High Current Hollow Cathode Research Articles

Observation of rotating magnetohydrodynamic modes in the plume of a high-current hollow cathode

High-current hollow cathodes are widely used in electric propulsion as well as for laboratory plasma generation applications. The plasma region just outside of high-current cathodes is characterized by the presence of many fluctuations, which can develop coherent plasma structures affecting the operation and life of the cathode. The properties of plasma oscillations have been investigated in the exterior region of a high-current hollow cathode operating at 25–150 A of discharge current with an applied axial magnetic field. Electrostatic and magnetic probes were used to measure the electromagnetic fluctuations, and correlation analysis between each of the probes signals provides the spatiotemporal characterization of the generated waves. The results of this investigation reveal the presence of a coherent magnetohydrodynamic (MHD) azimuthal mode with a fundamental frequency of 58 kHz. This mode has features of a helical kink instability with azimuthal wave number m=1 and axial wavenumber kz=50m−1 that is readily observed in high-speed Fast-Cam images. The occurrence of this mode, the frequency, and the wavenumber at onset are found to be predicted well by the ideal MHD theory, considering the boundary condition of a non-line-tying cylindrical anode. Investigation of the MHD mode properties at different discharge currents and mass flow rate shows additional features that could qualitatively agree with resistive MHD modes. In particular, when the plasma has a finite conductivity associated with anomalous resistivity in the cathode plume, a gradual emergence of modes at relatively low discharge currents are observed.

Absolute radiometric calibration of a VUV spectrometer in the wavelength range 46–300 nm

Vacuum ultraviolet (VUV) radiation corresponds to photon energies exceeding 6.2 eV and provides access to atomic and molecular resonant transitions in various plasmas. Determining population densities of electronic levels directly connected to the ground state requires an absolute intensity calibration of the VUV spectrometer. For this purpose, synchrotron radiation from electron storage rings is the typical primary standard source associated with transport effort and operating costs. Alternatively, a complex procedure using several secondary standards and discharges directly at the setup in use can be applied. In this work, an absolute intensity calibration avoiding synchrotron radiation to cover the wavelength range 46–300 nm accessible with two detectors—a photomultiplier tube and a channel electron multiplier—is presented. A combination of different standard sources—two deuterium arc lamps, branching-ratios of a nitrogen plasma and a high current hollow cathode—is applied for the relative calibration. The absolute calibration is based on simultaneous measurements of atomic lines in a low pressure helium discharge with the VUV spectrometer and an absolutely calibrated optical emission spectrometer. Special focus is laid on the uncertainties of the relative and absolute calibration depending on the wavelength range. The applicability of the intensity calibration is demonstrated at a hydrogen plasma in the wavelength range 80–300 nm. Using the emissivity of the Lyman series, population densities of the first six excited atomic states are calculated considering opacity effects. The excellent agreement with results from the Balmer series by optical emission spectroscopy manifests the high validity of the performed absolute intensity calibration.

Генерация плазмы с повышенной степенью ионизации в импульсном сильноточном тлеющем разряде низкого давления с полым катодом

Генерация плазмы с повышенной степенью ионизации в импульсном сильноточном тлеющем разряде низкого давления с полым катодом

Transient non-classical transport in the hollow cathode plume I: measurements of time-varying electron collision frequency

Electrostatic probes are employed to measure the time-variations in electron collision frequency due to a large-scale, low-frequency, plasma instability in a high-current hollow cathode plasma discharge (plume mode oscillation). Time-resolved measurements of ion acoustic turbulence are used to infer the effective collision frequency on the timescale of this underlying wave. Through a direct comparison, it is shown that the observed variation in the electron collision frequency cannot be described by classical collisional processes, i.e. Coulomb and neutral collisions, but rather is well represented by the changes in the anomalous collision frequency due to the turbulence.

Generating large, tunable plasmas for practical applications

A new method can vary operating parameters for a pulsed plasma created by combining a high-current hollow cathode discharge with a low-voltage glow discharge.

Recent progress in research and development of hollow cathodes for electric propulsion

Electric thrusters are finding increasing usage worldwide in spacecraft applications. Significant progress has been made in recent years in the modeling and performance of thermionic hollow cathodes used in flight thrusters, such as Hall and ion thrusters, or other types of plasma sources, such as for technological plasmas. The recent progress is surveyed in this paper through the discussion of six areas: hollow cathode modeling and simulation, low-current hollow cathodes, high-current hollow cathodes, heaterless hollow cathodes, new thermionic insert materials, and plasma oscillations. This includes descriptions of hollow cathode designs capable of < 1 A to over 300 A, advances in electron emitter and heating/starting technologies, and modeling and simulation of the plasma properties, thermal behavior and instabilities in the discharge. Advances in the understanding and technology in these areas and challenges that still need to be addressed and solved are discussed.

