Effect of Working Current on C12A7 Hollow Cathode
Round 1
Reviewer 1 Report
This is a rather well written paper. I have only a few minor comments:
1) In some places, you call the work function "escape work". This is not a standard term. Please change these to "work function" (or, if you wish, "escape work function"). Or, if you wish to retain the term, please define it before first usage.
2) What is "Bide length"? Perhaps you mean Debye length?
3) "Since the emitter of C12A7 electron compound is a smooth, pure black object, its emissivity is roughly estimated to be about 0.9." Do you mean infrared emissivity or electron emissivity or something else? Please qualify.
4) "The short working life under high current conditions means that C12A7 hollow cathode is difficult to have practical applications under high current conditions." At the end, can you please quantify what "high current conditions" means (e.g., if it means 'larger than 4 A') and add it to the sentence, for example in parentheses at the end of the sentence.
Author Response
Responses to the questions raised by reviewer 1
Reviewer #1: This is a rather well written paper. I have only a few minor comments:
- In some places, you call the work function "escape work". This is not a standard term. Please change these to "work function" (or, if you wish, "escape work function"). Or, if you wish to retain the term, please define it before first usage.
Answer 1
We appreciate you pointing out our mistakes.
We have replaced "escape work" with "work function".
- What is "Bide length"? Perhaps you mean Debye length?
Answer 2
We appreciate your suggestion.
We have replaced " Bide length " with " Debye length ".
- "Since the emitter of C12A7 electron compound is a smooth, pure black object, its emissivity is roughly estimated to be about 0.9." Do you mean infrared emissivity or electron emissivity or something else? Please qualify.
Answer 3
Thank you for your suggestion.
In our case，0.9 is the thermal radiation emissivity coefficient.It affects the heating power consumption and infrared radiation released to other elements of the cathode. We have changed it in the article to “its thermal radiation emissivity coefficient was roughly estimated to be about 0.9.”
- "The short working life under high current conditions means that C12A7 hollow cathode is difficult to have practical applications under high current conditions." At the end, can you please quantify what "high current conditions" means (e.g., if it means 'larger than 4 A') and add it to the sentence, for example in parentheses at the end of the sentence.
Answer 4
We appreciate your suggestion.
The performance of hollow cathode is affected by many factors such as structure and material. Under the current structure, the safe working current of C12A7 hollow cathode is 4A. If the safe current needs to be improved, copper material can be considered. We consider that above 10A belongs to large current and add a comment at the end.
"The voltage transition phenomenon of C12A7 hollow cathode under high current conditions (More than 10A) has changed some key structures of C12A7 hollow cathode"
Author Response File: 
Author Response.doc
Reviewer 2 Report
The paper “Effect of working current on C12A7 hollow cathode” by Huang et al provides a set of measurements with varying currents to identify a good working regime for C12A7 hollow cathodes. Although the work is interesting, the quality of the paper is poor and the conclusions derived are not generally valid. The critic is summarized as follows:
The English grammar must be significantly improved, it is partly un-readable, full of too short sentences and missing words. Use a translation office or tools like Grammarly. Please replace “escape work” with “work function”.
Introduction: A number of references are outdated, please use up-to-date work.
Reference [10] is wrong – it’s an overview paper from Bock et al from 2016 and not from Drobny et al. The latest papers are e.g. (including a 1000 h endurance test without insert degradation, also copper emitter holders and much improved thermal conductivity by using a C12A7 compound)
Wätzig, K., Drobny, C. and Tajmar, M., "Improved Thermal and Mechanical Properties of [Ca24Al28O64]4+(4e-) Electride Ceramic by Adding Mo Metal", Advanced Engineering Materials, 2201286 (2022)
Reference [10] is from McDonald and Caruso and not from McDonald and Natalie
Many other working groups are missing e.g. Airbus Friedrichshafen, University of Giessen, etc., please provide a much more up-to-date review.
Testing Facility: Vacuum Chamber – wrong symbol before 400 mm. Also it is unclear what e.g. 0.05%/0.2% means. Is this a range of uncertainties?
The exact dimensions of the C12A7 insert are missing. Provide a sketch.
Page 3: “The heater is turned on and the power is set at a low level.” What does this mean? How much power?
Page 3: “Carry out replacement gas for the gas pipeline …”. What does this mean?
Page 4: What is the Bide length?
Page 5: The abbreviation “Q_emitt” for the electron thermal emission does not make sense.
Page 6: Model verification with two-color thermometer. It is not clear how this works. Where is the thermometer located? Please provide a skwtch.
