Free Access
Issue
Eur. Phys. J. Appl. Phys.
Volume 91, Number 1, July 2020
Article Number 10901
Number of page(s) 6
Section Physics of Energy Transfer, Conversion and Storage
DOI https://doi.org/10.1051/epjap/2020190213
Published online 27 July 2020

© EDP Sciences, 2020

1 Introduction

Ion thrusters of various types, e.g. Hall effect thrusters (HET) or gridded ion thrusters (GIT), which are the prevailing devices in the commercial market [1,2], can be used for generating the thrust for a multitude of tasks such as north south station keeping, drag compensation or spiral-up maneuvers. All ion thrusters eject mainly positively charged ions to generate the required thrust. This leads to a charging of the spacecraft, unless one compensates the extracted ion beam current with an electron current of the same magnitude. The usual approach to overcome this challenge is to use hollow cathodes. The core element of this type of neutralizers is an insert with a low work function, such as lanthanum hexaboride LaB6 (2.7 eV) [3] or barium oxide BaO in a tungsten matrix (1.56 eV) [4]. However, to generate a sufficient electron emission, the inserts must be heated up to temperatures exceeding 1000 K. At such high temperatures, the inserts become prone to reactions with the constituents of the propellant gas used, e.g. oxygen impurities in case of inert propellants or in case of iodine as propellant the reactive halogen itself. Furthermore, such high temperatures cause a thermal strain and can lead to an embrittlement of the insert. Therefore, an alternative technology, which uses an insert-free RF-discharge, has been developed in the recent past to circumvent such issues.

RF-neutralizers operating with inert gases such as xenon or argon have been either or both tested and modeled by various groups in the recent past [58]. Scholze and coworkers [7] developed a plasma bridge neutralizer and showed a good agreement between experimental results and a simulation based on the so called Lieberman model introduced in reference [9] for the neutralizer operated with argon in a plasma bridged mode. Jahanbaksh and coworkers discussed the influence of the geometry of both the discharge vessel and the extraction orifice on neutralizer operation with argon. Furthermore, they showed the transition between different modes of operation (anode spot formation) [6]. The company Busek has also recognized the potential of an rf neutralizer operated with iodine [10] for space applications.

This paper is structured as follows: We first introduce the experimental setup in Section 2. We then give a brief description of the global model used in Section 3. The results of both, experiments and simulation, are shown and discussed in Section 4. Section 5 summarizes the paper and gives a brief outlook.

2 Experimental setup

For all tests with the rf neutralizer (RFN) a cylindrical shaped vacuum chamber was used. The chamber possesses a diameter of 1 m and a height of 0.68 m. The chamber is equipped with corrosion resistant pumps. The nominal pumping speed of the installed turbo pump is 1400 ls−1 for nitrogen. The typical base pressure is in the high 10−5 Pa regime, the maximum operation pressure is approximately 10−3 Pa. As shown in Figure 1, the rf neutralizer itself consists of a cylindrical discharge vessel made of quartz. It is 4 cm in diameter and 3.5 cm in length. A copper coil with seven windings is wrapped around the vessel and two ion collectors with an effective total surface of 23.4 cm2 are placed inside the vessel. Both collectors are biased by the same power supply. We used two collectors to increase the available surface for absorbing ions without placing an electric conductor close to the windings of the coil, since otherwise significant eddy current losses may occur. An anode (not shown in the figure) for mimicking the potential of the ion beam of an ion thruster is placed approximately 5 cm downstream of the exit of the cathode. The geometric shape, e.g. cylindrical or plain, of the anode and its distance to the neutralizer influences the distribution of the electric field between anode and the collectors and therefore may influence the coupling between anode and the collectors. A coupling is present when the ion current absorbed by the collectors depends on the voltage applied to the anode. However, in our experiments, we could not observe any influence of the voltage applied to the anode on the collector currents measured in a range from ground potential to 50 V, which is similar to the potential of an ion beam of a thruster. Rehn and Kaufman discussed the effect of the anode on hollow cathodes’ performance in comparison to a coupled test with a thruster [11]. Unfortunately, literature about the influence of the anode is generally rare and systematic investigations for RF-neutralizers have apparently not been published yet.

