Richard Hofer

Verified

Richard verified their affiliation via an institutional email.

Learn more

Verified

Richard verified their affiliation via an institutional email.

Learn more

• PhD
• Principal Engineer & Supervisor at Jet Propulsion Laboratory

An experimental evaluation is presented of a two-equation model for the low frequency ( < 25 kHz), large amplitude ( > 100 % of mean) discharge oscillations exhibited by a 9-kW class magnetically shielded Hall thruster. The model is based on a theoretical treatment of the “breathing mode” oscillations in Hall thrusters (Barral and Peradzyński, “A n...

A stability criterion is derived for mode transitions in the discharge current oscillations of a magnetically shielded Hall thruster. The two-equation model evaluated in Paper I for these large-amplitude ( > 100 % background), low-frequency ( < 25 kHz) current oscillations is generalized and then validated with measurements from a 9 kW class test a...

Magnetic shielding technology has extended the operating life of Hall thrusters to timescales enabling them to expand beyond missions in Earth orbit, which typically require less than 10 kh of operation, to deep-space missions requiring 10-30 kh of operation. While this has expanded potential mission capture, the specific impulse of flight Hall thr...

Recent literature has shown a variety of benefits towards Hall thrusters operating at higher thrust and power density, including more efficient ionization with alternate propellants , higher specific impulse capability, higher power throttling ranges, and smaller form factors. These benefits have largely remained untapped due to concerns with thrus...

Experimental and theoretical studies of magnetic shielding of Hall thrusters have shown significant enhancement of lifetime through reduced erosion of the thruster discharge channel. This enabling technology provides the capability for Hall thrusters to operate in deep space missions. A low power variant of a thruster with this technology - the Mag...

A model of system mass and life-cycle costs is used to determine the optimal number of thrusters for electric propulsion systems. The model is generalized for application with most electric propulsion systems and then applied to high-power Hall thruster systems in particular. Mass and cost models were constructed for individual thruster strings usi...

Hexagonal boron nitride (h‐BN) and graphite have similar crystal structures, comparable lattice parameters, and coefficients of thermal expansion, but vastly different electrical and thermal transport. Despite their key differences, it is possible to couple h‐BN and graphite in a bimaterial system allowing the unique properties of both materials to...

NASA and Aerojet Rocketdyne are developing and qualifying a 12-kW Hall thruster for deep-space mission applications through the Advanced Electric Propulsion System (AEPS) project. The core technology of the thruster derives from the NASA-developed Hall Effect Rocket with Magnetic Shielding (HERMeS), which had a design lifetime of 50 kh at specific...

A magnetically shielded Hall thruster with graphite walls electrically tied to the anode was operated for the first time at 800 V discharge voltage and 3000 s specific impulse. Electrical isolation issues were absent with the conducting channel and the operating characteristics were virtually indistinguishable from boron nitride walls. Thrust effic...

View Video Presentation: https://doi.org/10.2514/6.2022-1353.vid This work presents a summary of a detailed performance assessment of the Hall Effect Rocket with Magnetic Shielding (HERMeS) Technology Demonstration Unit 3 (TDU-3) thruster at an expanded set of throttle conditions. First, an assessment was performed of TDU3 performance and stability...

The microstructure and magnetic performance of Fe49Co2V (Hiperco50) manufactured via laser‐directed energy deposition are determined. In the as‐printed form, the material displays a fine, equiaxed microstructure and magnetically “hard” behavior. With a customized post‐process annealing treatment, significant grain growth occurs, resulting in soft m...

This work presents the results of over 6500 h of wear testing completed during the maturation of the NASA 12.5 kW Hall Effect Rocket with Magnetic Shielding. Erosion of the thruster front pole covers was found to be the primary life-limiting mechanism and exhibits strong dependencies on thruster operating condition and material properties. Specific...

View Video Presentation: https://doi.org/10.2514/6.2021-3406.vid This work adapts manufacturing and metrology industry standards to create a risk-based approach for the determination of electric propulsion performance specifications. The developed process is applied to 208 total thrust measurements acquired using three different NASA Hall Effect Ro...

View Video Presentation: https://doi.org/10.2514/6.2021-3431.vid This work presents a summary of the first detailed performance assessment of the Advanced Electric Propulsion System (AEPS) Engineering Test Unit 2 (ETU-2) thruster produced by Aerojet Rocketdyne at the throttle conditions most relevant for AEPS application on the Gateway Power and Pr...

