arXiv subjects

D. Mitchell

Publications and source records attributed to D. Mitchell.

16 recordsLinked to original sources

arXiv2025

Design and Performance of the SPT-SLIM Receiver Cryostat

The South Pole Telescope Shirokoff Line Intensity Mapper (SPT-SLIM) is a millimeter-wavelength line-intensity mapping experiment, which was deployed on the South Pole Telescope (SPT) during the 2024-2025 Austral summer season. This pathfinder experiment serves to demonstrate the on-sky operation of multi-pixel on-chip spectrometer technology. We report on the cryogenic performance of the SPT-SLIM receiver for the first year of commissioning observations. The SPT-SLIM receiver utilizes an Adiabatic Demagnetization Refrigerator (ADR) for cooling the focal plane of superconducting filterbank spectrometers to a temperature of 150 mK. We demonstrate stable thermal performance of the focal plane module during observations consistent with thermal modeling, enabling a cryogenic operating efficiency above 80%. We also report on the receiver control system design utilizing the Observatory Control System (OCS) platform for automated cryogenic operation on the SPT.

astro-ph.IM↗

arXiv2022

Interchange reconnection as the source of the fast solar wind within coronal holes

The fast solar wind that fills the heliosphere originates from deep within regions of open magnetic field on the Sun called coronal holes. The energy source responsible for accelerating the plasma to high speeds is widely debated, however there is evidence that it is ultimately magnetic in nature with candidate mechanisms including wave heating^(1,2) and interchange reconnection^(3,4,5). The coronal magnetic field near the solar surface is structured on scales associated with supergranulation convection cells, where descending flows create intense fields. The energy density in these network magnetic field bundles is a likely candidate as an energy source of the wind. Here we report measurements of fast solar wind streams from the Parker Solar Probe (PSP) spacecraft^6 which provides strong evidence for the interchange reconnection mechanism. We show that supergranulation structure at the coronal hole base remains imprinted in the near-Sun solar wind resulting in asymmetric patches of magnetic 'switchbacks'^(7,8) and bursty wind streams with power law-like energetic ion spectra to beyond 100 keV. Computer simulations of interchange reconnection support key features of the observations, including the ion spectra. Important characteristics of interchange reconnection in the low corona are inferred from the data including that the reconnection is collisionless and that the energy release rate is sufficient to power the fast wind. In this scenario, open magnetic flux undergoes continuous reconnection and the wind is driven both by the resulting plasma pressure and the radial Alfvenic flow bursts.

astro-ph.SR↗

arXiv2020

The magnetic structure of the subsolar MPB current layer from MAVEN observations: Implications for the Hall electric force

We report on the local structure of the Martian subsolar Magnetic Pileup Boundary (MPB) from minimum variance analysis of the magnetic field measured by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft for six orbits. In particular, we detect a well defined current layer within the MPB and provide a local estimate of its current density which results in a sunward Hall electric force. This force accounts for the deflection of the solar wind ions and the acceleration of electrons which carry the interplanetary magnetic field through the MPB into the Magnetic Pileup Region. We find that the thickness of the MPB current layer is of the order of both the upstream (magnetosheath) solar wind proton inertial length and convective gyroradius. This study provides a high resolution view of one of the components of the current system around Mars reported in recent works.

physics.space-ph↗

arXiv2019

The Near-Sun Dust Environment: Initial Observations from Parker Solar Probe

The Parker Solar Probe (PSP) spacecraft has flown into the most dense and previously unexplored region of our solar system's zodiacal cloud. While PSP does not have a dedicated dust detector, multiple instruments onboard are sensitive to the effects of meteoroid bombardment. Here, we discuss measurements taken during PSP's first two orbits and compare them to models of the zodiacal cloud's dust distribution. Comparing the radial impact rate trends and the timing and location of a dust impact to an energetic particle detector, we find the impactor population to be consistent with dust grains on hyperbolic orbits escaping the solar system. Assuming PSP's impact environment is dominated by hyperbolic impactors, the total quantity of dust ejected from our solar system is estimated to be 1-14 tons/s. We expect PSP will encounter an increasingly more intense impactor environment as its perihelion distance and semi-major axis are decreased.

physics.space-ph↗

arXiv2017

Installation Progress At The PIP-II Injector Test At Fermilab

A CW-compatible, pulsed H- superconducting linac "PIP-II" is being planned to upgrade Fermilab's injection complex. To validate the front-end concept, a test accelerator (The PIP-II Injector Test, formerly known as "PXIE") is under construction. The warm part of this accelerator comprises a 10 mA DC, 30 keV H- ion source, a 2 m-long Low Energy Beam Transport (LEBT), a 2.1 MeV Radio Frequency Quadrupole (RFQ) capable of operation in Continuous Wave (CW) mode, and a 10 m-long Medium Energy Beam Transport (MEBT). The paper will report on the installation of the RFQ and the first sections of the MEBT and related mechanical design considerations.