The required flows for driving the neutralizer are in the order of several μg s−1. Since commercial mass flow controllers are unable to deliver such low flows of iodine we decided to specially design and calibrate an customized flow regulation system. The iodine reservoir is connected to a heatable regulation valve. Downstream of the valve, an orifice welded onto one end of a stainless steel pipe is installed close to a capacitive pressure sensor. The orifice used is much smaller in diameter than the piping, at least by a factor of 40. This leads to the occurrence of a choked flow due to the strong drop of pressure at the orifice. In this situation, the flow is proportional to the stagnation pressure p in front of the orifice. The volumetric flow Q can be calculated using the following equation [12]: (1)

where A0 is the cross section of the orifice, the mean thermal velocity of the gas and Ψ the outlet-function. In case of a choked flow Ψ can be calculated as (2)

and solely depends on the heat capacity ratio κ. For diatomic gases, such as iodine, κ is 1.4. More general, the outlet function also depends on the pressures upstream and downstream of the orifice, if the latter is not significantly smaller than the former. One has to consider, that, at rather low inlet pressures, the choked flow assumption might no longer be valid, since the flow will undergo a transition to the molecular flow regime.

A sketch of our final setup for controlling the flows of iodine and rare gases is given in Figure 2. All elements of the iodine branch have to be heated to avoid desublimation. The temperature of the reservoir is typically about 350 K. All other components need to be at least 20 K hotter to avoid desublimation.

We have calibrated our flow control unit for iodine using nitrogen. For that purpose we used a commercial mass flow controller and measured the stagnation pressure in front of the orifice with a capacitive pressure sensor as a function of the nitrogen flow provided by the flow controller. The mass flow controller is a device from Bronkhorst with a maximum flow of 1 sccm xenon or about 0.7 sccm nitrogen, the pressure sensor is a MKS 631D with a full range of 4 kPa. Prior to its use we calibrated the mass flow controller from Bronkhorst with the weighing method using a precise scale from Mettler-Toledo (resolution of 1 mg). In short, the weighing method is performed by accumulating the gas-flow provided by the MFC into an previously evacuated gas plenum. The weight of the plenum is measured as function of the time and the massflow can directly be determined by the slope of the linearrising mass.

We calibrated the flow control for an orifice of 144 μm in diameter. Figure 3 shows the results for the orifice used, which show a very good linearity. The flow foriodine can be calculated by correcting the values obtained for nitrogen by a simple correction factor , which is determined by the relative thermal velocities of nitrogen and iodine. The temperature was assumed to be 373.15 K during operation with iodine and 300 K with nitrogen. Furthermore, the volumetric flows were converted to mass flows. In terms of application of the neutralizer in electric propulsion, mass flow is the commonly chosen unit for specifying the propellant consumption. Finally, one obtains the following linear equation for the determination of the iodine flow through the orifice: (3)

The offset of the linear fit in the pressure range from 130 to 260 Pa is caused by a transition from the choked flow regime towards first a laminar and then a molecular flow at pressures below 130 Pa. Therefore, the calibration fit cannot be extrapolated towards significantly smaller pressures than about 120 Pa. The RFG used is composed ofa half-bridge topology operating in the near-resonant mode and is controlled by a digital implemented phase-locked-loop/frequency-locked-loop (PLL/FLL) pattern. This approach facilitates zero-current switching of the transistors for high power transfer efficiencies towards the plasma (also known as impedance bridging) [13]. The frequency is essentially fixed by the inductance of the neutralizer and the capacitance of the capacitor-board installed in the generator. We used an rf frequency of about 2.4 MHz.

thumbnail Fig. 1

Schematic drawing of the rf neutralizer. For a better visualization, the components dimensions are not entirely to scale.

thumbnail Fig. 2

Sketch of the setup used for controlling the flow of iodine and noble gases. The gas-isolator is a ceramic tube filled with quartz wool to separate the grounded flow control unit from the floating plasma.

thumbnail Fig. 3

Flow of nitrogen as function of the stagnation pressure. The effective diameter of the orifice has been determined to be 144 μm.

