An Alternative To Giant Cyclotrons

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 means 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.