Abstract This paper describes present status of magnetic measurements in the Large Helical Device (LHD). The magnetic measurements have been mainly applied for estimation of global parameters and for magnetohydrodynamic (MHD) study rather than the equilibrium control because of net current-free plasmas. The techniques of diamagnetic flux measurement and MHD mode analysis are introduced. The estimation of the diamagnetic flux strongly depends on the plasma currents inducing an eddy current on continuous helical coils. The obtained diamagnetic energy is almost consistent with kinetic energy within the measurement error. The MHD modes have been identified through comparison of magnetic probe signals with virtual perturbation generated by multifilament currents on Boozer coordinates based on three-dimensional MHD equilibria. The validity of this technique was considered through the pressure gradient control experiments.

Magnetic Measurements in LHD

/pdf/progress-in-modelling-fast-ion-d-alpha-spectra-and-neutral-4jsgarcahb.pdf

Progress in modelling fast-ion D-alpha spectra and neutral particle analyzer fluxes using FIDASIM

At ASDEX upgrade (AUG) a new framework for the evaluation of impurity densities based on measurements from charge exchange recombination spectroscopy (CXRS) diagnostics has been developed. The charge exchange impurity concentration analysis code, or CHICA, can perform these calculations for all of the beam-based CXRS diagnostics at AUG and is equipped with the atomic data for all of the regularly measured charge exchange spectral lines (He, Li, B, C, N, O, and Ne). CHICA includes four different methods for the evaluation of the neutral density populations, which feature different implementations and contain varying levels of sophistication. These methods have been thoroughly benchmarked against one another, enabling the important processes for the evaluation of neutral densities to be identified as well as the neutral populations that are most critical to the accurate interpretation of the measured CXRS intensities. For the AUG neutral beams, charge exchange with the ground state of the first energy component is typically the dominant contribution to the measured CXRS intensities, but emission from reactions with the n = 2 beam halo population can contribute up to 35% to the total signal and must be included in the analysis. Neglect of this population leads to incorrect magnitudes and incorrect profile shapes of the calculated impurity density profiles. The edge lines of sight (LOS) of the core CXRS diagnostics at AUG intersect the edge pedestal inside of the neutral beam volume. Therefore, the impurity density is not constant along the LOS, complicating the interpretation of the measured intensities. Within CHICA a forward model for the edge impurity densities has been implemented, enabling the reconstruction of accurate edge profiles.

Evaluation of impurity densities from charge exchange recombination spectroscopy measurements at ASDEX Upgrade

The neutral beam (NB) fast ion confinement in the Large Helical Device (LHD) is studied for several full field ( in the standard configuration It is clarified that the NB fast ion confinement improves when the plasmas are shifted inward Moreover, it is also found that the simulation, which considers the deuteron dilution effect due to the presence of impurity ions, can describe a neutron emission rate consistent with the measurement

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022377820000525

Studies of the fast ion confinement in the Large Helical Device by using neutron measurement and integrated codes

https://iopscience.iop.org/article/10.1088/1741-4326/abf953/pdf

Stability of beta-induced Alfvén eigenmodes (BAE) in DIII-D

Understanding the effect of Alfven eigenmodes (AEs) and neoclassical tearing modes (NTMs) on fast ions is highly important for fusion reactors due to potentially strong resonant interactions between the fast ions and the modes. Here, we use the four-view fast-ion D-alpha (FIDA) diagnostic installed in the DIII-D tokamak to reconstruct the fast-ion velocity distribution at two radial positions during two sequential discharges with strong and weak mode activity, respectively. The velocity-space coverage of the diagnostics, however, only allows reliable reconstructions of fast ions with positive pitches. Therefore, we suggest new tomographic inversion methods relying on prior information outside the well-diagnosed region. We find that within the population of fast ions with positive pitches, ions at all energies are transported away from the measurement volumes. Comparisons between the reconstructions and kick model simulations, where the mode activity is considered, reveals that low-frequency modes such as the NTMs and low-frequency AEs contribute significantly to the positive-pitch fast-ion transport in the central measurement volume, whereas TAEs and EAEs become important farther out and are responsible for decreased fast-ion confinement.

/pdf/tomography-of-the-positive-pitch-fast-ion-velocity-3h5c95jc35.pdf

Tomography of the positive-pitch fast-ion velocity distribution in DIII-D plasmas with Alfvén eigenmodes and neoclassical tearing modes

Fast-ion D alpha diagnostic with 3D-supporting FIDASIM in the Large Helical Device

Evaluation of an Energetic Particle Profile Using a Tangential-FIDA Diagnostic in the Large Helical Device

The weight function W(X) is a diagnostic sensitivity to phase-space variables X that relates the measured signal C to the distribution function F(X) through the equation C=∫W(X)F(X)dX . In the present work, an algorithm to calculate W for a diagnostic that measures 3 MeV protons produced in d(d,p)t fusion reactions between a fast ion and a thermal deuteron is developed. The emitted protons escape the tokamak on curved orbits and are detected. These curved orbits constitute effective diagnostic ‘sightlines’. The presented algorithm accounts for the complications associated with these curved sightlines. An initial calculation of time-reversed proton orbits computes effective solid angles and sightlines for the relevant range of incident proton velocity vectors. These precomputed orbits are then used within the framework of FIDASIM (2020 Plasma Phys. Cont. Fusion 62 105008) to calculate the reactivity averaged over the thermal distribution of the ‘target’ deuterons and the probability that a fast ion of specified energy and pitch has a gyroangle that is consistent with the kinematic equations along each of the sightlines. Comparisons with analytic formulas and with independent calculations for the Mega Amp Spherical Tokamak 3 MeV proton diagnostic verify the algorithm.

Alvin Garcia

Papers

Progress in modelling fast-ion D-alpha spectra and neutral particle analyzer fluxes using FIDASIM

Tomography of the positive-pitch fast-ion velocity distribution in DIII-D plasmas with Alfvén eigenmodes and neoclassical tearing modes

Fast-ion D alpha diagnostic with 3D-supporting FIDASIM in the Large Helical Device

Evaluation of an Energetic Particle Profile Using a Tangential-FIDA Diagnostic in the Large Helical Device

Phase-space sensitivity (weight functions) of 3 MeV proton diagnostics