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Brown, G. V.

Publications and source records attributed to Brown, G. V..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2010

High Performance Non-Dispersive X-Ray Spectrometers for Charge Exchange Measurements

Currently, the only measurements of cosmological charge exchange have been made using low resolution, non-dispersive spectrometers like the PSPC on ROSAT and the CCD instruments on Chandra and XMM/Newton. However, upcoming cryogenic spectrometers on Astro-H and IXO will add vast new capabilities to investigate charge exchange in local objects such as comets and planetary atmospheres. They may also allow us to observe charge exchange in extra-solar objects such as galactic supernova remnants. With low spectral resolution instruments such as CCDs, x-ray emission due to charge exchange recombination really only provides information on the acceptor species, such as the solar wind. With the new breed of x-ray calorimeter instruments, emission from charge exchange becomes highly diagnostic allowing one to uniquely determine the acceptor species, ionization state, donor species and ionization state, and the relative velocity of the interaction. We will describe x-ray calorimeter instrumentation and its potential for charge exchange measurements in the near term. We will also touch on the instrumentation behind a decade of high resolution measurements of charge exchange using an x-ray calorimeter at the Lawrence Livermore National Laboratory.

Porter Frederick↗
NASA NTRS2006

Laboratory Measurements of the X-ray Line Emission from Neon-like Fe XVII

We have conducted a systematic study of the dominant x-ray line emission from Fe XVII. These studies include relative line intensities in the optically thin limit, intensities in the presence of radiation from satellite lines from lower charge states of iron, and the absolute excitation cross sections of some of the strongest lines. These measurements were conducted at the Lawrence Livermore National Laboratory electron beam ion trap facility using crystal spectrometers and a NASA-Goddard Space Flight Center microcalorimeter array.

Brown, G. V.↗
NASA NTRS2006

Electron Impact Excitation Cross Section Measurement for n=3 to n=2 Line Emission in Fe(17+) to Fe(23+)

We have measured the electron impact excitation cross sections for the strong iron L-shell 3 --> 2 lines of Fe XVIII to Fe XXIV at the EBIT-I electron beam ion trap using a crystal spectrometer and NASA-Goddard Space Flight Centers 6 x 6 pixel array microcalorimeter. The cross sections were determined by direct normalization to the well established cross section of radiative electron capture through a sophisticated model analysis which results in the excitation cross section for the strong Fe L-shell lines at multiple electron energies. This measurement is part of a laboratory X-ray astrophysics program utilizing the Livermore electron beam ion traps EBIT-I and EBIT-II.

Chen, H.↗
NASA NTRS2006

Magnetic-Field Sensitive Line Ratios in EUV and Soft X-ray Spectra

We discovered a class of lines that are sensitive to the strength of the ambient magnetic field, and present a measurement of such a line in Ar IX near 49 A. Calculations show that the magnitude of field strengths that can be measured ranges from a few hundred gauss to several tens of kilogauss depending on the particular ion emitting the line.

Beiersdorfer, P.↗
NASA NTRS2005

Analysis of the XRS background

Background counts on the XRS Calorimeter spectrometer of Astro-E2 have several sources, including primary cosmic rays and secondary particles interacting with the pixels and with the silicon structure of the array. After rejecting events coincident between pixels or between a pixel and the anti-coincidence detector behind the calorimeter array, the residual background on the ground in the 0.1 - 10 keV band is 1e-3 counts/s (8e-3 counts/s/sq cm). We will present the details of the ground background events and the rejection criteria required lo remove them while minimizing deadtime. We will also present preliminary analysis of the in-orbit background.

Kilbourne, Caroline A.↗
NASA NTRS2004

Measurements of K Shell Emission of Highly Charged Ions with the XRS at EBIT

The XRS/EBIT is a 32 channel microcalorimeter spectrometer operating at the electron beam ion trap (EBIT) facility at Lawrence Livermore National Laboratory. The system contains a flight candidate detector array from the XRS instrument for the Astro-E2 mission. The detector array in the XRS/EBIT is functionally identical to the flight array integrated into the XRS instrument and benefits from the enormous amount of calibration data from the XRS program. Since the XRSEBIT was upgraded with the new detector array in October 2003, the system has been used for a number of experiments including a survey of the K shell emission from He-like and H-like O, Ne, Ar, Fe, Ni and Kr as well as for L shell emission to measure the 3C/3D line ratio in Ni. Here we present some basic operational parameters of the instrument as well as direct excitation and simulated maxwellian spectra of He-like and E-like Fe. We show that the XRS instrument can resolve the Lyal and Lya2 lines from each other as well as from the dielectronic recombination satellites in thermal H-like Fe. We also show exactly how well the XRS instrument on Astro-E2 will resolve the thermal He-like triplet in collisionally excited Fe. This work was funded under NASA's Research Opportunities in Space Science program.

