Rex Booth

Tritium target for intense neutron source

Abstract A rotating target is described for an intense source of 14 MeV neutrons. For a fresh target the source strength is 2 × 10 12 neutros/s. The source strength decreases to half its initial value in about 100 h of continuous operation.

PLEIADES: A picosecond Compton scattering x-ray source for advanced backlighting and time-resolved material studies

The PLEIADES (Picosecond Laser-Electron Inter-Action for the Dynamical Evaluation of Structures) facility has produced first light at 70 keV. This milestone offers a new opportunity to develop laser-driven, compact, tunable x-ray sources for critical applications such as diagnostics for the National Ignition Facility and time-resolved material studies. The electron beam was focused to 50 μm rms, at 57 MeV, with 260 pC of charge, a relative energy spread of 0.2%, and a normalized emittance of 5 mm mrad horizontally and 13 mm mrad vertically. The scattered 820 nm laser pulse had an energy of 180 mJ and a duration of 54 fs. Initial x rays were captured with a cooled charge-coupled device using a cesium iodide scintillator; the peak photon energy was approximately 78 keV, with a total x-ray flux of 1.3×106 photons/shot, and the observed angular distribution found to agree very well with three-dimensional codes. Simple K-edge radiography of a tantalum foil showed good agreement with the theoretical divergence-...

Rotating target neutron generators

Abstract The rotating target neutron source at the Lawrence Livermore Laboratory (RTNS-I) is the most intense dc 14 MeV neutron source now in operation. The source consists of a 400 keV air-insulated accelerator which bombards a titanium tritide target rotating at 1100 rpm. A maximum source strength of 6 × 10 12 n/s can be produced by bombarding the target with a 20 mA beam of deuterons. Useful target life is ∼ 100 h. By extrapolation of the technology of the present source two new neutron sources (RTNS-II) have been designed to produce 4 × 10 13 n/s. These sources and the research facility to house them are now under construction. The facility will be operated by LLL as a national center for fusion reactor materials research.

A Mössbauer spectrometer

BS>The detector and velocity transducer components of a Mossbauer spectrometer are described. The energy resolution of the detecting system and the velocity resolution of the spectrometer are discussed in terms of the 14-kev line of Fe/sup 57/. Special techniques used in conjunction with studies of the 35-kev transition of Te/sup 125/ are described. The hyperfine splittirgs of the ground and first excited states of Fe/sup 57/ are reported as an indication of the possible utility of the spectrometer in the study of narrow line resonances. (auth)

Ultrafast x‐ray streak camera for use in ultrashort laser‐produced plasma research

In recent years there has been growing interest in energetic (≳100 eV), temporally short (<10 ps) x rays produced by ultrashort laser‐produced plasmas. The detection and temporal dispersion of the x rays using x‐ray streak cameras has been limited to a resolution of 2 ps, primarily due to the transit time dispersion of the electrons between the photocathode and the acceleration grid. The transit time spread of the electrons traveling from the photocathode to the acceleration grid is inversely proportional to the accelerating field. By increasing the field by a factor of 7, we have minimized the effects of transit time dispersion in the photocathode/accelerating grid region and produce an x‐ray streak camera with subpicosecond temporal resolution (≊900 fs). The streak camera has been calibrated using a Michelson interferometer and 100 fs, 400 nm laser light. The characteristics of the streak camera, along with the most recent x‐ray streak data will be presented.

Space charge lens for high current ion beams

Abstract A space charge or plasma lens consists of an axial electron trap produced with biased ring electrodes and an axial magnetic field to inhibit radial motion of electrons. It is useful for high current ion beams because it does not remove the neutralizing electrons from the beam. In this paper the design of the space charge trap is discussed qualitatively, the focal length is derived for a simple geometry, and a space charge lens in use in a 400 keV−25 mA deuteron beam transport system is described.

