Summary form only given. This paper describes a remote sensing concept that involves active thermal imaging with a millimeter-wave source (ATIMS). High frequency (HF) microwave or millimeter wave radiation can be beamed onto a target, thus generating rapid transient temperature increases in different portions of the target area. The time-dependent thermal contrast can be measured using sensitive infrared (IR) imagers. The technique can in principle be used in many situations where passive infrared imaging is currently used. This talk will include a description of the concept, a discussion of potential advantages and issues, model calculations of the predicted time-dependent temperature profile for various source intensities and materials, and results from preliminary laboratory proof-of-principle experiments. The experiments were performed at the HF microwave materials processing facility at NRL, where an 83 GHz gyrotron has been used to rapidly heat a variety of simple and complex targets. Thermal imaging with a sensitive mid-wavelength IR camera reveals clear signatures in a variety of target configurations. Potential applications include long range detection of explosive devices. Although a variety of heating sources have been used for active thermal imaging, including lasers, flashlamps, and longer wavelength microwaves, millimeter waves appear to be particularly well-suited for long range applications
This paper reports on a study of the statistical properties of single photons emitted by a nitrogen vacancy centre using a confocal microscope and Hanbury-Brown and Twiss interferometer. High signal-to-noise photon correlation histograms from a single NV centre have been recorded for various pump laser powers. The form of these signals can be described by modelling the nitrogen vacancy as a three-level system. The parameters that characterize the model have been measured as a function of pump laser power.
To achieve multi GeV electron energies in the laser wakefield accelerator (LWFA) it is necessary to propagate an intense laser pulse long distances in a plasma without disruption. A three-dimensional envelope equation for the laser field is derived that includes nonparaxial effects, wakefields, and relativistic nonlinearities. In the broad beam, short pulse limit the nonlinear terms in the wave equation that lead to Raman and modulation instabilities cancel. Long pulses (several plasma wavelengths) experience substantial modification due to these instabilities. The short pulse LWFA, although having smaller accelerating fields, can provide acceleration for longer distances in a plasma channel. By allowing the plasma density to increase along the propagation path electron dephasing can be deferred, increasing the energy gain. A simulation example of a GeV channel guided LWFA accelerator is presented. Simulations also show that multi-GeV energies can be achieved by optimally tapering the plasma channel.
Summary form only given. The authors have discovered a new mechanism for pinched transport of a subrelativistic positive ion beam within a narrow plasma channel formed in a gas at a density of (/spl les/100 mtorr), in a regime where the plasma electrons are collisionless. In the simplest implementation, it may be assumed that the channel is preionized to a charge density greater than that of the beam. The space charge of the beam induces a radial inflow of plasma electrons, which provides space charge neutralization. In this regime, the axial dynamics of the plasma electrons are initially dominated by electrostatic (rather than inductive) forces, which leads to a large axial inflow of plasma electrons from the region in front of the beam. The resulting axial current reinforces the beam current, and generates a strong pinch force at the beam head, which is "frozen in" and persists for the remainder of the beam pulse. The authors are investigating the extent to which this mechanism is effective in an initially underdense plasma channel (or even in the absence of a preionized channel), where the build-up of the plasma channel is due to beam-impact ionization.
Ionization processes limit the accelerating gradient and place an upper limit on the pulse duration of the electromagnetic driver in the inverse Cherenkov accelerator (ICA). Group velocity slippage, i.e., pulse lethargy, on the other hand, imposes a lower limit on the pulse duration. These limits are obtained for two ICA configurations in which the electromagnetic driver (e.g., laser or millimeter wave source) is propagated in a waveguide that is (i) lined with a dielectric material or (ii) filled with a neutral gas. In either configuration the electromagnetic driving field is guided and has an axial electric field with phase velocity equal to the speed of light in vacuum, c. The intensity of the driver in the ICA, and therefore the acceleration gradient, is limited by tunneling and collisional ionization effects. Partial ionization of the dielectric liner or gas can lead to significant modification of the dispersive properties of the waveguide, altering the phase velocity of the accelerating field and causing particle slippage, thus disrupting the acceleration process. An additional limitation on the pulse duration is imposed since the group velocity of the driving pulse is less than c and the pulse slips behind the accelerated electrons. Hence for sufficiently short pulses the electrons outrun the pulse, terminating the acceleration. Limitations on the driver pulse duration and accelerating gradient, due to ionization and pulse lethargy, are estimated for the two ICA configurations. Maximum accelerating gradients and pulse durations are presented for a 10 \ensuremath{\mu}m, 1 mm, and 1 cm wavelength electromagnetic driver. The combination of ionization and pulse lethargy effects impose severe limitations on the maximum energy gain in inverse Cherenkov accelerators.
The evolution of longitudinal electron density and temperature profiles in plasma channel produced by a low-current Plexiglas capillary discharge with laser ignition was investigated by spectroscopic methods. The plasma was produced by an electric discharge using a 0.5mm diameter, 15mm long Plexiglas capillary. The electron density measured in near-outlet region was found to be lower by 30%. Simulations show that this variation of the plasma density near the entrance of the capillary can pose substantial difficulties for external injection of electrons for laser wakefield accelerator applications.
A theory is presented for the guiding of relativistic electron beams by rarefied gaseous channels. The analysis is based on analytic computations of the transverse force felt by a rigid‐rod beam propagating off axis from a channel of reduced gas density. The density gradients produce an attractive channel force that can be surprisingly robust, even though it develops from relatively subtle gas chemistry properties. Static numerical calculations support the analytic work. Longitudinal beam coupling and effects that degrade channel guidance are discussed as well.
Ionization processes and pulse lethargy limit the pulse duration, accelerating gradient, and maximum energy gain in the inverse Cherenkov accelerator (ICA). These limits are obtained for two ICA waveguide configurations: (i) lined with a dielectric or (ii) filled with a neutral gas. Ionization of the dielectric or gas, by the electromagnetic driver, can modify the dispersive properties, altering the accelerating field phase velocity, causing particle slippage, and disrupting the acceleration. In addition, the electrons can outrun the pulse (pulse lethargy), terminating the acceleration. Limitations due to ionization and pulse lethargy on the accelerating gradient, pulse duration, and maximum energy gain are obtained and illustrated by several examples.
Possible plasma-based techniques for controlling or modifying the laser pulse in a laser wakefield accelerator (LWFA) are examined, using laser propagation simulation codes. Laser pulse shortening or chopping prior to injection into a plasma guiding channel occurs when the peak laser power is slightly above the critical power for relativistic focusing. A channel-guided LWA may undergo further compression due to relativistic modulation effects. Multi-stage capillary discharges allow the channel density to be extended and tapered for optimal acceleration in future LWFA experiments.
