M. Albergante

Neutron spectroscopy measurements and modeling of neutral beam heating fast ion dynamics

The energy spectrum of the neutron emission from beam-target reactions in fusion plasmas at the Joint European Torus (JET) has been investigated. Different beam energies as well as injection angles were used. Both measurements and simulations of the energy spectrum were done. The measurements were made with the time-of-flight spectrometer TOFOR. Simulations of the neutron spectrum were based on first-principle calculations of neutral beam deposition profiles and the fast ion slowing down in the plasma using the code NUBEAM, which is a module of the TRANSP package. The shape of the neutron energy spectrum was seen to vary significantly depending on the energy of the beams as well as the injection angle and the deposition profile in the plasma. Cross validations of the measured and modeled neutron energy spectra were made, showing a good agreement for all investigated scenarios.

Comparison of measured and simulated fast ion velocity distributions in the TEXTOR tokamak

Here we demonstrate a comprehensive comparison of collective Thomson scattering (CTS) measurements with steady-state Monte Carlo simulations performed with the ASCOT and VENUS codes. The measurements were taken at a location on the magnetic axis as well as at an off-axis location, using two projection directions at each location. The simulations agree with the measurements on-axis, but for the off-axis geometries discrepancies are observed for both projection directions. For the near perpendicular projection direction with respect to the magnetic field, the discrepancies between measurement and simulations can be explained by uncertainty in plasma parameters. However, the discrepancies between measurement and simulations for the more parallel projection direction cannot be explained solely by uncertainties in plasma parameters. Here anomalous fast ion transport is a possible explanation for the discrepancy.

Neutron spectroscopy measurements of tritium beam transport at JET

A detailed description of the 14 MeV neutron emission in plasmas heated by neutral beam injection has been carried out by coupling Monte Carlo calculations of the neutron emission spectrum with TRANSP modelling of the beam ion energy distributions. The model is used to study tritium beam injection experiments of the JET trace tritium campaign for internal transport barrier (ITB) and H-mode discharges. For ITB discharges, the measured neutron emission spectrum is well described by modelling using as input the beam ion distribution calculated with TRANSP. For H mode discharges the neutron spectrum can be reproduced only if high energy tritons are lost from the plasma, suggesting the possible role of low frequency tearing modes on the beam ions. The presented results are of relevance for tritium beam transport studies in trace tritium experiments and, more generally, for deuterium and tritium transport studies in high power experiments using neutron emission spectroscopy.

Anomalous transport of energetic particles in ITER relevant scenarios

The anomalous transport of energetic ions in the presence of turbulent fields is investigated. Nonlinear simulations of a steady-state ITER [R. Aymar et al., Nucl. Fusion 41, 1301 (2001)] scenario have been carried out using the gyrokinetic turbulence code GENE [F. Jenko et al., Phys. Plasmas 7, 1904 (2000)], modeling the suprathermal particles as high temperature Maxwellian distributions in the passive tracer limit. Velocity space analysis shows that single particle diffusivities of ions above the critical energy are significantly larger than their neoclassical counterpart.

Numerical modelling of electromagnetic turbulent transport of energetic ions in burning plasmas

We investigate the redistribution of the neutral beam driven current in the presence of small scale turbulence in the ITER steady-state scenario. Gyrokinetic simulations show that anomalous transport of beam ions can be larger than collisional estimates. The impact on the beam driven current in ITER is studied with a single particle following code. The results indicate that the current driven by the 1MeV neutral beam injection is not significantly redistributed by the microturbulent fields. The numerical investigation shows that a larger impact is expected for lower energy neutral beams.

Microturbulence driven transport of energetic ions in the ITER steady-state scenario

Modelling of microturbulence-driven transport of energetic ions in an ITER steady-state scenario is presented. Results indicate that a significant fraction of the velocity space distribution of alpha particles and deuterium ions can be transported above neoclassical predictions. Turbulent magnetic fluctuations are found to significantly enhance the fast ion diffusivity. Overall, the conclusion that turbulent fields have a limited influence on the transport of fusion born alpha particles is drawn. At the same time, anomalous fast ion diffusion could have an impact on the neutral beam driven current in ITER.

Assessment of turbulent beam ion redistribution in tokamaks through velocity space-dependent gyrokinetic analyses

We present the interface between a gyrokinetic code and a guiding centre code dedicated to the study of fast ion turbulent transport. A set of velocity space-dependent (kinetic) transport quantities, representing the link between the two codes, is presented. The code suite is applied to DEMO and TCV plasmas. While negligible alpha particle transport is observed for both tokamaks, important beam ion redistribution is obtained for simulations of DEMO. Results for the TCV tokamak demonstrate that the influence of turbulent fields on fast ion transport strongly depends on the plasma scenario.

Helical core tokamak MHD equilibrium states

Bifurcated magnetohydrodynamic tokamak equilibrium states with axisymmetric or helical core structure are computed. When a peaked pressure profile is chosen, the helical core structures appear like the {em snakes} that are observed in the JET tokamak. They also have the allure of saturated ideal internal kinks. The existence of a magnetic island is not a requisite condition. Novel equilibrium states that can model the snake are obtained for a JET configuration when the

Evidence for a new path to the self-sustainment of thermonuclear fusion in magnetically confined plasmas

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Observations on turbulence and beam-ion driven modes in TEXTOR

-profile has weak reversed magnetic shear with minimum