Porter, F. Scott↗
NASA NTRS2003

Energy Scales in X-Ray Microcalorimeters

Microcalorimeter pulse shape characteristics, such as pulse height, decay time and rise time, are dependent on the detector temperature and bias as well as the photon energy and flux. We examine the nature of the temperature dependency by illuminating the ASTRO-E2 X-ray Spectrometer (XRS) microcalorimeter array with X-rays generated by electron impact on a range of foil targets. The resulting pulses are collected for a range of detector temperatures. We observe and model the temperature dependence of the pulse shape characteristics by fitting the data with non-linear pulse models. Our aim is to determine a robust method for correcting the energy scale obtained in ground calibration for slight differences in the operating conditions while in orbit.

Tillotson, W. A.↗
NASA NTRS2002

Laboratory Studies of the X-ray Emission Produced by the Interaction of Solar Wind Heavy Ions with Comets

The process of X-ray emission following charge exchange between solar wind heavy ions and cometary gases is studied in the laboratory. The emission is recorded with the spare ASTRO-E 6x6 microcalorimeter array. The microcalorimeter affords a resolution of better than 10 eV in the range of X-ray energies of interest arid thus individual emission lines can be resolved. Our present measurements focus on the most abundant K-shell heavy ions found in the solar wind. In particular, we measure the K-shell emission of bare C, N, O, and Ne, and their hydrogenlike counter parts interacting with such gases as CO2, N2, and CH4. Several results are noted that had not been considered in the early cometary X-ray models.

Beiersdorfer, P.↗
NASA NTRS2002

Laboratory Study of the Diagnostic Utility of the 3C/3D Line Ratio in Fe XVII

Fe XVII X-ray emission is present in a multitude of sources, such as the corona of the Sun, Capella, and Procyon. Two of the strongest lines observed in these spectra are the resonance and intercombination lines located at 15.01 and 15.26 A, respectively. As part of the laboratory astrophysics program at the electron beam ion traps EBIT-I & EBIT-II located at the Lawrence Livermore National Laboratory we have measured this line ratio for the case where the relative abundance of Fe XVI to Fe XVII is approx. 1. Our results show that an Fe XVI innershell satellite line coincides with the intercombination line and can significantly reduce the relative intensity, R, of the resonance to intercombination line. The fact that the apparent relative intensity of the resonance and intercombination line in Fe XVII is sensitive to the strength of an Fe XVI innershell satellite, and therefore, tho relative abundance of Fe XVI to Fe XVII, makes the line ratio a diagnostic of temperature, and explains the anomalously low ratios observed in the solar and stellar coronae.

Brown, G. V.↗
NASA NTRS2002

Laboratory Astrophysics at the LLNL Electron Beam Ion Traps: EBIT-I and EBIT-II

In order to provide a complete, accurate set of atomic data for interpreting spectra provided by missions such as XMM-Newton, the Chandra X-Ray Observatory, and Astro-E2, we have harnessed the Lawrence Livermore National Laboratory's electron beam ion traps EBIT-I. EBIT-II, and Super-EBIT for laboratory astrophysics. In support of this work we have developed a number of unique techniques, including the ability to experimentally simulate a Maxwellian distribution of electron energies and measuring low-energy charge exchange cross sections using the magnetic trapping mode. We have also built and operated a full suite of spectrometers spanning the 1-7000 Angstrom wavelength band, the most recent being a spectrometer based on a spare Astro-E (6 x 6) microcalorimeter array. Results of our efforts include a complete list of wavelengths of the Fe L-shell transitions, measurements of absolute and relative cross sections for direct impact, dielectronic, and resonance excitation, and measurements of low energy charge transfer reactions. A brief overview of the LLNL ebit facility, its capabilities, and some results will be discussed.

Brown, G. V.↗
NASA NTRS2000

Laboratory Data for X-Ray Astronomy

Laboratory facilities have made great strides in producing large sets of reliable data for X-ray astronomy, which include ionization and recombination cross sections needed for charge balance calculations as well as the atomic data needed for interpreting X-ray line formation. We discuss data from the new generation sources and pay special attention to the LLNL electron beam ion trap experiment, which is unique in its ability to provide direct laboratory access to spectral data under precisely controlled conditions that simulate those found in many astrophysical plasmas. Examples of spectral data obtained in the 1-160 A wavelength range are given illustrating the type of laboratory X-ray data produced in support of such missions as Chandra, X-Ray Multi-Mirror telescope (XMM), Advanced Satellite for Cosmology and Astrophysics (ASCA) and Extreme Ultraviolet Explorer Satellite (EUVE).

