Search Results (14 total)

No monochromatic (Δω_x/ω_x<1%), high peak brightness [>10²⁰   photons/(mm²×mrad²×s×0.1%   bandwidth)], tunable light sources currently exist above 100 keV. Important applications that would benefit from such new hard x-ray and γ-ray sources include the following: nuclear resonance fluorescence spectroscopy and isotopic imaging, time-resolved positron annihilation spectroscopy, and MeV flash radiography. In this paper, the peak brightness of Compton scattering light sources is derived for head-on collisions and found to scale quadratically with the normalized energy, γ; inversely with the electron beam duration, Δτ, and the square of its normalized emittance, ε; and linearly with the bunch charge, eN_e, and the number of photons in the laser pulse, N_γ:   B-^ _x∝γ²N_eN_γ/ε²Δτ. This γ² scaling shows that for low normalized emittance electron beams (1 nC, 1  mm·mrad, <1  ps, >100  MeV), and tabletop laser systems (1–10   J, 5 ps) the x-ray peak brightness can exceed 10²³   photons/(mm²×mrad²×s×0.1%   bandwidth) near ℏω_x=1   MeV; this is confirmed by three-dimensional codes that have been benchmarked against Compton scattering experiments performed at Lawrence Livermore National Laboratory. The interaction geometry under consideration is head-on collisions, where the x-ray flash duration is shown to be equal to that of the electron bunch, and which produce the highest peak brightness for compressed electron beams. Important nonlinear effects, including spectral broadening, are also taken into account in our analysis; they show that there is an optimum laser pulse duration in this geometry, of the order of a few picoseconds, in sharp contrast with the initial approach to laser-driven Compton scattering sources where femtosecond laser systems were thought to be mandatory. The analytical expression for the peak on-axis brightness derived here is a powerful tool to efficiently explore the 12-dimensional parameter space corresponding to the phase spaces of both the electron and incident laser beams and to determine optimum conditions for producing high-brightness x rays.

We present a detailed comparison of the measured characteristics of Thomson backscattered x rays produced at the Picosecond Laser-Electron Interaction for the Dynamic Evaluation of Structures facility at Lawrence Livermore National Laboratory to predicted results from a newly developed, fully three-dimensional time and frequency-domain code. Based on the relativistic differential cross section, this code has the capability to calculate time and space dependent spectra of the x-ray photons produced from linear Thomson scattering for both bandwidth-limited and chirped incident laser pulses. Spectral broadening of the scattered x-ray pulse resulting from the incident laser bandwidth, perpendicular wave vector components in the laser focus, and the transverse and longitudinal phase spaces of the electron beam are included. Electron beam energy, energy spread, and transverse phase space measurements of the electron beam at the interaction point are presented, and the corresponding predicted x-ray characteristics are determined. In addition, time-integrated measurements of the x rays produced from the interaction are presented and shown to agree well with the simulations.

Electron thermal pressure fluctuations measured in the edge plasma of the Texas Experimental Tokamak Upgrade are a fundamental component of plasma turbulence on both sides of the velocity shear layer. The ratio of specific heats, estimated from fluctuations in electron temperature and electron number density measured simultaneously at the same electrode, indicates that observed fluctuations are adiabatic. The observations are made by means of a novel Langmuir probe technique, the time domain triple-probe method, which concurrently measures multiple plasma properties at each of two electrodes with the temporal and the spatial resolution required to estimate thermodynamic properties in a turbulent plasma.

Two distinct broadband modes of density turbulence propagating in the electron drift direction are simultaneously observed, in the first complete measurement of the wave number frequency spectrum S(k,ω) in the interior of the TEXT-U tokamak plasma. A drift wave-type mode with a peak in the wave number spectrum at k_θ∼4.5 cm ^-1 is found on both sides of the magnetic axis. A long-wavelength mode with k_θ≤2 cm ^-1 is found only on the low field side of the tokamak. The results resolve apparent discrepancies in previous turbulence measurements. Confinement properties of TEXT-U are consistent with the transport predicted by the observed drift wave-type k̅ _θ  =  3 cm ^-1.

Experimental evidence of the nonlinear nature of the broadband edge fluctuations has been obtained in edge turbulence in the Advanced Toroidal Facility torsatron. Whereas little nonlinear wave interaction is found in the scrape-off layer region, three-wave coupling is enhanced in the plasma edge region (r<a_shear). The degree of three-wave coupling strongly depends on the plasma conditions; it decreases in the temperature range (T_e≊10 eV) where the ionization rates depend strongly on T_e suggesting a link between ionization source and turbulence.

Bispectral analysis of Langmuir probe data indicates that coherent nonlinear coupling, in addition to the noncoherent turbulent interactions, exists in the edge plasma of the tokamak TEXT. Not all the modes involved reside within the spectral region of the usual broadband turbulence. At a major resonant surface the small-scale turbulent activity interacts coherently with a localized long-wavelength mode; a signature of regular or coherent structure. By the observed coupling to the transport related turbulence, the long-wavelength mode can influence plasma confinement indirectly. These observations signify the influence of low-order resonant surfaces on the edge turbulence in tokamaks.

Modulation of the gas feed to a tokamak induces a density modulation and also a temperature modulation. Although the density change is fractionally small and the response is linear, the perturbed particle flux can exceed the equilibrium flux. The particle flux Γ thus cannot be a simple homogeneous function of ∇n. The ratio of thermal to particle flux is quite different for the equilibrium and perturbations, precluding a simple linkage of particle and thermal transport. The modulation of the amplitude of density turbulence does not correlate with any of the modulated fluxes.

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.

The electrostatic fluctuations are decorrelated in the region of a naturally occurring E_r×B velocity shear close to the outermost closed flux surface of regular Ohmic TEXT discharges. The concomitant local steepening of the density profile and suppression of the fluctuations are consistent with theoretical predictions. The high-confinement mode found in other tokamaks shows, in exaggerated form, similar characteristics, and could thus be related to the same mechanism leading to a locally improved confinement.

A quantitative comparison of the fluctuation-induced energy flux with the total energy flux has been made in the edge region of the TEXT tokamak using fluctuation measurements from Langmuir, heavy-ion-beam, and magnetic probes. At all but the lowest densities the convected energy flux due to electrostatic fluctuations dominates the energy losses caused by plasma transport. Energy loss through magnetic fluctuations is insignificant in the edge region.

The fluctuating plasma potential and electron density has been measured in Ohmic and neutral-beam–heated tokamak discharges. Radial profiles are presented in the outer two-thirds of the plasma, and the E×B transport calculated. The transport is found to be an order of magnitude larger for beam driven plasmas. Measurements indicate the linearized Boltzman equation is satisfied in the interior, but not at the plasma edge.

Plasma potentials have been measured for the first time in neutral-beam-heated tokamak discharges, with a heavy-ion beam probe. Radial profiles of these potentials are presented for coinjection, counterinjection, and balanced injection, and for plasmas having only Ohmic heating. They are found to be very dependent on the direction of rotation with respect to the plasma current. They are also found qualitatively consistent with those inferred from radial momentum balance with use of the measured toroidal rotation velocity.