The LLNL Electron-Positron Accelerator Facility

The ongoing research programs at the LLNL Electron-Positron Accelerator (LINAC) encompass basic research projects, technology transfer activities, and defense-related programmatic research. These include experiments in fundamental nuclear, atomic, solid-state, plasma and particle physics.  Also in progress are  fundamental experiments in laser-electron interactions and applied research in materials science. Some unique facility capabilities are used in the development of diagnostic and analytical techniques for industrial applications. Major equipment in the facility includes two electron accelerators and a high-power short-pulse laser. The primary accelerator is a 100 MeV electron RF-linear accelerator.

The Facility consists of a complex of above and below ground rooms adjunct to the electron accelerator, as shown in the pictures above. The accelerator and experimental target areas are located underground and include six experimental "caves", four of which can receive the primary electron beam. Individual caves are identified in the picture shown above. The LINAC is a variable energy RF linear accelerator, that produces electron beams with energies from 10 MeV to 165 MeV at average beam power up to 45 kW (600 mA at 75 MeV). The pulse structure of the beam is variable, ranging from a short-pulse mode (2-20 ns long pulses, 10 A peak current, 1440 pulses per second) to a long-pulse mode (3 µs pulses, 700 mA peak, at 300 pps). Some additional details may be found in references cited in the following sections and their associated web sites.

Secondary beams

The 0° Cave has provisions for the generation of intense secondary beams of neutrons, bremsstrahlung photons, and positrons. The secondary neutron and photon beams may be allowed to propagate through time-of-flight tubes to target areas in the Inner and Outer Detector Caves. At 45 kW of electron beam power incident on a neutron production target, the neutron beam flux in the Outer Detector Cave is 9x10⁶ cm-2 s-1, corresponding to a dose rate of 1200 R/hr. The bremsstrahlung photon energy flux is 2x10⁹ MeV cm-2 s-1 (2800 R/hr). The neutron and photon fluxes in the Inner Detector Cave are a factor of ten higher.

Much of the early work done at this facility focussed on neutron reaction cross section measurements including fission cross sections using the broad spectrum neutrons produced by photonuclear reactions in a tungsten target [1-2]. An experimental program provided neutron cross sections at several flight paths up to 250 m in length [3-5]. However, the neutron source and flight paths have now been decommissioned.

Laser beams

An intense, short-pulse laser facility has been developed where the optical pulse is compressed to 30 fs (3x10^-14 seconds) with focal intensities up to 10⁺²⁰ W/cm² at 820 nm wavelength. Laser light is transported below ground. There is an interaction chamber in one of the below ground caves where the compressed and focussed beam will be used for experiments that study interaction between the laser and the accelerated electron beam.

Technical data

Technical descriptions of the on-going research programs undertaken at the facility in Materials Science, Advanced X-ray source, Accelerator Technology, and Laser-Plasma Interaction studies, are given in the Technical Program section.  Technical points of contact are given there and in the Who's Who.

References to early work done at the 100 MeV LINAC facility

[1] B. L. Berman, S. C. Fultz, "Measurements of the giant dipole resonance with monoenergetic photons", Rev. Mod. Phys. 47, 713-761 (1975).

[2]  T. W. Phillips, R. G. Johnson, "Evidence for the isovector giant dipole resonance in ¹⁶O from the ¹⁶O(q,n₀)¹⁵O reaction", Phys. Rev. C20, 1689-99 (1979).

[3] J. W. Behrens, J. C. Browne, "Structure in the subthreshold ²³²Th(n,f) cross section", Phys. Lett. 69B, 278-80, (1978).

[4]  R. E. Howe, T. W. Phillips, C. D. Bowman, "Prompt fission neutrons from eV resonances in ²³⁵U: Measurement and correlation with other fission properties", Phys. Rev. C13, 195-205 (1976).

[5]  H. S. Camarda, T. W. Phillips, R. M. White, "Neutron absolute and total cross section difference measurements in the mass-140 region", Phys. Rev. C29, 2106-17 (1984).