Life test research of a high specific impulse Hall thruster HEP-140MF

Wei MAO(毛威),Nan WU(吳楠),Yanlin HU(扈延林),Yan SHEN(沈巖),Zhaopu YAO (姚兆普),Xuhui LIU (劉旭輝) and Yanming WEI (魏延明)

1 Beijing Institute of Control Engineering,Beijing 100190,People’s Republic of China

2 Advanced Space Propulsion Laboratory of BICE,Beijing 100190,People’s Republic of China

3 Beijing Engineering Research Center of Efficient and Green Aerospace Propulsion Technology,Beijing 100190,People’s Republic of China

Abstract

Keywords:Hall thruster,high specific impulse,life test,accelerated test

1.Introduction

A high specific impulse Hall thruster HEP-140MF has been developed to satisfy the needs of all-electric propulsion satellite platforms [1].Currently,the flight prototypes SPT-100,PPS1350-G,BPT-4000,and SPT-140,employing the Hall thruster,have low discharge voltages ranging from 300 to 400 V.These thrusters have demonstrated degradation during life tests.The SPT-100 thruster has a discharge voltage of 300 V,a discharge current of 4.5 A,an anode flow rate of 5.3 mg s?1,and a specific impulse of 1600 s.During the life test of the thruster,the thrust and specific impulse initially decreased in a gradual manner until it reached its minimum value during the first 1000 h.After another 1000 h,the thrust and specific impulse recovered gradually.After 3000 h,the performance stabilized,and the thrust and specific impulse during the time period of 3000-7000 h remained unchanged[2-5].The PPS1350-G thruster has a discharge voltage of 350 V,a discharge current of 4.28 A,an anode flow rate of 5.3 mg s?1,and a specific impulse of 1700 s.During the life test of the thruster,there was a small increase in the amplitude of the discharge current oscillation over the first few hundred hours to approximately 1000 h;later,the amplitude decreased after approximately 2000 h,and stabilized at a low level after approximately 3000 h[6,7].The BPT-4000 thruster has four power/discharge voltage operating points:4.5 kW/300 V,4.5 kW/400 V,3 kW/300 V,and 3 kW/400 V.The thrust and specific impulse of the thruster initially decreased and then increased during the life test.Specifically,they reached the minima at approximately 1000 h,and recovered and stabilized after 2000 h [8,9].The SPT-140 thruster has a discharge voltage of 300 V and two operating powers of 4.5 and 3 kW.The thrust and specific impulse of the thruster demonstrated a downward trend within the first 2000 h of the life test.After 2000 h,the thrust and specific impulse stabilized [10].

For a high specific impulse Hall thruster,the discharge voltage and consequently the ion energy are high,and the ion sputtering on the wall of the discharge channel is severe.To quantitatively study the effect of a high specific impulse on the lifetime of Hall thrusters,we use the wall erosion rate formula of the discharge channel and the basic physical relationship of Hall thrusters [11]to obtain the following formulas (1)-(10):

where h,t,ji,w,andY(E,θ)denote the wall thickness of the discharge channel,time,ion current density toward the channel wall,and sputtering yield,respectively.

whereY(E) denotes the energy sputtering yield andVddenotes the discharge voltage.

wherePddenotes the discharge power.

wheretlifedenotes the accumulated operational time.

whereItotaldenotes the total impulse and F denotes the thrust.

whereηa,Ispa,andgdenote the anode efficiency,anode specific impulse,and gravitational acceleration,respectively.

Therefore,it can be inferred that for a certain thruster,the total impulse is inversely proportional to the specific impulse.For spacecraft tasks having a certain velocity increase,a thruster with a high specific impulse can reduce the propellant consumption and increase the payload rate;however,the high specific impulse introduces challenges to the lifetime and total impulse.Therefore,lifetime is the key to engineering applications of high specific impulse Hall thrusters;hence,new design technologies,which are different from traditional Hall thrusters,must be adopted.Moreover,life test data regarding Hall thrusters having a high specific impulse are still lacking.The long-term evolution of the wall profile of the discharge channel of a high specific impulse Hall thruster is unknown.A 10 000 h 1:1 full-life test is expected to incur 13 million RMB considering only the xenon consumption,power consumption,equipment maintenance,and labor costs.Hence,accelerated life tests of Hall thrusters are preferred to predict their lifetimes.The Moscow Aeronautical Institute of Russia proposed an accelerated test method to study the wall erosion of the discharge channel of a Hall thruster and performed validations using the SPT-100[12].The Russia Keldysh Research Center proposed a semi-empirical life prediction method and verified it on the 325 W KM-45 and 900 W EM-900 thrusters [13].Pagnon,from France,proposed a method for the life estimation of different wall chamber deposition rates combined with an optical emission spectrum[14].The Beijing Institute of Control Engineering (BICE) established a wall profile prediction method that was based on the measured wall profile data during a short-duration life test.This method was proven to be effective based on the published discharge channel wall profile data for a 200 W-10 kW power range,and hence,can be used to accelerate the life test.For the traditional Hall thruster,the accuracy of lifetime predictions can be as low as 5% [15].However,the above-mentioned lifetime prediction methods have not been experimentally verified for high specific impulse Hall thrusters,and their accuracies are still unclear.