Beiersdorfer, P.↗
NASA NTRS2000

Emission Line Spectra from Low-Density Laboratory Plasmas

Using spectroscopic equipment optimized for laboratory astrophysics, we are performing systematic measurements of the line emission from astrophysically relevant ions in the wavelength band between 1 and 400 Angstroms important to X-ray missions such as Chandra, XMM, Astro-E, and EUVE. Obtained in a controlled laboratory setting at electron densities similar to those found in stellar coronae, the data are used to test spectral modeling codes for accuracy and completeness. Our effort includes the compilation of the iron L-shell emission lines from 6-18 Angstroms and the iron M-shell emission lines from 50-200 Angstroms. Many lines have been identified for the first time, and the fluxes from lines missing in the spectral modeling codes are assessed. Our measurements also assess the accuracy of line excitation calculations, including direct electron-impact excitation, dielectronic recombination, and resonance excitation. These measurements yield a calibration of specific diagnostic line ratios. Examples of our current measurements are given.

Beiersdorfer, P.↗
NASA NTRS1995

An active homopolar magnetic bearing with high temperature superconductor (HTS) coils and ferromagnetic cores

A proof-of-feasibility demonstration showed that high temperature superconductor (HTS) coils can be used in a high-load, active magnetic bearing in liquid nitrogen. A homopolar radial bearing with commercially wound HTS (Bi 2223) bias and control coils produced over 200 lb (890 N) radial load capacity (measured non-rotating) and supported a shaft to 14000 rpm. The goal was to show that HTS coils can operate stably with ferromagnetic cores in a feedback controlled system at a current density similar to that in Cu in liquid nitrogen. Design compromises permitted use of circular coils with rectangular cross section. Conductor improvements will eventually permit coil shape optimization, higher current density and higher bearing load capacity. The bias coil, wound with non-twisted, multifilament HTS conductor, required negligible power to carry its direct current. The control coils were wound with monofilament HTS sheathed in Ag. These dissipated negligible power for direct current (i.e. for steady radial load components). When an alternating current (AC) was added, the AC component dissipated power which increased rapidly with frequency and quadratically with AC amplitude. In fact at frequencies above about 2 hz, the effective resistance of the control coil conductor actually exceeds that of the silver which is in electrical parallel with the oxide superconductor. This is at least qualitatively understandable in the context of a Bean-type model of flux and current penetration into a Type II superconductor. Fortunately the dynamic currents required for bearing stability are of small amplitude. These results show that while twisted multifilament conductor is not needed for stable levitation, twisted multifilaments will be required to reduce control power for sizable dynamic loads, such as those due to unbalance.

Brown, G. V.↗
NASA NTRS1995

An active magnetic bearing with high T(sub c) superconducting coils and ferromagnetic cores

A proof-of-feasibility demonstration showed that high-T(sub c) superconductor (HTS) coils can be used in a high-load, active magnetic bearing in LN2. A homopolar radial bearing with commercially wound HTS (Bi 2223) bias and control coils produced over 890 N (200 lb) radial load capacity (measured non-rotatings) and supported a shaft to 14,000 rpm. The goal was to show that HTS coils can operate stably with ferromagnetic cores in a feedback controlled system at a current density similar to that for Cu in LN2. The bias coil, wound with non-twisted, multifilament HTS conductor, dissipated negligible power for its direct current. The control coils, wound with monofilament HTS sheathed in Ag, dissipated negligible power for direct current. AC losses increased rapidly with frequency and quadratically with AC amplitude. Above about 2 Hz, the effective resistance of the control coils exceeds that of the silver which is in electrical parallel with the oxide superconductor. These results show that twisted multifilament conductor is not needed for stable levitation but may be desired to reduce control power for sizable dynamic loads.

Brown, G. V.↗
NASA NTRS1992

Stiffness of magnetic bearings subjected to combined static and dynamic loads

This paper investigates the stiffness of a magnetic bearing that is subjected to the combined action of static and dynamic loads. Since their sum cannot exceed the saturation load, a large static load will imply that the bearing can carry only a small dynamic load. This smaller dynamic load together with the practical vibration amplitude define a practical upper bound to the dynamic stiffness. This paper also presents approximate design formulas and curves for this stiffness capacity as a function of the ratio of dynamic and static loads. In addition, it indicates that vibrations larger than a certain gap fraction can destabilize the magnetic bearing. This gap fraction, called the critical gap fraction, depends on the dynamic and static load ratio. For example, if the dynamic load is half of the static load, the use of more than 25 percent of gap can destabilize the bearing.

Rao, D. K.↗
NASA NTRS1991

Development of a vibration isolation prototype system for microgravity space experiments

The presence of small levels of low-frequency accelerations on the Space Shuttle orbiters has degraded the microgravity environment for the science community. Growing concern about this microgravity environment has generated interest in systems that can isolate microgravity science experiments from vibrations. This interest has resulted primarily in studies of isolation systems with active methods of compensation. The development of a magnetically suspended, six-degree-of-freedom active vibration isolation prototype system capable of providing the needed compensation to the orbital environment is presented. A design for the magnetic actuators is described, and the control law for the prototype system that gives a nonintrusive inertial isolation respone to the system is also described. Relative and inertial sensors are used to provide an inertial reference for isolating the payload.

Logsdon, K. A.↗