In fusion devices, the X-ray plasma emissivity contains essential information on the magnetohydrodynamic activity, the magnetic equilibrium and on the transport of impurities, in particular for tokamaks in the soft X-ray (SXR) energy range of 0.1–20 keV. In this context, tomography diagnostics are a key method to estimate the local plasma emissivity from a given set of line-integrated measurements. Unfortunately, the reconstruction problem is mathematically ill-posed, due to very sparse and noisy measurements, requiring an adequate regularization procedure. The goal of this paper is to introduce, with a didactic approach, some methodology and tools to develop an X-ray tomography algorithm. Based on a simple 1D tomography problem, the Tikhonov regularization is described in detail with a study of the optimal reconstruction parameters, such as the choice of the emissivity spatial resolution and the regularization parameter. A methodology is proposed to perform an in situ sensitivity and position cross-calibration of the detectors with an iterative procedure, by using the information redundancy and data variability in a given set of reconstructed profiles. Finally, the basic steps to build a synthetic tomography diagnostics in a more realistic tokamak environment are introduced, together with some tools to assess the capabilities of the 2D tomography algorithm.

A multi-machine study has been carried out to investigate the impact of a strongly bounded wave propagation domain on the Lower Hybrid current drive, a condition which occurs principally in high aspect ratio tokamaks. In this regime, the condition of kinetic resonance can be far above the upper boundary of the propagation domain, and may not be achieved by the usual toroidal upshift. Therefore no tail of fast electrons can be pulled out from the thermal bulk. Nevertheless, while tokamak plasmas are in principle almost transparent to the wave in this regime so-called "unbridgeable spectral gap", full current drive is well achieved for the two tokamaks considered in this study, TRIAM-1M [H. Zushi, et al., Nucl. Fusion 43 (2003) 1600] and WEST [C. Bourdelle, et al., Nucl. Fusion 55 (2015) 063017], both characterized by a very large aspect ratio R/a > 5.5. The case of the high aspect ratio tokamak HL-2A [Y. Liu et al. Nucl. Fusion 45 (2005) S239] for which the wave propagation domain has also an upper boundary, but close to the resonance condition, is considered by comparison. First principles modeling of the rf-driven current and the fast electron bremsstrahlung using the ALOHA/C3PO/LUKE/R5-X2 chain of codes shows unambiguously that the spectral gap must be already filled at the separatrix in order to reproduce quantitatively observations and some important parametric dependencies. This result is an important milestone in the physics understanding of the Lower Hybrid current drive, highlighting the existence of a powerful and likely universal alternative mechanism to bridge the spectral gap, that is not related to toroidal magnetic refraction. With an initially broad power spectrum, lobes with low parallel refractive indexes that carry most of the plasma current can be absorbed in almost single pass, restoring the full validity of the ray-tracing approximation for describing the propagation of the Lower Hybrid wave in cold plasmas.

Tomography diagnostics represent an essential tool in tokamaks to infer the local plasma properties using line-integrated measurements from one or several cameras. In particular, soft X-rays (SXR) in the energy range 0.1-20 keV can provide valuable information on magnetohydrodynamic activity, magnetic equilibrium or impurity transport. Heavy impurities like tungsten (W) are a major source of concern due to significant radiation losses in the plasma core, thus they have to be kept under acceptable concentrations. Therefore, 2D SXR tomography diagnostics become crucial to estimate the W concentration profile in the plasma, quantify the W poloidal distribution and identify relevant impurity mitigation strategies. In this context, a synthetic diagnostic becomes a very valuable tool (1) to study the tomographic reconstruction capabilities, (2) to validate diagnostic design as well as (3) to assess the error propagation during the reconstruction process and impurity transport analysis. The goal of this contribution is to give some highlights on recent studies related to each of these three steps, for the development of SXR synthetic diagnostic tools in tokamak plasmas.

With the choice of tungsten as a material for the ITER plasma facing components, the suprathermal electron interaction with non-fully ionized impurities emerged as an important issue in plasma modeling. Microwave heating and current drive systems, especially lower hybrid current drive, can generate a significant population of suprathermal electrons in the plasma. Also, in the case of the runaway electron generation and mitigation by massive gas injection, the collisions with impurities can have a significant impact on the electron drag force. A correct description of the fast electrons collisions with non-fully ionized impurities requires calculation of the atomic form factor. This can be done with ab initio models that are accurate, though time consuming in practical applications. In this paper, we compare existing approximations of the form factors, based on the Thomas–Fermi or Pratt–Tseng models. Ab initio density functional theory (DFT) calculations are used as a reference method to determine the accuracy of the compared models. Based on this analysis, we propose some modifications of the existing models, tuned with numerical parameter optimization, which provide a higher accuracy while maintaining a short computation time. These modifications include multiple exponents in the Pratt–Tseng model and fitting the parameters of the form factor equation to the DFT-based results. Some applications of the presented models to the calculation of the elastic and inelastic collision frequencies for Fokker–Planck equation are presented, showing a good agreement between the results obtained with the DFT method and the proposed models.

Soft X-ray (SXR) radiation emitted from tokamak plasmas contains very useful information about plasma stability, shape and impurity content, all key parameters to improve plasma performance. In the deuterium-tritium phase of ITER, the high neutron fluxes, gamma and hard X-ray emission will constitute too harsh an environment to permit the use of classical semiconductor detectors. New SXR detector technologies, more robust to such environments, should thus be investigated. First GEM (Gas Electron Multiplier) measurements performed at WEST were successful and showed that both spatially and spectrally resolved calibrated data could be acquired. Strategies to reconstruct tungsten (W) impurity radiation synthetic diagnostics, modelling and real measurements based on multiple diagnostics are proposed.