Number 662, November 18, 2003
by Phillip F. Schewe, Ben Stein, and James Riordon
A Liquid Wall in a Fusion Energy Device
A liquid wall in a fusion energy device has improved the performance
of fusion fuel, and may lead to more resilient fusion devices. At last
month's American Physical Society Division of Plasma Physics meeting
in Albuquerque, Princeton Plasma Physics Laboratory researchers (Dick
Majeski, PPPL, 609-243-3112 and Bob
Kaita, PPPL, 609-243-3275) described how they tested this idea on
the Current Drive Experiment-Upgrade (CDX-U). CDX-U is a spherical torus,
a more rotund version of the well-known tokamak. Like the donut-shaped
tokamak, the device uses magnetic fields to confine hot plasma. At the
bottom of their tubby tokamak, the researchers placed a pool of liquid
lithium. Meanwhile, they applied electrical current to the plasma both
to create strong magnetic fields that confine it and also to heat the
plasma to desired hot temperatures. In contact with the fusion plasma,
the liquid lithium increased the efficiency of transferring the current
to the plasma, leading to less wasted energy. It also does an excellent
job of absorbing impurities, such as carbon and oxygen, which could
otherwise cool the plasma. What's more, it absorbs hydrogen plasma that
reaches it, requiring the researchers continually to pump in hydrogen
gas. This is actually a good thing, as it prevents an undesirable buildup
of cool hydrogen at the plasma boundary which could return to the plasma
and lower its temperature. Finally, since the liquid surface can be
continually replenished, the liquid wall is not subject to the same
degradation and damage that would occur by neutrons that bombard a solid
metal wall. The liquid wall can conceivably be applied to future magnetic
fusion reactors, whether a spherical torus, a tokamak, or another design.
(Meeting paper RI1.004; see
picture; also see R.
Majeski et al., Journal of Nuclear Materials, March
2003.)
Electron Spins Can Control Nuclear Spins
Electron spins can control nuclear spins in a semiconductor when trapped
in a very confined space, a recent experimental development which calls
upon laser science, solid-state physics, and nuclear magnetic resonance.
David Awschalom and his colleagues at the Center for Spintronics and
Quantum Computation at UC Santa Barbara begin by lithographically creating
a quantum well, an extremely thin, practically two-dimensional region
inside a semiconductor capable of trapping electrons. First, a laser
pulse injects polarized electrons (their spins have a definite orientation
determined by the laser's polarization) into the well. Once in the well,
the tiny disk of electrons (with a radius of about 20 microns but a
thickness of only 20 nm) can be controllably moved along one axis, much
as an abacus bead can be slid along a wire, by simply changing a voltage.
In this case, the disk can be positioned with nm-accuracy. The nuclei
of atoms residing within the thin volume occupied by the spin-polarized
electrons will in turn be polarized; that is, the spin of these nuclei
will tend to align themselves with the spin of the electrons. The result
is an extremely thin---equivalent to the thickness of several tens of
atoms--- region of polarized nuclei which can be precisely positioned
by changing a single voltage. These thin sheets of nuclear polarization
could constitute the basic elements of an information storage device
in which nuclear spin determines the logical state of the system. One
may ask, why not take out the "middle man" and just use the
electron spin to encode information? The answer: nuclear spins have
a weaker interaction with the surrounding environment than electron
spins. While harder to flip, once oriented, nuclear spins preserve their
state longer than do electrons. One may also wonder, why not just use
some large magnet to orient the nuclear spins? Why use electrons as
intermediaries? The answer: all-electronic control of spin is desirable
because electric fields are so much easier to control and create on
a small scale than magnetic fields. They are scalable and easy to implement,
while it is notoriously hard to produce large and localized magnetic
fields. In addition, all of our current integrated circuit technology
is based on charge and electric field; it would certainly be helpful
to manipulate spin using "knobs" which are well developed
and familiar to engineers. Awschalom
(805-893-2121) believes this current result is the first step toward
the establishment of an all-electrical manipulation of countable numbers
of nuclear spins.(Poggio
et al., Physical Review Letters, 14 November 2003)
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