407 research outputs found
Low and High-Power Inductive Pulsed Plasma Thruster Development Testing at NASA-MSFC
The inductive pulsed plasma thruster (IPPT) is an electromagnetic plasma accelerator that has been identified in NASA roadmaps as an enabling propulsion technology for some niche low-power missions and for high-power in-space propulsion needs. The IPPT is an electrodeless space propulsion device where a capacitor is charged to an initial voltage and then discharged producing a high current pulse through a coil. The field produced by this pulse ionizes propellant, inductively driving current in a plasma located near the face of the coil. Once the plasma is formed it can be accelerated and expelled at a high exhaust velocity by the electromagnetic Lorentz body force arising from the interaction of the induced plasma current and the magnetic field produced by the current in the coil. Thrusters of this type possess many demonstrated and potential benefits that make them worthy of continued investigation. The electrodeless nature of these thrusters eliminates the lifetime and contamination issues associated with electrode erosion in conventional electric thrusters. Also, a wider variety of propellants are accessible when compatibility with metallic electrodes in no longer an issue. IPPTs have been successfully operated using propellants like ammonia, hydrazine, and CO2, and there is no fundamental reason why they would not operate on other in situ propellants like H2O. It is well-known that pulsed accelerators can maintain constant specific impulse (I(sub sp)) and thrust efficiency (eta(sub t)) over a wide range of input power levels by adjusting the pulse rate to hold the discharge energy per pulse constant. It has also been demonstrated that an inductive pulsed plasma thruster can operate in a regime where eta(sub t) is relatively constant over a wide range of I(sub sp) values (3000-8000 s). Finally, thrusters in this class have operated in single-pulse mode at high energy per pulse, and by increasing the pulse rate they offer the potential to process very high levels of power using a single thruster. There has been significant previous research on IPPTs designed around a planar-coil (flat-plate) geometry. The most notable of these was the Pulsed Inductive Thruster (PIT), with the PIT MkV presently representing the state-of- the-art in pulsed high-power IPPT technological development. In this paper, we focus on two planar-geometry devices that operate at significantly different power levels. Most work performed at NASA-Marshall Space Flight Center (MSFC) has, to date, focused on lower power thruster operation (approx. = 10s to 100s of J/pulse, up to 2-2.5 kW average power throughput) and previously described. The most recent work aimed to assemble a device that could be tested in cyclic mode on a thrust-stand, and which could augment the existing data set for IPPTs. In addition, the thruster was designed to serve as a test-bed for solid state switching circuitry and pulsed gas valves, with the modular design of the device allowing for variation in or upgrades to test configuration. Recently, MSFC obtained on loan from the Georgia Institute of Technology (Atlanta, GA) the PIT MkVI, successor to the PIT MkV. The MkV and MkVI are similar in design with much of the hardware from the former, specifically the capacitors and spark-gap switches, being reused in the latter. The coil is similar in geometry but has bent copper rods used in the latest iteration in place of the Litz wire windings found in the MkV. The MkVI master switch for the spark gaps is located in the vacuum chamber contained within a sealed, pressurized vessel fastened to the back of the thruster. This is different from the MkV where many capacitor charging lines and spark gap-triggering delay lines ran to the thruster from a master trigger located outside the vacuum chamber. The MkVI was damaged during testing soon after its fabrication was completed. The thruster arrived at MSFC still-damaged and mostly disassembled into many individual pieces. The device has been repaired, with a few additional design changes implemented after discussions with the late Prof. Lovberg regarding the initial testing results and issues encountered. In the present work, we present results from testing of both the small IPPT and the larger MkVI thruster. The smaller device (Fig. 1) is tested on a thrust stand on multiple gases to demonstrate its capability to operate in a repetition-rate mode and serve as a IPPT technology-development testbed. The larger MkVI (Fig. 2) is operated for the first time in its newly reconstituted state, demonstrating full-power pulsed operation and, for the first time, repetition-rate operation of a high-power IPPT. The additional upgrades required for synchronous operation of all the pulsed systems in single-pulse and repetition-rate mode are described in detail
Inductive Pulsed Plasma Thruster Development Testing at NASA-MSFC
