Search Results (38 total)

Electron gyroscale fluctuation measurements in National Spherical Torus Experiment H-mode plasmas with large toroidal rotation reveal fluctuations consistent with electron temperature gradient (ETG) turbulence. Large toroidal rotation in National Spherical Torus Experiment plasmas with neutral beam injection generates E×B flow shear rates comparable to ETG linear growth rates. Enhanced fluctuations occur when the electron temperature gradient is marginally stable with respect to the ETG linear critical gradient. Fluctuation amplitudes decrease when the E×B flow shear rate exceeds ETG linear growth rates. The observations indicate that E×B flow shear can be an effective suppression mechanism for ETG turbulence.

Measurements with coherent scattering of electromagnetic waves in plasmas of the National Spherical Torus Experiment indicate the existence of turbulent fluctuations in the range of wave numbers k_⊥ρ_e=0.1–0.4, corresponding to a turbulence scale length nearly equal to the collisionless skin depth. Experimental observations and agreement with numerical results from a linear gyrokinetic stability code support the conjecture that the observed turbulence is driven by the electron-temperature gradient.

The suppression of (neoclassical) tearing modes is of great importance for the success of future fusion reactors like ITER. Electron cyclotron waves can suppress islands, both by driving noninductive current in the island region and by heating the island, causing a perturbation to the Ohmic plasma current. This Letter reports on experiments on the TEXTOR tokamak, investigating the effect of heating, which is usually neglected. The unique set of tools available on TEXTOR, notably the dynamic ergodic divertor to create islands with a fully known driving term, and the electron cyclotron emission imaging diagnostic to provide detailed 2D electron temperature information, enables a detailed study of the suppression process and a comparison with theory.

High temporal and spatial resolution two-dimensional (2D) images of electron temperature fluctuations were employed to study the sawtooth oscillation in the Toroidal Experiment for Technically Oriented Research tokamak plasmas. The 2D images are directly compared with the expected 2D patterns of the plasma pressure (or electron temperature) from various theoretical models. The observed experimental 2D images are only partially in agreement with the expected patterns from each model: The image of the initial reconnection process is similar to that of the ballooning mode model. The intermediate and final stages of the reconnection process resemble those of the full reconnection model. The time evolution of the images of the hot spot or island is partially consistent to those from the full reconnection model but is not consistent with those from the quasi-interchange model.

High resolution (temporal and spatial), two-dimensional images of electron temperature fluctuations during sawtooth oscillations were employed to study the crash process and heat transfer in magnetically confined toroidal plasmas. The combination of kink and local pressure driven instabilities leads to a small poloidally localized puncture in the magnetic surface at both the low and the high field sides of the poloidal plane. This observation closely resembles the “fingering event” of the ballooning mode model with the high-m mode only predicted at the low field side.

Small-scale structures with high poloidal mode numbers (m=10–20) have been observed in the TEXTOR tokamak plasma with pulsed radar reflectometry and an electron cyclotron emission diagnostic, in conjunction with large 2/1 and 1/1 islands. The small islands have a peaked density profile, similar to that of the simultaneously observed large-scale 2/1 islands. This together with the observation that high-frequency density and temperature fluctuations are very pronounced near the X points of the large islands hints to a strongly perturbed magnetic topology around the X points.

The absolute instability is a subject of considerable physics interest as well as a major source of self-oscillations in the gyrotron traveling-wave amplifier (gyro-TWT). We present a theoretical study of the absolute instabilities in a TE₀₁ mode, fundamental cyclotron harmonic gyro-TWT with distributed wall losses. In this high-order-mode circuit, absolute instabilities arise in a variety of ways, including overdrive of the operating mode, fundamental cyclotron harmonic interactions with lower-order modes, and second cyclotron harmonic interaction with a higher-order mode. The distributed losses, on the other hand, provide an effective means for their stabilization. The combined configuration thus allows a rich display of absolute instability behavior together with the demonstration of its control. We begin with a study of the field profiles of absolute instabilities, which exhibit a range of characteristics depending in large measure upon the sign and magnitude of the synchronous value of the propagation constant. These profiles in turn explain the sensitivity of oscillation thresholds to the beam and circuit parameters. A general recipe for oscillation stabilization has resulted from these studies and its significance to the current TE₀₁-mode, 94-GHz gyro-TWT experiment at UC Davis is discussed.

Detailed experimental studies of the first operation of an X-band (8.547 GHz) rf photoinjector are reported. The rf characteristics of the device are first described, as well as the tuning technique used to ensure operation of the 11 / 2-cell rf gun in the balanced π-mode. The characterization of the photoelectron beam produced by the rf gun includes: measurements of the bunch charge as a function of the laser injection phase, yielding information about the quantum efficiency of the Cu photocathode ( 2×10^-5 for a surface field of 100 MV/m); measurements of the beam energy (1.5–2 MeV) and relative energy spread ( Δγ/γ₀  =  1.8±0.2%) using a magnetic spectrometer; measurements of the beam 90% normalized emittance, which is found to be ɛ_n  =  1.65π mm mrad for a charge of 25 pC; and measurements of the bunch duration ( <2 ps). Coherent synchrotron radiation experiments at Ku-band and Ka-band confirm the extremely short duration of the photoelectron bunch and a peak power scaling quadratically with the bunch charge.

