Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session GI3: Thrusters, Plasma Expansion, and Laboratory AstrophysicsInvited
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Chair: Vadim Khayms, Lockheed Martin Corporation Room: 210 ABEF |
Tuesday, November 1, 2016 9:30AM - 10:00AM |
GI3.00001: The X3: A 200 kW Class Nested Channel Hall Thruster Invited Speaker: J. P. Sheehan Electric propulsion has seen rapid adoption in recent years for commercial, scientific, and exploratory space missions. The X3 is a three channel nested channel Hall thruster, designed to push the boundaries of high power electric propulsion for cargo transfer to Mars and large military assets. It has been operated at thermal steady state up to 30 kW of power. Thrust measurements were made on an inverted pendulum thrust stand, indicating over 2000 s specific impulse and 65 mN/kW thrust to power ratio. Detailed plume measurements were made with Faraday and Langmuir probes. The multiple concentric channels provide better performance than the sum of the individual channel operations due to superior propellant utilization from its compact design. Using a high speed camera, the breathing and spoke mode instabilities were captured in all three channels. Spoke and breathing instabilities couple between the channels, indicating that complex plasma and neutral interactions are at play. Electron transport, both cross field and in the cathode plume, are well suited to be explored in a thruster of this size. [Preview Abstract] |
Tuesday, November 1, 2016 10:00AM - 10:30AM |
GI3.00002: Theory and Numerical Simulation of Plasma-wall Interactions in Electric Propulsion Invited Speaker: Ioannis Mikellides Electric propulsion (EP) can be an enabling technology for many science missions considered by NASA because it can produce high exhaust velocities, which allow for less propellant mass compared to typical chemical systems. Over the last decade two EP technologies have emerged as primary candidates for several proposed science missions, mainly due to their superior performance and proven record in space flight: the Ion and Hall thrusters. As NASA looks ahead to increasingly ambitious science goals, missions demand higher endurance from the propulsion system. So, by contrast to the early years of development of these thrusters, when the focus was on performance, considerable focus today is shifting towards extending their service life. Considering all potentially life-limiting mechanisms in Ion and Hall thrusters two are of primary concern: (a) the erosion of the acceleration channel in Hall thrusters and (b) the erosion of the hollow cathode. The plasma physics leading to material wear in these devices are uniquely challenging. For example, soon after the propellant is introduced into the hollow cathode it becomes partially ionized as it traverses a region of electron emission. Electron emission involves highly non-linear boundary conditions. Also, the sheath size is typically many times smaller than the characteristic physical scale of the device, yet energy gained by ions through the sheath must be accounted for in the erosion calculations. The plasma-material interactions in Hall thruster channels pose similar challenges that are further exacerbated by the presence of a strong applied magnetic field. In this presentation several complexities associated with plasma-wall interactions in EP will be discussed and numerical simulation results of key plasma properties in two examples, Hall thrusters and hollow cathodes, will be presented. [Preview Abstract] |
Tuesday, November 1, 2016 10:30AM - 11:00AM |
GI3.00003: Instabilities and transport in Hall plasmas with ExB drift Invited Speaker: Andrei Smolyakov Low temperature plasma with moderate magnetic field, where the ions are not or just weakly magnetized, i.e. the ion Larmor radius being larger or comparable to the characteristic length scale of interest (e.g. the size ofthe system), have distinctly different properties from strongly magnetized plasmas such as that for fusion applications. Such parameters regimes are generally defined here as Hall plasmas. The natural scale separation between the ion and electron Larmor radii in Hall plasma, further exploited by the application of the external electric field, offers unique applications in various plasma devices for material processing and electric propulsion. Plasmas in such devices are in strongly non-equilibrium state making it prone to a number of instabilities. This talk presents physics description of the dominant unstable modes in ExB Hall plasmas resulting in highly turbulent state with nonlinear coherent structures and anomalous electron current. Since ions are un-magnetized, fundamental instabilities operating in low temperature Hall plasmas are very different from much studied gradients (density, temperature and magnetic field) driven drift-wave turbulence in strongly magnetized plasmas for fusion applications. As a result the nonlinear saturation mechanisms, role of the ExB shear flows are also markedly different in such plasmas. We review the basic instabilities in these plasmas which are related to the ion-sound, low-hybrid and anti-drift modes, discuss nonlinear saturation and anomalous transport mechanisms. The advanced nonlinear fluid model for such plasmas and results of nonlinear simulations of turbulence and anomalous transport performed within a modified BOUT++ framework will be presented. [Preview Abstract] |
Tuesday, November 1, 2016 11:00AM - 11:30AM |
