Search Results (9 total)

In a first beam dynamics validation experiment for a new Pulse Line Ion Acceleration (PLIA) concept, the predicted energy amplification and beam bunching were experimentally observed. Beam energy modulation of -80 to +150  keV was measured using a PLIA input voltage waveform of -21 to +12  kV. Ion pulses accelerated by 150 keV, and bunching by a factor of 4 were simultaneously achieved. The measured longitudinal phase space and current waveform of the accelerated beam are in good agreement with 3D particle-in-cell simulations.

The pulse line ion accelerator concept was motivated by the desire for an inexpensive way to accelerate intense short pulse heavy ion beams to regimes of interest for studies of high energy density physics and warm dense matter. A pulse power driver applied at one end of a helical pulse line creates a traveling wave pulse that accelerates and axially confines the heavy ion beam pulse. Acceleration scenarios with constant parameter helical lines are described which result in output energies of a single stage much larger than the several hundred kilovolt peak voltages on the line, with a goal of 3–5  MeV/meter acceleration gradients. The concept might be described crudely as an “air core” induction linac where the pulse-forming network is integrated into the beam line so the accelerating voltage pulse can move along with the ions to get voltage multiplication.

A concept for collective acceleration and focusing of a high-energy electron bunch is presented. The scheme combines an intense relativistic electron beam propagating in low-density gas with a trailing picosecond laser pulse for prompt photoionization of the gas to create the intense collective fields.

A new and relatively simple method has been developed to focus and guide electron beams without the use of a magnetic field. The scheme relies on the electrostatic charging of a highly resistive wire in the presence of a beam. The beam is then strongly guided and focused by the oppositely charged wire. In addition, the highly anharmonic nature of the wire potential leads to rapid phase-mix damping of transverse beam displacements and radial pulsations.

The first-order Raman spectrum of α-quartz, consisting of four totally symmetric A₁ lines, four E lines with "unresolved" LO-TO components, and four LO-TO split E doublets, have been investigated under uniaxial stress. A compressive force F→ was applied along the trigonal axis z-^, along the binary axis x-^, or along z-^^′=(1/√2)(-y-^+z-^). All splittings and/or shifts were found to be linear in stress and hence describable in terms of a linear-deformation-potential theory. From measurements using these three force directions the two deformation-potential constants characterizing each A₁ and the four constants characterizing each E line have been deduced. The extreme sharpness of the 128-cm^-1 E line at liquid-helium temperature allowed a Fabry-Perot interferometer to be used in the study of its behavior. The LO-TO splittings of the E lines at 263, 695, and 1160 cm^-1 were measured to be 1.25 ± 0.09, 2.39 ± 0.10, and -3.02 ± 0.45 cm^-1, respectively, for the phonon wave vector, q→ along y-^; the LO-TO splittings are comparable to the observed linewidths at liquid-helium temperature and were deduced from the zero-stress intercepts of the least-squares fits characterizing the stress dependence of the components. The LO-TO splittings of the 695- and 1160-cm^-1 lines were just resolved at liquid-helium temperature for q→∥y-^. The LO-TO splittings calculated from the reststrahlen spectra for all eight E lines are in satisfactory agreement with those measured in the present Raman study, even as to the unusual negative sign for the splitting of the 1160-cm^-1 line.

Ion current densities up to several kiloamperes per square centimeter and total ion currents ranging from 50 to 150 kA have been produced at voltages of 100-300 kV in a modified relativistic-electron-beam diode. The experiments are in general accord with the predictions of a recent reflex-triode theory.

The effect of uniaxial stress on the one-phonon Raman lines of CdS having A₁, E₁, and E₂ symmetries are studied for compressive force F→, either along the a axis (x-^) or the c axis (z-^) and using a low-temperature stress cryostat. The scattering geometries chosen resulted in the phonon propagation direction q→ being either parallel or perpendicular to z; this in turn allowed the observation of pure E₁(TO), E₁(LO), A₁(TO), and A₁(LO) lines. For F→∥x-^, the lines or their components shift linearly with stress. The E₂ line at 256 cm^-1 splits into two clearly resolved components, the polarization features being consistent with the deformation-potential theory. The E₁(TO) and the E₁(LO) lines separate further, whereas the A₁(TO) and the A₁(LO) lines increase at the same rate. For F→∥z-^, all the lines exhibit shifts linear in stress; the E₁(TO) and the E₁(LO) lines increase at the same rate, and the E₂ line at 256 cm^-1 does not split. From the results for F→∥x-^ and F→∥z-^, the deformation-potential constants characterizing each line have been deduced. A comparison of the piezospectroscopy of the doubly degenerate E₁ line of CdS with that of the E lines of α-quartz is presented along with some new results for the latter. The LO overtones observed in y̅(zz)y geometry shift linearly for F→∥z-^, the rates of shift increase linearly with increasing order. The magnitudes of these shifts suggest that all overtones are of E₁ nature.

Localized coupling structures, such as open-ended wave guides located close to the plasma boundary, radiate a spectrum of electrostatic waves that are accessible to the lower hybrid layer in an inhomogeneous plasma. The energy flow is confined within narrow channels, similar to the resonance cone effect discussed by Fisher and Gould. Radical changes in the energy-flow picture result if the constant-density contours are aligned at a small angle [∼(m_e / m_i)^{1 / 2}] to the magnetic field.

By considering the evolution of an initially localized disturbance, we show that collisionless trapped-particle modes in a Tokamak geometry remain highly localized in the radial direction up to the point of nonlinear saturation. The inferred localization dimension of only a few "banana" widths is considerably less than the normal-mode picture has indicated. We discuss the implications of our results on estimates of the turbulent diffusion coefficient.