2,473 research outputs found
High power operation of an X-band gyrotwistron
We report the first experimental verification of a gyrotwistron amplifier. The device utilized a single 9.858 GHz, TE011 cavity, a heavily attenuated drift tube, and a long tapered output waveguide section. With a 440 kV, 200-245 A, 1 ÎĽs electron beam and a sharply tapered axial magnetic field, peak powers above 21 MW were achieved with a gain near 24 dB. Performance was limited by competition from a fundamental TE11 mode. A multimode code was developed to analyze this system, and simulations were in good agreement with the experiment
A variational principle for stationary, axisymmetric solutions of Einstein's equations
Stationary, axisymmetric, vacuum, solutions of Einstein's equations are
obtained as critical points of the total mass among all axisymmetric and
symmetric initial data with fixed angular momentum. In this
variational principle the mass is written as a positive definite integral over
a spacelike hypersurface. It is also proved that if absolute minimum exists
then it is equal to the absolute minimum of the mass among all maximal,
axisymmetric, vacuum, initial data with fixed angular momentum. Arguments are
given to support the conjecture that this minimum exists and is the extreme
Kerr initial data.Comment: 21 page
Expansion Tube Investigation of Shock Stand-Off Distances in High-Enthalpy CO_2 Flow Over Blunt Bodies
The shock standoff distance in front of a blunt body is sensitive to the thermochemical
state of the free stream. Recently, experimental and numerical studies
have reported significantly different bow shock profiles in high-enthalpy carbon
dioxide flows, a discrepancy that may result from non-equilibrium processes during
flow acceleration in ground-based facilities. In this work, an expansion tube is used
to create a Mach 5.7 carbon dioxide flow, matching the stagnation enthalpy and
the velocity of previous studies. Images of shock layers are obtained for spherical
geometries and a scaled model of the Mars Science Lander. Different sphere
diameters are used in order to access non-equilibrium and equilibrium stagnation
line shock profiles predicted by theory. Mars Science Lander profiles at zero angle
of attack are in good agreement with available data from the LENS X expansion
tunnel facility, confirming results are facility-independent for the same type of flow
acceleration, and indicating the flow velocity is a suitable first-order matching parameter
for comparative testing. Heat transfer measurements on the Mars Science
Lander are also presented for the three different angle of attacks, and the results
are consistent with previous studies. Initial results from a proposed organo-metallic
based emission spectroscopy technique for bow shock layer interrogation are also
presented
Evaluation of Hypervelocity Carbon Dioxide Blunt Body Experiments in an Expansion Tube Facility
This work represents efforts to study high-enthalpy carbon dioxide flows in anticipation
of the upcoming Mars Science Laboratory (MSL) and future missions. The
current study extends the previous presentation of experimental results by the comparison
now with axisymmetric simulations incorporating detailed thermochemical
modeling. The work is motivated by observed anomalies between experimental
and numerical studies in hypervelocity impulse facilities. In this work, experiments
are conducted in the Hypervelocity Expansion Tube (HET) which, by virtue of its
flow acceleration process, exhibits minimal freestream dissociation in comparison
to reflected shock tunnels. This simplifies the comparison with computational result
as freestream dissociation and considerable thermochemical excitation can be
neglected. Shock shapes of the Laboratory aeroshell and spherical geometries are
compared with numerical simulations. In an effort to address surface chemistry
issues arising from high-enthalpy carbon dioxide ground-test based experiments,
spherical stagnation point and aeroshell heat transfer distributions are also compared
with simulation. The shock stand-off distance has been identified in the
past as sensitive to the thermochemical state and as such, is used here as an experimental
measureable for comparison with CFD and two different theoretical
models. For low-density, small-scale experiments it is seen that models based upon
assumptions of large binary scaling values are unable to match the experimental
and numerical results. Very good agreement between experiment and CFD is seen
for all shock shapes and heat transfer distributions fall within the non-catalytic and
super-catalytic solutions
Experimental and Numerical Investigation of Hypervelocity Carbon Dioxide Flow over Blunt Bodies
This paper represents ongoing efforts to study high-enthalpy carbon dioxide flows in anticipation of the upcoming
Mars Science Laboratory and future missions. The work is motivated by observed anomalies between experimental
and numerical studies in hypervelocity impulse facilities. In this study, experiments are conducted in the
hypervelocity expansion tube that, by virtue of its flow acceleration process, exhibits minimal freestream dissociation
in comparison with reflected shock tunnels, simplifying comparison with simulations. Shock shapes of the laboratory
aeroshell at angles of attack of 0, 11, and 16 deg and spherical geometries are in very good agreement with simulations
incorporating detailed thermochemical modeling. Laboratory shock shapes at a 0 deg of attack are also in good
agreement with data from the LENS X expansion tunnel facility, confirming results are facility-independent for the
same type of flow acceleration. The shock standoff distance is sensitive to the thermochemical state and is used as an