physics.acc-ph↗

arXiv2016

The Australian Square Kilometre Array Pathfinder: Performance of the Boolardy Engineering Test Array

We describe the performance of the Boolardy Engineering Test Array (BETA), the prototype for the Australian Square Kilometre Array Pathfinder telescope ASKAP. BETA is the first aperture synthesis radio telescope to use phased array feed technology, giving it the ability to electronically form up to nine dual-polarization beams. We report the methods developed for forming and measuring the beams, and the adaptations that have been made to the traditional calibration and imaging procedures in order to allow BETA to function as a multi-beam aperture synthesis telescope. We describe the commissioning of the instrument and present details of BETA's performance: sensitivity, beam characteristics, polarimetric properties and image quality. We summarise the astronomical science that it has produced and draw lessons from operating BETA that will be relevant to the commissioning and operation of the final ASKAP telescope.

astro-ph.IM↗

arXiv2015

Ionospheric modelling using GPS to calibrate the MWA. 1: Comparison of first order ionospheric effects between GPS models and MWA observations

We compare first order (refractive) ionospheric effects seen by the Murchison Widefield Array (MWA) with the ionosphere as inferred from Global Positioning System (GPS) data. The first order ionosphere manifests itself as a bulk position shift of the observed sources across an MWA field of view. These effects can be computed from global ionosphere maps provided by GPS analysis centres, namely the Center for Orbit Determination in Europe (CODE), using data from globally distributed GPS receivers. However, for the more accurate local ionosphere estimates required for precision radio astronomy applications, data from local GPS networks needs to be incorporated into ionospheric modelling. For GPS observations, the ionospheric parameters are biased by GPS receiver instrument delays, among other effects, also known as receiver Differential Code Biases (DCBs). The receiver DCBs need to be estimated for any non-CODE GPS station used for ionosphere modelling, a requirement for establishing dense GPS networks in arbitrary locations in the vicinity of the MWA. In this work, single GPS station-based ionospheric modelling is performed at a time resolution of 10 minutes. Also the receiver DCBs are estimated for selected Geoscience Australia (GA) GPS receivers, located at Murchison Radio Observatory (MRO1), Yarragadee (YAR3), Mount Magnet (MTMA) and Wiluna (WILU). The ionospheric gradients estimated from GPS are compared with the ionospheric gradients inferred from radio source position shifts observed with the MWA. The ionospheric gradients at all the GPS stations show a correlation with the gradients observed with the MWA. The ionosphere estimates obtained using GPS measurements show promise in terms of providing calibration information for the MWA.

astro-ph.IM↗

arXiv2014

Titan interaction with the supersonic solar wind

After 9 years in the Saturn system, the Cassini spacecraft finally observed Titan in the supersonic solar wind. These unique observations reveal that Titan interaction with the solar wind is in many ways similar to un-magnetized planets Mars and Venus in spite of the differences in the properties of the solar plasma in the outer solar system. In particular, Cassini detected a collisionless, supercritical bow shock and a well-defined induced magnetosphere filled with mass-loaded interplanetary magnetic field lines, which drape around Titan ionosphere. Although the flyby altitude may not allow the detection of an ionopause, Cassini reports enhancements of plasma density compatible with plasma clouds or streamers in the flanks of its induced magnetosphere or due to an expansion of the induced magnetosphere. Because of the upstream conditions, these observations are also relevant for unmagnetized bodies in the outer solar system such as Pluto, where kinetic processes are expected to dominate.

astro-ph.EP↗

arXiv2012

First Light for the First Station of the Long Wavelength Array

The first station of the Long Wavelength Array (LWA1) was completed in April 2011 and is currently performing observations resulting from its first call for proposals in addition to a continuing program of commissioning and characterization observations. The instrument consists of 258 dual-polarization dipoles, which are digitized and combined into beams. Four independently-steerable dual-polarization beams are available, each with two "tunings" of 16 MHz bandwidth that can be independently tuned to any frequency between 10 MHz and 88 MHz. The system equivalent flux density for zenith pointing is ~3 kJy and is approximately independent of frequency; this corresponds to a sensitivity of ~5 Jy/beam (5sigma, 1 s); making it one of the most sensitive meter-wavelength radio telescopes. LWA1 also has two "transient buffer" modes which allow coherent recording from all dipoles simultaneously, providing instantaneous all-sky field of view. LWA1 provides versatile and unique new capabilities for Galactic science, pulsar science, solar and planetary science, space weather, cosmology, and searches for astrophysical transients. Results from LWA1 will detect or tightly constrain the presence of hot Jupiters within 50 parsecs of Earth. LWA1 will provide excellent resolution in frequency and in time to examine phenomena such as solar bursts, and pulsars over a 4:1 frequency range that includes the poorly understood turnover and steep-spectrum regimes. Observations to date have proven LWA1's potential for pulsar observing, and just a few seconds with the completed 256-dipole LWA1 provide the most sensitive images of the sky at 23 MHz obtained yet. We are operating LWA1 as an open skies radio observatory, offering ~2000 beam-hours per year to the general community.

