Examination of a 5 A-class cathode with a LaB6 flat disk emitter in the 2 A–20 A current range
Introduction
Within the electric space propulsion field, a cathode is a source of electrons. Along with the magnetizing coils, it is a critical component of a Hall Thruster (HT) assembly [1]. The cathode (the term “hollow cathode” is also used in the literature) has two roles during thruster operation. Firstly, it provides electrons towards the anode to counterbalance losses and maintain the plasma discharge. Secondly, it furnishes electrons for ion beam neutralization downstream the thruster exhaust. The electron stream in the plume typically amounts for 80% of the total cathode current [2].
Over the past decades, intensive R&D efforts have been undertaken to produce reliable, high-efficiency cathodes with a life span that is fully compatible with the HT lifetime. In particular, lanthanum hexaboride (LaB6) is considered as a reliable electron-emitting material for HT cathodes. More than 300 HTs (mostly Russian) have been operated, or are currently operated, in space with LaB6 cathodes over the last 45 years [3]. The first HT cathode with a LaB6 insert was proposed in 1963 in Russia [4] and was first operated in space [5] in 1972 coupled with a SPT-60, aboard of a Meteor-1 satellite. Lanthanum hexaboride is currently considered as an insert material due to its significantly higher poisoning resistance [6] than porous tungsten emitters impregnated with a barium calcium aluminate mixture (BaO-W). A LaB6 cathode therefore allows for simplified handling and startup procedures [7]. This material also shows longer simulated lifetimes than dispenser cathode emitters [7]. Cathodes using a LaB6 emitter are therefore perfectly suited for high power HTs. However, the LaB6 insert must be operated at higher temperatures than a BaO-W insert, requiring a high temperature heater which may lead to greater risk of failures [8]. This point is currently under consideration by several research teams [8], [9], [10] developing high power HTs.
Current trends regarding the Hall thruster technology indicate a clear interest for high-power electric propulsion devices able to operate at an input power level between 20 kW and 100 kW [11]. In addition to NSSK and orbit transfer maneuvers of geosynchronous communication satellites, Hall thrusters are foreseen as high potential and reliable candidates for missions such as celestial body exploration, trips to far-off planets as well as space cargo and space tug propulsion [2], [11]. In this context, the French space agency (CNES) presently supports a research program that aims at developing, building and investigating a high-power cathode able to deliver up to 100 A of electron current, exceeding by far what is available in France at the present time.
This contribution presents results obtained during the first phase of the high-power cathode development project. This phase consists in collecting and analyzing several physical quantities during operation of a 5 A-class cathode. An existing laboratory model of a heated cathode with a LaB6 emitter was studied in a diode configuration, i.e. without HT. The originality of this work is that the cathode design is based on a LaB6 flat disk emitter. During the last two decades, most of the published works studied HT cathodes using a cylindrical design for the emitting insert (the term “hollow cathode” is often used for such a design). Even though one of first cathode designs that was used with closed drift thrusters in the early 1970–1980s is based on a flat disk shape for the electron emitter [12], such a shape is rarely studied in the literature. The majority of published works with a flat disk shape for the electron emitter is dedicated to cathodes coupled with Hall thrusters [13], [14], [15], [16], [17]]. To the best of the authors' knowledge only the works published by the group of L.B. King (Michigan Technological University, U.S.A.) were dedicated to study a cathode design based on a flat disk emitter in a diode configuration [18], [19], [20]. These works of King's group were mainly focused on bismuth-fed cathode and a few experimental I-V characteristics were published, for a different cathode configuration and at different operating conditions to what is reported in the present study. Even though a flat disk emitter requires a continuous heating to maintain the proper temperature for the LaB6 insert to produce sufficient electrons [21], this shape ensures an homogeneous utilization of the emitter surface in spot mode [22], [23]. This is an advantageous in comparison with a cylindrical insert for which, in some cases, only a fraction of the overall axial length of the cylindrical insert is used to emit electrons [24]. In addition, the plasma density distribution in the vicinity of a flat disk emitter is expected to be rather uniform over a large part of the insert surface, inducing a uniform wear of the insert.
This work reports measurements of electrical parameters and temperatures of several cathode parts, including the LaB6 emitter, during heating of the cathode (i.e., the pre-ignition phase) and normal operation of the cathode (i.e., with the plasma discharge). Our purpose is to study experimentally this cathode design in a diode configuration in order: 1- to help address the lack of knowledge on a cathode design not often studied in the literature; 2- to check if this cathode design can be operated in spot mode at a discharge current up to 20 A; 3- to provide real test cases for comparison with the numerical simulations performed by our colleagues from LAPLACE in Toulouse (France) with their home-designed cathode code [25]. With the aim of testing HT of different power classes, it is important to investigate whether this 5 A-class cathode can be operated in spot mode up to a discharge current of 20 A. It is well known that HT cathodes run in diode configuration exhibit two main discharge modes [26], [27]: the spot mode and the plume mode, the latter being detrimental to the cathode lifetime [28] (wear of the cathode body).
Section snippets
The NExET test bench
Experiments were performed in the NExET (New Experiments on Electric Thrusters) vacuum vessel (Fig. 1). This test bench was commissioned at the ICARE laboratory in November 2008 and is used in conjunction with the PIVOINE-2g chamber [21] to carry out experiments in the field of electric propulsion. The NExET test-bench is a stainless steel chamber, 1.8 m in length and 0.8 m in diameter. It is equipped with a pump stack composed of a large dry pump, a 350 L.s−1 turbomolecular pump to evacuate
Pre-ignition measurements
In this part, the cathode is studied without the plasma discharge. The heating current is gradually increased from 2 A to 15 A with a 2 A-step, except for the last step (14 A–15 A). During each current step, the temperatures of the emitter , the external body of the cathode , the cathode base , the igniter , and the anode are monitored. The duration of each step is set to 600 s which is long enough to ensure a steady-state temperature of the LaB6 emitter at a given
Operating envelope
Once the heating phase of the LaB6 emitter has been achieved (i.e., when °C), the discharge is ignited by applying a positive potential to the igniter. The ignition voltage typically ranges between 200 V and 300 V. The mass flow rate is set to 0.2 mg.s−1 of xenon and the discharge current is limited to 2 A. As soon as the plasma is present downstream of the cathode orifice, the ignition potential is applied to the anode and the igniter is allowed to float. Because the power supply of
Temperatures
Fig. 13 shows the steady-state temperatures measured with the thermocouples. Data for each mass flow rate are plotted versus the discharge current. At each current increment, the temperatures were allowed to reach steady state. In order to easily compare the different cases (0.2 mg.s−1, 0.4 mg.s−1, and 0.6 mg.s−1), a temperature offset of a few °C (positive or negative) is applied to the cases with 0.2 mg.s−1 and 0.6 mg.s−1 of Xe, equalizing the temperatures at .
As expected, the higher the
Conclusion
A laboratory model of a 5 A-class cathode (nominal condition: 5 A at 0.4 mg.s−1 of Xe) has been experimentally studied in the diode configuration with a flat metal anode set instead of a HT. The cathode is used as electron source for gas ionization and plume neutralization in HTs. The cathode design studied here is based on the heating of a LaB6 flat disk to emit electrons. The data gathered during the test campaign were used to investigate whether this design allows the cathode to be operated
Acknowledgments
This work is supported by the CNES Direction des Lanceurs Research and Technology Program. R. Joussot benefits from a CNES post-doctoral fellowship and financial support from Safran. L. Grimaud benefits from a CNES-Région Centre Ph.D. Grant. The authors would furthermore like to acknowledge the constructive feedback from the reviewers.
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