Search Results (14 total)

Evidence of relaxation has appeared, for the first time, in the extremely high-β, steady-state field-reversed configuration plasma states driven by rotating magnetic fields (RMF) in the translation, confinement, and sustainment experiment. The plasma self-organizes into a near-force-free state in the vicinity of the magnetic axis, with significant improvement in confinement. Associated with this change in magnetic topology is the appearance of an axial RMF component; this would, in turn, generate a current drive in the poloidal direction, thus sustaining the magnetic helicity. A newly developed two-dimensional “equilibrium-lite” model is employed to analyze the magnetic properties of the final high-confinement state, and shows a large q and a significant magnetic shear in the core.

A one-dimensional, quasistatic model of a capillary discharge plasma has been developed. Such a plasma is useful as a medium to generate plasma waves for acceleration of electrons via processes such as laser wakefield acceleration or plasma wakefield acceleration. Another important characteristic of the plasma is its intrinsic parabolic density distribution near the center of the capillary, which can channel a laser beam along the capillary. The model is intended to be a design tool to aid in the selection of the capillary parameters in order to obtain desired plasma characteristics, e.g., plasma density and matched laser beam radius for guiding. An optional external axial magnetic field can be included, which improves the laser-channeling effect in some cases. The model also enables a measure of the potential for laser damage of the capillary wall. Results are presented for the design of a gas-filled capillary that will be tested during the staged electron laser acceleration–laser wakefield (STELLA-LW) experiment.

A new approach to laser-wakefield acceleration (LWFA) has been analyzed. A seed electron beam bunch precedes the laser pulse into the plasma. This seed bunch initiates formation of plasma waves via a plasma wakefield acceleration mechanism. The amplitude of the plasma waves is subsequently amplified by the laser pulse via a self-modulated LWFA (SM-LWFA) process. This method enables the generation of strong wakefields even when the laser pulse by itself has characteristics that are insufficient for driving resonant LWFA or SM-LWFA. Another advantage is the wakefield formation begins at the seed bunch and does not start from noise as typically occurs in SM-LWFA. This feature may be helpful when the phase of the wakefield must be accurately controlled, for example, when staging multiple LWFA devices in series.

An extremely high-β (over 85%) self-organized field-reversed configuration (FRC) with a spherical-torus- (ST-)like core is produced in the translation, confinement, and sustainment experiment by highly super-Alfvénic translation of a spheromaklike plasmoid. Substantial flux conversion from toroidal into poloidal occurs during the capture process, resulting in the ST-like core. This plasma state exhibits a remarkable stabilizing property for the ubiquitous n=2 centrifugally driven interchange modes present in θ-pinch formed FRCs. This is explained, for the first time, by a simple model taking into account magnetic shear and centrifugal effects. The FRC-ST configuration has up to 4 times improvement in flux confinement times over the scaling of conventional θ-pinch formed FRCs and, thus, a significant improvement in the resistivity and transport.

Presented are details of the staged electron laser acceleration (STELLA) experiment, which demonstrated high-trapping efficiency and narrow energy spread in a staged laser-driven accelerator. Trapping efficiencies of up to 80% and energy spreads down to 0.36% (1σ) were demonstrated. The experiment validated an approach that may be suitable for the basic design of a laser-driven accelerator system. In this approach, a laser-driven modulator together with a chicane creates a train of microbunches spaced apart by the laser wavelength. These microbunches are sent into a second laser-driven accelerator designed to efficiently trap the microbunches in the ponderomotive potential well of the laser electric field while maintaining a narrow energy spread. The STELLA scientific apparatus and procedures are described in detail. In-depth comparisons between the data and model are given including the predicted energy spectrum, energy-phase plot, and microbunch length profile. Data and model comparisons as a function of the phase delay between the microbunches and the accelerating wave are presented. The model is exercised to reveal how the high-trapping efficiency process evolves during the acceleration process.

High-β plasmoids can survive the violent dynamics of supersonic reflection off mirror structures, producing a stable high-β field-reversed configuration (FRC). This shows both the robustness of FRCs and their tendency to assume a preferred plasma state, possibly conforming to a relaxation principle. The key observations are (1) approximate preservation of the magnetic helicity, (2) substantial conversion from toroidal to poloidal magnetic flux, (3) substantial toroidal flow, and (4) a high-β quiescent final state. These results are from the Translation, Confinement, and Sustainment experiment where a disorganized plasmoid is injected at super-Alfvenic speed into a confinement chamber. After successive reflections from end mirrors, the plasmoid settled into a near-FRC state with high β and low toroidal magnetic field. The flux conversion and helicity preservation are inferred by an interpretive model.

Laser-driven electron accelerators (laser linacs) offer the potential for enabling much more economical and compact devices. However, the development of practical and efficient laser linacs requires accelerating a large ensemble of electrons together (“trapping”) while keeping their energy spread small. This has never been realized before for any laser acceleration system. We present here the first demonstration of high-trapping efficiency and narrow energy spread via laser acceleration. Trapping efficiencies of up to 80% and energy spreads down to 0.36% (1σ) were demonstrated.

