January 29, 2003
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Here come the T-rays
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The nation's premier source of low-energy or "soft" x-rays for scientific and technological purposes may also become a premier source of T-rays— far-infrared or terahertz frequency (trillion-cycle-per-second) radiation.

Plans to build a new synchrotron ring dedicated to generating high-powered T-ray beams at Berkeley Lab's Advanced Light Source (ALS) are moving forward thanks in part to a recent collaborative experiment at the Thomas Jefferson Accelerator Facility. In that experiment, a linear accelerator or linac was used to produce beams of terahertz frequency radiation thousands of times more powerful than beams obtained from tabletop and even free electron lasers.

Michael Martin at the Advanced Light Source's existing infrared beamline.

"With this experiment, we've shown that the basic physics predictions behind the production of high-powered terahertz radiation in a synchrotron are correct," says Michael Martin, a Berkeley Lab physicist who, along with colleague Wayne McKinney, was a key participant in the Jefferson Lab experiment. "We're now confident we can design a terahertz ring for the ALS and make it work."

Unless they're at a temperature of absolute zero, all objects, animate and inanimate, give off terahertz radiation, the heat from molecular vibrations. This "black-body" radiation is emitted at such low intensities — typically less than a millionth of a watt per square centimeter — that we're unaware of it. However, T-rays have immense scientific and technological importance because their spectral range embraces the vital interface between electronics and photonics.

Using ultra-fast lasers and nonlinear crystals to generate coherent light beams, scientists have already put T-rays to good use for a variety of purposes that include the nondestructive imaging of biological and other materials and the manipulation of the electronic properties of semiconductors. It is widely believed that T-rays could be put to even better use if the power of T-ray beams could be substantially boosted.

"With high-powered coherent terahertz beams we could make full-field, real-time, video-rate movies, which could be very useful in medical imaging," says Martin. "They should also be useful for security inspections, because terahertz radiation goes through most everything except metal and water, and you don't have the shielding issues you do for X-rays."

In recent years, scientists have used femtosecond lasers and semiconductors or nonlinear crystals to generate coherent picosecond pulses of T-rays at about one-ten-thousandth of a watt of power. In the experiment at Jefferson Lab, using that facility's Energy Recovery Linac (ERL), researchers were able to sustain coherent T-ray beams at an average power of 20 watts over a broad bandwidth of far-IR frequencies.

"Coherent synchrotron radiation has been measured at linacs before," says Martin, "but this is the first time it's been measured at a linac that runs its electron beams at a repetition rate and current comparable to what you'd get in a synchrotron ring."

Jefferson Lab's ERL is a superconducting radio-frequency electron accelerator that recovers the energy of spent electron bunches. The combination of superconducting accelerator cavities and energy recovery enable it to operate at a repetition rate as high as 75 megahertz, which means the average current of its electron beams, up to 5 milliamps, is much higher than the current of conventional electron linacs.

In the T-ray experiment at Jefferson Lab, led by Gwyn Williams and reported in the November 14 issue of the journal Nature, very short bunches of electrons — about 500 femtoseconds in pulse length — were accelerated to energies of about 40 million electron volts (MeV). These 40 MeV femtosecond pulses were then passed through the powerful magnetic field of a bending magnet with a one meter radius. This produced a sideways shove in their trajectory which caused the electrons to shed energy in the form of high-powered T-rays

"Because this was a linac rather than a synchrotron, each pass of the electron beam through the bend magnet field carried new electrons, which meant a lot of timing jitter and current fluctuations that resulted in a low signal to noise ratio," says Martin. "With a synchrotron, where you have the same electrons passing through the bend magnets over and over again, the signal-to-noise ratio will be much higher. Nonetheless, this experiment was a proof of principle for our proposed coherent terahertz synchrotron ring, because it enabled us to make the optical measurements that needed to be made."

This schematic shows how the proposed T-ray synchrotron ring would piggyback atop the existing booster ring at the ALS, inside the facility's electron storage ring.

Berkeley Lab's ALS is an electron synchrotron facility consisting of three main components — a linac that can lift the energies of electrons to 50 MeV; a booster synchrotron ring for raising those electrons to nearly two billion electron volts; and a storage ring approximately 200 meters in circumference (660 feet) that can circulate the energized electrons for hours in a tightly constrained, ribbon-shaped beam no thicker than a human hair.

Beams of light are extracted from this stored electron beam through the use of bending, wiggler, or undulator magnetic devices. While the ALS is optimized for the extraction of x-ray and ultraviolet light — its best X-ray beams are a hundred million times brighter than the light from X-ray tubes — its bend magnets also generate intense beams of photons at mid-infrared frequencies, between 20 and 300 terahertz.

Martin is the manager and McKinney is the spokesperson for the infrared beamlines at the ALS. They are also leaders in proposing that a dedicated T-ray synchrotron ring be built atop the ALS booster ring. This ring would measure about 66 meters in circumference, be stocked with 30 quadrupole and a dozen dipole or bend magnets, and would make use of the same electron linac and booster ring used to fill the ALS storage ring. It would be optimized to operate as a coherent source of T-ray beams.

"Our terahertz ring would generate electron beams at much higher repetition rate (1.5 gigahertz) and higher current than the ERL at Jefferson Lab," says Martin. "It would produce terahertz light beams at an average of 50 watts of power to serve many beamlines simultaneously."

The projected cost for constructing this new synchrotron ring runs between $10 and $20 million, which is about the cost of one to two new premier X-ray beamlines at the ALS.

"We've never had such powerful terahertz beams before, so it's difficult to say what the most important applications for it will be," Martin says.

However, he adds, the scientific interest in high-powered T-ray beams is intense. He and his colleagues expect to submit to the U.S. Department of Energy a formal proposal for a dedicated T-ray ring at the ALS later this year.

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