Measuring fast ions in fusion plasmas with neutron diagnostics at JET

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.

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

Étude du transport d'énergie thermique dans les plasmas du tokamak à configuration variable au moyen de chauffage électronique cyclotronique

Yann Camenen

The development of controlled thermonuclear fusion, a quasi-unlimited energy source suitable for large scale electricity production, is one of the main goals of plasma physics research. Among the directions explored to date, the use of toroidal devices called tokamaks to create and confine hot plasmas using strong magnetic fields is particularly promising. The energy used to heat the plasma must remain well confined in order to achieve plasma temperatures higher than one hundred millon degrees for a sufficiently long period to obtain numerous fusion reactions. In the frame of efficient electricity production, maximising the energy confinement is also essential to achieve the required temperature with the lowest heating power. In tokamak plasmas, energy losses are mainly due to radiation and radial energy transport from the plasma core to the edge. A significant fraction of plasma physics research is therefore dedicated to the study of radial transport in tokamaks and the exploration of new operation regimes characterised by a low transport level. The development and optimisation of diagnostics used to observe plasmas is also part of this work. This thesis work, performed on the Tokamak à Configuration Variable (TCV) in Lausanne, covers the implementation and exploitation of a multi-channel soft X-ray detector with high spatial and temporal resolution, together with the development of the tomographic inversion routines used for data analysis. The detector, comprised of two superposed wire chambers, has been tested and calibrated using an X-ray source and then installed onto the tokamak. The position of the detector was chosen such as to observe the whole plasma cross-section with maximum spatial resolution leading to high quality tomographic inversions. A mobile absorber holder was installed between the plasma and the wire chambers. The energy range of the soft X-ray emission observed by the detector was thus chosen by selecting the appropriate absorber. These various features have made possible the use of the detector for numerous studies and in particular for the spatial and temporal characterisation of the plasma internal transport barrier formation. Plasma shaping abilities covering a wide range of plasma elongations and triangularities, including negative values, are one of the strengths of the TCV tokamak. For instance, plasmas with elongated cross-sections offer higher energy confinement as well as higher plasma current and pressure limits. However, the increase of the plasma vertical instability growth rate with elongation makes the vertical control of elongated plasmas difficult, in particular if the plasma current profile is too peaked. As the current profile is usually peaked for low plasma currents, current profile broadening is required there to achieve high elongation. During this thesis, a current profile broadening method based on temperature profile modification by localised EC heating has been studied in detail. The mechanism of this method has been documented and the optimal conditions for the EC power deposition determined. Using these conditions, the TCV operational space has been extended towards higher elongation at low current. The highest elongation obtained at low current has been increased by over 25% permitting the exploration of the plasma transport properties in this regime. The flexibility of the TCV EC heating system has also been used to investigate radial electron heat transport in L-mode plasmas. For the first time, the normalised temperature gradient has been varied by a factor of four and its influence on electron heat transport has been separated from that of the electron temperature. Electron heat transport increases strongly with the normalised temperature gradient, for values between 6 and 10, and then becomes independent of this parameter. In addition, electron heat transport increases with increasing electron temperature, decreasing density and increasing effective charge. The electron heat transport dependence on these three parameters can be cast as a single dependence on the plasma collisionality. TCV shaping abilities have then been used to test the influence of plasma triangularity. The main variations of the level of electron heat transport are described by a decrease of the electron heat diffusivity towards negative triangularity and high collisionality. At constant collisionality, electron heat transport is two times lower at a negative triangularity of –0.4 than at a positive triangularity of +0.4. Concerning micro-instabilities, gyro-fluid and gyro-kinetic simulations indicate that TEM and ITG instabilities are at play in these plasmas. The good qualitative agreement between the observed experimental dependencies and the predictions of simulations suggests strongly that the TEM instabilities are involved in the transport of electron heat. The experimental study provides dependable scaling of the electron heat transport on plasma parameters that can now be used to test the prediction of transport simulations. New elements such as the saturation of electron heat transport at high values of the normalised temperature gradient and the decrease of electron heat transport towards negative triangularities have been demonstrated.

EPFL2006

A Fast Ion Loss Detector for the TCV Tokamak

Lorenzo Stipani

A Fast Ion Loss Detector (FILD) was designed, assembled, installed and commissioned for TCV. This is a radially positionable, scintillator based detector that provides information on the 2D fast-ion velocity space lost at the probe's location. The collected particles are collimated inside the probe head and impinge upon a plate coated with a scintillator material that emits light. The photon flux is relayed to two acquisition systems: a sCMOS camera for high spatial resolution measurements of the emission locations in the scintillator, and fast photo-multipliers that allow time-correlation studies of fast-ion losses with high-frequency electromagnetic fluctuations. From its position with respect to the confined plasma and the fast-ion sources on TCV, FILD probes a small portion of the lost fast-ion phase space with high velocity-space and temporal resolution. Therefore, it naturally complements other available diagnostics such as neutral particle analysers, spectroscopy techniques for light emission from energetic particles following CX reactions, or neutron counters, which feature a broader, but less resolved, spatial coverage of fast-ion dynamics.The TCV-FILD design introduces some novelties for exploring new ranges of operation that may be adopted in similar systems for other Tokamaks, such as ITER. Two entrance slits can collect particles that circulate in co- and cntr-plasma current directions. This will be particularly useful when the second NBH system, injecting in the opposite toroidal direction, with particle energies that can excite strong Alfvénic modes, will be operated on TCV. A controlled pneumatic linear actuator radially positions the detector to expose the slits to the particle flux up to 9mm inward of the vessel wall. A plug-in design was conceived to facilitate diagnostic installation. The diagnostic was installed and commissioned during TCV experiments in 2020. The sensitivity of the detector to the local magnetic field line direction was investigated. The direction of the plasma current was found to select which of the two slits may be traversed by lost particles. The operational limits in discharges with NBH with total delivered energies up to 1MJ were assessed with the help of sensors monitoring the temperature of the probe head and cameras detecting visible light emissions resulting from the graphite shield heating by particle fluxes. Using FILD, fast-ion losses were detected for the first time on TCV in plasma discharges exhibiting strong MHD modes. Ejections of energetic ions were found correlated in time with Sawtooth crashes, as a sequence of individual events, and in strong phase coherence, as a continuous loss in time, with magnetic perturbations of a saturated and toroidally rotating magnetic island of a NTM. These observations, in addition to providing initial results on the relevant physics phenomena, stimulating further experiments and comparisons with theory, demonstrate the ability of TCV-FILD to provide valuable information in plasma discharges of interest for fast-ion studies. These measurements and additional information from other TCV diagnostics may now be combined to reconstruct, with tomographic inversion techniques, more of the fast-ion phase space. This will be used to identify the conditions for the excitation/suppression of magnetic instabilities, develop methods for their real-time control with heating and/or shaping actuators and investigate their dependencies on plasma parameters.

EPFL2021