Text 673, 87 rader
Skriven 2005-12-10 09:20:31 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 755
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Physics News Update
Number 755, November 23, 2005
by Phil Schewe, Ben Stein, and Davide Castelvecchi

Optical Vortex - Trying to Look at Extrasolar Planets Directly

A new optical device might allow astronomers to view extrasolar planets
directly without the annoying glare of the parent star. It would do this by
"nulling" out the light of the parent star by exploiting its wave nature,
leaving the reflected light from the nearby planet to be observed in
space-based detectors.

About ten years ago, the presence of planets around stars other than our sun
was first deduced by the very tiny wobble in the star's spectrum of light
imposed by the mutual tug between the star and its satellite. Since then, more
than 100 extrasolar planets have been detected in this way. Also, in a few
cases the slight diminution in the star's radiation caused by the transit of
the planet across in front of the star has been observed. Many astronomers
would, however, like to view the planet directly, a difficult thing to do.

Seeing the planet next to its bright star has been compared to trying to
discern, from a hundred meters away, the light of a match held up next to the
glare of an automobile's headlight. The approach taken by Grover Swartzlander
and his colleagues at the University of Arizona is to eliminate the star's
light by sending it through a special helical-shaped mask, a sort of lens whose
geometry resembles that of a spiral staircase turned on its side.

The process works in the following way: light passing through the thicker and
central part of the mask is slowed down. Because of the graduated shape of the
glass, an "optical vortex" is created: the light coming along the axis of the
mask is, in effect, spun out of the image. It is nulled, as if an opaque mask
had been placed across the image of the star, but leaving the light from the
nearby planet unaffected.

The idea of an optical vortex has been around for many years, but it has never
been applied to astronomy before. In lab trials of the optical vortex mask,
light from mock stars has been reduced by factors of 100 to 1000, while light
from a nearby "planet" was unaffected (see figure).

Attaching their device to a telescope on Mt. Lemon outside Tucson, Arizona, the
researchers took pictures of Saturn and its nearby rings to demonstrate the
ease of integrating the mask into telescopic imaging system. This is, according
to Swartzlander (520-626-3723, grovers@optics.arizona.edu), a more practical
technique than merely attempting to cover the star's image, as is done in
coronagraphs, devices for observing our sun's corona by masking out the disk of
the sun. It could fully come into its own on a project like the Terrestrial
Planet Finder, or TPF, a proposed orbiting telescope to be developed over the
coming decade and designed to image exoplanets.

Foo et al., Optics Letters, 15 December 2005
Summary of articles related to optical vortex on Swartzlander's Web page

First Steps Toward Fusion at NIF

Laser pulses shot into a cavity can produce the conditions required to trigger
nuclear fusion reactions, scientists at Lawrence Livermore National Laboratory
in California report. The finding was a crucial test of principle for
Livermore's National Ignition Facility (NIF), the $3.5 billion machine now
under construction and expected to start full operations in 2009.

NIF will produce fusion reactions by focusing 192 powerful ultraviolet laser
beams through small holes into the hollow interior of a gold cavity called a
hohlraum. The laser light quickly heats up the cavity's inner walls, which
generate x rays, in a few nanosecond-long bursts of energy more than 60 billion
times as bright as the surface of the sun. The outer shell of a small capsule
containing frozen deuterium and tritium placed inside this mini-oven will be
heated by these x rays and rapidly expand, resulting in heating and compression
of its core (to 1000 times its initial density) which will become as dense as
the sun's center, triggering nuclear fusion.

During the first hohlraum experiments at NIF, a large team of physicists,
engineers and technicians (contact: Eduard Dewald, dewald3@llnl.gov,
925-422-7087) used the four existing NIF laser beams to prove NIF's x-ray
production capability. NIF was operating at just 1 percent of its full design
energy, and the cavity contained no fusion materials. However, the x-ray flux
inside the cavity---the amount of energy per unit area and per unit time---has
been shown to agree with expectations, and is similar to those required for
future fusion experiments.

Uncertainties over the continued funding of NIF seemed to be resolved in a
recent House-Senate conference agreement over the 2006 energy bill (see FYI No.
162, November 11).

Dewald et al., Physical Review Letters, 18 November 2005

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 * Origin: Big Bang (1:106/2000.7)