The National Science Foundation's (NSF) Tokyo Office periodically receives and disseminates reports on research developments in Japan that are related to the Foundation's mission. NSF-sponsored researchers currently working in Japan prepare many of these reports. These reports present information for use by NSF program managers and policy makers; they are not statements of NSF policy.
Special Scientific Report #00-02 (September 01, 2000)
Mr.
Christopher D. Barnes, a graduate student in Accelerator Research at Stanford University,
prepared the following report. Mr.
Barnes was a participant in the 2000
Summer Institute sponsored in the United States by NSF/NIH/USDA and the
Science and Technology Agency and Japan Science and Technology Corporation in
Japan. Dr. Kazuhisa Nakajima of the
Accelerator Laboratory at the High Energy
Accelerator Research Organization (KEK)
in Tsukuba, hosted Mr. Barnes. Mr.
Barnes can be reached via email at: cbarnes@leland.stanford.edu
This
summer has been very interesting, as it provided a counterpoint and extension to
my normal research in the United States. At
Stanford, my research focuses on the acceleration of electrons by laser light.
Our mechanism uses the electric fields produced by an oscillating laser
wave to modulate the kinetic energy of a relativistic electron beam.
My
purpose in coming to Japan and the High Energy Accelerator Research Facility (KEK)
in Tsukuba was to learn about other possible laser-electron acceleration
techniques, as well as to learn about the many very high power laser systems in
Japan which could have great implications for my research back at Stanford.
With Dr. Hirohisa Nakajima, I have investigated a new acceleration
mechanism which we think holds great promise for the future.
Most
of my work this summer was theoretical and was motivated by the first portion of
the Summer Institute, which consisted of a number of site visits to labs in the
Kanto Area. Particularly, I visited
labs in Tokai, Hayama, Nara, and in Tsukuba itself. These labs generally focused on ultra high power laser
systems. The existence of such high
power lasers allows a novel mechanism for the acceleration of electrons, or
indeed, any charged particle.
The
current state of the art is represented by the 100 Terawatt laser at JAERI-Kansai.
When this laser is focused to a 10-micron waist, power densities of well
over ten to the twentieth Watts per square centimeter are achievable, something
difficult to imagine just a decade ago. With
such enormous intensities available, a wide variety of applications present
themselves. One is the
circumvention of some of the limits in current charged particle acceleration
techniques.
Traditionally,
radiofrequency acceleration has used the strong electric fields generated in a
copper cavity from an oscillating electromagnetic field to give energy to
electrons for the purposes of high energy physics research, medical isotope
generation, and any number of applications.
As
these traditional techniques reach their limits in terms of power sources and
accelerating gradients, interest has moved toward lasers as the best sources of
energy for accelerators.
My
research at Stanford focuses on a direct scaling of some of these traditional
techniques to optical wavelengths, and has the advantage of being able to draw
on many of the lessons learned from radio wave accelerators, while avoiding some
of the problems inherent in the older designs.
For
example, the best radiofrequency accelerators cannot give more than about 100
million electron Volts of energy to charged particles per meter, but my current
research hopes to achieve 1 billion electron Volts per meter soon.
Although
there are some significant departures from tradition, we still use the electric
field of our laser to accelerate electrons.
However,
extremely intense lasers can interact with electrons through a different
mechanism than has been used before, the ponderomotive force.
In its simplest description, this effect uses both the electric and
magnetic fields of an electromagnetic wave to accelerate charged particles.
The result is that the energy gain of a charged particle increases with
increasing laser power linearly, rather than as the square root, which is the
case for any technique relying solely on the electric field for acceleration.
Despite
the obvious scaling benefits, this technique has not been studied much until
recently because it is not feasible without extremely high intensities.
A measure of when ponderomotive acceleration will be useful is the
normalized field strength of the laser. This
quantity compares the peak electric field strength of a laser pulse to the rest
mass of an electron. When this
normalized field strength becomes greater than one, ponderomotive acceleration
is possible, and becomes very desirable.
What
I have studied this summer is this powerful mechanism, as well as the
theoretical basis for a further refinement which, in the absence of a better
name, I will call quantum acceleration. With
Dr. Nakajima and Dr. Igor Smetanin, a member of the group, I investigated
several aspects of this mechanism.
Our
new insight is that under certain circumstances, it is more convenient to think
of the laser-electron interaction as a quantum scattering problem, rather than
as a directly electromagnetic effect.
Basically,
if the phase velocity of an intense laser field can be reduced below the speed
of light in vacuum as it moves toward a stationary electron, its ponderomotive
potential will act like a racquet hitting a tennis ball.
After the interaction, the electron will move significantly faster than
the laser pulse which hit it.
In
quantum mechanics, no wall is completely solid, so an electron has a chance of
“tunneling” through any barrier. With
low power lasers, the chance of tunneling through the weak barriers they present
is essentially 100%, but we believe that modern lasers can present a wall which
is hard enough to push the electrons with high probability.
This regime should start when intensities approach ten to the twentieth
Watts per square centimeter, as in current lasers.
Dr.
Smetanin, building on calculations I made, used relativistic quantum mechanics
to confirm that modern lasers are intense enough to accelerate electrons to
relativistic speeds in very short distances using this technique.
The
reason that quantum scattering is an improvement even over ponderomotive
acceleration is that it allows greater flexibility and it scales differently.
The energy of the accelerated electrons depends on the phase velocity of
the laser pulse rather than its intensity, once that intensity is high enough to
ensure a high probability of scattering. This
also allows the trade off between the number of scattered electrons and the
energy they carry, a very useful feature.
Moreover,
as lasers become more powerful, the energy that can be imparted to the
accelerated electrons will increase as the square of the laser intensity, rather
than just linearly, as in ponderomotive acceleration, or as the square root,
which is the case with traditional mechanisms.
Although
the effect has not yet been observed, we think that it should be relatively easy
to investigate. This is because
there are a number of experiments here in Japan which already have most of the
features that would be required to search for quantum acceleration.
Currently,
Dr. Nakajima’s group at JAERI-Kansai is conducting a series of experiments on
laser-plasma interactions, using a so-called Z-pinch cell.
We believe that relatively small changes in the apparatus could enable us
to check whether our idea can work in practice.
We hope to be able to perform such experiments soon.
Although
I am normally an experimental physicist, this summer has been an interesting
chance to spend time learning about and participating in the theory work in my
field, as well as a chance to see several experimental setups around Japan and
get a sense of where research is going in this country. If quantum acceleration proves feasible, it could be very
exciting.
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(NSF/Tokyo SSR#00-02)
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