Improvements at Free-Electron Laser Laboratory Will Enhance Research

Research in areas ranging from ultra precise laser surgery to the physics of exploding stars will be aided by a $5.2 million equipment upgrades underway within Duke's Free-Electron Laser Laboratory (DFELL). The upgrades, to be completed by mid 2006, will substantially increase power levels and deliver novel new arrangements of laser and gamma ray beams.

Key components for these enhancements are completed or are being built by Russia's Budker Institute of Nuclear Physics in Novosibirsk.

The improvements are supported by a $3.2 million equipment grant from the U.S. Department of Energy. About $2 million in additional funding has been provided by various units of the university as well as the Air Force Office of Scientific Research and the Triangle Universities Nuclear Laboratory (TUNL). TUNL is a Duke-headquartered consortium of three major universities in the Research Triangle area.

The changes include:

• An improved radio frequency cavity system for cycling speeding bunches of electrons around the 353 ft.-in-circumference storage ring.
• A new and more powerful accelerator to inject electrons into that ring.
• A new generation of free-electron laser that will use those electrons to make larger varieties of ultraviolet light and ultra-energetic gamma ray beams.

"All three of these upgrades are large-scale efforts for a university campus," said DFELL director and physics professor Glenn Edwards.

"It really broadens the types of research that we can do with these instruments," added Calvin Howell, a physics professor and TUNL staff member who serves as project manager under a partnership between DFELL and TUNL.

Free-electron lasers -- such as the infrared and ultraviolet lasers now operating at the DFELL -- produce light by using powerful magnetic fields to "wiggle" electrons freed of their normal bonding within atoms.

The oscillation of such unfettered electrons can be varied to create laser light in a wide range of wavelengths. And this flexibility makes free-electron lasers especially useful as research tools.

DFELL's ultraviolet laser design allows it to also make unusual kinds of pencil-sized beams of gamma rays -- the most energetic form of light -- by colliding the electrons with particles of light called photons.

Those gamma ray beams will enable physicists to probe atomic nuclei with a precision and selectivity unmatched at other facilities -- a promise that attracted TUNL researchers to form a collaboration with DFELL.

Ying Wu, an assistant physics professor who is DFELL's associate director for accelerators and light sources, is overseeing the phased installation of his laboratory's next generation ultraviolet laser -- the OK-5. Wu received his Ph.D. at Duke while setting up DFELL's first ultraviolet free-electron laser -- the OK-4.

Under their partnership, DFELL and TUNL are both soliciting research groups to set up experiments using the free-electron laser and gamma beams. As new components are added under the upgrade, those beams will become more powerful and useful.

"We really designed it in a way that we can work on the upgrades for a few months at a time and have extended operations in between for users to carry out research," Wu said.

Ultraviolet laser and gamma light are both made in DFELL along a large oval ring that stores streams of electrons accelerated to nearly the speed of light. These energized electrons are bunched into pulses that ride like surfers atop radio frequency waves also generated along the ring.

The upgrades include installing a new radio frequency system that Wu said should allow the ring to store more current. That enhanced current will feed electrons to the new OK-5 ultraviolet laser, which is being built in stages by adding new components to replace the existing OK-4. "The area that the OK-5 opens up that the OK-4 doesn't have is circular polarization," said Wu.

Polarization provides the ability to deliver both laser and gamma light oriented in different directions, Wu explained. Circularly polarized beams are particularly useful probes that can distinguish between left and right handed orientations of the molecules of chemistry and biology, or the fundamental particles of physics.

Because the OK-5's light can be delivered in pulses as short as tens of trillionths of a second, these differentially oriented beams will also be able to act as strobes to study changes that occur extremely rapidly in biological molecules.

Biological molecules such as proteins are sometimes envisioned as tiny machines that operate in cycles. "We would like to understand the cycles they go through as they mechanically rearrange themselves in repetitive ways," said Edwards.

The other major upgrade will be a new oval-shaped electron booster injector, to be assembled in a bunker-like extension to the DFELL building that is already completed.

Right now, electrons are pumped into DFELL's big electron storage ring for laser and gamma production with a 270 million electron volt straight-line accelerator. The additional oval booster ring will allow operations at energy ranges between 270 million and 1.2 billion electron volts.

The OK-5 will also be able to generate ultraviolet light at even shorter wavelengths than the OK-4, which once set a record for the shortest wavelength for a tunable laser.

TUNL researchers are excited about the new opportunities that alternative polarization, shorter pulses and higher energies will bring to their nuclear physics research program.

The special gamma ray beam, for instance, may help resolve whether fundamental particles called quarks are really fundamental or actually have an underlying structure, some key TUNL researchers said.

Such gammas may also help resolve uncertainties in steller evolution by enabling measurements of key nuclear reactions that that determine whether collapsing large stars become highly condensed neutron stars or instead further devolve into black holes, they added.

The gammas could additionally open new lines of medical research for DFELL investigators like Edwards and Duke Medical Center assistant research professor Robert Pearlstein. Pearlstein is also DFELL's associate director for medical affairs.

That pair is currently investigating the ability of OK-4 gamma beams to arrest inflammation in nerve tissue scarring following surgery, Edwards said.