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The current research program at the LINAC facility includes a variety of basic science and programmatic missions. There is a major initiative underway in materials science using positron annihilation techniques to characterize defects and materials surface properties. Also, very high intensity, ultra short pulse lasers are being harnessed for studies of intense laser-electron interaction phenomena. Such phenomena include: photonuclear reactions driven by intense laser excitation of solids; nuclear reactions caused by the acceleration of low-mass ions in the laser-plasma interaction; and the generation of ultra short X-ray pulses from the Thomson scattering of laser light off the relativistic electron bunches of the LINAC's beam.
Point of contact: Richard Howell
A low-energy intense positron beam production target in the 0o Cave can generate a beam of > 1010 e+/s that can be transported to target areas in the South Cave. At present this is the world's most intense beam of low energy positrons. It is used for a wide variety of fundamental and applied research in materials characterization and basic materials science. High intensity beams of positrons are moderated to produce slow positrons. The slow positrons are then used to probe materials, determining defect type and defect density from the time interval between positron injection and positron annihilation. Spectroscopy of reflected positrons is also utilized to study surface properties of materials. Additional details are given in the Positron microprobe discussion.
A positron microprobe is under development that will allow scanning materials with well-focussed beams of slow positrons. This instrument is expected to provide positron beams in the energy range 0.5-50 keV with spot size as small as 0.5 mm. By varying the incident energy the depth at which positrons stop in a material can be controlled and the result is a very high resolution 3-D scan of materials using the high intensity positron beam available at this facility. Sample volumes as small as 0.0025 mm3 may be sampled using this instrument.
An electrostatic 3 MV Pelletron Accelerator, NEC model 9SDH-2e, has been installed in the North Cave. The Pelletron generates very-high-resolution electron beams in the energy range 0.6 - 3.2 MeV, and at DC currents up to 100 µA. The Pelletron beam can be transported through the Magnet Cave to a target site in the South Cave. The Pelletron can be configured to generate a high-resolution positron beam, using a moderated radioactive source (22Na) in the high voltage terminal. These beams may be used in conjunction with certain positron and plasma physics experiments.
A photonuclear activation set-up in the 0° Cave includes a remote-fill and recovery gas target. A future sample irradiation station may be installed in the Accelerator Cave for low-energy high-current electron irradiation of materials. This facility is not yet functioning but could be implemented expeditiously for an interested sponsor.
Point of contact: Todd Ditmire
The above ground portion of the facility includes a laser laboratory containing several high-power lasers. 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 to an interaction chamber in one of the caves where the optical pulse is compressed and focused for experiments on the interaction between the laser and the accelerated electron beam.
High intensity laser beams are also used to study nuclear reactions induced in the high temperature plasmas generated in solid density targets. Neutron production and photo-nuclear reactions may be studied in solid or gas targets illuminated by the high intensity laser light.
Other lasers in this lab include solid state amplifiers producing 5 W average power and peak pulse energies up to 4 J at wavelengths 527, 532, and 1053 nm.
T. Ditmire, J. Zweiback, V. P. Yanovsky, T. E. Cowan, G. Hays, K. B Wharton, "Nuclear fusion from explosions of femtosecond-laser heated deuterium cluster", Nature 398, 492 (1999).
Point of contact: Tom Cowan
Goals of the Intense Laser-Electron Interactions research effort include producing intense sub-picosecond x-rays, advancing ultrafast science, and other advanced accelerator research activity. Outside users from universities and other national laboratories and corporate partners participate in some aspects of the research program.
Point of contact: Greg LeSage
Advanced photocathode development
A photoinjector project at the 100 MeV LINAC Facility is under development. It is based on a laser-driven photocathode source in a novel RF structure which will enable ~1 ps long beam pulses at a 10 Hz repetition rate, but having very high beam quality (low emittance and narrow energy spread).
Optical transition radiation is being utilized as a prompt diagnostic to characterize the emittance and intensity profile of accelerated electron beams. This diagnostic allows study and production of very high quality electron beams needed for basic studies of laser-electron interactions.
UCRL-MI-135159
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