Figure 7,8,9: Graphs have a different style, should be consistent. From the text it is not clear where this data is coming from. Is this a model or a measurement? How is it measured?
Page 10: What is a “reaming type”?
General statement: The correct behaviour of a hollow cathode is influenced by many parameters like geometry, supporting materials, mass flow rate and emitter current. The conclusions are therefore too general – the results are only valid for the exact configuration/mass flow rate tested. This has to be made very clear.
Author Response
Responses to the questions raised by reviewer 2
Reviewer #2: The paper “Effect of working current on C12A7 hollow cathode” by Huang et al provides a set of measurements with varying currents to identify a good working regime for C12A7 hollow cathodes. Although the work is interesting, the quality of the paper is poor and the conclusions derived are not generally valid. The critic is summarized as follows:
- The English grammar must be significantly improved, it is partly un-readable, full of too short sentences and missing words. Use a translation office or tools like Grammarly. Please replace “escape work” with “work function”.
Answer:
We appreciate you pointing out our mistakes. We have improved the English grammar of this article and corrected the wrong phrases and words.We have changed “escape work” to “work function”.
- The Introduction: A number of references are outdated, please use up-to-date work.
Reference [10] is wrong – it’s an overview paper from Bock et al from 2016 and not from Drobny et al. The latest papers are e.g. (including a 1000 h endurance test without insert degradation, also copper emitter holders and much improved thermal conductivity by using a C12A7 compound)
Wätzig, K., Drobny, C. and Tajmar, M., "Improved Thermal and Mechanical Properties of [Ca24Al28O64]4+(4e-) Electride Ceramic by Adding Mo Metal", Advanced Engineering Materials, 2201286 (2022)
Reference [10] is from McDonald and Caruso and not from McDonald and Natalie
Many other working groups are missing e.g. Airbus Friedrichshafen, University of Giessen, etc., please provide a much more up-to-date review.
Answer:
We appreciate your suggestion.
We updated the citations and corrected some errors.
|
Before |
12CaO–7Al2O3 (C12A7) was first reported by S. Matsuishi in 2003 [7]. L.P. Rand et al. from Colorado State University took the lead in the test experiment of the electron compound hollow cathode in 2011 [8]. The work function of this electron compound can be as low as 0.6eV theoretically. The low working temperature and chemical stability make it possible for C12A7 to replace the existing BaO-W cathode and LaB6 cathode and become a new generation of hollow cathode for space. Although the actual work function of C12A7 emitter measured by subsequent experiments is higher than the theoretical value, its lower working temperature and stronger anti-poisoning characteristics are gradually verified by experiments. In 2013, Rand established a zero position model, which combined with the operating temperature measured by experiments, calculated the plasma properties of cathodes with three different pore sizes, and found that reducing the pore plate size could increase the internal cathode pressure and reduce the electron temperature, but the pore size had no significant effect on the potential[9]. In 2016, Drobny et al. from the University of Dresden in Germany successfully processed a hollow cylindrical C12A7 emitter and filled it in a stainless steel cathode. [10] Finally, the emitter was overheated and melted, resulting in the complete failure of the cathode. McDonald and Natalie et al. improved the cathode material in 2017 and used a large number of copper heat dissipation structures [11]. After the experiment, it was found that the emitter degraded to a certain extent after discharge, and the cylindrical emitter also decomposed and broken. In addition, the copper cathode top and wrapped copper foil also have obvious traces of melting, indicating that the actual working temperature of the hollow cathode of C12A7 may be higher than the melting point of copper material. In 2022, Hua Zhiwei conducted microscopic analysis on the surface of electronic compounds, and the results showed that overheating would melt and decompose the surface layer of C12A7 [12]. |
|
Revised |