The inductive pulsed plasma thruster (IPPT) is an electromagnetic plasma accelerator that has been identified in NASA roadmaps as an enabling propulsion technology for some niche low-power missions and for high-power in-space propulsion needs. The IPPT is an electrodeless space propulsion device where a capacitor is charged to an initial voltage and then discharged producing a high current pulse through a coil. The field produced by this pulse ionizes propellant, inductively driving current in a plasma located near the face of the coil. Once the plasma is formed it can be accelerated and expelled at a high exhaust velocity by the electromagnetic Lorentz body force arising from the interaction of the induced plasma current and the magnetic field produced by the current in the coil. Thrusters of this type possess many demonstrated and potential benefits that make them worthy of continued investigation. The electrodeless nature of these thrusters eliminates the lifetime and contamination issues associated with electrode erosion in conventional electric thrusters. Also, a wider variety of propellants are accessible when compatibility with metallic electrodes in no longer an issue. IPPTs have been successfully operated using propellants like ammonia, hydrazine, and CO2, and there is no fundamental reason why they would not operate on other in situ propellants like H2O. It is well-known that pulsed accelerators can maintain constant specific impulse (I(sub sp)) and thrust efficiency ((sub t)) over a wide range of input power levels by adjusting the pulse rate to hold the discharge energy per pulse constant. It has also been demonstrated that an inductive pulsed plasma thruster can operate in a regime where (sub t) is relatively constant over a wide range of I(sub sp) values (3000-8000 s). Finally, thrusters in this class have operated in single-pulse mode at high energy per pulse, and by increasing the pulse rate they offer the potential to process very high levels of power using a single thruster. There has been significant previous research on IPPTs designed around a planar-coil (flat-plate) geometry. The most notable of these was the Pulsed Inductive Thruster (PIT), with the PIT MkV presently representing the state of- the-art in pulsed high-power IPPT technological development. In this paper, we focus on two planar-geometry devices that operate at significantly different power levels. Most work performed at NASA-Marshall Space Flight Center (MSFC) has, to date, focused on lower power thruster operation ( 10s to 100s of J/pulse, up to 2-2.5 kW average power throughput) and previously described in Refs. [5,6]. The most recent work aimed to assemble a device that could be tested in cyclic mode on a thrust-stand, and which could augment the existing data set for IPPTs. In addition, the thruster was designed to serve as a test-bed for solid state switching circuitry and pulsed gas valves, with the modular design of the device allowing for variation in or upgrades to test configuration. Recently, MSFC obtained on loan from the Georgia Institute of Technology (Atlanta, GA) the PIT MkVI, successor to the PIT MkV. The MkV and MkVI are similar in design with much of the hardware from the former, specifically the capacitors and spark-gap switches, being reused in the latter. The coil is simliar in geometry but has bent copper rods used in the latest iteration in place of the Litz wire windings found in the MkV. The MkVI master switch for the spark gaps is located in the vacuum chamber contained within a sealed, pressurized vessel fastened to the back of the thruster. This is different from the MkV where many capacitor charging lines and spark gap-triggering delay lines ran to the thruster from a master trigger located outside the vacuum chamber. The MkVI was damaged during testing soon after its fabrication was completed. The thruster arrived at MSFC still-damaged and mostly disassembled into many individual pieces. The device has been repaired, with a few additional design changes implemented after discussions with the late Prof. Lovberg regarding the initial testing results and issues encountered
HI distribution and kinematics of NGC 1569
We present WSRT observations of high sensitivity and resolution of the
neutral hydrogen in the starburst dwarf galaxy NGC 1569. Assuming a distance of
2.2 Mpc, we find a total HI mass of 1.3 * 10^8 M_sun to be distributed in the
form of a dense, clumpy ridge surrounded by more extended diffuse HI containing
a few additional discrete features, such as a Western HI Arm and an HI bridge
reaching out to a small counterrotating companion cloud. About 10% by mass of
all HI in NGC 1569 is at unusually high velocities. Some of this HI may be
associated with the mass outflow evident from H-alpha measurements, but some
may also be associated with NGC 1569's HI companion and intervening HI bridge,
in which case, infall rather than outflow might be the cause of the discrepant
velocities. No indication of a large bubble structure was found in
position-velocity maps of the high-velocity HI. The galaxy as a whole is in
modest overall rotation, but the HI gas lacks any sign of rotation within 60''
(0.6 kpc) from the center, i.e. over most of the optical galaxy. Here,
turbulent motions resulting from the starburst appear to dominate over
rotation. In the outer disk, the rotational velocities reach a maximum of 35
\pm 6 km/s, but turbulent motion remains significant. Thus, starburst effects
are still noticeable in the outer HI disk, although they are no longer dominant
beyond 0.6 kpc. Even excluding the most extreme high-velocity HI clouds, NGC
1569 still has an unusually high mean HI velocity dispersion of sigma_v=21.3
