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{at}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{at}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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