THOMAS JEFFERSON NATIONAL ACCELERATOR FACILITY

Scientists from across the country and around the world visit the U.S. Department of Energy's (DOE) Thomas Jefferson National Accelerator Facility— Jefferson Lab, in Newport News, Virginia—to further their knowledge of the structure of atomic nuclei. They investigate the boundary between nuclear and particle physics, seeking to determine how the neutrons and protons (more generally, the hadrons) are "constructed" from the more fundamental quarks and gluons of quantum chromodynamics (QCD), and how the forces between the hadrons arise from QCD. They also seek to identify and expand the limits of the understanding of the behavior of nuclei by high-precision studies of their properties.

The Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab shows the transition region between two views of the nucleus. In the traditional view, the nucleus appears as a cluster of nucleons—protons and neutrons. The more detailed view, which began to emerge around 1970, reveals nucleons as composite objects, made from quarks and gluons. Ultimately, the process of bridging these two views will yield a complete understanding of nuclear matter—99.5 percent of the observable universe. We will learn both how matter itself is constructed and how it obtains its characteristic properties.

The experimenters probe nuclei using continuous-wave (CW) beams of electrons from CEBAF, a 6-GeV (giga electron volt) research instrument that stretches through a racetrack-shaped tunnel nearly a mile long. It delivers beams for simultaneous experiments in three cavernous experimental halls, where advanced particle-detection equipment observes the probing and where ultra-high-speed data-acquisition equipment gathers the resulting data.

The CEBAF accelerator has its roots in the decades-old tradition of electromagnetic nuclear physics, which exploits two fundamental advantages of electrons: pointlike structure and a well understood interaction with other particles. Since the 1960s, it has been recognized that CW, high-energy beams of electrons would constitute a unique, powerful new tool. The 100 percent duty factor of CEBAF's CW electron beams allows the extension of electromagnetic interaction studies to a broad range of reactions in which the probing electron is observed in coincidence with the particles emitted as a consequence of its interaction with the nuclear target. CEBAF's 6-GeV energy is optimized for probing spatial

scales ranging from the size of a large nucleus down to a fraction of the size of a nucleon.

The construction of CEBAF for the U.S. Department of Energy began in 1987 under the inspired leadership of Hermann Grunder, with the Southeastern Universities Research Association (SURA) serving as the prime contractor. The decision to have SURA build the laboratory in the southeast was motivated, in part, by a strong desire to strengthen science in that region. In 1996, with experiments getting under way, the new laboratory was named after Thomas Jefferson (1743–1826), the United States president and statesman of science who fostered American optimism about science, technology, and the future.

The CEBAF accelerator consists of two antiparallel linear accelerators (linacs) interconnected by 180° beam-recirculation arcs, which give the accelerator its racetrack shape. A 1,500-MHz train of polarized electron bunches is preaccelerated before injection into the first linac "straightaway." The beam then makes as many as five acceleration passes around the racetrack, with each pass raising the energy by about 1 GeV. After any pass, every third bunch—constituting a 500-MHz CW beam—can be directed to a particular experimental hall. In each linac, superconducting radio-frequency (SRF) accelerating cavities transfer rf energy to the beam. Liquid helium at 2 K, supplied by the world's largest refrigeration plant for that temperature, cools the SRF components for superconducting operation. Electron bunches at first-pass through fifth-pass energy travel together through the linacs, but the bunches at each of the energies in the linac are separated into individual recirculation arc beam lines for transport through a 180° arc and then recombined in preparation for further acceleration. CEBAF overall incorporates more than 2,200 beam-transport magnets.

The electron beams from CEBAF are sent to three experimental halls. The first hall is equipped with two high-resolution spectrometers for precision electron-scattering measurements; the second has a large-acceptance, lower-resolution spectrometer for studying reactions with many particles in the final state; and the third is a multipurpose hall used mainly for one-of-a-kind experiments. The laboratory has developed intense, laser-driven sources of polarized electrons that produce high-current, highly polarized beams of electrons. The unique, new combination of high-energy, continuous, highly polarized electron beams gives scientists, for the first time, sufficient precision to test key theoretical predictions about nucleon structure.

Research at Jefferson Lab began in earnest in 1997, and although the research program is in its early stages, the laboratory has already made significant contributions to the understanding of nuclear and nucleon structure. Precise measurements have been made on the charge and magnetization distributions of nucleons. Since the proton and neutron are each made up of quarks bound together by the exchange of gluons, and since each quark carries its charge in an extraordinarily small volume, mapping the charge and magnetization of the nucleon tells how the quarks are organized within it. The data for the proton indicate that its magnetization density peaks in the center and falls off rapidly near the edges. The charge density, however, is significantly lower than the magnetization density at the center and drops off somewhat more slowly with increasing distance from the center. Similar data for the neutron show that although its total charge is zero, the distribution of charge is not; it is positive near the center and becomes negative near its outer edge. While this general feature of the neutron has been known for some time, the new data from CEBAF provide a precise map of exactly how the charge is distributed. These data provide a kind of "X-ray" picture of the internal structure of the nucleons that helps to guide theorists who are trying to explain how they are constructed from quarks and gluons.

Considerable progress has also been made in understanding the transition between the classical nuclear physics description of nuclei (in which they are described in terms of nucleons acting as fundamental particles held together by the exchange of mesons) and the modern view, in which the underlying quark structure is accounted for explicitly. Data indicate that the classical description works well down to a scale of about one-half the size of the proton. Quarks are tightly bound inside the nucleon, and only their aggregate properties are observed until one probes more deeply. The quark substructure becomes evident on distance scales smaller than approximately one-tenth the size of the proton. Understanding the transition region where the individual quark properties start to appear is an important goal of nuclear physics.

As a secondary mission, Jefferson Lab has used its SRF electron-accelerating technology to take the lead in developing a powerful, versatile new kind of laser for science, applied research, and industry: the free-electron laser (FEL). All lasers convert electron energy into laser light, although usually with only one fixed choice of wavelength. However, a beam of electrons from a CEBAF-style SRF accelerator can be manipulated to cost-effectively generate immensely powerful laser light. Moreover, the wavelength can be precisely selected over a broad range of wave-lengths—a crucial feature for making light perform useful work. The first Jefferson Lab FEL produced infrared (IR) wavelengths at over 2 kilowatts—more than two orders of magnitude higher average power than any predecessor. As of 2002, following FEL's use by over twenty-five research groups in biology, physics, chemistry, and materials science, an upgrade was under way to 10 kilowatts in the IR and 1 kilowatt in the ultraviolet (UV).

The recirculating SRF linac that drives the Jefferson Lab FEL has served as the first proof at significant power of the energy-recovery principle, in which the electron beam is returned through the accelerating structures for deceleration, enabling the recycling of its unspent energy. As of 2002, groups in the United States and Europe were envisioning, proposing, and developing energy-recovery linacs for a variety of accelerators and light sources.

See also:Benefits of Particle Physics to Society; Funding of Particle Physics

Bibliography

Leemann, C.; Douglas, D.; and Krafft, G. "The Continuous Electron Beam Accelerator Facility: CEBAF at the Jefferson Laboratory." Annual Reviews of Nuclear and Particle Science51 , 413–450 (2001).

Thomas Jefferson National Accelerator Facility. <http://www.jlab.org/>.

Lawrence S. Cardman