View Video Presentation: https://doi.org/10.2514/6.2021-3432.vid During development testing of the 12.5-kW Advanced Electric Propulsion System engineering unit Hall thruster, which is magnetically shielded, a laser-induced fluorescence test was performed. During this test, a third medium-energy ion population was found near the inner front pole cov...

This work presents a summary of the first wear test of the 12.5 kW Advanced Electric Propulsion System (AEPS) Engineering Test Unit 2 (ETU-2) thruster produced by Aerojet Rocketdyne. The ETU-2 Wear Test accumulated approximately 730 hours of operation split between the nominal 600 V/12.5 kW condition and the 300 V/6.25 kW condition previously ident...

NASA’s Psyche mission will launch in 2022 and begin a 3.6-year cruise to the metallic asteroid Psyche, where it will examine this unique body. The baseline spacecraft design is a hybrid of JPL’s deep-space heritage subsystems with commercial partner Maxar’s electric propulsion, power, and structure subsystems. All primary propulsion will be done wi...

This work presents a summary of the detailed characterization test of the 12.5 kW Advanced Electric Propulsion System (AEPS) Engineering Test Unit 2 (ETU-2) thruster produced by Aerojet Rocketdyne. This test campaign had two major goals: to assess the risk of design compliance with thruster requirements and provide a comparison to the previously-te...

The authors and co-signers of the Terrestrial Planets Comparative Climatology (TPCC) mission concept white paper advocate that planetary science in the next decade would greatly benefit from comparatively studying the fundamental behavior of the atmospheres of Venus and Mars, contemporaneously and with the same instrumentation, to capture atmospher...

We present time-resolved laser-induced fluorescence measurements of ion velocity distributions in a 12.5 kW Hall Effect Rocket with Magnetic Shielding (HERMeS) operating in both quasi-periodic and aperiodic oscillation regimes. Transfer function averaging in Fourier space is used to obtain useable signal-to-noise ratios and synchronize data traces...

We present time-resolved laser-induced fluorescence measurements of ion velocity distributions in a 12.5 kW Hall Effect Rocket with Magnetic Shielding (HERMeS) operating in both quasi-periodic and aperiodic oscillation regimes. Transfer function averaging in Fourier space is used to obtain useable signal-to-noise ratios and synchronize data traces...

We present time-resolved laser-induced fluorescence measurements of ion velocity distributions in a 12.5 kW Hall Effect Rocket with Magnetic Shielding (HERMeS) operating in both quasi-periodic and aperiodic oscillation regimes. Transfer function averaging in Fourier space is used to obtain useable signal-to-noise ratios and synchronize data traces...

We present time-resolved laser-induced fluorescence measurements of ion velocity distributions in a 12.5 kW Hall Effect Rocket with Magnetic Shielding (HERMeS) operating in both quasi-periodic and aperiodic oscillation regimes. Transfer function averaging in Fourier space is used to obtain useable signal-to-noise ratios and synchronize data traces...

We present time-resolved laser-induced fluorescence measurements of ion velocity distributions in a 12.5 kW Hall Effect Rocket with Magnetic Shielding (HERMeS) operating in both quasi-periodic and aperiodic oscillation regimes. Transfer function averaging in Fourier space is used to obtain useable signal-to-noise ratios and synchronize data traces...

We present time-resolved laser-induced fluorescence measurements of ion velocity distributions in a 12.5 kW Hall Effect Rocket with Magnetic Shielding (HERMeS) operating in both quasi-periodic and aperiodic oscillation regimes. Transfer function averaging in Fourier space is used to obtain useable signal-to-noise ratios and synchronize data traces...

We present time-resolved laser-induced fluorescence measurements of ion velocity distributions in a 12.5 kW Hall Effect Rocket with Magnetic Shielding (HERMeS) operating in both quasi-periodic and aperiodic oscillation regimes. Transfer function averaging in Fourier space is used to obtain useable signal-to-noise ratios and synchronize data traces...

We present laser-induced fluorescence measurements of acceleration zone scaling with discharge voltage (Vd), magnetic field strength (B), and facility background pressure (PBG) in NASA’s 12.5 kW Hall Effect Rocket with Magnetic Shielding. At fixed discharge current, the plasma potential profiles at discharge voltages from 300 to 600 V approximately...

The use of electric propulsion for commercial and research purposes has been gaining terrain in the past few years. The focus of this paper is the technological and commercial development of electric propulsion systems since 1993, the year when the first commercial satellite with an EP system was launched. Since then, 248 satellites with EP systems...

The successful application of a fully shielding magnetic field topology in a low-power Hall thruster is demonstrated through the testing of the MaSMi-60 Hall thruster (an improved variant of the original Magnetically Shielded Miniature Hall thruster). The device was operated at discharge powers from 160 to 750 W at discharge voltages ranging from 2...

