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.

Measuring fast ions in fusion plasmas with neutron diagnostics at JET

Otto Asunta, Patrick Blanchard, Yann Camenen, Stefano Coda, Basil Duval, Ambrogio Fasoli, Jonathan Marc Philippe Faustin, Davide Galassi, Javier García Hernández, Jonathan Graves, Jan Horacek, Samuel Lanthaler, Yves Martin, Mikhail Maslov, Federico Nespoli, Hamish William Patten, Alessandro Pau, Antonio José Pereira de Figueiredo, David Pfefferlé, Richard Pitts, Olivier Sauter, Cristian Sommariva, Duccio Testa, Nicola Vianello, Henri Weisen, Dalziel Joseph Wilson, Marco Wischmeier, Liang Yao, Yao Zhou

Fast ions in fusion plasmas often leave characteristic signatures in the neutron emission from the plasma. In this paper, we show how neutron measurements can be used to study fast ions and give examples of physics results obtained on present day tokamaks. The focus is on measurements with dedicated neutron spectrometers and with compact neutron detectors used in each channel of neutron profile monitors. A measured neutron spectrum can be analyzed in several different ways, depending on the physics scenario under consideration. Gross features of a fast ion energy distribution can be studied by applying suitably chosen thresholds to the measured spectrum, thus probing ions with different energies. With this technique it is possible to study the interaction between fast ions and MHD activity, such as toroidal Alfven eigenmodes (TAEs) and sawtooth instabilities. Quantitative comparisons with modeling can be performed by a direct computation of the neutron emission expected from a given fast ion distribution. Within this framework it is also possible to determine physics parameters, such as the supra-thermal fraction of the neutron emission, by fitting model parameters to the data. A detailed, model-independent estimate of the fast ion distribution can be obtained by analyzing the data in terms of velocity space weight functions. Using this method, fast ion distributions can be resolved in both energy and pitch by combining neutron and gamma-ray measurements obtained along several different sightlines. Fast ion measurements of the type described in this paper will also be possible at ITER, provided that the spectrometers have the dynamic range required to resolve the fast ion spectral features in the presence of the dominating thermonuclear neutron emission. A dedicated high-resolution neutron spectrometer has been designed for this purpose.

2019

Particle transport in JET and TCV H-mode plasmas

Mikhail Maslov

Understanding particle transport physics is of great importance for magnetically confined plasma devices and for the development of thermonuclear fusion power for energy production. From the beginnings of fusion research, more than half a century ago, the problem of heat transport in tokamaks attracted the attention of researchers, but the particle transport phenomena were largely neglected until fairly recently. As tokamak physics advanced to its present level, the physics community realized that there are many hurdles to the development of fusion power beyond the energy confinement. Particle transport is one of the outstanding issues. The aim of this thesis work is to study the anomalous (turbulence driven) particle transport in tokamaks on the basis of experiments on two different devices: JET (Joint European Torus) and TCV (Tokamak à Configuration Variable). In particular the physics of particle inward convection (pinch), which causes formation of peaked density profiles, is addressed in this work. Density profile peaking has a direct, favorable effect on fusion power in a reactor, we therefore also propose an extrapolation to the international experimental reactor ITER, which is currently under construction. To complete the thesis research, a comprehensive experimental database was created on the basis of data collected from on JET and TCV during the duration of the thesis. Improvements of the density profile measurements techniques and careful analysis of the experimental data allowed us to derive the dependencies of density profile shape on the relevant plasma parameters. These improved techniques also allowed us to dispel any doubts that had been voiced about previous results. The major conclusions from previous work on JET and other tokamaks were generally confirmed, with some minor supplements. The main novelty of the thesis resides in systematic tests of the predictions of linear gyrokinetic simulations of the ITG (Ion Temperature Gradient) mode against the experimental observations. The simulations were performed with the GS2 code. The parameter dependencies of plasma density gradient, as observed on JET, are a good agreement with those from the simulations over a wide range. The simulations done for the TCV case, which are in a different physics parameter domain, are also in partial agreement with the experiment. Complete agreement is not out of the question, but will remain a goal for the future, when measurements of one of the most important parameters will be available. The good agreement between the experiment and the simulations suggests that the ITG instability may be responsible for the majority of the anomalous inward particle convection observed in tokamaks. Both the simulations and the empirical data extrapolations predicting a peaked density profile in ITER plasma conditions, instead of a flat one, as was assumed during the concept design period.

EPFL2009

Experimental investigation of fast ion dynamics in TORPEX toroidal plasmas

Boris Roulet

The goal of the present diploma work is to experimentally investigate the fast ion physics in TORPEX plasmas, in which interchange modes with flute characteristics and intermittent transport events, i.e. blobs, are observed to dominate the plasma dynamics. During the first part of the diploma, the Candidate will test different fast ion emitters on a test bench, which will be then mounted into the BN casing to form the fast ion source. In parallel, the Candidate will design an innovative in-vessel movable system, which will displace toroidally the fast ion source, thus allowing 3D measurements of the fast ion beam. In the second part of the diploma, the detector will be mounted onto the existing 2D position system and the Candidate will focus on energy-resolved 2D measurements of the fast ion beam. Provided that the in-vessel movable system is timely and successfully constructed, the measurements will be repeated at different toroidal positions of the source, thus providing first 3D and energy resolved measurements of fast ion profiles. The measurements will be performed together with sets of movable Langmuir probe arrays, which provide complete monitoring of the plasma dynamics with fine spatial and temporal resolution.

2010

Measuring fast ions in fusion plasmas with neutron diagnostics at JET

2019

Particle transport in JET and TCV H-mode plasmas

Mikhail Maslov

EPFL2009