Institute acronyms: Naval
Research Laboratory
(NRL), Lawrence Livermore National Laboratory
(LLNL),
University of Leicester (UL), Mullard Space Science Laboratory
(MSSL)
The
Joint Astrophysical Plasmadynamic
Experiment (J-PEX) launched
successfully on a NASA sounding rocket
from White Sands Missile
Range, New Mexico, on February 21. A
collaborative effort
between the Naval Research Laboratory and
Lawrence Livermore
National Laboratory in the U.S., and the University
of
Leicester and Mullard Space Science Laboratory in the U.K.,
the
J-PEX objective is to produce the first high-resolution spectrum
of a white dwarf star at extreme ultraviolet (EUV)
wavelengths.
White
dwarfs are important, notes
the scientific team, because they
are the end product of evolution
for most stars in our galaxy.
(For example, our sun will become
a white dwarf in about 5
billion years.) White dwarfs are very
dense objects, with the
mass of the sun squeezed into a volume
typically the size of
the Earth. During the course of its evolution
into a white
dwarf, a star will shed some of its mass into the
interstellar
medium, seeding it with helium and other heavy elements.
Since
clouds of interstellar matter eventually collapse to form
new
stars and planetary systems, understanding white dwarf evolution
impacts directly on our knowledge of the galaxy and ultimately
the universe.
Previous studies of the chemical
composition of
white dwarf stars have shown that they fall into
two distinct
categories. One type has evidence for only hydrogen
in its
atmosphere, and the second type is rich in helium. The
J-PEX
target was the white dwarf G191-B2B, a member of the latter
category.
However,
earlier observations
of the target at visible and
far-ultraviolet wavelengths have
produced only upper limits to
the amount of helium, and measurements
by NASA's Extreme
Ultraviolet Explorer satellite do not have
enough spectral
resolution to separate and identify the helium
lines from those
of heavier elements, such as iron. J-PEX is
the first
instrument with enough sensitivity and resolution to
make such
observations in the EUV wavelength range 225-245Å,
says
the investigative team.
Secondarily, the mission served
as a testbed for
technical innovations. The primary J-PEX instrument
is a
high-resolution spectrometer, the design heritage going
back to
the NRL S-082A instrument flown on Skylab in 1973, but
with critical
improvements. The new spectrometer consists of
four identical
spherical diffraction gratings that collect light
from the star
and focus wavelength-dispersed images onto the
detector. Each
grating forms a separate image, and these spectra
will be added
together during analysis. The gratings were produced
using a
special technique, unavailable at the time of Skylab.
This
technique, which involves holography and ion-etching, results
in superior quality groove profiles and ultra-smooth surfaces.
However, note the scientists, the high-quality gratings would
be useless without high-reflectance multilayer coatings. The
coatings, which were developed at NRL and LLNL, consist of alternate
layers of molybdenum and silicon, and enhance grating efficiency
by a factor of a hundred.
The NRL AMCORS group (Application
of Multilayer
Coated Optics to Remote Sensing) supports the research
and
development of multilayer gratings, with the J-PEX gratings
being the finest examples to date. Observing times in a sounding
rocket flight are typically limited to only 300 seconds above
the atmosphere, and therefore high efficiency is necessary to
obtain sufficient counts in the spectrum for the desired scientific
result. J-PEX grating efficiencies were calibrated at the NRL
beamline X24C at the National Synchrotron Light Source, Brookhaven
National Laboratory, and resulted in the highest values yet
published
at EUV wavelengths.
High spectral resolution also
places
strong demands on detector spatial resolution and efficiency
and on instrument pointing. The Skylab instrument detector was
photographic film of limited sensitivity. J-PEX uses a
photon-counting
microchannel plate (MCP) detector, which has a
high-efficiency
Cesium-Iodide photocathode and a
state-of-the-art vernier anode
of high spatial resolution. In
addition, a totally new attitude
control system (ACS) was flown
on this mission to provide ultra-low
thrust levels for payload
station-keeping on target, gyros of
low drift rate, and a
digital control loop. The tiny residual
motions from the ACS
were tracked by imaging the target star
field with a co-aligned
optical telescope that included a CCD-readout.
The CCD was a
spare unit developed by the LLNL for the Lunar
Imaging Star
Tracker onboard the NRL Clementine spacecraft. The
ACS residual
motions will be separated from the spectrometer
detector data
to obtain the highest possible spectral resolving
power.
The J-PEX
project was managed
in NRL's X-ray Astronomy Branch by a team
that includes: Dr.
Raymond G. Cruddace (Principal
Investigator), Dr. Michael P.
Kowalski (Project Scientist and
Principal Investigator on AMCORS),
Dr. Daryl J. Yentis (Data
Processing and Analysis), Mr. Gilbert
G. Fritz (Management and
Technical Oversight), Mr. William R
Hunter, (SFA, Inc.; Optical
Design), Mr. Don Woods (DBW Enterprises;
Mechanical Design),
Mr. Greg Clifford (SEI; Electrical Design)
and Dr. Herbert
Gursky. In keeping with the branch's philosophy
of training the
next generation of scientists and engineers,
the team also
included co-op and SEAP students Cara Golembiewski,
Jason
Thrasher, Naim Darghouth, and Steven Titus .
Team members from NRL's Solar
Physics Branch assisted in spectrometer calibration: Dr. Dennis
G. Socker, Mr. Randy S. Waymire, and Mr. Ed Shepler, all of NRL;
and Mr. Don Lilly, Mr. Robert Moye, Mr. Don Robertson, and Mr.
Richard Rogers, all of ARTEP. Also from NRL's Solar Terrestrial
Relationships Branch, Dr. Charlie Brown, Dr. Uri Feldman and
Dr. John Seely participated in developing the instrument concept
and the development of the gratings.
LLNL team members include Dr.
Troy W.
Barbee, Jr. (multilayer coatings), Dr. William H. Goldstein
and
Mr. Joseph F. Kordas (CCD camera). Team members at UL include
Dr. Martin A. Barstow, Dr. George W. Fraser, and Mr. Nigel P.
Bannister (MCP detector and Data Analysis) and team members at
MSSL include Dr. Jon Lapington, Mr. Jason Tandy, and Mr. Ben
Sanderson (MCP detector and Data Analysis).
The U.S. Naval Research Laboratory is the Navy's full-spectrum corporate laboratory, conducting a broadly based multidisciplinary program of scientific research and advanced technological development. The Laboratory, with a total complement of nearly 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 85 years and continues to meet the complex technological challenges of today's world. For more information, visit the NRL homepage or join the conversation on Twitter, Facebook, and YouTube.
Comment policy: We hope to receive submissions from all viewpoints, but we ask that all participants agree to the Department of Defense Social Media User Agreement. All comments are reviewed before being posted.