In this study,to understand the evolution characteristics of the wall profile of the discharge channel and change the characteristics of thrust,specific impulse,and other performance indexes during the entire lifetime of the high specific impulse Hall thruster,a life test study was performed on a HEP-140MF thruster.The HEP-140MF is a Hall thruster specially designed for high specific impulse applications.In addition,the measured wall profiles were used to justify our lifetime prediction method in a high specific impulse situation.This is the first study to investigate the lifetime characteristics of a high specific impulse Hall thruster.

2.Experimental equipment and method

The nominal performance parameters of the HEP-140MF thruster are listed in table 1.It can operate in three modes:5,4,and 3 kW.The discharge voltages and anode flow rates corresponding to the three modes are 540,540,and 600 V,and 98,80,and 55 sccm,respectively.The HEP-140MF thruster can be used for orbit transfer,station keeping,and deorbit tasks of GEO satellites.

To reduce sputtering erosion in the discharge channel wall and to ensure a long lifetime of the HEP-140MF thruster,the maximum radial magnetic field strength Brmaxin the discharge channel centerline was shifted to the downstream of the channel exit by adjusting the relative positions and sizes of the magnetic screens and magnetic poles.The topography of the magnetic field in the HEP-140MF thruster is depicted in figure 1.

Two HEP-140MF thrusters were prepared for this experimental study.One HEP-140MF thruster (labeled #1)was subjected to a 1:1 720 h full life test,whereas a 420 h accelerated life test was conducted on the other thruster(labeled#2)according to the method proposed by BICE[15]for comparison.At the beginning of the life test,a 20 h fire test was conducted on both the HEP-140MF thrusters (#1 and #2) for full degassing.

Table 1.Nominal performance parameters of the HEP-140MF thruster.

Figure 1.Topography of the magnetic field in the HEP-140MF thruster.

To study the performance change rule of the HEP-140MF thruster and the change in the wall profile of the discharge channel during its lifetime,a hot fire test was performed on the two thrusters mentioned above.The thrust and wall profile of the discharge channel were measured several times during the test.

The hot fire test was performed in a ?3 m×6 m vacuum environment simulation device,which was equipped with three ?500 mm cryogenic pumps and six ?500 mm xenon pumps,as shown in figure 2.Under a xenon flow rate of 95 sccm,the pressure in the vacuum chamber could be maintained at ～5×10?3Pa (air).

Several commercial DC power supplies were used in the life test to power the thruster,namely,one anode power supply,two coil power supplies,one ignition power supply,and one heating power supply,as shown in figure 3.Two commercial flow controllers,in which the range of the anode flow controller was 150 sccm,were used;the range of the cathode flow controller was 10 sccm,and the flow control accuracy was 1%.

During the life test of the HEP-140MF thruster,the discharge parameters,including the anode current,anode voltage,coil current,coil voltage,and flow rates,were measured online in real time;the measurement accuracies of the voltage and current were 0.5%.

Figure 2.Vacuum environment simulation device.

A three-wire twist pendulum thrust stand was used to measure the thrust.The measurement principle of this thrust stand is depicted in figure 4.Its basic principle is to convert the rotation angle of the platform caused by thrust into the linear displacement of the light spot on the scale;subsequently,a weight of known mass is used to calibrate the position of the light spot.Finally,the light spot position can be converted into thrust.The thrust calibration accuracy was within 2%.A thermal insulation pad was installed between the thruster and thrust stand,and a heat radiation screen was installed around the thruster to reduce heat radiation from the thruster.