GI3.00004: Collisionless coupling of a high-$\beta$ expansion to an ambient plasma Invited Speaker: Jeffrey Bonde We report on an experimental study of collisionless coupling between a high-$\beta$ ($\sim 10^6$) expansion and a quiescent, magnetized plasma via laminar electric fields. The dynamic, 3D structure of the electrostatic field, $-\nabla\phi$, and the induced electric field, $-\partial_t \vec{A}$, of a laser-produced plasma (LPP) were measured in situ [1] within the uniform plasma provided by the Large Plasma Device at UCLA. The LPP was generated using a graphite target and oriented to provide Alfvénic expansion speeds along the initially uniform magnetic field, $\vec{B}_0$. The strongest measured electric field was an inward, radial electrostatic field established by charge separation of the ions and electrons across $\vec{B}_0$. The imbalance between the charge layers also resulted in a radially outward electrostatic field that appeared to be responsible for the observed magnetic compression. Global neutralization of this charge distribution is connected with previously observed energetic electrons and subsequent whistler wave radiation [2]. The cumulative effect of the total electric field is to pull ambient ions inward against the expansion. This is in contrast to models that neglect $-\nabla\phi$ in favor of $-\partial_t \vec{A}$ due to the large-scale motion of the magnetic field lines. Planar laser-induced fluorescence imaging in an argon background plasma confirmed this effect by measuring directly the velocity of the ambient ions. A simple model is presented of these results and other high-$\beta$ expansions that is similar to Lord Rayleigh's work on gaseous bubble cavitation [3]. It predicts relative magnitudes of $-\nabla\phi$ and $-\partial_t\vec{A}$ as well as an approximate description of their ability to couple energy and momentum to the ambient plasma. This model also provides scaling quantities appropriate for similar collisionless plasma expansions including explosions near young stellar objects, magnetospheric chemical releases, and high-altitude nuclear explosions. [1] Bonde, J et al., Phys. Rev. E 92, 051102 (2015). [2] Vincena, S et al., Phys. Plasmas 15, 072114 (2008). [3] Rayleigh, L. Philos. Mag. 34 (200), 94 (1917). [Preview Abstract] |
Tuesday, November 1, 2016 11:30AM - 12:00PM |
GI3.00005: Cathode-less gridded ion thrusters for small satellites Invited Speaker: Ane Aanesland Electric space propulsion is now a mature technology for commercial satellites and space missions that requires thrust in the order of hundreds of mN, and with available electric power in the order of kW. Developing electric propulsion for SmallSats (1 to 500 kg satellites) are challenging due to the small space and limited available electric power (in the worst case close to 10 W). One of the challenges in downscaling ion and Hall thrusters is the need to neutralize the positive ion beam to prevent beam stalling. This neutralization is achieved by feeding electrons into the downstream space. In most cases hollow cathodes are used for this purpose, but they are fragile and difficult to implement, and in particular for small systems they are difficult to downscale, both in size and electron current. We describe here a new alternative ion thruster that can provide thrust and specific impulse suitable for mission control of satellites as small as 3 kg. The originality of our thruster lies in the acceleration principles and propellant handling. Continuous ion acceleration is achieved by biasing a set of grids with Radio Frequency voltages (RF) via a blocking capacitor. Due to the different mobility of ions and electrons, the blocking capacitor charges up and rectifies the RF voltage. Thus, the ions are accelerated by the self-bias DC voltage. Moreover, due to the RF oscillations, the electrons escape the thruster across the grids during brief instants in the RF period ensuring a full space charge neutralization of the positive ion beam. Due to the RF nature of this system, the space charge limited current increases by almost a factor of 2 compared to classical DC biased grids, which translates into a specific thrust two times higher than for a similar DC system. This new thruster is called Neptune and operates with only one RF power supply for plasma generation, ion acceleration and electron neutralization. We will present the downscaling of this thruster to a 3cm diameter unit well adapted for a CubeSat or SmallSat mission. [Preview Abstract] |
Tuesday, November 1, 2016 12:00PM - 12:30PM |
GI3.00006: Studying astrophysical particle acceleration with laser-driven plasmas Invited Speaker: Frederico Fiuza The acceleration of non-thermal particles in plasmas is critical for our understanding of explosive astrophysical phenomena, from solar flares to gamma ray bursts. Particle acceleration is thought to be mediated by collisionless shocks and magnetic reconnection. The microphysics underlying these processes and their ability to efficiently convert flow and magnetic energy into non-thermal particles, however, is not yet fully understood. By performing for the first time ab initio 3D particle-in-cell simulations of the interaction of both magnetized and unmagnetized laser-driven plasmas, it is now possible to identify the optimal parameters for the study of particle acceleration in the laboratory relevant to astrophysical scenarios. It is predicted for the Omega and NIF laser conditions that significant non-thermal acceleration can occur during magnetic reconnection of laser-driven magnetized plasmas. Electrons are accelerated by the electric field near the X-points and trapped in contracting magnetic islands. This leads to a power-law tail extending to nearly a hundred times the thermal energy of the plasma and that contains a large fraction of the magnetic energy. The study of unmagnetized interpenetrating plasmas also reveals the possibility of forming collisionless shocks mediated by the Weibel instability on NIF. Under such conditions, both electrons and ions can be energized by scattering out of the Weibel-mediated turbulence. This also leads to power-law spectra that can be detected experimentally. The resulting experimental requirements to probe the microphysics of plasma particle acceleration will be discussed, paving the way for the first experiments of these important processes in the laboratory. As a result of these simulations and theoretical analysis, there are new experiments being planned on the Omega, NIF, and LCLS laser facilities to test these theoretical predictions. [Preview Abstract] |
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