km/s, more than double that of other dwarf galaxies.Comment: Figure 11a,b and Figure 14 separately in jpg forma
The Arecibo Arp 220 Spectral Census I: Discovery of the Pre-Biotic Molecule Methanimine and New Cm-wavelength Transitions of Other Molecules
An on-going Arecibo line search between 1.1 and 10 GHz of the prototypical
starburst/megamaser galaxy, Arp 220, has revealed a spectrum rich in molecular
transitions. These include the ``pre-biotic'' molecules: methanimine
(CHNH) in emission, three direct l-type absorption lines of
HCN, and an absorption feature likely to be from either OH or formic
acid (HCOOH). In addition, we report the detection of two, possibly three,
transitions of 4-cm excited OH not previously detected in Arp~220
which are seen in absorption, and a possible absorption feature from the
6.668-GHz line of methanol. This marks the first distant extragalactic
detection of methanimine, a pre-biotic molecule. Also, if confirmed, the
possible methanol absorption line presented here would represent the first
extragalactic detection of methanol at a distance further than 10 Mpc. In
addition, the strong, previously undetected, cm-wave HCN direct
l-type lines will aid the study of dense molecular gas and active star-forming
regions in this starburst galaxy.Comment: 25 pages, 10 figures. Accepted for publication in A
Gemini GMOS/IFU spectroscopy of NGC 1569 - II: Mapping the roots of the galactic outflow
We present a set of four Gemini-North GMOS/IFU observations of the central
disturbed regions of the dwarf irregular starburst galaxy NGC 1569, surrounding
the well-known super star clusters A and B. This continues on directly from a
companion paper, in which we describe the data reduction and analysis
techniques employed and present the analysis of one of the IFU pointings. By
decomposing the emission line profiles across the IFU fields, we map out the
properties of each individual component identified and identify a number of
relationships and correlations that allow us to investigate in detail the state
of the ionized ISM. Our observations support and expand on the main findings
from the analysis of the first IFU position, where we conclude that a broad (<
400 km/s) component underlying the bright nebular emission lines is produced in
a turbulent mixing layer on the surface of cool gas knots, set up by the impact
of the fast-flowing cluster winds. We discuss the kinematic, electron density
and excitation maps of each region in detail and compare our results to
previous studies. Our analysis reveals a very complex environment with many
overlapping and superimposed components, including dissolving gas knots,
rapidly expanding shocked shells and embedded ionizing sources, but no evidence
for organised bulk motions. We conclude that the four IFU positions presented
here lie well within the starburst region where energy is injected, and, from
the lack of substantial ordered gas flows, within the quasi-hydrostatic zone of
the wind interior to the sonic point. The net outflow occurs at radii beyond
100-200 pc, but our data imply that mass-loading of the hot ISM is active even
at the roots of the wind.Comment: 21 pages, 23 figures, 3 tables. Accepted for publication in MNRA
Deviations from He I Case B Recombination Theory and Extinction Corrections in the Orion Nebula
We are engaged in a comprehensive program to find reliable elemental
abundances in and to probe the physical structure of the Orion Nebula, the
brightest and best-resolved H II region. In the course of developing a robust
extinction correction covering our optical and ultraviolet FOS and STIS
observations, we examined the decrement within various series of He I lines.
The decrements of the 2^3S-n^3P, 2^3P-n^3S and 3^3S-n^3P series are not in
accord with caseB recombination theory. None of these anomalous He I decrements
can be explained by extinction, indicating the presence of additional radiative
transfer effects in He I lines ranging from the near-IR to the near-UV. CLOUDY
photoionization equilibrium models including radiative transfer are developed
to predict the observed He I decrements and the quantitative agreement is quite
remarkable. Following from these results, select He I lines are combined with H
I and [O II] lines and stellar extinction data to validate a new normalizable
analytic expression for the wavelength dependence of the extinction. In so
doing, the He+/H+ abundance is also derived.Comment: 42 pages, 10 figures. To be published in Ap
Far Ultraviolet Observations of Molecular Hydrogen in the Diffuse Interstellar Medium of Starburst Galaxies
(Abridged) We have used the Far Ultraviolet Spectroscopic Explorer (FUSE) to
search for H_2 absorption in five starburst galaxies: NGC 1705, NGC 3310, NGC
4214, M83 (NGC 5236), and NGC 5253. We tentatively detect weak absorption by
H_2 in M83 and NGC 5253, and set upper limits on the H_2 column density in the
other galaxies. Conservative upper limits on the mass of molecular gas detected
with FUSE are many orders of magnitude lower than the H_2 mass inferred from CO
emission. This indicates that almost all of the H_2 is in the form of clouds
with N(H_2)>10^20 cm^-2 that are opaque to far-UV light and therefore cannot be
probed with far-UV absorption measurements. The far-UV continuum visible in the
FUSE spectra passes between the dense clouds, which have a covering factor <1.
