[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 225-245A, says the investigative

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

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).