The applicability of a fully shielding magnetic field topology to a low-power xenon Hall thruster was demonstrated through testing of the MaSMi-60. Although the discharge channel lifetime was significantly increased, performance testing of the device revealed a peak anode efficiency of under 30%, which was lower than expected given the available da...

The NASA Hall Effect Rocket with Magnetic Shielding (HERMeS) 12.5 kW Technology Demonstration Unit-1 (TDU-1) Hall thruster has been the subject of extensive technology maturation in preparation for development into a flight ready propulsion system. Part of the technology maturation was to test the TDU-1 thruster in several ground based electrical c...

The Asteroid Redirect Robotic Mission is a Solar Electric Propulsion Technology Demonstration Mission (ARRM) whose main objectives are to develop and demonstrate a high-power solar electric propulsion capability for the Agency and return an asteroidal mass for rendezvous and characterization in a companion human-crewed mission. This high-power sola...

Mode transitions in a 6 kW laboratory Hall-effect thruster were induced by varying the magnetic field intensity while holding all other operating parameters constant. Ultrafast imaging, discharge current, and thrust measurements were used to characterize the change in discharge channel current density and thruster performance through mode transitio...

Magnetic shielding has been shown to dramatically reduce discharge channel wall erosion of high powered Hall thrusters, thereby increasing their useful lifetimes. However, unique challenges exist for developing a low power magnetically shielded Hall thruster. A previously tested 4 cm magnetically shielded miniature Hall thruster demonstrated low pe...

Discharge channel wall erosion of high power Hall thrusters can be eliminated as a life-limiting mechanism by use of magnetic shielding. However, unique challenges exist for developing a low-power magnetically shielded Hall thruster. The results from an experimental and computational investigation on a 4 cm magnetically shielded miniature Hall thru...

Spokes are azimuthally propagating perturbations in the plasma discharge of Hall effect thrusters (HETs) that travel in the E × B direction. The mechanisms for spoke formation are unknown, but their presence has been associated with improved thruster performance in some thrusters motivating a detailed investigation. The propagation of azimuthal spo...

The scaling of magnetically shielded Hall thrusters to low power is investigated through the development and fabrication of a 4-cm Hall thruster. During initial testing, the magnetically shielded miniature Hall thruster was operated at 275 V discharge voltage and 325-W discharge power. Inspection of the channel walls after testing suggests that the...

This paper offers a user-centric consolidation and comparison of the full range of government and commercial solar electric propulsion options available in the near term for primary propulsion on deep-space science missions of the class commonly proposed to NASA's Discovery program. Unlike previous papers, this work does not emphasize feasibility f...

A series of numerical simulations and experiments have been performed to assess the effectiveness of magnetic shielding in a Hall thruster operating in the discharge voltage range of 300–700 V (Isp ≈ 2000–2700 s) at 6 kW, and 800 V (Isp ≈ 3000) at 9 kW. At 6 kW, the magnetic field topology with which highly effective magnetic shielding was previous...

The X3 is a 100-kW class nested-channel Hall thruster developed by the Plasmady- namics and Electric Propulsion Laboratory at the University of Michigan in collaboration with the Air Force Research Laboratory and NASA. The cathode, magnetic circuit, boron nitride channel rings, and anodes all required specific design considerations during thruster...

Since 2006, JPL has investigated many different commercially-available electric thruster systems for application to deep-space missions. The 4.5-kW SPT-140 Hall thruster system, now under qualification by SSL for communications satellites, carries significant heritage to SSL’s successful SPT-100 system and is attractive for many types of NASA missi...

A mode transition study is conducted in magnetically shielded thrusters where the magnetic field magnitude is varied to induce mode transitions. Three different oscillatory modes are identified with the 20-kW NASA-300MS-2 and the 6-kW H6MS: Mode 1) global mode similar to unshielded thrusters at low magnetic fields, Mode 2) cathode oscillations at n...

We describe the effects of finite pressure in a large vacuum facility on a magnetically shielded Hall thruster using internally and externally mounted cathodes. A numerical model of the pumping system was first used to guide the placement of an ionization gauge and an auxiliary propellant injector used for increasing the backpressure. The laborator...

NASA is developing mission concepts for a solar electric propulsion technology demonstration mission. A number of mission concepts are being evaluated including ambitious missions to near Earth objects. The demonstration of a high-power solar electric propulsion capability is one of the objectives of the candidate missions under consideration. In s...