A complete, three-dimensional theory of Compton scattering is described, which fully takes into account the effects of the electron beam emittance and energy spread upon the scattered x-ray spectral brightness. The radiation scattered by an electron subjected to an arbitrary electromagnetic field distribution in vacuum is first derived in the linear regime, and in the absence of radiative corrections; it is found that each vacuum eigenmode gives rise to a single Doppler-shifted classical dipole excitation. This formalism is then applied to Compton scattering in a three-dimensional laser focus, and yields a complete description of the influence of the electron beam phase-space topology on the x-ray spectral brightness; analytical expressions including the effects of emittance and energy spread are also obtained in the one-dimensional limit. Within this framework, the x-ray brightness generated by a 25 MeV electron beam is modeled, fully taking into account the beam emittance and energy spread, as well as the three-dimensional nature of the laser focus; its application to x-ray protein crystallography is outlined. Finally, coherence, harmonics, and radiative corrections are also briefly discussed.

The validity of the concept of laser-driven vacuum acceleration has been questioned, based on an extrapolation of the well-known Lawson-Woodward theorem, which stipulates that plane electromagnetic waves cannot accelerate charged particles in vacuum. To formally demonstrate that electrons can indeed be accelerated in vacuum by focusing or diffracting electromagnetic waves, the interaction between a point charge and coherent dipole radiation is studied in detail. The corresponding four-potential exactly satisfies both Maxwell’s equations and the Lorentz gauge condition everywhere, and is analytically tractable. It is found that in the far-field region, where the field distribution closely approximates that of a plane wave, we recover the Lawson-Woodward result, while net acceleration is obtained in the near-field region. The scaling of the energy gain with wave-front curvature and wave amplitude is studied systematically.

The relativistic dynamics of an electron submitted to the three-dimensional field of a focused, ultrahigh-intensity laser pulse are studied numerically. The diffracting field in vacuum is modeled by the paraxial propagator and exactly satisfies the Lorentz gauge condition everywhere. In rectangular coordinates, the electromagnetic field is Fourier transformed into transverse and longitudinal wave packets, and diffraction is described through the different phase shifts accumulated by the various Fourier components, as constrained by the dispersion relation. In cylindrical geometry, the radial dependence of the focusing wave is described as a continuous spectrum of Bessel functions and can be obtained by using Hankel’s integral theorem. To define the boundary conditions for this problem, the beam profile is matched to a Gaussian-Hermite distribution at focus, where the wave front is planar. Plane-wave dynamics are verified for large f numbers, including canonical momentum invariance, while high-energy scattering is predicted for smaller values of f at relativistic laser intensities.

Results are obtained, for the first time in a major tokamak plasma, of the spatial electric field structure of a launched fast magnetosonic wave (FW). The FW launch directionality necessary for current drive has been confirmed at a position close to the FW antenna. The FW electric field profile is determined using X-mode reflectometry. Electric field profiles, calculated using a full-wave numerical code, exhibit many features common to the experimental results. However, differences remain regarding density dependence that suggest alternative damping mechanisms may exist.

The relativistic dynamics of an electron subjected to the classical electromagnetic field of an ultrashort laser pulse is studied theoretically at arbitrary intensities. Frequency modulation effects associated with the nonlinear relativistic Doppler shift induced on the backscattered radiation are analyzed in detail. For circular polarization, an exact analytical expression for the full nonlinear spectrum is derived. For linear polarization, it is found that the scattering of coherent light by a single electron describing a well-behaved trajectory can yield anharmonic spectra when the laser ponderomotive force strongly modulates the electron’s proper time. At ultrahigh intensities, these nonlinear relativistic spectra exhibit complex structures. In addition, the temporal laser pulse shapes best suited to generate narrow Compton backscattered spectral lines at ultrahigh intensities are discussed. © 1996 The American Physical Society.

A recent theory for achieving stability in high-power gyrotron traveling wave (gyro-TWT) amplifiers has been verified. By keeping the interaction length shorter than the critical oscillation length for the competing modes, stability was achieved. The experiment also verified the theoretical prediction that harmonic gyro-TWT's can produce higher power because their weaker interaction yields a higher threshold electron current for oscillation. The second-harmonic gyro-TWT single-stage amplifier stably generated 207 kW, nearly twice the level achieved by fundamental-harmonic gyro-TWT's at a comparable voltage.

