A collaboration of physicists in California has shown that a rarified
plasma, a million times less dense than air, can bend or "refract"
an intense, high-energy electron beam that can ordinarily bore through
several millimeters of steel. This demonstration opens new possibilities
for "plasma" wires that guide electric current or novel alternatives
to magnets that guide charged particles in accelerators.
The above figure shows refraction of a portion of the beam. Figure
(a) is an image of the beam without the plasma (i.e., the laser which
creates the plasma is off). Figure (b) is an image with the plasma (laser
on). The beam crossing the plasma/vacuum boundary causes a deflection
of about 1 milliradians in the beam. Cross-hairs show the undeflected
beam location. This shows a characteristic splitting of the beam into
two: The head of the beam travels straight, whereas the core of the
beam is deflected, in qualitative agreement with the simulation shown
below it. Figure (c) is a computer simulation of electron beam, side
view with plasma shown in blue. The head of the beam (two groups at
the bottom right of the image) travels straight, whereas the core of
the beam (cylinder following the head) is deflected toward the plasma.
The inward motion of the plasma electrons is visible as a depression
in the blue plasma surface behind the beam core. (d) Simulation, head-on
view corresponding to (b).
The researchers propose a mechanism by which the charged particle beam
is refracted. This mechanism is shown in Figure 2, with (a) a side view
and (b) a front view of the beam and plasma. When the electron beam
enters the plasma, it first repels plasma electrons, creating a positively
charged "ion channel" through which the latter part of the
beam travels. The channel first focuses the beam; but when the beam
nears the plasma/vacuum boundary, the ion channel becomes asymmetric,
resulting in a deflecting force which bends the electrons in the latter
part of the beam towards the plasma. This deflection is made possible
by the plasma's collective interaction, which refracts the beam by orders
of magnitude more than would be predicted by considering the plasma
as made of independent electric charges. The figure illustrates how
an asymmetric blowout creates a net deflection force.
This research is reported by Patric Muggli, Seung Lee, Thomas Katsouleas,
Ralph Assmann, Franz-Joseph Decker, Mark J. Hogan, Richard Iverson,
Pantaleo Raimondi, Robert H. Siemann, Dieter Walz, Brent Blue, Christopher
E. Clayton, Evan Dodd, Ricardo A. Fonseca, Roy Hemker, Chandrashekhar
Joshi. Kenneth A. Marsh, Warren B. Mori, Shoquin Wang in the 3 May 2001
issue of Nature.
Thanks to the researchers for providing the figures and much of the
caption text. All figures are courtesy Patrick Muggli (muggli@usc.edu)
and collaborators
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