32 research outputs found
Optimization study of high power static inverters and converters Final report
Optimization study and basic performance characteristics for conceptual designs for high power static inverter
A Kinematic, Flexure-based Mechanism for Precise, Parallel Motion for the Hertz Variable-delay Polarization Modulator (VPM)
We describe the design of the linear motion stage for a Variable-delay Polarization Modulator (VPM) and of a grid flattener that has been built and integrated into the Hertz ground-based, submillimeter polarimeter. VPMs allow the modulation of a polarized source by controlling the phase difference between two linear, orthogonal polarizations. The size of the gap between a mirror and a very flat polarizing grid determines the amount of the phase difference. This gap must be parallel to better than 1% of the wavelength. A novel, kinematic, flexure-based mechanism is described that passively maintains the parallelism of the mirror and the grid to 1.5 pm over a 150 mm diameter, with a 400 pm throw. A single piezoceramic actuator is used to modulate the gap, and a capacitive sensor provides position feedback for closed-loop control. A simple device that ensures the planarity of the polarizing grid is also described. Engineering results from the deployment of this device in the Hertz instrument April 2006 at the Submillimeter Telescope Observatory (SMTO) in Arizona are presented
Properties of a Variable-delay Polarization Modulator
We investigate the polarization modulation properties of a variable-delay
polarization modulator (VPM). The VPM modulates polarization via a variable
separation between a polarizing grid and a parallel mirror. We find that in the
limit where the wavelength is much larger than the diameter of the metal wires
that comprise the grid, the phase delay derived from the geometric separation
between the mirror and the grid is sufficient to characterize the device.
However, outside of this range, additional parameters describing the polarizing
grid geometry must be included to fully characterize the modulator response. In
this paper, we report test results of a VPM at wavelengths of 350 microns and 3
mm. Electromagnetic simulations of wire grid polarizers were performed and are
summarized using a simple circuit model that incorporates the loss and
polarization properties of the device.Comment: 25 pages, 10 figures, accepted by Applied Optic
New Results on the Submillimeter Polarization Spectrum of the Orion Molecular Cloud
We have used the SHARP polarimeter at the Caltech Submillimeter Observatory
to map the polarization at wavelengths of 350 and 450 micron in a ~2 x 3 arcmin
region of the Orion Molecular Cloud. The map covers the brightest region of the
OMC-1 ridge including the Kleinmann-Low (KL) nebula and the submillimeter
source Orion-south. The ratio of 450-to-350 micron polarization is ~ 1.3 +/-
0.3 in the outer parts of the cloud and drops by a factor of 2 towards KL. The
outer cloud ratio is consistent with measurements in other clouds at similar
wavelengths and confirms previous measurements placing the minimum of the
polarization ratio in dusty molecular clouds at a wavelength ~ 350 micron.Comment: 7 pages, 6 figures, submitted to ApJ Let
Dust in Comet C/2007 N3 (Lulin)
We report optical imaging, optical and near-infrared polarimetry, and Spitzer
mid-infrared spectroscopy of comet C/2007 N3 (Lulin). Polarimetric observations
were obtained in R (0.676 micron) at phase angles from 0.44 degrees to 21
degrees with simultaneous observations in H (1.65 micron) at 4.0 degrees,
exploring the negative branch in polarization. Comet C/2007 N3 (Lulin) shows
typical negative polarization in the optical as well as a similar negative
branch near-infrared wavelengths. The 10 micron silicate feature is only weakly
in emission and according to our thermal models, is consistent with emission
from a mixture of silicate and carbon material. We argue that large,
low-porosity (akin to Ballistic Particle Cluster Aggregates) rather absorbing
aggregate dust particles best explain both the polarimetric and the
mid-infrared spectral energy distribution.Comment: 18 pages, 9 figures, 3 table
Stringent Limits on the Polarized Submillimeter Emission from Protoplanetary Disks
We present arcsecond-resolution Submillimeter Array (SMA) polarimetric
observations of the 880 um continuum emission from the protoplanetary disks
around two nearby stars, HD 163296 and TW Hydrae. Although previous
observations and theoretical work have suggested that a 2-3% polarization
fraction should be common for the millimeter continuum emission from such
disks, we detect no polarized continuum emission above a 3-sigma upper limit of
7 mJy in each arcsecond-scale beam, or <1% in integrated continuum emission. We
compare the SMA upper limits with the predictions from the exploratory Cho &
Lazarian (2007) model of polarized emission from T Tauri disks threaded by
toroidal magnetic fields, and rule out their fiducial model at the ~10-sigma
level. We explore some potential causes for this discrepancy, focusing on model
parameters that describe the shape, magnetic field alignment, and size
distribution of grains in the disk. We also investigate related effects like
the magnetic field strength and geometry, scattering off of large grains, and
the efficiency of grain alignment, including recent advances in grain alignment
theory, which are not considered in the fiducial model. We discuss the impact
each parameter would have on the data and determine that the suppression of
polarized emission plausibly arises from rounding of large grains, reduced
efficiency of grain alignment with the magnetic field, and/or some degree of
magnetic field tangling (perhaps due to turbulence). A poloidal magnetic field
geometry could also reduce the polarization signal, particularly for a face-on
viewing geometry like the TW Hya disk. The data provided here offer the most
stringent limits to date on the polarized millimeter-wavelength emission from
disks around young stars.Comment: 15 pages, 6 figures, accepted for publication in Ap
The Hertz/VPM polarimeter: Design and first light observations
We present first results of Hertz/VPM, the first submillimeter polarimeter
employing the dual Variable-delay Polarization Modulator (dual-VPM). This
device differs from previously used polarization modulators in that it operates
in translation rather than mechanical rotation. We discuss the basic theory
behind this device, and its potential advantages over the commonly used half
wave plate (HWP). The dual-VPM was tested both at the Submillimeter Telescope
Observatory (SMTO) and in the lab. In each case we present a detailed
description of the setup. We discovered nonideal behavior in the system. This
is at least in part due to properties of the VPM wire grids (diameter, spacing)
employed in our experiment. Despite this, we found that the dual-VPM system is
robust, operating with high efficiency and low instrumental polarization. This
device is well suited for air and space-borne applications.Comment: 31 pages, 11 figures, 2 table