The relativistic dynamics of electrons subjected to the electromagnetic field of an intense, ultrashort laser pulse in vacuum is studied theoretically. The effects of both finite pulse duration and beam focusing are taken into account. It is found that when the quiver amplitude of the electrons driven by the laser field exceeds the focal spot radius of a Gaussian beam, the restoring force acting on the charge decays exponentially, and the electrons are scattered away from the focus. This physical process, known as ponderomotive scattering, effectively terminates the interaction within a laser wavelength, and the electrons can escape with very high energy, as the normalized laser field is of the order of or greater than unity. The relation between the scattering angle and the escape energy is derived analytically from the conservation of canonical momentum and energy in the photon field. For a linearly polarized laser field, the interaction produces two jets of high energy electrons. The theory is supplemented by detailed two-dimensional computer simulations.

A covariant expression for the instantaneous radiation damping force acting on an accelerated charged particle is derived within the frame of classical electrodynamics. The radiation pressure of the wave emitted by the charge is averaged on a sphere of radius R to obtain the net force due to the photon momentum recoil, and the limit is taken when R tends to zero, assuming no internal structure of the particle. The relativistic Doppler effects break the symmetry of the instantaneous rest frame dipole radiation pattern, and the Abraham-Becker damping force is obtained.

This paper investigates the inadequacy of using the growth rate from the dispersion relation to represent the bandwidth of a cyclotron autoresonance maser (CARM) amplifier. It is shown that the dispersion relation can accurately predict the amplifier’s bandwidth, but at least three waves that the electron beam couples to must be retained in the calculation. Interference between these waves must be included. The bandwidth of the coupling coefficient for the forward growing wave can be much narrower than the bandwidth of this wave’s growth rate. In addition, the input signal has been observed to be almost totally absorbed by the electron beam at a specific frequency which is a function of the operating conditions. This phenomenon is caused by the beating between the forward constant amplitude wave and the forward growing wave. It is analogous to the Kompfner dip in a conventional linear beam traveling wave tube and can, therefore, become a useful diagnostic for determining the experimental parameters of a CARM amplifier.

The coherent synchrotron radiation process in a waveguide is theoretically investigated. A single, short bunch propagating through a wiggler is considered. In a waveguide, two very distinct regimes are possible. At grazing, where the beam velocity matches the wave group velocity, the bunch emits a single, ultrashort chirped pulse whose duration is determined by the interaction bandwidth and the waveguide dispersion. Away from grazing, where slippage dominates, two distinct pulses are radiated at the Doppler upshifted and downshifted frequencies. Both the time and frequency domain expressions for the radiation characteristics are derived.

Brillouin enhanced four-wave mixing and phase conjugation of microwaves in an unmagnetized hydrogen plasma are observed. Transient and steady-state responses of the plasma and the phase conjugate wave are presented. Low-power, low-density operation agrees well with predictions of simple two-fluid plasma theory.

The authors reply to the comment on confinement degradation and enhanced microturbulence as long-time precursors to high-density-limit text device disruptions.The phenomenological model described in the comment(1) represents a novel interpretation of the long-time precursors to high-density-limit disruptions on TEXT. (AIP)

Long-time precursors of order 5 times the global energy confinement time are observed for disrupting plasmas at the high-density limit on the Texas Experimental Tokamak. These precursors, occurring well in advance of any change in the plasma MHD activity, are reflected in the electron particle and heat transport along with the density fluctuation level. Enhanced microturbulence is proposed as the physical mechanism for the confinement degradation and subsequent disruption.

Detailed measurements of heat and particle transport are made on the TEXT tokamak by analyzing the dynamics of the postcrash sawtooth perturbation. Both the density and temperature perturbations are observed to be transported out through the confinement zone at the same rate and exhibit similar scaling with plasma parameters. The density and temperature sawteeth are in phase and highly correlated. These results suggest a strong coupling of plasma particle diffusion and heat flow.

Strong damping (depletion) of the driver wave (electron plasma wave) after electron acceleration has been observed in the v_p×B acceleration scheme in microwave-plasma interaction experiments. This depletion can be controlled by changing the rise time of the rf pump wave. Time- and space-resolved observations of high-energy electrons reveal the possible existence of different acceleration mechanisms near the resonance absorption layer.

The operation of a high harmonic gyro-traveling-wave-tube amplifier which is based on the synchronous interaction of a rotating beam of large orbit, axis-encircling electrons with a TE_n1 cylindrical waveguide mode is described. Principal results include amplification of the TE₈₁ mode at the eighth harmonic of the cyclotron frequency mode with a small signal gain of 10 dB, an instantaneous interaction bandwidth of 4.3%, and a saturated power transfer of 0.5 kW.

Electron density perturbations produced by sawtooth oscillations in the interior of the Texas Experimental Tokamak are measured with a high-resolution, multichannel, far-infrared interferometer. Abel inversion over the sawtooth perturbation indicates a space-time evolution which is diffusive in character. The diffusion coefficient is determined from a density-pulse–propagation model and found to be appreciably larger than equilibrium values.