The complex observational biases related to varying extinction across the
extended UV emission in the FUSE apertures prevent an unambiguous
characterization of the diffuse H_2 in these starbursts. However, the evidence
is suggestive that there is less H_2 in the diffuse interstellar medium between
the dense clouds compared to similarly reddened sight lines in the Milky Way.
This holds with the expectation that the destructive UV radiation field is
stronger in starbursts. However, previous UV observations of these starbursts
have shown that there is reddening caused by the diffuse interstellar medium.
This suggests that while diffuse H_2 may be destroyed in the starburst, dust
still exists.Comment: 10 pages, 8 figures. Accepted by ApJ. For higher resolution versions
of figures 1-6 please contact the primary autho
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Projecting Antarctic ice discharge using response functions from SeaRISE ice-sheet models
The largest uncertainty in projections of future sea-level change results from the potentially changing dynamical ice discharge from Antarctica. Basal ice-shelf melting induced by a warming ocean has been identified as a major cause for additional ice flow across the grounding line. Here we attempt to estimate the uncertainty range of future ice discharge from Antarctica by combining uncertainty in the climatic forcing, the oceanic response and the ice-sheet model response. The uncertainty in the global mean temperature increase is obtained from historically constrained emulations with the MAGICC-6.0 (Model for the Assessment of Greenhouse gas Induced Climate Change) model. The oceanic forcing is derived from scaling of the subsurface with the atmospheric warming from 19 comprehensive climate models of the Coupled Model Intercomparison Project (CMIP-5) and two ocean models from the EU-project Ice2Sea. The dynamic ice-sheet response is derived from linear response functions for basal ice-shelf melting for four different Antarctic drainage regions using experiments from the Sea-level Response to Ice Sheet Evolution (SeaRISE) intercomparison project with five different Antarctic ice-sheet models. The resulting uncertainty range for the historic Antarctic contribution to global sea-level rise from 1992 to 2011 agrees with the observed contribution for this period if we use the three ice-sheet models with an explicit representation of ice-shelf dynamics and account for the time-delayed warming of the oceanic subsurface compared to the surface air temperature. The median of the additional ice loss for the 21st century is computed to 0.07 m (66% range: 0.02–0.14 m; 90% range: 0.0–0.23 m) of global sea-level equivalent for the low-emission RCP-2.6 (Representative Concentration Pathway) scenario and 0.09 m (66% range: 0.04–0.21 m; 90% range: 0.01–0.37 m) for the strongest RCP-8.5. Assuming no time delay between the atmospheric warming and the oceanic subsurface, these values increase to 0.09 m (66% range: 0.04–0.17 m; 90% range: 0.02–0.25 m) for RCP-2.6 and 0.15 m (66% range: 0.07–0.28 m; 90% range: 0.04–0.43 m) for RCP-8.5. All probability distributions are highly skewed towards high values. The applied ice-sheet models are coarse resolution with limitations in the representation of grounding-line motion. Within the constraints of the applied methods, the uncertainty induced from different ice-sheet models is smaller than that induced by the external forcing to the ice sheets
HIFI spectroscopy of low-level water transitions in M82
We present observations of the rotational ortho-water ground transition, the
two lowest para-water transitions, and the ground transition of ionised
ortho-water in the archetypal starburst galaxy M82, performed with the HIFI
instrument on the Herschel Space Observatory. These observations are the first
detections of the para-H2O(111-000) (1113\,GHz) and ortho-H2O+(111-000)
(1115\,GHz) lines in an extragalactic source. All three water lines show
different spectral line profiles, underlining the need for high spectral
resolution in interpreting line formation processes. Using the line shape of
the para-H2O(111-000) and ortho-H2O+(111-000) absorption profile in conjunction
with high spatial resolution CO observations, we show that the (ionised) water
absorption arises from a ~2000 pc^2 region within the HIFI beam located about
~50 pc east of the dynamical centre of the galaxy. This region does not
coincide with any of the known line emission peaks that have been identified in
other molecular tracers, with the exception of HCO. Our data suggest that water
and ionised water within this region have high (up to 75%) area-covering
factors of the underlying continuum. This indicates that water is not
associated with small, dense cores within the ISM of M82 but arises from a more
widespread diffuse gas component.Comment: 5 pages, 4 figures. Accepted for publication in A&
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