Hi Jaro,
I don’t know how a 4000x increase in
power happens in a star, but that’s simply not going to happen in ITER. Here’s
a reference for common fusion reaction rates, including DT which will be used
in ITER:
http://w3.pppl.gov/~dcoster/nrl/45.html
ITER’s line averaged plasma
temperature will be in the 10 keV range, meaning doubling the temperature to 20
keV would raise the fusion reaction rate by ~4X.
You’re absolutely right, though - the
first wall of ITER couldn’t handle the increase in heat deposition this
much extra power would create. Within <10 ms, the extra power would ablate
surface layers of plasma-facing materials (mainly carbon, but possibly also beryllium
and tungsten), instantly cooling the plasma with impurities, poisoning the
fusion process, and shutting down the reactor.
This is actually expected to occur
however, not by sudden temperature increases but by disruptions, turbulence and
other MHD processes (processes that current machines are focused on solving). There
is quite a bit of margin built into the divertor especially in order to keep
the wall intact through 100’s-1000’s of these events. Finally, the
current 20 year operating schedule allows for 3 replacements of the entire first
wall to provide the opportunity to upgrade and improve the design/materials.
Lots more info / data available online in
the physics basis for ITER at:
www.iter.org/physics_text.htm
Adam
From:
cdn-nucl-l-admin@mailman1.cis.McMaster.CA
[mailto:cdn-nucl-l-admin@mailman1.cis.McMaster.CA] On Behalf Of Jaro
Sent: Wednesday, July 27, 2005
9:39 PM
To: multiple cdn
Subject: [cdn-nucl-l] fusion
instability
Quote: "deuterium
fusion is exquisitely sensitive to a star's temperature. The amount of energy
generated is proportional to the twelfth
power of a star's temperature, so doubling the temperature boosts the energy
output over 4,000 times."
.....it could get quite interesting if
ITER is ever cranked up, and a slight temperature change sends its 500MW
thermal power shooting up a few dozen times. Since most of the power
output is in the form of fast neutrons, the structures surrounding the plasma
torus should get a nice roasting.... (hope its all made to be quickly
replaceable !).
Jaro
^^^^^^^^^^^^^^^^^^^^^
The least massive stars may resemble Cepheids.
Brown dwarfs should pulsate
when they're young, predict astronomers in Italy
and France.
Such pulsations may already have been detected in Orion.
Brown dwarfs are born with too little mass — less than 8 percent of the
Sun's — to sustain the fusion of hydrogen-1, the nuclear reaction
powering main sequence stars like the Sun. Brown dwarfs still shine, though,
chiefly by transforming gravitational energy into heat. When young, they also
generate energy by converting deuterium into helium-3. Deuterium is
hydrogen-2, the rare heavy isotope of hydrogen.
Deuterium fusion should cause brown dwarfs and very-low-mass red dwarfs to
pulsate, say Francesco Palla of the Arcetri Astrophysical Observatory in Italy and Isabelle Baraffe at the Astronomical
Research Center of Lyon in France.
They will present their idea in a future issue of the journal Astronomy and Astrophysics.
Deuterium burns at a cooler temperature than ordinary hydrogen, which
explains why even brown dwarfs can ignite it. But deuterium fusion is
exquisitely sensitive to a star's temperature. The amount of energy generated
is proportional to the twelfth
power of a star's temperature, so doubling the temperature boosts the energy
output over 4,000 times.
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It's just this temperature sensitivity that should
induce pulsations in stars under 0.10 solar mass — brown dwarfs and
very-low-mass red dwarfs. A small temperature fluctuation can trigger rhythmic
expansions and contractions in such a star. A small temperature rise
increases the energy output; this makes the star expand and cool. The cooling
reduces the energy output, so the star shrinks and heats up. The cycle then
repeats.
The best known pulsating stars are Cepheids, yellow supergiants that expand
and contract every few days, weeks, or months, making their light wax and
wane with the same period. The bigger and brighter a Cepheid, the longer it
takes to pulsate — so merely measuring a Cepheid's period reveals the
star's intrinsic brightness and, hence, the distance to its parent galaxy.
Brown dwarfs should pulsate faster than Cepheids. Still, like Cepheids, the
more massive the star, the longer its period should be. Palla and Baraffe
calculate that a 0.02-solar-mass brown dwarf pulsates once an hour, whereas a
0.10-solar-mass red dwarf takes 5 hours to do the same.
The stars should pulsate only when young. That's because deuterium is so rare
— accounting for only one of every 50,000 hydrogen nuclei — that
the stars quickly exhaust their supply. A 0.02-solar-mass brown dwarf
consumes its deuterium in just 20 million years, while a 0.10-solar-mass red
dwarf does the same in just 2.5 million years. Thus, observers who seek
pulsating brown dwarfs should examine groups of young stars, such as those in
the constellations of Orion, Taurus, and Chamaeleon.
Intriguingly, say Palla and Baraffe, such pulsations may have been seen in
the Sigma (ó) Orionis cluster, just west of the Horsehead Nebula. The light
of two stars in the 3-million-year-old cluster varies every 3 hours. Another
star in the cluster has a period around 2 hours, and another varies every 46
minutes. Furthermore, the last star, named Sigma Orionis 45, has an estimated
mass of only about 0.02 solar mass.
These light variations had been attributed to starspots and stellar rotation.
But Palla and Baraffe say they may be the mark of pulsation.
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