L.P. Rand et al. first used C12A7 electride in a hollow cathode test in 2011. The experimental results were significant. Despite the irregular shape of the emitter, the cathode still discharged for several minutes, with the highest discharge current reaching 3.6A [5]. In 2016, Drobny et al. [6] successfully processed a hollow cylindrical C12A7 emitter and filled it in a stainless steel cathode. The discharge current ranged from 0.2A to 5A, and the voltage of the contact electrode varied greatly from 50V to 260V. McDonald and Caruso[7] used copper as a cathode tube in 2017. The cathode worked for several hours under low-current conditions ranging from 30 mA to 150 mA. After the experiment, it was found that the emitter degraded to a certain extent after discharge, and the cylindrical emitter was also decomposed and broken. In addition, the copper cathode top and wrapped copper foil also had obvious traces of melting, indicating that the actual working temperature of the hollow cathode of C12A7 may be higher than the melting point of copper material. In 2019, Drobny and Tajmar et al. [8] designed a copper saucer-type cathode that activates without heat. The shape of the emitter was a disc, which greatly reduces the difficulty of processing the emitter. The authors believed that the cylindrical emitter was conducive to reducing the temperature gradient of the emitter, and a large number of copper structures were used in the cathode body to further reduce the temperature of the emitter. The stability of the improved C12A7 hollow cathode was greatly improved; the maximum working time reached 350 hours, and the maximum current reached 5A. In 2021, Drobny et al. [9] carried out a long lifetime experiment on an improved disc-type C12A7 hollow cathode, which worked continuously for 950 hours under the 2A standard discharge condition, which is the longest working time of a C12A7 hollow cathode at present. In 2022, Watzig [10] improved the thermal and mechanical properties of the C12A7 electride by adding Mo metal, improving the performance of the material. Toledo [11]compared the cathode properties of C12A7: e- and Lab6 in different structures and operations. The results showed that C12A7: e- has a distinct advantage in Ar and Xe; above all, greater stability of the anode current was observed with lower mass flows and power cathode. Hua Zhiwei [12] conducted microscopic analysis on the surface of C12A7 electride, and the results showed that overheating melts and decomposes the surface layer of C12A7.
5.Rand L, Williams J, Hoyt R. Hollow Cathode with Electride Insert[C]. Aiaa/asme/sae/asee Joint Propulsion Conference & Exhibit, 2011. 6.C.Drobny and M.Tajmar. Development of a C12A7 Electride Hollow Cathode[C].52nd AIAA/SAE/ASEE Joint Propulsion Conference.2016:3124751. 7.Mcdonald M S, Caruso N R. Ignition and early operating characteristics of a low-current C12A7 hollow cathode[C]. 35th International Electric Propulsion Conference (IEPC), Atlanta, GA, USA, 2017: 8-12. 8.Drobny C, Wulfkühler J P, Tajmar M. Development of a C12A7 Electride Hollow Cathode and Joint Operation with a Plasma Thruster[C], 2019. 9.Drobny C, Wulfkühler J-P, Wätzig K, et al. Endurance Test of a Hollow Cathode using the Emitter Material C12A7 Electride[C]. Space Propulsion 2020+1, 2021. 10.Wätzig, K., Drobny, C. and Tajmar, M., "Improved Thermal and Mechanical Properties of [Ca24Al28O64]4+(4e-) Electride Ceramic by Adding Mo Metal", Advanced Engineering Materials, 2201286 (2022). 11.J.Toledo,J F Plaza, et al. Performance comparison of LaB6 and C12A7:e- emitters for space electric propulsion cathodes [J],Materials science and engineering, 1226 (2022)012093. 12.Zhiwei HUA,Pingyang Wang.An expermental study on the degradation of the C12A7 hollow cathode[C].Plasma Sci.Technol.24(2022)074010,2022.
|
- Testing Facility: Vacuum Chamber – wrong symbol before 400 mm. Also it is unclear what e.g. 0.05%/0.2% means. Is this a range of uncertainties?
Answer:
Thank you for your suggestion.
We have corrected the symbol, it is “ 400 mm × 1200 mm”.
The random uncertainty is taken as the standard deviation of the mean of the measured current/voltage values. The systematic uncertainty comes from DC power supplies, current/potential probes, and the oscilloscope.
- The exact dimensions of the C12A7 insert are missing. Provide a sketch.
Answer:
We appreciate your suggestion.
The size of the C12A7 insert is 2mm in diameter, 5mm in diameter and 10mm in length.Then loaded into a graphite barrel with a perforated graphite lid.We added relevant content in the article.
|
Revised |
In this paper, the C12A7 emitter was machined by special tools into a hollow cylinder with an inner diameter of 2mm, an outer diameter of 5mm, and a length of 10mm. The C12A7 hollow cathode in this paper improves on the hollow cathode which was successfully tested and ignited in Shanghai Jiao Tong University[13]. The old cathode working current was 1A-4A for a total of 10 hours, after which the emitter degraded. On this basis, the C12A7 hollow cathode was improved for high-current working conditions, and the cathode aperture was expanded from 1mm to 3mm.
Figure 1. C12A7 emitter and C12A7 hollow cathode. |
- Page 3: “The heater is turned on and the power is set at a low level.” What does this mean? How much power?
Page 3: “Carry out replacement gas for the gas pipeline …”. What does this mean?
