The upgraded CEBAF will go further down the path already trodden by the earlier deuterium experiments, exploring the transition from classical nuclear physics to the underlying quark and gluon structure in nuclei. It will also explore the transition of the quark-quark interaction from the strong values characteristic of nuclear distance scales to the weak character observed at high energies.
Accelerating innovation
CEBAF was a pioneer of SRF in a large-scale facility. When built, its cryomodules were designed to run at 5 MV/m. They currently operate about 50% higher, and some cavities have reached 12 MV/m. CEBAF engineers literally know each module by name; there is one called Atlantic, and another called Pacific. Phoenix is a module that proved difficult to commission, and Tranquillity was quite the opposite. The energy upgrade will be achieved by adding a further 10 cryomodules, each providing 100 MV of accelerating voltage at a gradient of 21 MV/m.
In 2001, CEBAF scientists and engineers broke with electron accelerator tradition and dismantled the accelerator's thermionic gun. All CEBAF beams now originate in one of two polarized gallium arsenide photocathode guns that produce electrons at 100 keV. These devices require extremely high vacuum, and this is provided by thin-film coating of a non-evaporable getter following a technique pioneered at CERN (see Molecular sticky tape). Fast diode lasers allow the accelerator's beam structure to be defined at the photocathode. Today, CEBAF routinely delivers precise electron beams that are spin-polarized above 80% at the full design current of 200 µA.
In 1999, CEBAF was joined by a second experimental facility, a free-electron laser (FEL), and the laboratory added a broad new strand to its research programme. An exercise in technology transfer, the FEL was built by applying the laboratory's SRF expertise, fresh from completing CEBAF, to the task of providing a facility with both scientific and industrial relevance. The FEL programme started in the mid-1990s as an R&D project to bring the cost per kilowatt of tunable sub-picosecond laser power down to a level at which new industrial and research processes would be possible. As the world's most powerful tunable laser by over two orders of magnitude, it was immediately successful and reliable enough to be run as a user facility. To date about 30 research groups in biology, physics, chemistry and materials science have used it.
One of the highlights of the FEL user programme has been the production of copious quantities of one of the new materials of the moment, carbon nanotubes, and researchers from industry are eagerly taking all they can. Potential applications include use as electron emitters for flat-panel displays. Nanotubes can also sustain current densities hundreds of times greater than common metals, and exist in semiconducting as well as metallic form. This opens up the possibility of using them as tiny circuit elements in future electronic devices.
The FEL has also been used for critical experiments in life science, studying energy flow in proteins to elucidate protein function. This is a departure from the work done at synchrotron light sources, where only structural information can be extracted from protein crystallography. Similar dynamics experiments have also been carried out on hydrogen defects in silicon, shedding light on crucial energy-loss mechanisms in one of today's most important technological materials. Other experiments have successfully used resonant pulsed laser deposition to make high-quality films for microelectronics, and explored the use of subpicosecond light pulses in surface modification experiments.
Like CEBAF, JLab's FEL is also being upgraded. Its current is set to double to 10 mA, and its energy to triple to 160 MeV. Improvements in efficiency will lead to an overall power of over 10kW, and the FEL will also be able to be rapidly tuned. Eventually it will cover the wavelength range from 250 nm to 15 µm. JLab has also acquired the compact HELIOS synchrotron, which was first commissioned around 1990 as a turnkey instrument for X-ray lithography. HELIOS was donated to JLab in 2000 and will be used in conjunction with the FEL for studies of nonlinear phenomena and protein dynamics using a pump-probe approach. Under this scheme, an FEL pulse will disturb the system under investigation and a HELIOS pulse will be used to investigate how it responds. This offers the potential for studying any dynamical system that can be driven out of equilibrium, with applications to materials as varied as proteins and superconductors.
Such a diverse programme, taking in aspects of pure and applied science, would surely have pleased statesman of science and US president Thomas Jefferson. It also makes JLab something of a mould-breaking institution, but then as Jefferson once remarked: "A little rebellion now and then is a good thing."
Further reading
Christoph W Leemann, David R Douglas and Geoffrey A Krafft 2001 "The Continuous Electron Beam Accelerator Facility: CEBAF at the Jefferson Laboratory" Ann. Rev. Nucl. Part. Sci. 51 413-450.
The JLab website is here.