3 Simulation using a global model

We have implemented the global model of Chabert and coworkers [14] for xenon. For krypton, the model was adapted by replacing the rate coefficients using the cross sections provided by the Plasma Data Exchange Project1. The models calculate four fundamental plasma parameters, the densities and temperatures of both neutral gas and electrons, respectively, by solving a set of four coupled differential equations. The global model of Grondein and coworkers was implemented for analyzing the iodine plasma [15]. The model yields the densities of molecular and atomic ions and neutrals and also considers negative charged atomic ions. Besides the difference, that more processes have to be taken into account for iodine, the models are very similar. While both global models are originally designated for simulations of gridded ion thrusters, they, in a first approach, may be adapted for investigations of neutralizers by setting the effective extraction area for ions to zero. Both global models require sets of input parameters. One important quantity is the total probability for neutrals exiting the discharge volume via the orifice in the ion collector. This probability PTotal is calculated as the product of the geometric transparency TCollector of the circular collector and the probability to transverse the 1 mm thick orifice of 6 mm diameter POrifice. TCollector is 0.0225, POrifice has been determined to be 0.858, assuming a molecular flow, using the empirical formula for pipes of arbitrary lengths given in the handbook edited by Jousten [12]. Therefore, the total probability for neutrals approaching the circular collector and leaving the neutralizer is less than two percent. All parameters that were used as input in all simulations are listed in Table 1. Simulating iodine, we assumed a mean excitation energy of the halogen atom of 5 eV. The value is arbitrary in principle, since no accurate literature is available, and is obtained by comparing the results of the experiments with those with the global models. The paper of Grondein and coworkers [15] gives rate coefficients instead of cross sections and gives no information about the mean excitation energy. Therefore, the uncertainty about the excitation energy is an possible source of error in our modeling of the iodine driven neutralizer. We considered, that iodine is injected at a temperature of about 400 K in the discharge vessel, whereas both noble gases are injected at about 300 K.

The collector current is calculated by multiplying the absorbing area of both collectors with the Bohm current density derived from electron temperature and ion density [16].

Table 1

Parameters used in all simulations.

4 Results

The performance of the neutralizer was analyzed for three different emission currents for each propellant. The voltage of both ion collectors was set to − 30 V. In analogy to an radio-frequency ion-thruster (RIT), we characterized the performance of each propellant by measuring the required DC-input-power of the RFG at a given mass flow to sustain a fixed collector current. The anode current is about 80% of the collectorcurrent, the remaining electron current is assumed to be absorbed by grounded surfaces in the chamber. In contrast to other rf-neutralizers known from the literature, for instance by Jianwu and coworkers [17], the collector current is independentof the voltage applied to the anode. The most likely reason is the comparatively large orifice diameter of 6 mm, while Jianwu and coworkers used orifices of up to 2.5 mm in diameter,which allows the electrons to leave the neutralizer without the need of an attractive anode potential. The performancecurves shown in Figure 4 exhibit for all propellants essentially the same behavior, a decrease of PRFG with increasing mass flow. However, foriodine, a rise of PRFG is observed at higher mass flows somewhat similar to the performance curves of RITs with iodine [18]. The performance with iodine is similar to that with xenon for low flows of up to ≈ 12 μgs−1. The apparently odd shape of the iodine performance curve is mainly caused by excitation of neutral atomic iodine. At electron temperatures lower than 6 eV the rate coefficient for atomic excitation exceeds those for ionization of both atom and molecule [15]. The lower the electron temperatureis, the larger is the ratio of those rate coefficients. Therefore, high flows respectively high pressures in the discharge have to be avoided using iodine.