Answer: Thank you for your suggestion.
In order to prevent the interference of unexpected factors, we heated the hollow cathode to remove impurities. Specific operation: The heater was activated at 1.5A and 2.4V for half an hour, 2.5A and 3.9V for half an hour, and 3.5A and 6.1V for half an hour to remove different impurities inside the cathode at different stages.
There may be residual gas in the gas line, and xenon gas may be passed through the line for a period of time to remove the impurity gas that may exist in the line.
|
Revised |
(1) The electride emitter was cleaned with ethanol, removing the grease and dust on the surface, and put into the hollow cathode. A stainless steel air pipe was installed, and the air pipe and bulkhead insulated with Teflon hose or porcelain seal alloy. In order to prevent the interference of unexpected factors, we heated the hollow cathode to remove impurities. After the background pressure of the vacuum chamber was reduced to 10-3Pa, The heater was activated at 1.5A and 2.4V for half an hour, 2.5A and 3.9V for half an hour, and 3.5A and 6.1V for half an hour to remove different impurities inside the cathode at different stages. (2) The C12A7 hollow cathode was injected with 2sccm Xe for 1min to exclude impurities that may exist in the pipeline. Then, the ignition parameters of the keeper were set to Vk=500 V, Ik=2 A, and the xenon flow rate to 18 sccm.
|
- What is the Bide length
Answer:
We appreciate your suggestion.
We have changed " Bide length " to " Debye length ".
- Page 5: The abbreviation “Q_emitt” for the electron thermal emission does not make sense.
Answer: Thanks for your kind suggestion. We have made correction of symbols. We have replaced " Q_emitt " with " ", is heat lost by electron thermal emission.
- 8. Page 6: Model verification with two-color thermometer. It is not clear how this works. Where is the thermometer located? Please provide a skwtch.
Answer: Thanks for your kind suggestion. The two-color thermometer collects the infrared radiation energy of two bands in the adjacent channel and calculates the ratio of them to determine the temperature of the measured object. To avoid misunderstanding, we changed “the two-color thermometer” to “the infrared double-wave temperature thermometer”.
Figure 1. Schematic of the temperature measurement of the electride insert.
- Figure 7,8,9: Graphs have a different style, should be consistent. From the text it is not clear where this data is coming from. Is this a model or a measurement? How is it measured?
Answer : Thank you for your suggestion. We have unified the styles of Figure 7, 8, 9 and added a grid to the graph.
Electron number density distribution, atomic number density distribution and emission temperature axial distribution are derived from COMSOL simulation results. We have added the explanation.
|
Revised |
The atomic number density reflects the amount of gas in the cathode and affects the frequency of ionization collision. Fig. 9 shows the simulation distribution results of the atom number density of the small-orifice cathode (1mm) and the open-orifice hollow cathode (3mm cathode) under the state of steady discharge, where the anode current is 2A and the xenon flow rate is 10sccm. As can be seen from the electron number density simulation distribution cloud Fig.10. Fig. 11 shows the temperature simulation distribution of the C12A7 electride along the axial direction. |
10.Page 10: What is a “reaming type”?
Answer : Thank you for your suggestion. It should be “the open-orifice hollow cathode (3mm cathode)”
|
Revised |
The open-orifice hollow cathode (3mm) was 50k lower than the small-orifice cathode (1mm). |
11.General statement: The correct behaviour of a hollow cathode is influenced by many parameters like geometry, supporting materials, mass flow rate and emitter current. The conclusions are therefore too general – the results are only valid for the exact configuration/mass flow rate tested. This has to be made very clear.
Answer : Thank you for your suggestion. We totally agree with your point of view.
The performance of C12A7 hollow cathode is affected by many factors. In our current configuration, the safe operating current of the hollow cathode is 4A. Changing the cathode structure and using better materials can improve its safe working limit to some extent. We have added this to the conclusion.For the C12A7 hollow cathode working state in high current conditions, this paper provides some interesting phenomena, C12A7 hollow is not suitable for high current conditions.
|
Revised |
1. The C12A7 hollow cathode works well under low-current conditions. The C12A7 hollow cathode of the current configuration has a maximum safe current of 4A. It should be pointed out that the working performance of the hollow cathode is affected by many factors, such as cathode structure, orifice size, working current, and cathode material. If it is necessary to improve the safe working current, it is necessary to consider optimizing the heat dissipation structure and materials. Compared with the traditional Lab6 and Bao-W cathode, the expansion of the cathode orifice can reduce the risk of overheating to a certain extent, and may be more suitable for the open-orifice hollow cathode configuration. |
Author Response File: Author Response.doc
Reviewer 3 Report
The paper presents a computational study on the effect design parameters and an experimental study on high and low current operating conditions of a C12A7 hollow cathode which may be an essential component in future ion thrusters. The paper is well-written, clear, and contains valuable experimental results.
