Problems related to the development of diagnostics for steady state fusion plasma experiments are discussed. The paper concentrates on those necessities already appearing in current non-burning plasma fusion experiments when extending pulse lengths beyond 10 s, i.e. thermal load, erosion, deposition and long-time signal integration in magnetic diagnostics. Problems arising from high power ECRH under conditions of incomplete absorption are outlined. Individual standard diagnostic systems are discussed to identify their specific problems as well as the opportunities connected with long pulse operation. Burning plasma experiments characterized by intense n- and γ-radiation are briefly reviewed for reasons of completeness, dealing with radiation induced processes in windows, fibres, cables and mirrors. Methods of data handling, real time monitoring and plasma control are outlined.

/pdf/diagnostics-for-steady-state-plasmas-3yh19dtxc0.pdf

Diagnostics for steady state plasmas

By injecting a tracer-encapsulated solid pellet (TESPEL) with triple tracers, V (Z = 23), Mn (Z = 25) and Co (Z = 27), into a plasma in the Large Helical Device (LHD), it was found in the high-density case (typically ne = (5–7) × 1019 m−3) that the Kα emissions from the intrinsic impurities were strongly suppressed, while those from all the three tracers were retained for a long time. In the medium-density case (typically ne = (3–4) × 1019 m−3), Kα emissions from the intrinsic impurities were observed clearly, while those of the tracer impurities were observed to decay much faster than the high-density case. When the intrinsic impurities penetrate into the plasma core through the plasma periphery in the plasma build-up phase under relatively low-density conditions, then such impurities are found to be kept for a long time in the later phase under high-density conditions. By implementing supersonic Ar gas puffing in addition to the TESPEL injection, Ar Kα emission was clearly observed together with Kα emissions from the tracers in the medium-density case. In contrast to this, Ar Kα emission was completely suppressed in the high-density case. This result shows that the suppression of the intrinsic impurity coming from outside the plasma is indeed working in the high-density case, while the impurities deposited inside the plasma are kept for a long time.

https://iopscience.iop.org/article/10.1088/0741-3335/55/9/095014/pdf

Transport characteristics of tracer and intrinsic impurities depending on the density of LHD plasmas

A new diagnostic method with tracer-encapsulated solid pellet (TESPEL) injection with multiple tracers is developed to study impurity behaviour in a magnetically confined plasma If a pellet contains multiple tracers, it becomes possible to compare the behaviour of different impurities simultaneously under the same plasma conditions We injected a TESPEL into the Large Helical Device mainly with triple tracers: vanadium (V), manganese (Mn) and cobalt (Co) The Li-like lines in the vacuum ultraviolet range and the K? lines in the soft x-ray range from these tracers are simultaneously observed with a time resolution of 50?ms As the charges of the nuclei of intrinsic impurities, chromium (Cr) and iron (Fe), are in between those of the tracers, the behaviour of Cr and Fe can be studied quantitatively by knowing the number of tracer particles and also by comparing the emission intensity change due to the electron temperature change It is observed that the tracer impurities remain in the plasma core region when the plasma density is higher than 5???1019?m?3 It is also observed that the intrinsic impurities cannot enter the core region when the plasma density is higher than the same level, although the two phenomena appear to be independent

https://iopscience.iop.org/article/10.1088/0029-5515/52/6/063012/pdf

Multiple-tracer TESPEL injection for studying impurity behaviour in a magnetically confined plasma

Recent progress in the Large Helical Device (LHD) experiment during the last 2 yr is reviewed The LHD has been extending its operational regime toward fusion-relevant conditions while taking advantage of the net-current-free heliotron concept employing a superconducting coil system Heating capability has exceeded 10 MW, and the central ion and electron temperatures have reached 7 and 10 keV, respectively The maximum values of beta and pulse length have been extended to 32% and 150 s, respectively Several encouraging physics observations have been obtained, ie, simultaneous achievement of the mitigation of the magnetohydrodynamic instability criteria and good confinement, and formation of an internal transport barrier The initial results have been obtained using a local island divertor, which shows the possibility of particle control at the plasma edge

Review on the Progress of the LHD Experiment

We present cross section experimental data required for modeling beam production, attenuation and energy loss in the heavy-ion beam probe technique, namely, electron-loss cross sections for anionic gold projectiles and Ar and N2 targets in the energy range from 30 keV to 1 MeV. The results agree well with low-energy measured values from the literature for the Ar target. The present results are also compared with literature results for the H− projectile and the Ar target, displaying similar velocity dependences. This similarity led to the use of a simple semi-classical model to calculate the projectile-electron-loss cross sections in a wide energy range. This model employs the total scattering cross sections for free electrons and the momentum distribution of the Au− electrons. The present experimental results are in good agreement with this model, allowing a reliable estimate of the cross section values up to 100 MeV. Our results indicate that previously calculated cross sections are overestimated by at least a factor of two. This previous overestimation of electron-loss cross sections leads to significant errors in the modeling of charge-state fractions of gold beams, with consequences in the optimization of beam energy resolution and intensity for HIBP with megaelectronvolts gold beams.

https://iopscience.iop.org/article/10.1088/0741-3335/51/4/045007/pdf

Electron-loss cross sections for heavy-ion beam probe using gold beams: experiment and theory

Standard diagnostics for fundamental plasma parameters and for plasma physics are routinely utilized for daily operation and physics studies in the large helical device (LHD) with high reliability. Diagnostics for steady-state plasma are under intensive development, especially for Te, ne (yttrium–aluminium garnet (YAG) laser Thomson, CO2 laser polarimeter), data acquisition in steady-state and heat-resistant probes. To clarify the plasma properties of the helical structure, two- or three-dimensional diagnostics are being aggressively developed: tangential cameras (fast SX TV, photon counting CCD, Hα TV); tomography (tangential SX CCD, bolometer); imaging (bolometer, ECE, reflectometer). Divertor and edge physics are important key issues for steady-state operation. Diagnostics for neutral flux (Hα array, Zeeman spectroscopy) and ne (fast scanning probe, Li beam probe, pulsed radar reflectometer) are also in advanced stages of development. In addition to these, advanced diagnostics are being intensively developed in LHD through domestic and international collaborations.

/pdf/recent-diagnostic-developments-on-lhd-3fg3jwioiv.pdf

Recent diagnostic developments on LHD

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Recent diagnostic developments on LHD