Progress in Space Charge Lens Development

A number of space charge lens electrode geometries have been studied for use at very high beam currents at low energy, and for precision optics at MeV energies. One such lens mounted very near a Duo-plasmatron extractor delivered a 30-keV beam of H+ ions through an analyzing magnet at a current of 175 mA for a period of six hours. Another produced a focal spot of diameter 0.2 mm when a 1-cm diameter beam of 1.2-MeV protons was focussed 90 cm from the lens. This paper also describes observations on self-sustaining discharge in the lenses. A very interesting rotation of the lens optic axis about the geometric axis at a frequency near the drift frequency occurs when the vacuum is spoiled by electrode outgassing.

Characterization of a bright, tunable, ultrafast Compton scattering X-ray source

The Compton scattering of a terawatt-class, femtosecond laser pulse by a high-brightness, relativistic electron beam has been demonstrated as a viable approach toward compact, tunable sources of bright, femtosecond, hard X-ray flashes. The main focus of this article is a detailed description of such a novel X-ray source, namely the PLEIADES (Picosecond Laser–Electron Inter-Action for the Dynamical Evaluation of Structures) facility at Lawrence Livermore National Laboratory. PLEIADES has produced first light at 70 keV, thus enabling critical applications, such as advanced backlighting for the National Ignition Facility and in situ time-resolved studies of high- Z materials. To date, the electron beam has been focused down to σ x = σ y = 27 μm rms, at 57 MeV, with 266 pC of charge, a relative energy spread of 0.2%, a normalized horizontal emittance of 3.5 mm·mrad, a normalized vertical emittance of 11 mm·mrad, and a duration of 3 ps rms. The compressed laser pulse energy at focus is 480 mJ, the pulse duration 54 fs Intensity Full Width at Half-Maximum (IFWHM), and the 1/ e 2 radius 36 μm. Initial X rays produced by head-on collisions between the laser and electron beams at a repetition rate of 10 Hz were captured with a cooled CCD using a CsI scintillator; the peak photon energy was approximately 78 keV, and the observed angular distribution was found to agree very well with three-dimensional codes. The current X-ray dose is 3 × 10 6 photons per pulse, and the inferred peak brightness exceeds 10 15 photons/(mm 2 × mrad 2 × s × 0.1% bandwidth). Spectral measurements using calibrated foils of variable thickness are consistent with theory. Measurements of the X-ray dose as a function of the delay between the laser and electron beams show a 24-ps full width at half maximum (FWHM) window, as predicted by theory, in contrast with a measured timing jitter of 1.2 ps, which contributes to the stability of the source. In addition, K -edge radiographs of a Ta foil obtained at different electron beam energies clearly demonstrate the γ 2 -tunability of the source and show very good agreement with the theoretical divergence-angle dependence of the X-ray spectrum. Finally, electron bunch shortening experiments using velocity compression have also been performed and durations as short as 300 fs rms have been observed using coherent transition radiation; the corresponding inferred peak X-ray flux approaches 10 19 photons/s.

Level structure of A40

Abstract The energy spectrum of inelastically scattered protons leading to excited levels in A 40 has been measured at bombarding energies of 7.32 and 9.39 MeV using the Livermore variable-energy cyclotron. The argon target was contained in a 7 cm diameter gas cell with maylar window extending uninterruptedly for about 300° except for the region within 15° of the beam line. The protons were observed in a “particle-namer” detector incorporating a gas proportional counter and p-n junction solid-state detector. Twenty-one levels were observed up to an excitation energy of 6.65 MeV.

Gated x-ray intensifier for large format simultaneous imaging

Some applications of gated x-ray imagers, pulsed x-ray spectroscopy, for example, benefit if image capture is simultaneous and gain is uniform over the frame. Simultaneity and uniformity are both improved when the voltage gate pulse propagation distance is as short as practical across the micro-channel-plate. This article describes a micro-channel-plate intensifier that captures a 40×100mm2 image in <300ps. A simple transmission line loss model is proposed to explain voltage loss across the micro-channel-plate. The voltage loss exponent was measured to be 0.05∕cm±20%, and used to predict spatial and temporal gain dependence. The spatial and temporal gain profile was measured in detail by capturing images of ∼1ps x-ray bursts created with a short pulse laser. The measured profile is consistent with that predicted using the loss model.