The computational and experimental evidence indicates that channel wall erosion, which has limited the life of Hall thrusters in the past, has been solved by advent of Magnetic Shielding. The two to three order-of-magnitude reductions in the discharge chamber wall erosion rate in magnetically shielded Hall thrusters has eliminated wall sputtering a...

The magnetically shielded miniature Hall thruster, originally tested at the University of California, Los Angeles, underwent performance validation experiments at the Jet Propulsion Laboratory. The thruster was operated over a range of discharge voltages, from 150 V – 300V, and currents, from 1 A – 2.3 A. It was discovered that the thruster operate...

We demonstrate a technique by which erosion of the acceleration channel in Hall thrusters can be reduced by at least a few orders of magnitude. The first principles of the technique, now known as “magnetic shielding,” have been derived based on the findings of 2-D numerical simulations. The simulations, in turn, guided the modification of an existi...

The physics of magnetic shielding in Hall thrusters were validated through laboratory experiments demonstrating essentially erosionless, high-performance operation. The magnetic field near the walls of a laboratory Hall thruster was modified to effectively eliminate wall erosion while maintaining the magnetic field topology away from the walls nece...

Studies of high-power solar electric propulsion systems (i.e., tens to hundreds of kilowatts) suggest that significant mass savings may be realized by implementing direct-drive power systems. The National Direct-Drive Testbed was established to address issues associated with implementation of direct drive, and experimental results at power levels u...

The scaling of magnetically shielded Hall thrusters to low-power is investigated through the development and fabrication of a 4 cm Hall thruster. During initial testing, the magnetically shielded miniature Hall thruster was operated at 275 V discharge voltage and 325 W discharge power. Inspection of the channel walls after testing suggests that the...

Plasma oscillations from 0–100 kHz in a 6-kW magnetically shielded Hall thruster are experimentally characterized with a high-speed, optical camera. Two modes are identified at 7–12 kHz and 70–90 kHz. The low frequency mode is found to be azimuthally uniform across the thruster face, while the high frequency oscillation is peaked close to the cente...

A unique configuration of the magnetic field and channel geometry near the wall of Hall thrusters, called magnetic shielding, has recently demonstrated the ability to significantly reduce the erosion of the boron nitride (BN) walls and extend the life of Hall thrusters by orders of magnitude. The ability of magnetic shielding to minimize interactio...

Mode transitions have been commonly observed in Hall Effect Thruster (HET) operation where a small change in a thruster operating parameter such as discharge voltage, magnetic field or mass flow rate causes the thruster discharge current mean value and oscillation amplitude to increase significantly. Mode transitions in a 6-kW-class HET called the...

High-power Hall thrusters capable of throughput on the order of 100 kW are currently under development, driven by more demanding mission profiles and rapid growth in on-orbit solar power generation capability. At these power levels the nested Hall thruster (NHT), a new design that concentrically packs multiple thrusters into a single body with a sh...

The plasma plume of a 6 kW Hall Effect Thruster (HET) has been investigated in order to determine time-averaged and time-resolved plasma properties in a 2-D plane. HETs are steady-state devices with a multitude of kilohertz and faster plasma oscillations that are poorly understood yet impact their performance and may interact with spacecraft subsys...

We demonstrate by numerical simulations and experiments that the unmagnetized ion beam formed in a Hall thruster can be controlled by an applied magnetic field in a manner that reduces by 2–3 orders of magnitude deleterious ion bombardment of the containing walls. The suppression of wall erosion in Hall thrusters to such low levels has remained elu...

A series of numerical simulations and experiments have been performed to assess the effectiveness of magnetic shielding in a Hall thruster operating in the discharge voltage range of 300-700 V (Isp≈2000-2700 s) at 6 kW, and 800 V (Isp≈3000) at 9 kW. In this paper we report on the simulation results and their validation with laboratory measurements....

Several Hall thrusters (e.g. BPT-4000, BHT-600, SPT-100, etc.) are able that operate with useful thrust at the sub-kilowatt power levels that would be available from solar arrays at Jupiter distance. We have found that a combination of a multi-kilowatt thruster (e.g. the BPT-4000) for the interplanetary trajectory with a sub-kilowatt thruster (e.g....

Magnetic shielding in Hall thrusters significantly reduces the transport of high-energy ions to the channel walls such that erosion is effectively eliminated. The physics of magnetic shielding were validated through laboratory experiments demonstrating essentially erosionless, high-performance Hall thruster operation for the first time. The magneti...