The thruster efficiency is defined as follows:

whereIsp,c,η,andPcoildenote the thruster specific impulse,cathode mass flow rate,thruster efficiency,and coil power,respectively.

Figure 3.Connection between the thruster and power supplies.

Figure 4.Principle of thrust measurement of a three-wire torsion pendulum.

The main reason for the lifetime failure of the Hall thrusters is that the discharge channel wall becomes increasingly thin owing to high-energy ion sputtering [2].Eventually,the magnetic pole loses the protection of the discharge channel wall and is directly exposed to ion bombardment.Thus,the magnetic field of the thruster is damaged and the thruster cannot function normally.During the life test,the change in the wall profile of the discharge channel must be monitored.It was measured using a profile meter every 100 h,as shown in figure 5,and the accuracy of the profile measurement was 0.1 mm.

Owing to the similar anode voltages in the 5,4,and 3 kW modes,the erosion corresponding to the 5 kW mode was the worst and most representative.To reduce the test cost and time,only the 5 kW mode was selected for the life test.The life test of thruster#1 was conducted in the 5 kW mode,switched to the 4 and 3 kW modes for the performance test every 100 h,and returned to the 5 kW mode after the performance test.Most of the time,the thruster was shut down after 12 h of operation during the life test;however,it was shut down after 1 h of operation on rare occasions.The thruster cooled for at least half an hour before the next start.The thrust,specific impulse,and efficiency were measured during the life test at the three operating modes of 3,4,and 5 kW.Thruster#2 was tested only in 5 kW mode without switching to the 4 and 3 kW modes.

Figure 5.Wall profile measurement of HEP-140MF discharge channel.

The magnetic field of the thruster,optimized by experiments in the previous design stage,could adapt to the three different working modes.The coil current in the three modes was constant at 3.4 A,and the maximum radial magnetic field intensity of the discharge channel centerline was 200 G.

The wall profiles of the thruster discharge channel were measured at each of the four circumferential positions of the discharge channel to obtain the circumferential sputtering erosion differences,as shown in figure 6.The No.3 and 7 positions correspond to a 0° azimuth angle,and No.4 and 8 correspond to a 90° azimuth angle.The average of the measured erosion depths at the No.1,2,3,and 4 positions is taken as the erosion depth of the outer wall,and the average of the measured erosion depths at the No.5,6,7,and 8 positions is taken as the erosion depth of the inner wall.

3.Results and discussion

3.1.Performance change of thruster

Figure 6.Four circumferential positions for measuring the wall profiles (view from the exit of the discharge channel).

Figure 7.Thrust change trend of HEP-140MF (#1) during its life test.

The changes in the curves of the thrust,specific impulse,and efficiency during the life test of the HEP-140MF thruster are depicted in figures 7-9.A performance test was conducted on the HEP-140MF thruster(#1)for every 100 h of the life test;the seven data points on the performance curve corresponded exactly to the performances at 120,220,…,720 h.The HEP-140MF thruster (#1) underwent a 720 h life test and 120 starts.The thrust range in the 3 kW mode was 126-148 mN;that in the 4 kW mode was 184-207 mN;and that in the 5 kW mode was 227-253 mN.The specific impulse range in the 3 kW mode was 2286-2558 s;that in the 4 kW mode was 2310-2570 s;and that in the 5 kW mode was 2344-2630 s.The range of efficiency in the 3 kW mode was 0.47-0.62;that in the 4 kW mode was 0.53-0.65;and that in the 5 kW mode was 0.52-0.65.

Figure 8.Specific impulse change trend of HEP-140MF(#1)during its life test.

Figure 9.Efficiency change trend of HEP-140MF (#1) during its life test.

Figure 10.Photograph of the discharge channel of the HEP-140MF thruster (#1).

The thrust,specific impulse,and efficiency trends of the HEP-140MF thruster exhibited an initial decrease and subsequent increase.Before the life test,the color of the entire discharge channel was fresh white.During the test,the discharge channel turned black except for an ～4 mm long area near the exit.The black film of the discharge channel began to peel off during the later stage of the test.The appearance of the discharge channel during the test is illustrated in figure 10.The change in the deposited film in the channel was related to the performance of the thruster.The sediments in the channel were sampled and analyzed using a NanoLab 600i scanning electron microscope.The results of the energy spectrum analysis are presented in table 2.The film deposited on the wall of the discharge channel contained primarily B,C,N,O,and Si elements,which were from the deposition of the discharge channel ceramic material (BN-SiO2) and the deposition of anti-sputtering graphite targets in the vacuum chamber.