astro-ph.IM↗

arXiv2012

Recent advances in Ti and Nb explosion welding with stainless steel for 2K operating (ILC Program)- To the proceedings of LCWS11

The world first samples 0f Ti+SS and Nb+SS joints were manufactured by an explosion welding technology demonstrating a high mechanic properties and leak absence at 4.6 x 10^{-9} atm-cc/sec. Residual stresses in bimetallic joints resulting from explosion welding measured by neutron diffraction method are quite high (~1000 MPa). Thermal tempering of explosion welded Ti+SS and Nb+SS specimens leads to complete relaxation of internal stresses in Ti,Nb and Stainless steel and makes the transition elements quite serviceable.

physics.ins-det↗

arXiv2009

GPUs for data processing in the MWA

The MWA is a next-generation radio interferometer under construction in remote Western Australia. The data rate from the correlator makes storing the raw data infeasible, so the data must be processed in real-time. The processing task is of order ~10 TFLOPS. The remote location of the MWA limits the power that can be allocated to computing. We describe the design and implementation of elements of the MWA real-time data processing system which leverage the computing abilities of modern graphics processing units (GPUs). The matrix algebra and texture mapping capabilities of GPUs are well suited to the majority of tasks involved in real-time calibration and imaging. Considerable performance advantages over a conventional CPU-based reference implementation are obtained.

astro-ph.IM↗

arXiv2005

A dearth of planetary transits in the direction of NGC 6940

We present results of our survey for planetary transits in the field of NGC 6940. We think nearly all of our observed stars are field stars. We have obtained high precision (3-10 millimags at the bright end) photometric observations of 50,000 stars spanning 18 nights in an attempt to identify low amplitude and short period transit events. We have used a matched filter analysis to identify 14 stars that show multiple events, and four stars that show single transits. Of these 18 candidates, we have identified two that should be further researched. However, none of the candidates are convincing hot Jupiters.

astro-ph↗

arXiv2005

A Survey for Planetary Transits in the Field of NGC 7789

We present results from 30 nights of observations of the open cluster NGC 7789 with the WFC camera on the INT telescope in La Palma. From ~900 epochs, we obtained lightcurves and Sloan r-i colours for ~33000 stars, with ~2400 stars with better than 1% precision. We expected to detect ~2 transiting hot Jupiter planets if 1% of stars host such a companion and that a typical hot Jupiter radius is ~1.2RJ. We find 24 transit candidates, 14 of which we can assign a period. We rule out the transiting planet model for 21 of these candidates using various robust arguments. For 2 candidates we are unable to decide on their nature, although it seems most likely that they are eclipsing binaries as well. We have one candidate exhibiting a single eclipse for which we derive a radius of 1.81+0.09-0.00RJ. Three candidates remain that require follow-up observations in order to determine their nature.

astro-ph↗

arXiv2003

Searching for Planetary Transits in the Field of Open Cluster NGC 6819 - I

We present results from our survey for planetary transits in the field of the intermediate age (~2.5 Gyr), metal-rich ([Fe/H]~+0.07) open cluster NGC 6819. We have obtained high-precision time-series photometry for over 38,000 stars in this field and have developed an effective matched-filter algorithm to search for photometric transits. This algorithm identified 8 candidate stars showing multiple transit-like events, plus 3 stars with single eclipses. On closer inspection, while most are shown to be low mass stellar binaries, some of these events could be due to brown dwarf companions. The data for one of the single-transit candidates indicates a minimum radius for the companion similar to that of HD 209458b.

astro-ph↗

arXiv2002

University of St.Andrews Open Cluster Survey for Hot Jupiters

We are using the Isaac Newton Telescope Wide Field Camera to survey open cluster fields for transiting hot Jupiter planets. Clusters were selected on the basis of visibility, richness of stars, age and metallicity. Observations of NGC 6819, 6940 and 7789 began in 1999 and continued in 2000. We have developed an effective matched-filter transit-detection algorithm which has proved its ability to identify very low amplitude eclipse events in real data. Here we present our results for NGC 6819. We have identified 7 candidates showing transit-like events. Colour information suggests that most of the companion bodies are likely to be very-low-mass stars or brown dwarfs, intrinsically interesting objects in their own right.

astro-ph↗

arXiv1996

Topological Dependence of Universal Correlations in Multi-Parameter Hamiltonians

Universality of correlation functions obtained in parametric random matrix theory is explored in a multi-parameter formalism, through the introduction of a diffusion matrix $D_{ij}(R)$, and compared to results from a multi-parameter chaotic model. We show that certain universal correlation functions in 1-d are no longer well defined by the metric distance between the points in parameter space, due to a global topological dependence on the path taken. By computing the density of diabolical points, which is found to increases quadratically with the dimension of the space, we find a universal measure of the density of diabolical points in chaotic systems.

chao-dyn↗