Bibliothek

Notizen: We present the first experimental results of a uniformly insulated extraction applied-B ion diode. The diode typically attained beam generating efficiencies of 70% at the time of peak beam power. This is a significant improvement over previous extraction applied-B ion diodes, demonstrating the importance of uniform insulation as suggested by theory [J. Appl. Phys. 59, 2685 (1986)]. Furthermore, we found that the beam current is nearly independent of the ion-emitting area down to a minimum area. At the minimum area, the beam current density was enhanced by 150 times the monopolar Child–Langmuir value. This data supports a recently proposed theory [Phys. Rev. Lett. 59, 2295 (1987)] that predicts large enhancements due to the electron diamagnetic effect on the virtual cathode.

Notizen: Particle-in-cell simulations of applied-B ion diodes using the QUICKSILVER code [D. B. Seidel et al., in Proceedings of the Europhysics Conference on Computational Physics, Amsterdam, 1990, edited by A. Tenner (World Scientific, Singapore, 1991), p. 475] have been augmented with Monte Carlo calculations of electron–anode interactions (reflection and energy deposition). Extraction diode simulations demonstrate a link between the instability evolution and increased electron loss and anode heating. Simulations of radial and extraction ion diodes show spatial nonuniformity in the predicted electron loss profile leading to hot spots on the anode that rapidly exceed the 350 °C–450 °C range, known to be sufficient for plasma formation on electron-bombarded surfaces. Thermal desorption calculations indicate complete desorption of contaminants with 15–20 kcal/mole binding energies in high-dose regions of the anode during the power pulse. Comparisons of parasitic ion emission simulations and experiment show agreement in some aspects, but also highlight the need for better ion source, plasma, and neutral gas models. © 1999 American Institute of Physics.

Notizen: Magnetically insulated ion diodes are being developed to drive inertial confinement fusion. Ion beam microdivergence must be reduced to achieve the very high beam intensities required to achieve this goal. Three-dimensional particle-in-cell simulations [Phys. Rev. Lett. 67, 3094 (1991)] indicate that instability-induced fluctuations can produce significant ion divergence during acceleration. These simulations exhibit a fast growing mode early in time, which has been identified as the diocotron instability. The divergence generated by this mode is modest, due to the relatively high-frequency ((approximately-greater-than)1 GHz). Later, a low-frequency low-phase-velocity instability develops with a frequency that is approximately the reciprocal of the ion transit time. This instability couples effectively to the ions, and can generate unacceptably large ion divergences ((approximately-greater-than)30 mrad). Linear stability theory reveals that this mode has structure parallel to the applied magnetic field and is related to the modified two-stream instability. Measurements of ion density fluctuations and energy-momentum correlations have confirmed that instabilities develop in ion diodes and contribute to the ion divergence. In addition, spectroscopic measurements indicate that lithium ions have a significant transverse temperature very close to the emission surface. Passive thin-film lithium fluoride (LiF) anodes have larger transverse beam temperatures than laser-irradiated active sources. Calculations of the ion beam source divergence for the LiF film due to surface roughness and the possible loss of adhesion and fragmentation of this film are presented. © 1996 American Institute of Physics.

Notizen: Particle Beam Fusion Accelerator II is a light-ion fusion accelerator that is presently capable of irradiating a 6-mm-diam sphere with ∼50 kJ of 5.5-MeV protons in ∼15 ns. An array of particle and x-ray diagnostics fielded on proton Inertial Confinement Fusion target experiments quantifies the incident particle beam and the subsequent target response. An overview of the ion and target diagnostic setup and capabilities will be given in the context of recent proton beam experiments aimed at studying soft x-ray emission from foam-filled targets and the hydrodynamic response of exploding-pusher targets. Ion beam diagnostics indicate ∼100 kJ of proton beam energy incident within a 1.2-cm radius of the center of the diode with an azimuthal uniformity which varied between 6% and 29%. Foam-filled target temperatures of 35 eV and closure velocities of 4 cm/μs were measured.

Notizen: The Laser Evaporation Ion Source (LEVIS) active lithium ion source has been developed for use on the focusing ion diode operated on the 10 TW Particle Beam Fusion Accelerator-II (PBFA-II) [J. P. VanDevender and D. L. Cook, Science 232, 831 (1986)] at Sandia National Laboratories. The source configuration consists of two laser pulses impinging on a heated (200 °C) thin-film LiAg layer on the anode surface. A short-pulse Nd:YAG laser creates a high-density vapor, which is then ionized by a long-pulse dye laser using the LIBORS (laser ionization based on resonant saturation) ionization method. Small-scale experiments determined that this dual laser-based approach can produce a source plasma of adequate density and confinement for acceleration and transport. Hardware modifications were undertaken to correct problems of premature impedance collapse and lack of beam lithium seen on previous PBFA-II experiments. As much as 85 kJ of Li is measured at the beam focus, but the source may not have been operating in a fully active (i.e., preformed) manner. Focusing performance appears superior to a passive LiF ion source operated on PBFA-II with the same magnetic field topology. © 1999 American Institute of Physics.

