[en] The UO2 in a spent nuclear reactor fuel element is separated from the metal cladding of Zr, a Zr alloy or stainless steel in a 0.01-14N HNO3 solution by electrolysis in a cell where the metal or metal alloy part is connected as a cathode and current flows through the cell. The electrolytic cell has at least 1 anode of Pt, Ti, Ti coated with Pt or Nb coated with Pt. (orig./PW)

[en] Full text: It is shown that gradual (more than a factor of two, in some cases - down to zero in the lab frame) reduction of the mode frequency (the so called frequency chirping) can be attributed to the reactive torque exerted on the plasma during the fishbone instability burst, which slows down the plasma rotation inside the q = 1 surface and reduces the mode frequency in the lab frame, while frequency in the plasma frame remains constant. This torque arises due to imbalance between the power transfered to the mode by energeric ions and the power of the mode dissipation by thermal species. Estimates show that the peak value of this torque exceeds the neutral beam torque in modern tokamaks and in ITER. The line-broadened quasilinear burst model, properly adapted for the fishbone case, is capable of reproducing the key features of the bursting mode. (author)

[en] The frequency splitting between m=n=1 eigenmodes in tokamak discharges with two q=1 surfaces and sheared toroidal rotation has been calculated. The instability domain of the ''doublet'' in the parameter space has been established for the first time. The results are consistent with observations of the frequency splitting in ASDEX Upgrade [O. Gruber et al., Nucl. Fusion 39, 1321 (1999)] and DIII-D [J. Luxon, Nucl. Fusion 42, 614 (2002)] tokamaks.

[en] It is suggested that relatively rare, but challenging for the existing theory Alfven cascades with downward frequency sweeping are actually the infernal Alfven eigenmodes (IAEs). Such modes exist in discharges with flat or weakly reversed q-profile in the broad central region, when the value of the safety factor in this region is slightly above the integer or low-order rational. Similar to the toroidal Alfven eigenmode, but in contrast to the ''conventional'' Alfven cascade with upward frequency sweeping, the spectrum of IAE is almost degenerate with respect to the mode numbers. Both features mentioned above are consistent with experimental observations.

No abstract available

[en] It is shown that, in the tokamak plasmas with broad low-shear central core and safety factor q₀ > or approx. 1, there exists a low-frequency global Alfven eigenmode capable of resonating with precession of the trapped energetic ions. This mode has the dominant numbers m=n=1, but the coupling with the upper toroidal sideband is crucial both for the eigenmode formation and its excitation by energetic ions. The properties of this mode are consistent with observations of the low-frequency n=1 mode driven by energetic ions in the ''hybrid'' discharges with perpendicular injection on the JT-60U tokamak [N. Oyama, A. Isayama, G. Matsunaga et al., Nucl. Fusion 49, 065026 (2009)].

[en] The current in a tokamak reactor can be sustained by fusion α particles in a noninductive manner, by virtue of the asymmetry of the region from which fast ions are lost. The conditions under which the α particles can sustain the current in the plasma are determined. Two types of fusion reactors are proposed, with structural differences which stem from different impurity levels near the magnetic axis

[en] The main difference among fusion alphas born in thermal-thermal, beam-thermal and beam-beam DT reactions is the significant energy broadening as well as the anisotropy induced by non-thermal reactants. JET experiments dedicated to detect these peculiarities were performed with blips of tritium NBI into deuterium plasmas, with and without deuterium NBI. Measurements of the decay of γ-rays (emitted in interactions of fusion-born alpha particles with beryllium impurity ions present in the plasma) after a tritium NBI blip into the deuterium plasma demonstrated higher rates of the γ-decay rates in current hole and in low current dischargers while some monotonic high current discharges demonstrated anomalously slow γ-decay. These higher rates were attributed to a reduced quality of alpha-particle confinement resulting form Fokker-Plan modelling. In the present paper we extended this model, incorporating first orbit los and convective radial transport of alphas by, accommodating the effects of anisotropy and of alpha-source broadening in energy. Furthermore we consider the effect of alpha losses induced by pitch-angle scattering, and also investigate the relaxation of alpha distributions as dependent on the position in the plasma cross section. The latter is important for the purposes of the envisaged multichannel γ-ray diagnostics of fast ions in JET and in ITER. For both beam-thermal and beam-beam fusion spectra, modelling results demonstrate a significant dependence of the relaxation of the alpha distribution on the spatial position in the plasma cross section. This dependence becomes even more essential in the case of low current and current hole plasmas due to the enhanced effect of first orbit loss like shown in Fig. 1 Accounting of the beam-beam alphas helps to explain the anomalously slow γ-decay rates observed in some higher current (≥2MA) plasmas with monotonic q-profile and thus yields satisfactory agreement of the modelling with JET measurements. (Author)

[en] We consider the impact of transition particles on the Alfvén waves and electrostatic trapped ion modes in a stellarator with closed contours of the second adiabatic invariant, which do not perfectly coincide with magnetic surfaces (a so-called quasi-isodynamic (QI) stellarator). Due to orbit transitions, the damping rate of Alfvén eigenmodes on localized electrons becomes independent of the collision frequency in the broad range of low-collision rates. Dissipative trapped ion modes (Kadomtsev–Pogutse (KP) modes) induced by the ion temperature gradient (ITG) are strongly stabilized by collisionless orbit transitions. (paper)

[en] Energetic ions, which are used for plasma heating in modern tokamaks, as well as fusion alphas, which will sustain fusion burn in future reactors, have a gyro-radius comparable with the half-width of magnetic islands, which are often present in discharges with high plasma pressure. When such ions are injected inside the island, they encircle the magnetic axis of the island, which is formed by magnetic field lines with the island O-points. As a result, a population of particles with strong azimuthal flow around the axis of the island is formed. In the present work it is shown that such population excites high-frequency modes, which are localized inside and move together with the island. The mode frequency (growth rate) can considerably exceed the frequency of the lower hybrid resonance (fast ion gyro-frequency). The possible relation of these modes with observed anomalous scattering of the gyrotron beams off magnetic islands is discussed. (paper)