Number 614, November 20, 2002
by Phil Schewe, James Riordon, and Ben Stein
Record-High Magnetic Fields in the Lab
Record-high magnetic fields in the lab, almost a Gigagauss in magnitude,
have been achieved by aiming intense laser light at a dense plasma,
expanding the possibilities for laboratory re-creations of astrophysical
events.
At last week's
APS Division of Plasma Physics Meeting in Orlando, researchers from
Imperial College, London, and the Rutherford Appleton Lab in the UK
announced evidence of super-strong magnetic fields that are hundreds
of times more intense than any previous magnetic field created in an
Earth laboratory and up to a billion times stronger than our planet's
natural magnetic field. Such intense magnetic fields may soon enable
researchers to recreate extreme astrophysical conditions, such as the
atmospheres of neutron stars and white dwarfs, in their very own laboratories.
At the Rutherford Appleton Laboratory near Oxford in the UK, researchers
at the VULCAN facility aimed intense laser pulses, lasting only picoseconds
(trillionths of a second), at a dense plasma. The resulting magnetic
fields in the plasma were on the order of 400 Megagauss.
To determine the magnitude of the fields, the researchers made polarization
measurements of high-frequency light emitted during the experiment.
Recent measurements presented at the APS/DPP conference suggested that
the peak magnetic field in the densest region of the plasma approaches
1 Gigagauss.
Due to technological advances peak laser intensities are likely to
increase still further and consequently even higher magnetic fields
may soon be possible, making it possible to put models of extreme astrophysical
conditions to the test. (Poster
CP1.125, November 11, contact Karl Krushelnick, Imperial College,
University of London, 011-44-20-7594-7635, kmkr@ic.ac.uk; for background
see Tatarakis et al., Nature,
17 January 2002)
Megagauss in Picoseconds
The item above describes the creation of high fields; this item describes
the rapid measurement of high fields. Physicists from the Tata Institute
and the Institute for Plasma Research in India have recorded in detail,
for the first time, the huge magnetic spike encountered by atoms in
a sample bearing the brunt of an intense laser shot.
Fields as great as 27 megagauss, roughly 50 million times the strength
of Earth's magnetic field, come about very quickly in the following
way: the 1016-watt/cm2 pump laser beam strikes
an aluminum target, the surface layer of atoms is quickly ionized, and
a stream of very fast electrons is released into the body of the target,
inducing the huge field.
Many high-power lasers around the world study the effects of intense
light upon a solid sample. The chief achievement of the Indian researchers
is to look at this process with unprecedented temporal precision, monitoring
the rising magnetic field in femtosecond intervals by watching the polarization
of a delayed secondary laser beam reflected from the particle plasma
engulfing the sample.
Femtosecond knowledge of megagauss fields might have a bearing on designs
for nuclear fusion reactions, and for studying other subjects where
high magnetic fields are important—NMR, Hall effect, and perhaps
even fast magnetic information storage and switching devices. (Sandhu
et al., Physical Review Letters, 25 November 2002;
contact G. Ravindra Kumar, Tata Institute, grk@tifr.res.in; 91-22-2152971
x 2381; www.tifr.res.in )
Nu Approach to CP Violation
The measured abundance of helium in the universe (about 25% of all
normal matter) suggests that there is about one proton for every 1010
photons. This in turn suggests that at some earlier phase of the universe
an almost equal number of protons and anti-protons existed and gradually
annihilated, but that because of some fundamental asymmetry (at the
level of one part per ten billion) in the way that the weak nuclear
force treats matter and antimatter, protons but not anti-protons survived
to the present time.
The standard model of particle physics usually enshrines this asymmetry
in the form of "CP violation," a mathematical convention concerning
the interaction of particles in which one imagines what happens when
the charge of all the particles is reversed (charge conjugation, abbreviated
as C) and the coordinates of all particles is reversed (the parity operation,
or P).The standard model is successful in predicting how CP violation
works out in the decay of K mesons or B mesons (see Update
600) but not so good at predicting where the abundance of baryons
(protons plus neutrons) comes from.
Now physicists at Hiroshima University, Niigata University (Japan)
and Seoul National University (Korea) have proposed an explanation in
which the proton excess comes (at least in part) from the decay of hypothetical
heavy neutrinos (in addition to the electron, muon, and tau neutrinos
already known). One testable prediction of this theory is that there
should be a slight preponderance of anti-neutrinos over neutrinos, a
disparity that could be studied in the next round of neutrino oscillation
experiments being planned. (Endoh
et al., Physical Review Letters, 2 December 2002;
contact Takuya Morozumi, Hiroshima University, morozumi@hiroshima-u.ac.jp,
81-824-24-7364.) (Also, see Frampton, Glashow and Yanagida, Phys. Lett. B548, 119, November 21, 2002.)
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