ANN ARBOR— University of Michigan researchers have
announced a new method to accelerate ions, using powerful
light from a table-top laser instead of the radio-frequency
waves that have been used for ion acceleration ever since
Ernest O. Lawrence invented the cyclotron more than 60 years ago. The new technique allows them to accelerate ions in almost a million-times shorter distance than a cyclotron, which uses huge electro-magnets to accelerate atomic particles around a circular path before releasing them at a target.

This new technique may eventually make ion accelerators much more affordable to clinics and hospitals as well as providing doctors with new cancer-treatment capabilities.
Besides pioneering the modern era in experimental nuclear
and high-energy physics by creating the first controlled
beams of energetic ions, Lawrence also used his cyclotron
for treating his mother’s cancer, which is now the most
common application of cyclotrons.

The discovery was made by a research group led by Donald
Umstadter, associate professor of electrical engineering and nuclear engineering at the U-M. Earlier this year, the same researchers had first demonstrated ion acceleration by mean of focusing their high-power laser beam into a helium gas jet (published in the journal Physical Review E). However, the ions in these early experiments were accelerated in a direction that was perpendicular to the direction of the laser beam and so the ions were not tightly focused into a beam. By replacing the gas with a sheet of aluminum foil, they are now able to accelerate the ions into a confined beam that points almost in the same direction as the laser beam.

“Based on our previous results on gases, we reasoned—as it turns out, correctly—that a solid-density target would
make a better ion beam,” Umstadter said.

The acceleration process involves several steps that occur
when the laser beam is focused onto the foil. The much
lighter electrons are first accelerated by the enormous
oscillating electric field of the laser light. As these
electrons leave the ions behind, a steady electrostatic
field is generated, like that of a capacitor. It is this
latter field that accelerates protons from the surface of
the foil. (A proton is the same as a hydrogen ion.) The
protons are accelerated perpendicular to the surface of the
foil regardless of the angle at which the laser hits the
foil. More than 10 billion ions were accelerated with each
laser shot.

This new finding will be announced by U-M research scientist Anatoly Maksimchuk during the annual meeting of the American Physical Society (APS) Division of Plasma Physics, Nov. 15-19, at the Westin Hotel in Seattle.

The announcement coincides with a similar finding to be
announced at the same conference by the Lawrence-Livermore
National Laboratory, but Umstadter points out that the
Livermore laser was the size of a large building instead of
a table top. While the U-M team achieved ion energies that
were only one-tenth as much as the national lab, they did so with a laser beam that is only one-thousandth the power.
This size reduction will be required to make the technique
practical for real-world medical applications, such as the
preparation of short-lived medical isotopes and tumor
treatment.

Another difference is that the area of the region from which the ions originate, which is called the laser focal spot-size, is a 1,000 times smaller in the Michigan experiment than it was in the Livermore experiment or in an ordinary cyclotron. This smaller source may permit the irradiation of a small group of cells, enabling biological research on the early stages of the growth of tumors. Also unlike a cyclotron, the laser-produced protons naturally diverge at a 40-degree angle from their source, which might make it easier to evenly treat a large volume of the body of a cancer patient.

Maksimchuk said, “With our beam of several million-volts
energy, we can now produce nuclear reactions and study
radiation chemistry with a table-top device.” Since the
protons in the U-M accelerator are generated in less than a
picosecond, or a trillionth of a second, they can be used
for radiology on ultrashort time scales.

The Michigan group is now investigating these research
applications while building an even smaller laser system,
one with the same high peak power but a higher average
power, so that they can accelerate a greater number of ions
in each second with higher energies. They hope to make the
technique practical for clinical applications within the
next few years.