Triple Langmuir Probes Measurements of LaB6 Hollow Cathodes Plume

Lanthanum hexaboride hollow cathodes represent a viable option for high power Hall effect thruster applications, under development for the next generation of manned and robotic interplanetary missions. In this scenario, SITAEL and the University of Pisa are actively developing high current hollow cathodes capable of providing discharge current in the range 10-100 A to be coupled with high power Hall effect thrusters. The cathode design is based on an in-house theoretical model of the internal sections of the cathode, recently integrated with a simplified model of the cathode plume. Despite the application of hollow cathodes on flight and laboratory model Hall effect thrusters, many questions remain unsolved. In particular, issues related to onset of instabilities, due to plume mode or ion acoustic turbulence, are still unclear, while it is known that they can affect the overall performance of the cathode and thruster unit. This paper focuses on the experimental investigation of the cathode plume by means of measurements of the main plasma parameters, at different operating conditions and for different cathode geometry. Two cathodes were investigated, namely HC20 and HC60, designed to be coupled with SITAEL’s HT5k and HT20k (5 kW- and 20kW-class) Hall effect thrusters. The cathodes were mounted in stand-alone configuration with an auxiliary cylindrical anode. The experimental campaign was performed using triple Langmuir probes as plasma diagnostic system. The probes were mounted on scanning mechanisms to measure the plume parameters at various radial and axial distances from the keeper exit. General trends of electron temperature, plasma potential and plasma density are reported in terms of discharge current, mass flow rate and cathode orifice geometry. The results highlight that the cathode plate orifice selection affects the plume mode onset, giving the possibility to extend the stable mode of cathode operation in the current range required by the thruster.

Plasma Diagnostics in High Current Hollow Cathode

Plasma Diagnostics in High Current Hollow Cathode

Ion Heating Measurements on the Centerline of a High-Current Hollow Cathode Plume

An experimental investigation into the correlation between ion acoustic turbulence (IAT) and anomalous ion heating in the plume of a 100 A-class hollow cathode is presented. Laser-induced fluorescence is employed to measure the ion velocity distribution function, and a translating ion saturation probe is used to quantify the spatial dependence of the IAT wave energy. It is found that over a range of flow rates and operating currents both the ion temperature and IAT energy increase downstream of the cathode in qualitatively similar ways. Both parameters also are shown to be impacted by operating conditions: the IAT energy and ion temperature decrease at higher flow rates and lower discharge currents. It is shown that the ratio between ion temperature and wave energy is related by a scaling parameter that depends on the background plasma parameters, and this relation is examined in the context of previous analytical work on IAT-induced ion heating.

High-Current Lanthanum Hexaboride Hollow Cathode for High-Power Hall Thrusters

NASA is continuing to develop high-power Hall thrusters in the range of 20 to 100 kW for future cargo and manned missions. The cathodes for these thrusters will be required to produce discharge currents in the 50 to 300 A range with lifetimes in excess of 10 kh. A prototype high-current hollow cathode with a 2-cm-diam lanthanum hexaboride () insert was previously developed for these applications. The original design featured a graphite cathode tube to interface with the insert and an insulated sheath heater capable of heating the higher temperature emitter to ignition temperatures. A new version of this cathode has been designed and built that uses a refractory metal cathode tube and features a robust design similar to the small cathode used for the H6 Hall thruster. The new cathode has been successfully tested at steady-state discharge currents from 25 to 300 A. This cathode is intended to be used in an 80 kW nested Hall thruster being built at the University of Michigan at discharge currents of over 250 A.

Detached Glow Above a Titanium Hollow Cathode Sputter Source

A simple argon hollow cathode (HC) was investigated as a plasma source and a sputtering source for titanium. The HC consists of a Ti cylinder. There are two different modes of operation: a low-current discharge with the typical pink bluish argon glow and the high-current HC discharge mode. In the latter case, an intensive glow was observed at the upper end of the Ti cathode. Under certain conditions, this glow was observed to float freely between cathode and anode ring.

Single-Channel Hollow Cathodes in 5–20-eV Argon Discharge for Spacecraft Thruster Applications

High-power magnetoplasmadynamic (MPD) thrusters are an attractive option for providing primary propulsion on flagship-class spaceflight missions beyond the Low Earth Orbit (LEO), such as piloted Mars and Mars cargo, and more near-term cargo missions supporting a crewed lunar outpost due to their unique ability to process large amounts of power in a relatively small footprint size. This results in significant savings in system mass and volume over competing propulsion options such as ion or Hall-effect thrusters. This paper presents a partial summary of data from recent research at the University of Southern California, in collaboration with NASA's Jet Propulsion Laboratory, investigating several concerns of the mechanisms influencing the performance and lifetime of high-current single-channel hollow cathodes (SCHCs), the central electrode, and the primary life-limiting component in MPD thrusters. The overall objective of such research is to extend the operational lifetime of the cathode to mission-enabling lengths, generally considered to be in the range of 5-10 000 h. Specifically covered are the trends seen in the discharge efficiency and power, the size of the plasma attachment to the cathode (the active zone), the cathode exit plume plasma density and energy, and the plasma property distributions of the internal plasma column of an SCHC.

