Biomedical subjects

S E Sharapov

Publications and source records attributed to S E Sharapov.

7 recordsLinked to original sources

PubMed2004

Monitoring Alfvén cascades with interferometry on the JET Tokamak.

A microwave interferometry technique is applied for the first time for detecting a discrete spectrum of Alfvén cascade (AC) eigenmodes excited with fast ions in reversed magnetic shear plasmas of the Joint European Torus. The interferometry measurements of plasma density perturbations associated with ACs show an unprecedented frequency and time resolution superior to that obtained with external magnetic coils. The measurements of ACs are used for monitoring the evolution of the safety factor and density of rational magnetic surfaces in the region of maximum plasma current.

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PubMed2004

First gamma-ray measurements of fusion alpha particles in JET trace tritium experiments.

Gamma-ray spectra from nuclear reactions between fusion-born alpha (alpha) particles and Be impurities were measured for the first time in deuterium-tritium plasmas in the Joint European Torus. The time dependence of the measured spectra allowed the determination of the density evolution of fast alpha particles. Correlation between the decay time of the gamma-ray emission and the plasma parameters in different plasma scenarios was established. Results are consistent with classical slowing down of the alpha particles in discharges with high plasma currents and monotonic q-profiles. In low plasma current discharges and in the discharges with large on-axis current holes (extreme reversal central magnetic shear), the gamma-ray emission decay times are shorter than the classical slowing down times, indicating an alpha-particle confinement degradation in such discharges in line with theoretical predictions.

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PubMed2004

Observation of odd toroidal Alfvén eigenmodes.

Experimental evidence is presented for the existence of the theoretically predicted odd toroidicity induced Alfvén eigenmode (TAE) from the simultaneous appearance of odd and even TAEs in a normal shear discharge of the joint European torus. The modes are observed in low central magnetic shear plasmas created by injecting lower hybrid current drive. A fast ion population was created by applying ion cyclotron heating at the high-field side to excite the TAEs. The odd TAEs were identified from their frequency, mode number, and timing relative to the even TAEs.

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PubMed2003

New interpretation of alpha-particle-driven instabilities in deuterium-tritium experiments on the Tokamak Fusion Test Reactor.

The original description of alpha particle driven instabilities in the Tokamak Fusion Test Reactor in terms of toroidal Alfvén eigenmodes (TAEs) remained inconsistent with three fundamental characteristics of the observations: (i) the variation of the mode frequency with toroidal mode number, (ii) the chirping of the mode frequency for a given toroidal mode number, and (iii) the antiballooning density perturbation of the modes. It is now shown that these characteristics can be explained by observing that cylindrical-like modes can exist in the weak magnetic shear region of the plasma that then make a transition to TAEs as the central safety factor decreases in time.

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PubMed2002

Controlling the profile of ion-cyclotron-resonant ions in JET with the wave-induced pinch effect.

Experiments on the JET tokamak show that the wave-induced pinch in the presence of toroidally asymmetric waves can provide a tool for controlling the profile of ion-cyclotron-resonant 3He ions. Direct evidence for the wave-induced pinch has been obtained from the measured gamma-ray emission profiles. Concurrent differences in the excitation of Alfvén eigenmodes (AEs), sawtooth stabilization, electron temperatures, and fast-ion stored energies are observed. The measured location of the AEs and gamma-ray emission profiles are consistent with the fast-ion radial gradient providing the drive for AEs.

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PubMed2002

Alpha-tail production with ion-cyclotron-resonance heating of 4He-beam ions in JET plasmas.

Third-harmonic ion-cyclotron-resonance heating of 4He-beam ions has produced for the first time on the JET tokamak high-energy populations of 4He ions to simulate 3.5 MeV fusion-born alpha (alpha) particles. Acceleration of 4He ions to the MeV energy range is confirmed by gamma-ray emission from the nuclear reaction 9Be(alpha,ngamma) 12C and excitation of Alfvén eigenmodes. Concomitant electron heating and sawtooth stabilization are observed. The scheme could be used in next-step tokamaks to gain information on trapped alpha particles and to test alpha diagnostics in the early nonactivated phase of operation.

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PubMed2000

Chaotic regime of Alfvén Eigenmode wave-particle interaction.

The chaotic regime in Alfvén eigenmode wave-particle interaction is identified for the first time in the tokamak plasma of the Joint European Torus. The Alfvén modes are driven by energetic hydrogen minority ions produced by ion cyclotron resonance heating. The experimental signatures of the chaotic regime include spectral broadening, phase flips, and nonperiodic amplitude variations. These phenomena are found to be consistent with a general nonlinear theory of kinetic instabilities near stability threshold developed by Berk, Breizman, and Pekker.

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