Figure 11.Wall profile measurement results obtained during the HEP-140MF thruster life test.(a)Inner wall surface,(b)outer wall surface.

Table 2.Energy spectrum analysis results.

The performance decline in the initial stage of the lifetime was due to the combined effect of two factors.The first factor is the change in the electronic wall conduction characteristics due to the deposition and contamination on the discharge channel wall;the second factor is the increase in the discharge channel width due to ion sputtering [16,17].The performance of the thruster recovered by the end of the life test as the discharge channel wall had adapted to the bombardment of ions,resulting in a weakened deposition on the channel wall surface.Additionally,the films deposited on the discharge channel wall peeled off gradually owing to the thermal shock effect[18];therefore,the electronic conduction characteristics of the wall surface of the discharge channel were partially restored.The performance of the HEP-140MF thruster began to recover in less than 720 h,in contrast to those of SPT-100 and BPT-4000.The reason for this phenomenon needs to be further studied;however,this phenomenon may occur owing to the difference in the composition of the channel wall material and the difference in adhesion between the film and the channel.

3.2.Wall profile variation of discharge channel

The variation in the inner and outer wall profiles of the discharge channel with life test time is depicted in figure 11.The radial ordinate is the normalized value of wall erosion depth and equates the ratio of the radial erosion depth to the initial wall thickness.The sputtering erosion zone of the discharge channel wall was mainly concentrated in the area within 4 mm upstream of the channel exit.The radial erosion rates on the inner and outer wall outlet end faces are shown in figure 12.

The volume erosion rate of the discharge channel of the HEP-140MF thruster in the 5 kW mode was compared with that of the SPT-100 thruster,as shown in figure 13.Volume erosion rate is the integration of the measured erosion depth over the channel length and circumference.The discharge channels of the two thrusters exhibited similar deceleration erosion laws [19].The volume erosion rate decayed exponentially with the operating time of the thruster.The volume erosion rate of the HEP-140MF thruster was smaller than that of the SPT-100 calculated from the data published in reference[20].The main reason is that the acceleration zone of the HEP-140MF shifted toward the downstream of the discharge channel exit by pushing the maximum radial magnetic field intensity to 6 mm downstream of the channel exit,as shown in figure 1.Therefore,the erosion zone length of the discharge channel was approximately 4 mm,which was considerably shorter than that (～8 mm) of the SPT-100 [20].

It was observed that the wall erosion rate of the HEP-140MF thruster decelerated.Although the anode voltage was approximately twice as high as that of the SPT-100,the volume erosion rate was approximately half of that of the SPT-100.The erosion depth of the HEP-140MF was～4.2 mm at 720 h in the 5 kW mode,and that of SPT-100 was ～4.4 mm at 800 h.The radial erosion rates of the HEP-140MF in the 5 kW mode and the SPT-100 were almost the same.There was no evident increase in the radial erosion rate caused by the high specific impulse.The lifetime design method adopted in the HEP-140MF thruster was effective in ensuring the long lifetime of the thruster.

Figure 12.Erosion rate of the wall outlet end face of the HEP-140MF thruster.(a) Inner wall surface,(b) outer wall surface.

Figure 13.Volume erosion rates of the HEP-140MF in the 5 kW mode and the SPT-100.

Figure 14 depicts the circumferential measurement results of the wall profile of thruster #1 at 720 h.The differences in erosion depth between the inner and outer walls of the thruster discharge channel at different circumferential positions were within±0.1 mm.The erosion of the discharge channel wall showed circumferential symmetry and uniformity.The cathode position had little effect on the circumferential erosion profile.

The wall profiles of the discharge channels during the life tests of the two HEP-140MF thrusters (#1 and #2) are depicted in figure 15.The wall profiles of the two thrusters were similar at 120 h and 220 h,indicating that the wall erosion characteristics of the discharge channels were similar,the thrusters were of good consistency,and the data of the single-thruster life tests were representative.

3.3.Validation of accelerated life test method

The wall erosion rate formula of the discharge channel is given in [12,13,15].The detailed derivation process can be seen in the references.The ion source in the channel of Hall thruster can be simplified as a point source in the channel shown in figure 16.Formula (13) is given below.The mathematical relationship between the ion source model,wall profile,and running time is established in the formula.

wherer(z,t),rs,zs,Y(γ),andF(α)denote the wall profile of the discharge channel at time t,radial coordinate of ion source position,axial coordinate of ion source position,angular sputtering yield,and sputtering intensity,respectively.