I have only a few minor comments:
-The first sentence of the introduction should be revised.
-In the first sentence of section 2.1 it is not clear what the f in "with a size of f400mm x 1200mm" stands for.
-The title of section 2.2 should be on the same page as the rest of the section.
-The parenthesis in equation 6 are not well formatted.
-There are two sections 2.3.
-It is not clear how the values in Table 1 are obtained.
-The plots should have grids to facilitate the interpretation.
Author Response
Responses to the questions raised by reviewer 3
Reviewer #3: The paper presents a computational study on the effect design parameters and an experimental study on high and low current operating conditions of a C12A7 hollow cathode which may be an essential component in future ion thrusters. The paper is well-written, clear, and contains valuable experimental results.
I have only a few minor comments:
- The first sentence of the introduction should be revised.
(Answer 1)
We appreciate your suggestion. We have modified the introduction.
Introduction
|
Before |
With the micro satellite in communication, remote sensing, electronics, reconnaissance and other civilian and military fields play an increasingly important role. With its advantages of high specific impulse, long life and adjustable thrust, electric propulsion system has been increasingly widely applied and mature in the field of aerospace propulsion [1][2]. The Hollow cathode is an indispensable component of the main thruster. It not only provides the initial electrons of the ionization medium for the thruster during startup, but also continuously provides electrons to neutralize the ion plume during operation so as to maintain the electric neutrality of the entire spacecraft surface [3] [4]. Therefore, the performance and life of hollow cathode are closely related to the whole electric propulsion system, and the improvement of hollow cathode technology is crucial to the development and guarantee of electric propulsion technology [5] [6].
|
|
Revised |
The development of commercial spaceflight in the last decade has ignited great interest for space exploration. The electric propulsion system, with its advantages of high specific impulse, long life and adjustable thrust, has been applied increasingly widely in the field of aerospace propulsion [1][2]. Hollow cathodes, as an indispensable key component in mainstream thrusters [3], are constantly developing with electric propulsion technology and micro-satellites. Currently, hollow cathodes must provide a wide range of discharge power in a variety of conditions, from low-power Hall thrusters with sub-kilowatt range to large EP applications with over 100 kilowatts [4]. Therefore, it is vital to develop a new type of hollow cathode technology to make the cathode structure more compact, propellant more efficient and power consumption lower, in order to make the C12A7 cathode more widely recognized.
|
- In the first sentence of section 2.1 it is not clear what the f in "with a size of f400mm x 1200mm" stands for.
-The title of section 2.2 should be on the same page as the rest of the section.
-The parenthesis in equation 6 are not well formatted.
-There are two sections 2.3.
Answer 2
We appreciate your suggestion. We have corrected the symbol, it is " 400 mm × 1200 mm".
We have corrected the format of the article and other errors.
- It is not clear how the values in Table 1 are obtained.
Answer 3
Thank you for your suggestion. The data in Table 1 are measured experimentally.
We have added a note. "The measurements of the specific heat capacity and thermal diffusivity of the C12A7 electride at different temperatures are shown in Table 1. "
- The plots should have grids to facilitate the interpretation.
Answer 4
We appreciate your suggestion. We have added a grid to the line graph. For cloud images adding grid will affect viewing, we did not change.
Figure 9. Contour of atom number density distribution and atom density variation of the symmetry axis along Z-direction.
Figure 10. Contour of electron number density distribution and electron number density variation of the symmetry axis along Z-direction.
Figure 11. Axial temperature of the emitter along the axis of symmetry in the Z-direction.
Author Response File: Author Response.doc
Reviewer 4 Report
Good work! Please proceed your work in the direction of cathode life time providing. Target value is 10000 hours.
Author Response
Thank you for your appreciation of our work.
The performance of C12A7 hollow cathode is affected by many factors. In our current configuration, the safe operating current of the hollow cathode is 4A. Changing the cathode structure and using better materials can improve its safe working limit to some extent. For the C12A7 hollow cathode working state in high current conditions, this article provides some interesting phenomena, C12A7 hollow is not suitable for high current conditions.
Round 2
Reviewer 2 Report
The suggestions were appropriatly implemented, the paper is now OK.