Conventional metal-wall waveguides support waveguide modes with phase velocities exceeding the speed of light. However, for infrared frequencies and guide dimensions of a fraction of a millimeter, one of the waveguide modes can have a phase velocity equal to or less than the speed of light. Such a metal microchannel then acts as a slow-wave structure. Furthermore, if it is a transverse magnetic mode, the electric field has a component along the direction of propagation. Therefore, a strong exchange of energy can occur between a beam of charged particles and this slow-waveguide mode. Moreover, the energy exchange can be sustained over a distance limited only by the natural damping of the wave. This makes the microchannel metal waveguide an attractive possibility for high-gradient electron laser acceleration because the wave can be directly energized by a long-wavelength laser. Indeed the frequency of CO₂ lasers lies at a fortuitous wavelength that produces a strong laser-particle interaction in a channel of reasonable macroscopic size (e.g., ∼0.6  mm). The dispersion properties including phase velocity and damping for the slow wave are developed. The performance and other issues related to laser accelerator applications are discussed.

The upgraded Accelerator Test Facility (ATF) CO₂ laser located at Brookhaven National Laboratory offers a unique opportunity to investigate laser wakefield acceleration (LWFA) with a 10.6-μm laser, a wavelength where little experimental work exists. While long laser wavelengths have certain advantages over short wavelengths, our modeling analysis has uncovered another important effect. The upgraded ATF CO₂ laser will have a pulse length as short as 2 ps. At a nominal plasma density of ∼10¹⁶  cm^-3, this pulse length would normally be considered too long for resonant LWFA, but too short for self-modulated LWFA. However, our model simulations indicate that a well-formed wakefield is nevertheless generated with electric field gradients of E_z≳2  GV/m assuming 2.5 TW laser peak power. The model indicates pulse steepening is occurring due to various nonlinear effects. It is possible that this intermediate laser pulse length mode of operation may permit the creation of well-formed, regular-shaped wakefields, which would be needed for staging the LWFA process. Discussed in this paper are the model, its predictions for an LWFA experiment at the ATF, and the pulse steepening effect.

Detailed experimental results of staging two laser-driven, relativistic electron accelerators are presented. During the experiment called STELLA (staged electron laser acceleration), an inverse free-electron laser (IFEL) is used to modulate the electron energy, thereby, causing ∼3 fs microbunches to form separated by the laser wavelength at 10.6 μm (equivalent to a 35 fs period). A second IFEL accelerates the electrons depending upon the phase of the microbunches entering the second IFEL with respect to the laser beam driving the second IFEL. The data presented includes electron energy spectra as a function of the phase delay and laser power driving the first IFEL. Also shown is a comparison with the computer model, which includes space charge and misalignment effects.

Staging of two laser-driven, relativistic electron accelerators has been demonstrated for the first time in a proof-of-principle experiment, whereby two distinct and serial laser accelerators acted on an electron beam in a coherently cumulative manner. Output from a CO₂ laser was split into two beams to drive two inverse free electron lasers (IFEL) separated by 2.3 m. The first IFEL served to bunch the electrons into ∼3 fs microbunches, which were rephased with the laser wave in the second IFEL. This represents a crucial step towards the development of practical laser-driven electron accelerators.

Space charge debunching is a major issue for future high-gradient, high-frequency accelerator techniques. Space charge will set limits on the maximum six-dimensional phase space density obtainable in optical or plasma based accelerators. These accelerators will have short microbunches a fraction of an optical wavelength, in which space charge debunching is unmitigated by two-dimensional effects. The element of an accelerator system most vulnerable to space charge is the drift space between the prebuncher and the acceleration sections. A self-consistent model coupling the energy and phase modulation in the drift space is developed. It is shown that both space charge effects and coherent energy spread can be offset by adjusting the prebuncher and beam optics parameters. In the accelerator sections, the large relativistically corrected inertia together with two-dimensional effects combine to make space charge debunching unimportant. The analytical results compare well with PARMELA code simulations.

The relaxation theory of an ideal magnetofluid is developed for a multispecie magnetofluid. Its invariants are the self-helicities, one for each specie. Their “local” invariance in the ideal case follows from the helicity transport equation. The global forms of the self-helicities are investigated for a two-fluid (ion and electron), and their ruggedness in a weakly dissipative system is defended by cascade and selective decay arguments. In general the two-fluid theory predicts relaxed states with finite pressure and sheared flows. The familiar single-fluid relaxation theory, which admits only force-free states, is a reduced case of the present more general theory.

A 580-MW peak power, radially polarized CO₂ laser beam (λ=10.6 μm) focused by an axicon accelerated 40-MeV electrons by ≤3.7 MeV over a 12-cm interaction length (31 MeV/m), using the inverse Cherenkov effect in which a gas is used to slow the light wave. This represents the first direct observation of acceleration using this effect and demonstrates the effectiveness of the radially-polarized-axicon-focused geometry. The observed energy gain agrees with model predictions.