Based on formula (13),an accelerated life test method was proposed [15].The main process is outlined as follows:

1.The thruster runs for a period of timeΔt1under a hot fire test,and the discharge channel wall profiles are measured at three different times during the test.

2.According to formula (13),the ion source model (rs,zs,andF(α)) is obtained from the three wall profiles measured in step 1.

3.According to formula (13),the wall profile of the thruster after a long periodΔt2is predicted by the ion source model obtained in step 2.

4.The wall profile of the discharge channel is machined to the predicted one in step 3.

5.Steps 1-4 are repeated until the wall thickness of the discharge channel reaches zero.

To verify the adaptability of the accelerated life test method to the high specific impulse Hall thrusters,an accelerated life test study was conducted on the HEP-140MF thruster (#2).First,the HEP-140MF thruster (#2) was subjected to a 220 h hot fire test,and the wall profiles of the discharge channel were measured at three different times:20,120,and 220 h.The prediction method was used to extrapolate the wall profiles to that at 420 h,as shown in figure 17.The predicted wall profile at 420 h was consistent with the measured wall profile obtained during the 1:1 life test of the HEP-140MF thruster (#1).

The wall profile of the discharge channel of the HEP-140MF (#2) thruster was machined to the predicted one at 420 h,and the hot fire test was continued for 200 h up to the 620 h mark.The wall profile of the discharge channel was measured at three different times:420,520,and 620 h.The extrapolation by the wall profiles at these three times yielded the predicted wall profile at 720 h.It can be seen from figure 18 that it was also consistent with that measured during the 1:1 life test of the HEP-140MF thruster (#1).

It was observed that the discharge channel wall profile data obtained by the HEP-140MF thruster (#2) in the accelerated life test was consistent with that obtained by the HEP-140MF thruster(#1) in the 1:1 life test.The difference in the wall profiles at 720 h between thrusters #1 and #2 was less than 10%,which verified the fact that the acceleration life test method was effective for high specific impulse Hall thrusters.

Figure 14.Differences in wall erosion depth in the circumferential direction of the HEP-140MF thruster (#1) at 720 h.(a) Inner wall,(b) outer wall.

Figure 15.Inner wall profiles of HEP-140MF thrusters (solid line:#1,dotted line:#2).

Figure 16.Schematic of the ion source model in the discharge channel.

4.Conclusions

A high specific impulse Hall thruster HEP-140MF was subjected to a 720 h 1:1 life test verification.During the life test,the thrust,specific impulse,and efficiency of the thruster exhibited a slight downward trend but stabilized gradually during the later stage.In the early stage,the sputtering rate of ions on the discharge channel wall was high,and the deposition of sputtering products on the channel wall was also large.The performance degradation in the early stage is due to the joint action of sputtering and deposition.During the later stage,the wall of the discharge channel had adapted to the bombardment of ions;the wall surface had a low sputtering rate and wall deposition rate,and the characteristics of the channel wall surface were partially restored.Therefore,the performance of the thruster was gradually restored and then stabilized.

Although a high anode voltage was used in the HEP-140MF thruster,the wall erosion rate of the discharge channel demonstrated an exponentially decaying trend,indicating a decelerated erosion.Shifting the acceleration zone outside the discharge channel exit could shorten the erosion zone of the discharge channel and yield a low volume erosion rate,thereby effectively guaranteeing a long lifetime of the thruster.

Finally,the accelerated life test method was validated on the HEP-140MF thrusters.Thus,this method could effectively reduce the cost,number of cycles,and technical risk of the life tests of high specific impulse Hall thrusters and promote their flight application.

Figure 17.Predicted wall profile at 420 h by measured profiles at 20,120,and 220 h on the HEP-140MF thruster (#2).(a) Inner wall,(b) outer wall.

Figure 18.Predicted wall profile at 720 h by measured profiles at 420,520,and 620 h on the HEP-140MF thruster (#2).(a) Inner wall,(b) outer wall.

Acknowledgments

This work was supported by Space Advance Research program(No.D010509),National Natural Science Foundation of China (No.51806011) and National Defense Pre-Research Foundation of China (No.JSZL2016203C006).This manuscript is recommended by the 15th China Electric Propulsion Conference.