25 research outputs found
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Alpha particle loss in TFTR deuterium-tritium plasmas with reversed magnetic shear
The confinement and loss of fusion alpha particles are examined for reversed magnetic shear plasmas in TFTR. Such plasmas, with high central q and non-monotonic q profiles can exhibit remarkably reduced energy and particle transport of the thermal ions. However, these same conditions are theoretically predicted to produce high levels of stochastic ripple loss of suprathermal particles, which may reduce the efficiency of plasma heating by the alpha particles and other heating schemes involving fast ions. This paper presents calculations of guiding-center code alpha particle orbit loss from deuterium-tritium (DT) simulations of TFTR deuterium-only experiments. They are compared to results of measurements made in DT reversed shear plasmas of both the confined alpha particle distribution and the alpha particles lost from the plasma. Large fast particle losses have also been found in reversed shear ITER simulations (up to 20%) and from measurements of triton burnup in reversed shear experiments on JT-60U (12%)
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Confinement and the safety factor profile
The conjecture that the safety factor profile, q(r), controls the improvement in tokamak plasmas from poor confinement in the Low (L-) mode regime to improved confinement in the supershot regime has been tested in two experiments on the Tokamak Fusion Test Reactor (TFTR). First, helium was puffed into the beam-heated phase of a supershot discharge which induced a degradation from supershot to L-mode confinement in about 100 msec, far less than the current relaxation time. The q and shear profiles measured by a motional Stark effect polarimeter showed little change during the confinement degradation. Second, rapid current ramps in supershot plasmas altered the q profile, but were observed not to change significantly the energy confinement. Thus, enhanced confinement in supershot plasmas is not due to a particular q profile which has enhanced stability or transport properties. The discharges making a continuous transition between supershot and L-mode confinement were also used to test the critical-electron-temperature-gradient transport model. It was found that this model could not reproduce the large changes in electron and ion temperature caused by the change in confinement
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Core Poloidal Rotation and Internal Trnasport Barrier Formation in TFTR
Impurity poloidal rotation velocities have been measured in the core of TFTR plasmas using a new spectroscopic diagnostic. Two types of transitions to enhanced confinement in reversed shear plasmas are examined. A bifurcation in carbon poloidal rotation is observed to occur before the transition to enhanced confinement for one of these types, while other measured plasmas parameters remain constant. A narrow radial region with reversed poloidal rotation and rotational shear is established 60-100 ms before the transition, and is associated with a large negative radial electric field
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Neoclassical Tearing Modes in Tokamak Fusion Test Reactor Experiments Part 1. Measurements of Magnetic Islands and Delta Prime.
Tearing-type modes are observed in most high-confinement operation regimes in TFTR. Three different methods are used to measure the magnetic island widths: external magnetic coils, internal temperature fluctuation from the electron cyclotron emission (ECE) diagnostic, and an experiment where the plasma major radius is rapidly shifted ("Jog" experiments). A good agreement between the three methods is observed. Numerical and analytic calculations of delta prime (the tearing instability index) are compared with an experimental measurement of delta prime using the tearing-mode eigenfunction mapped from the jog data. The obtained negative delta prime indicates that the observed tearing modes cannot be explained by the classical current-gradient-driven tearing theory
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Measuring the coherence properties of light emission from laser-plasma interactions. Final report
Several detrimental instabilities can be excited when a high-intensity laser interacts with plasma. The temporal evolution and spectra of the scattered light emitted by many of these instabilities are used to characterize the instabilities and to benchmark theories. It has been difficult to image the emission region with sufficient resolution to make quantitative comparisons with theory. Direct measurement of the emission region would yield information on ponderomotive steepening phenomena, the true emission zone of convective instabilities, and on the saturation of absolute instabilities. The increase in laser intensity caused by the filamentation instability is conjectured to elevate the levels of parametric instabilities found in high-energy laser-plasma interactions. Because the diameter of the filaments is very small (on the order of 10 {micro}m), it is impossible to image the emission sites directly and either to prove or to disprove this conjecture. The research reported here examines an alternate method of measuring the emission region of scattered light from parametric instabilities. This report provides a brief background of coherence theory by defining the relevant parameters in Section 2. A concrete example of the effect that multiple scattering sites would have on the proposed measurement is provided in Section 3. The following section briefly describes experiments that might be able to demonstrate the proposed technique. The conclusion raises the issue of coherence and its effect on the expected angular distribution of scattering light from parametric instabilities
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Energy cascading in the beat-wave accelerator
A review is given of energy cascading in the beat-wave accelerator. The properties of the electromagnetic cascade and the corresponding plasma-wave evolution are well understood within the framework of an approximate analytic model. Based on this model, idealized laser-plasma coupling efficiencies of the order of 10% do not seem unreasonable. 28 refs