picture of the day
Wesley Doggett (left) and Willard
Bennett (right) with Störmertron tube.
Credit: NCSU Special Collections Archive
Jun 13, 2008
Happy Birthday Willard Bennett
Willard Harrison Bennett, a notable
scientist and inventor and for whom the Bennett pinch, or
“z-pinch,” is named, was born June 13, 1903.
As a jet of water flows, the
surface tension causes the stream to constrict, and the jet
forms beads or droplets. One sometimes sees this in a stream
of water from a garden hose.
In 1992, Chief Researcher at the Kurchatov Institute, Boris
Trubnikov, noted that water beading is a good analogy for
the observed pinching of plasma jets in the laboratory as
well as the pinching of cosmic plasma in nebulae. In plasma,
the pinching is due to the self-generated magnetic field
compressing the jet unevenly along its length. The pinch is
sometimes called a z-pinch because the magnetic field lies
along the z-axis and the beading is sometimes referred to as
a sausage instability because of its shape.
In 1905, James Arthur Pollock and Samuel Barraclough at the
University of Sydney proposed that the distortions in a
length of copper piping used as a lightning conductor were
due to the pinch effect. The phenomenon has also been
suggested to be the cause for pinching in bead lightning.
Pinching metal is demonstrated in the laboratory by placing
an aluminum can in a coil of wire and sending a short pulse
of high electric current through the coil. The magnetic
field that is generated will crush the can into a
characteristic hourglass shape.
As cells of cosmic plasma move relative to each other they
generate currents and magnetic fields that also cause them
to produce jets that pinch and bead.
It is perhaps no coincidence that when astronomer Walter Baade first
distinguished individual stars in the Andromeda Galaxy’s core he described them
as like "beads on a string". The
Ant Nebula, which glows like a plasma-filled fluorescent light tube, has a
characteristic hourglass pinch in its middle.
Willard Harrison Bennett first investigated pinches in plasmas in the 1930s. He
was able to work out a relationship between the plasma density and current (the
so-called Bennett relation), and pinches are sometimes called a Bennett pinch.
Bennett also invented a device called the Störmertron tube. With it, he created
analogues for all the plasma stream formations in space, including the toroids
of charged particles known as the Van Allen radiation belts that surround Earth.
Dr. Willard H. Bennett was inducted into the
National Inventors Hall of Fame in 1991 for his invention of the Bennett
radio frequency mass spectrometer.
Contributed by Ian Tresman
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