The gas utilization factor ν, which is defined by the ratio of the ion flow reaching the collector and the flow of injected neutral atoms exceeds 30 for operation with xenon and even 40 using iodine at a collector current of 200 mA. This is possible by the fact that the propellant atoms are ionized and neutralized at the collectors surface repeatedly before exiting the RFN. The work of Hatakeyama and coworkers [8] gives an overview over the achievements of several groups. None of the listed neutralizers in that publication can provide such high values for ν. However, the demand on RFG-power to achieve those gas utilization factors is still quite high in case of our neutralizer, which leads to high extraction costs. The extraction cost C is determined by the sum of the RFG-input-power and collector-power divided by the current absorbed by the collector. Using iodine, at ν = 42 an extraction cost of 370 WA−1 is given. The relatively high extraction costs can partially be explained by the lack of a plasma bridge between the collectors and the anode. A plasma bridge mode will be achieved, if either a double layer sheath or electron sheath is present. Those sheaths can only be attained using either smaller orifices or collectors of larger area as described by Jianwu and coworkers [17]. If one uses much smaller orifices, the required flows, especially for operation with iodine, will have to be significantly smaller to avoid high power losses due to excitation. For example, using an orifice of 2 mm instead of 6 mm in diameter will require approximately ten times lower flows to keep the discharge pressure at the same level of about 0.1 Pa. Controlling such small flows is expected to be very challenging. Therefore, the use of larger collectors appears to be the more viable option, at least if one uses iodine as propellant. To further characterize the neutralizer, we measured the current absorbed by the collectors as a function of the input power of the RFG as shown in Figure 5 and performed simulations for comparison. In contrast to the experiments, in the simulations, the output power of the RFG is varied instead of the input power. For all threepropellants, the experimental determined collector current rises linear with the input power of the RFG. The simulated collector currents in Figure 5 show the same linear dependence, but the estimated currents are higher compared to the experiment. The higher currents in the simulation compared to the experimental results can be explained, at least partially, by neglecting loss channels in the simulation such as eddy current losses in the collectors, in the neutralizers housing and in the rf feeding line as well as the not accounting for the efficiency of the RFG, i.e. assuming 100% efficiency. Reeh and coworkers have recently shown for an RF-thruster of similar size, that the RF-power losses are dominated by peripheral losses, especially in the low flow regime [19].

In a first approach, we assumed a homogeneous distribution of the aforementioned plasma parameters. This may lead to a rather large possible error of up to a few 10%. One of the collectors is mounted near the exit of the neutralizer, in a axial distance of about 1 cm from the coil wrapped around the vessel. Therefore it appears plausible, that the local ion density in this region of the plasma is much less than its averaged value and, thus, the ion current absorbed by the collector is significantly smaller than expected. The linear behavior of the collector current as function of the RFG-input-power may be explained by a linear rise of the ion density as function of the input power of the RFG. The impact on the absorbed collector by such loss channels is exemplarily shown in Figure 6 for operating the neutralizer with iodine. For that purpose, we have rescaled the simulated ion currents assuming that the losses in both RFG and periphery are proportional to the RFG’s input power. Assuming an efficiency of 85% of the RFG and a loss of20% of its outputpower in peripheral components, the agreement between simulation and experiment is already significantly better. If one additionally considers a drop of the ion density at the collector mounted at the exit of the neutralizer, the simulation shows a satisfactory agreement with experiment. However, those estimations for the loss channels are only a crude approach, for instance theefficiency of the RFG may as well depend on the plasma resistance, which in turn will also depend on the RF power actually coupled into the plasma. Another reason for over-estimating the absorbed collector current is the assumption of a molecular flow in the orifice region. The simulated pressure is in the order of about 0.1 Pa and may be slightly too high to neglect the influence of a partially viscous flow [20].