Experimental study of weak intersystem lines and related strong persistent lines of Ne II

We operated a high-current hollow cathode discharge in pure neon at pressures from 93 to 173 Pa and measured the branching fractions of Ne II emission lines originating from several 2p{sup 5} 3p, 3d, and 4f levels, for which lifetime data were available in the literature. This allowed the determination of transition probabilities for all downward transitions, which included a number of weak intersystem lines. We focused our study on these weak lines, using care to assure correct identifications and accurate intensity measurements, in spite of the presence of much stronger other lines in their vicinity. In contrast to an earlier experiment, we achieved close agreement with a recent multiconfiguration Hartree-Fock calculation.

Downstream Ion Energy Distributions in a Hollow Cathode Ring Cusp Discharge

The presence of energetic ions produced by a hollow cathodes operating at high-emission currents (>10 A) has been documented in the literature. As part of an ongoing effort to uncover the underlying physics of the formation of these ions, ion efflux from a high-current hollow cathode operating in an ion thruster discharge chamber was investigated. With use of a spherical sector electrostatic energy analyzer located downstream of the discharge cathode, the ion energy distribution over a 0-60-eV energy range was measured. The sensitivity of the ion energy distribution function to zenith angle was also assessed at three different positions: 0, 15, and 25 deg. The measurements suggest that the majority of the ion current at the measuring point falls into the analyzer with an energy approximately equal to the discharge voltage. The ion distribution, however, was found to be quite broad. The high-energy tail of the distribution function tended to grow with increasing discharge current. Sensitivity of the profiles to flow rate at fixed discharge current was also investigated. A simple model is presented that provides a potential mechanism for the production of ions with energies above the discharge voltage.

Low-pressure, high-current hollow cathode with a ferroelectric plasma source

We report the parameters of a hollow cathode with a ferroelectric plasma source incorporated in it. It was found that this source allows the ignition and sustaining of a high-current discharge (⩽1.4 kA, ⩽2×10−5 s) at N2 gas pressure of (3–5)×10−4 Torr. It was shown that ∼85% of the discharge current is emitted by the ferroelectric sample. The plasma in the cathode acquires a positive potential (⩽50 eV) with respect to the anode and the plasma density and temperature are ⩽8×1012 cm−3 and ⩽18 eV, respectively. Generation of an electron beam (0.3–1.6 kA, 300 ns) was demonstrated under an accelerating pulse ⩽300 kV.

ＴｉＮセラミック膜の伸びの形態

The elongated morphology of TiN films deposited on SUS430 stainless steel coil has been studied. TiN-coated stainless steel coil has been deposited using a new horizontal-type high current (1500 A) hollow cathode discharge (HCD) gun installed in a continuous ion plating apparatus of air-to-air type.In the scanning electron microscope (SEM) observation of TiN films after bulge press-forming and 180° bend-forming, the TiN films showed comparatively sharp, straight cracks characteristic of ceramics. However, very small areas across cracks of the TiN films showed the “bridgelike” elongation on TiN-TiN surface matrix. The formation of these bridges was associated with the formation of the continuous convex lines on the film surface.

New Energy Levels and Classifications of Spectral Lines from Neutral and Singly-Ionized Dysprosium (Dy I and Dy II)

We present new energy-level classifications for over 400 lines in the spectra of neutral and singly-ionized dysprosium. These lines were observed using Fourier transform spectrometry of high current hollow cathode lamps and cover the wavelength range 230 nm to 1.6 µm. They were classified by 43 new energy levels of Dy I and 24 new energy levels of Dy II. The standard uncertainty of the energy levels and the strongest lines is estimated at 0.005 cm-1.

W II branching ratios and oscillator strengths

Branching ratios of 280 W II lines from 19 excited levels in the wavelength range 204–750 nm are presented which were obtained by means of emission measurements on a high-current hollow cathode as well as a suitable Penning discharge lamp and by using a high-resolution Fourier-transform spectrometer. The branching ratios are combined with recently measured lifetimes to determine absolute transition probabilities. As a result the uncertainty of the f-values typically is between 7 and 9%. A comparison with literature data is done only on a very limited scale because the overlap with other authors is small. Good agreement was achieved with thoeretical calculations, especially for the strong transitions.

Measurements of Ions from High-Current Hollow Cathodes Using Electrostatic Energy Analyzer

Energy distributions of ions emanating from high-current, xenon hollow cathodes were measured using an electrostatic energy analyzer. Ions with energies much greater thanmaximumpotential differences between electrodes were observed. This result is consistent with previous retarding-potential-analyzer data, but the electrostaticenergy-analyzer data show much more complicated structures. To characterize these distributions, they were Ž t with simple Gaussian functions, and the effects of discharge conditions on high-energy-ion characteristics were investigated. The energy of ions emanating from cathodes appears to increase as discharge current is increased or cathode  ow rate is reduced. Higher discharge currents cause the ion populations in the high-energy tails of the distributions to increase in the plume mode while the high-energy tails decrease in the spot mode.

W I branching ratios and oscillator strengths

Branching ratios of 263 W I lines in the wavelength range 225–750 nm are presented which were obtained by means of emission measurements on a high-current hollow cathode and by using a high-resolution Fourier-transform spectrometer. The branching ratios are combined with recently measured lifetimes to determine absolute transition probabilities. As a result the uncertainty of the f-values typically is between 7 and 9%. A comparison with literature data is done only on a very limited scale because the overlap with other authors is small.