experimental measurable for comparison with simulations and two different theoretical models. For low-density
small-scale experiments, it is seen that models based upon assumptions of large binary scaling values do not match the
experimental and numerical results. In an effort to address surface chemistry issues arising in high-enthalpy groundtest
experiments, spherical stagnation point and aeroshell heat transfer distributions are also compared with the
simulation. Heat transfer distributions over the aeroshell at the three angles of attack are in reasonable agreement
with simulations, and the data fall within the noncatalytic and supercatalytic solutions
Molecular hydrogen jets and outflows in the Serpens south filamentary cloud
We aimed to map the jets and outflows from the Serpens South star forming
region and find an empirical relationship between the magnetic field and
outflow orientation. Near-infrared H2 v=1-0 S(1) 2.122{\mu}m -line imaging of
the \sim 30'-long filamentary shaped Serpens South star forming region was
carried out. K s broadband imaging of the same region was used for continuum
subraction. Candidate driving sources of the mapped jets/outflows are
identified from the list of known protostars and young stars in this region,
which was derived from studies using recent Spitzer and Herschel telescope
observations. 14 Molecular Hydrogen emission-line objects(MHOs) are identified
using our continuum-subtracted images. They are found to constitute ten
individual flows. Out of these, nine flows are located in the
lower-half(southern) part of the Serpens South filament, and one flow is
located at the northern tip of the filament. Four flows are driven by
well-identified Class 0 protostars, while the remaining six flows are driven by
candidate protostars mostly in the Class I stage, based on the Spitzer and
Herschel observations. The orientation of the outflows is systematically
perpendicular to the direction of the near-infrared polarization vector,
recently published in the literature. No significant correlation was observed
between the orientation of the flows and the axis of the filamentary cloud.Comment: Accepted by A&A for publication. 7 pages, 5 figure
Pre-main sequence stars with disks in the Eagle Nebula observed in scattered light
NGC6611 and its parental cloud, the Eagle Nebula (M16), are well-studied
star-forming regions, thanks to their large content of both OB stars and stars
with disks and the observed ongoing star formation. We identified 834
disk-bearing stars associated with the cloud, after detecting their excesses in
NIR bands from J band to 8.0 micron. In this paper, we study in detail the
nature of a subsample of disk-bearing stars that show peculiar characteristics.
They appear older than the other members in the V vs. V-I diagram, and/or they
have one or more IRAC colors at pure photospheric values, despite showing NIR
excesses, when optical and infrared colors are compared. We confirm the
membership of these stars to M16 by a spectroscopic analysis. The physical
properties of these stars with disks are studied by comparing their spectral
energy distributions (SEDs) with the SEDs predicted by models of T-Tauri stars
with disks and envelopes. We show that the age of these stars estimated from
the V vs. V-I diagram is unreliable since their V-I colors are altered by the
light scattered by the disk into the line of sight. Only in a few cases their
SEDs are compatible with models with excesses in V band caused by optical
veiling. Candidate members with disks and photospheric IRAC colors are selected
by the used NIR disk diagnostic, which is sensitive to moderate excesses, such
as those produced by disks with low masses. In 1/3 of these cases, scattering
of stellar flux by the disks can also be invoked. The photospheric light
scattered by the disk grains into the line of sight can affect the derivation
of physical parameters of ClassII stars from photometric optical and NIR data.
Besides, the disks diagnostic we defined are useful for selecting stars with
disks, even those with moderate excesses or whose optical colors are altered by
veiling or photospheric scattered light.Comment: Accepted for publication in A&
Infrared Variability of Evolved Protoplanetary Disks: Evidence for Scale Height Variations in the Inner Disk
We present the results of a multi-wavelength multi-epoch survey of five
evolved protoplanetary disks in the IC 348 cluster that show significant
infrared variability. Using 3-8micron and 24micron photometry along with
5-40micron spectroscopy from the Spitzer Space Telescope, as well as
ground-based 0.8-5micron spectroscopy, optical spectroscopy and near-infrared
photometry, covering timescales of days to years, we examine the variability in
the disk, stellar and accretion flux. We find substantial variations (10-60%)
at all infrared wavelengths on timescales of weeks to months for all of these
young stellar objects. This behavior is not unique when compared to other
cluster members and is consistent with changes in the structure of the inner
disk, most likely scale height fluctuations on a dynamical timescale. Previous
observations, along with our near-infrared photometry, indicate that the
stellar fluxes are relatively constant; stellar variability does not appear to
drive the large changes in the infrared fluxes. Based on our near-infrared
spectroscopy of the Pa-beta and Br-gamma lines we find that the accretion rates
are variable in most of the evolved disks but the overall rates are probably
too small to cause the infrared variability. We discuss other possible physical
causes for the variability, including the influence of a companion, magnetic
fields threading the disk, and X-ray flares.Comment: Accepted to ApJ. 33 pages, emulate apj forma
- …