Nevertheless, the global modeling of the inductively coupled plasma in the RFN allows us to correlate the observed dependence of collector current on output power of the RFG in Figure 5 to the microscopic plasma parameters. The ratio of the power coupled into the plasma is almost constant in the covered range of RFG output powers. For both noble gases the coupling efficiency is about 75%, for iodine about 80%. The simulation results in Figure 7 show the presumed dependence of the ion density for all propellants. The electron temperature is approx. 0.2 eV larger operating with iodine instead of xenon, while the krypton plasma possesses a 1 eV higher electron temperature compared to the halogen. The electrons heat up slightly with increasing rf power. The larger electron density leads to a heating of the neutral gas, which increases the conductance of the neutralizers orifice and, therefore, decreases the neutral density. The lower neutral density leads to a small increase of the electron temperature. However, the electron density exhibits a linear dependence on rf power and is the main reason for the almost linearly increasing collector current. The simulated plasma parameters are difficult to verify experimentally. For instance, the small dimensions of the discharge chamber do not allow an insertion of a Langmuir probe without perturbing the plasma significantly.

thumbnail Fig. 4

Performance of the neutralizer fed with krypton, iodine and xenon for collector currents of a) 100 mA b) 150 mA and c) 200 mA.

thumbnail Fig. 5

Simulated (lines) and experimental (symbols) measured collector currents as function of theRFG power for krypton, xenon and iodine. The mass flow was set to 10 μgs−1.

thumbnail Fig. 6

Influence of several possible loss channels on the absorbed collector-current.

thumbnail Fig. 7

Simulated plasma parameters of krypton, xenon and iodine as function of the RFG output power, a) ion density and b) electron temperature.

5 Summary and outlook

The performance of a rf neutralizer was investigated using three different propellants. A stable extraction over several tens of hours was achieved for all propellants. No issues concerning the reactivity of iodine have occurred. The performance of xenon and iodine is comparable at low flows. As expected, krypton shows a significantly worse performance due to its larger ionization threshold and smaller ionization cross section. The dependence of the collector current on the RFG power was investigated and compared with simulations using global models. Simulation and experiment show a good qualitative agreement and an already satisfying quantitative agreement, if one considers the neglect of several loss channels in the global modeling. Thus, the global models described in references [14] and [15] can be used with only minor modifications for optimizing rf neutralizers.

Future activities will focus on achieving an operation in the plasma bridge mode, which is strongly preferable in coupled tests with an ion thruster and is expected to lead to an even better performance. For yielding an better agreement between experiment and modeling in the future, one also has to model the important peripheral components and account for the spatial distribution of the plasma parameters such as ion density and electron temperature.

Author contribution statement

The manuscript was prepared by P. Dietz. The experimental setup was built by F. Becker and P. Dietz, who also performed the experiments. K. Keil implemented the global models for both noble gases and the halogen iodine. K. Holste and P.J. Klar contributed to the design of the experimental setup and supervised the work. All authors were involved in discussion and data interpretation.

Acknowledgements

This work has been supported by the Federal Ministry for Economic Affairs and Energy under contract 50RS1709.

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Cite this article as: Patrick Dietz, Felix Becker, Konstantin Keil, Kristof Holste, Peter J. Klar, Performance of a rf neutralizer operating with noble gases and iodine, Eur. Phys. J. Appl. Phys. 91, 10901 (2020)

All Tables

Table 1

Parameters used in all simulations.

All Figures

thumbnail Fig. 1

Schematic drawing of the rf neutralizer. For a better visualization, the components dimensions are not entirely to scale.

In the text
thumbnail Fig. 2

Sketch of the setup used for controlling the flow of iodine and noble gases. The gas-isolator is a ceramic tube filled with quartz wool to separate the grounded flow control unit from the floating plasma.

In the text
thumbnail Fig. 3

Flow of nitrogen as function of the stagnation pressure. The effective diameter of the orifice has been determined to be 144 μm.

In the text
thumbnail Fig. 4

Performance of the neutralizer fed with krypton, iodine and xenon for collector currents of a) 100 mA b) 150 mA and c) 200 mA.

In the text
thumbnail Fig. 5

Simulated (lines) and experimental (symbols) measured collector currents as function of theRFG power for krypton, xenon and iodine. The mass flow was set to 10 μgs−1.

In the text
thumbnail Fig. 6

Influence of several possible loss channels on the absorbed collector-current.

In the text
thumbnail Fig. 7

Simulated plasma parameters of krypton, xenon and iodine as function of the RFG output power, a) ion density and b) electron temperature.

In the text

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