Notizen: BaTiO3/SrTiO3 multilayered thin films were deposited on Si and Pt/Si substrates. X-ray diffraction clearly shows the formation of the superstructures. Phase transition properties were studied via dielectric measurements. Glassy behavior, characterized by a strong frequency dispersion of dielectric properties, was found in samples with a total thickness of 400 nm, while in samples with a total thickness of 800 nm, normal ferroelectric phase transitions with two dielectric peaks were observed. A preliminary interpretation assumes that size effects which frustrate long range ferroelectric ordering may lead to the relaxational behavior in BaTiO3/SrTiO3 superstructures. © 1998 American Institute of Physics.

Notizen: The concept of converting a multi-kA relativistic e beam into an ultrahigh-power rf pulse with a magnetically swept e-beam switch is studied. Two short VHF pulses are created by rapidly switching a 60-ns, 900-keV, 15-kA e beam between a pair of inverse e-beam diodes. The switching is done by a transverse 500-G, sinusoidal B field, oscillating at 50 MHz. The e-beam pulses are transported 1.6–4.6 cm in vacuum and ballistically or magnetically focused onto the collectors of the inverse diodes. The inverse diodes extract electrical energy from the deflected e-beam pulses and transport the energy to resistive loads that varied between 30 and 130 Ω. Numerical simulations of the e-beam transport with self-electric and magnetic fields are performed that show the sources of electrical inefficiency. These occur because the e beam is near the space-charge limit during transport. Peak unipolar powers of 2.0 and 0.5 GW are coupled to the resistive loads for the ballistically and magnetically transported e-beam geometries, respectively.

Notizen: A time-resolved magnetic spectrograph was used to measure the energy spectrum of an intense (0.7 TW/cm2) proton beam. A thin gold foil placed at the focus of an ion-diode Rutherford-scattered protons by 90° into the spectrograph, reducing the beam intensity to a level suitable for magnetic analysis. The scattered protons were collimated and then deflected in a samarium-cobalt permanent magnet. The deflected protons were recorded simultaneously on CR-39 and eight p-i-n diodes. A Monte Carlo computer code was used to calculate the sensitivity and resolution of the spectrograph. Data taken on the Proto I accelerator shows a 150–250-keV-wide proton-energy spectrum at each instant in time.

Notizen: A series of experiments was performed with an Applied-B ion diode on the Particle Beam Fusion Accelerator-I, with peak voltage, current, and power of approximately 1.8 MV, 6 MA, and 6 TW, respectively. The purpose of these experiments was to explore issues of scaling of Applied-B diode operation from the sub-TW level on single module accelerators to the multi-TW level on a low impedance, self-magnetically insulated, multimodule accelerator. This is an essential step in the development of the 100-TW level light ion beam driver required for inertial confinement fusion. The accelerator and the diode are viewed as a whole because the power pulse delivered by the 36 imperfectly synchronized magnetically insulated transmission lines to the single diode affects module addition, diode operation, and ion beam focusability. We studied electrical coupling between the accelerator and the diode, power flow symmetry, the ionic composition of the beam, and the focusability of the proton component of the beam. Scaling of the diode impedance behavior and beam quality with electrical drive power is obtained from comparison with lower-power experiments.The diode impedance lifetime was about 10 ns, several times shorter than for lower-power experiments. Azimuthal and top-to-bottom variations of the diode and ion currents were found to be approximately ±10%, compared with an estimated requirement of 5%–7% uniformity to avoid focal blurring by self-magnetic field effects. The ion production efficiency was 80%–90%. However, only 50%±10% of the ion current was carried by protons; the balance was carried by multiply charged carbon and oxygen ions. Activation measurements showed a proton beam energy of approximately 50 kJ. A gas cell filled with 5 Torr of argon was used for beam transport. The macroscopic divergence was 15±10 mrad and the microscopic divergence was 20±15 mrad, values that are similar to those from lower-power experiments. A model of beam focusing is formulated that predicts the proton charge focused onto 0.47-cm radius lithium targets, taking into account beam purity, magnetic bending, small-angle multiple scattering, and intrinsic divergence. The model results and activation measurements of the number of protons focused onto targets agree, and indicate that the spatially averaged (over about 3 cm2) peak focal power was about 0.5 TW/cm.2 The most important limitations on power concentration were found to be the low proton content of the beam, the short impedance lifetime of the diode, and the asymmetric current feed of the accelerator. The short impedance lifetime limited the power coupled to the diode, and caused the voltage at peak ion power to be low, which exacerbates the small-angle scattering problem. The asymmetric feed caused focal blurring through nonuniform self-magnetic bending. At least partly because of the experience gained with low impedance beams during these experiments, the next generation accelerator, the 100-TW Particle Beam Fusion Accelerator-II, has been configured to produce a 25–30-MV Li+ beam rather than a 5-MV proton beam. off