase in the discharge voltage and oscillation, and a high ion energy is always maintained in the plume.From low to high current there is a transition process of discharge oscillation. Combined with the temperature of the cathode orifice plate and the internal pressure of the cathode with the change in current,it is initially proposed that the change in the discharge position is the cause of the high voltage and high oscillation.Therefore,the proposed method of turning on the heater during cathode operation is only suitable for the extension of the cathode to a lowdischarge-current condition, and is not suitable for operating conditions with a higher discharge current.This provides more possibilities for the further extension of existing cathodes to low operation conditions.

Acknowledgments

This study was supported by National Natural Science Foundation of China (Nos. 61571166 and 11775063), the Key Fund under grant No. 51736003, and a laboratory fund under grant No. LabASP-2017-05.