A proof-of-principle effort to demonstrate a technique by which erosion of the acceleration channel in Hall thrusters can be eliminated has been completed successfully. The first principles of the technique, now known as "magnetic shielding, were derived based on the findings of numerical simulations with the Hall2De code. The simulations, in turn,...

Currently, there is great interest in the development of high-power electric propulsion (EP) devices that can be employed in missions requiring >100 kW levels of propulsive power. Of the candidates for such thrusters, the Nested-channel Hall thruster (NHT) has been shown to be particularly scalable to this mission requirement. To this end, the Univ...

The availability of affordable commercial electric propulsion fundamentally changes the option space for Mars sample return, leading to near-term architectures that are simpler and less expensive and enabling combined human-robotic missions.

The T6 ion engine is a 22-cm diameter, 4.5-kW Kaufman-type ion thruster produced by QinetiQ, Ltd., and is baselined for the European Space Agency BepiColombo mission to Mercury and is being qualified under ESA sponsorship for the extended range AlphaBus communications satellite platform. The heritage of the T6 includes the T5 ion thruster now succe...

The cathode coupling voltage in Hall thrusters, which is the voltage difference between the cathode and the thruster beam plasma potential, is considered an indicator of the ease with which electrons flow from cathode to anode. Historically, the coupling voltage has been minimized by increasing the amount of propellant injected through the hollow c...

In order to better understand interactions between the plasma and channel walls of a Hall thruster, the near-wall plasma was characterized within the H6 Hall thruster using five flush-mounted Langmuir probes. These probes were placed within the last 15% of the discharge channel and were used to measure plasma potential, electron temperature, and io...

Computer simulations using Hall2De identified the fundamental physical processes through which magnetic shielding can reduce channel wall erosion by orders of magnitude in Hall thrusters. Because magnetic shielding reduces significantly the energy and flux of the incident plasma a natural question is whether it has also a large effect on Hall thrus...

A model of system mass and life-cycle costs is used to determine the optimal number of thrusters for electric propulsion systems. The model is generalized for application with most electric propulsion systems and then applied to high-power Hall thruster systems in particular. Mass and cost models were constructed for individual thruster strings usi...

In a proof-of-principle effort to demonstrate the feasibility of magnetically shielded (MS) Hall thrusters, an existing laboratory thruster has been modified with the guidance of physics-based numerical simulation. When operated at a discharge power of 6 kW the modified thruster has been designed to reduce the total energy and flux of ions to the c...

We examine human missions to the Martian moons, Phobos and Deimos, that use hybrid chemical and solar-electric propulsion trajectories. Chemical propulsion is used where the orbit dynamics favor an impulsive maneuver and this efficiency offsets the inefficiency of the lower specific impulse of chemical systems. Solar-electric propulsion at power le...

Laser-induced fluorescence velocimetry measurements obtained from the interior of a 6-kW Hall thruster using the Xe I 6s‰3=2Š 0 2 ! 6p‰3=2Š 2 transition at 823.4 nm (vacuum) and the Xe I 6s 0 ‰1=2Š 0 1 ! 6p 0 ‰3=2Š 2 transition at 834.9 nm (vacuum) are presented. The thruster is operated under seven conditions, with discharge voltages ranging from...

In a qualification life test of a Hall thruster it was found that the erosion of the acceleration channel practically stopped after ∼ 5600 h. Numerical simulations using a two-dimensional axisymmetric plasma solver with a magnetic field-aligned mesh reveal that when the channel receded from its early-in-life to its steady-state configuration the fo...

In a Qualification Life Test (QLT) of the BPT-4000 Hall thruster that recently accumulated >10,000 h it was found that the erosion of the acceleration channel practically stopped after ∼5,600 h. Numerical simulations of this thruster using a 2-D axisymmetric, magnetic field-aligned-mesh (MFAM) plasma solver reveal that the process that led to this...

In preparation for in-depth internal wall measurements for erosion and electron mobility studies, a flush-mounted Langmuir probe and emissive probe were used to characterize the near-wall region near the exit plane of a 6-kW Hall thruster. Various plasma properties and electron energy distribution functions were measured at discharge voltages of 15...

Various methods for accurately determining ion species' current fractions using E x B probes in Hall thruster plumes are investigated. The effects of peak broadening and charge exchange on the calculated values of current fractions are quantified in order to determine the importance of accounting for them in the analysis. It is shown that both peak...

The effects of cathode position on the operation and plume properties of an 8-kW Hall thruster are discussed. Thruster operation was investigated at operating conditions ranging from 200 to 500 V of discharge voltage, 10-40 A of discharge current, and 2-8 kW of discharge power, with a cathode positioned either in the traditional externally mounted...