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In many fusion devices, such as tokamaks or stellarators like Wendelstein 7-X (W7-X), soft x-ray pulse height analysis (PHA) system diagnostics are routinely used during the experiments. The PHA system is dedicated to providing information about the impurity content, and average along line-of-sight electron temperature in the plasma conditions. Moreover, it is also able to estimate impurity density and an average effective charge from the comparison of experimental spectra with the modeled ones. However, the experimental x-ray spectra can be interpreted in terms of interesting plasma parameters only when the theoretical radiation models first identify and then take into account all the relevant factors that affect the spectrum. Therefore, for this purpose, a theoretical model has been applied. Flexible Atomic Code, which allows for calculation of various atomic properties such as energy levels, cross sections for excitation and ionization by electron impact, transition probabilities for radiative transitions and autoionization, and any others as needed in the collisional–radiative approximation. The chosen spectra collected during the W7-X campaign (OP1.2b) were examined, trying to obtain an agreement between the observed and simulated spectra. The analysis carried out allowed for a reliable interpretation of experimental x-ray spectra, estimation of the electron temperature, and obtaining information on the content of tungsten impurities. [ABSTRACT FROM AUTHOR]

This paper describes results of spectroscopic investigation of laser-produced tungsten plasma. The laser intensity on the target surface reached up to 30 GW/cm2 depending on the focusing conditions. Optical spectra emitted from plasma plumes which were formed under vacuum conditions in front of the tungsten target due to the interaction of Nd-YAG laser pulses (1.06 μm, 0.5 J), were characterised by means of an optical spectrometer (λ/Δλ= 900) in the wavelength range from 300 to 1100 nm. The spectra were recorded automatically with the use of a CCD detector with exposition time varied from 100 ns to 50 ms. On the basis of WI and WII lines it was possible to estimate electron temperature and electron density which corresponded to the expansion phase of the plasma. Te and Ne were measured as 1.1 eV and 8×1016 cm-3, respectively. The spectra collected by the ion energy analyser showed that the plasma included tungsten ions up to 6+ ion charge. Signals from the ion collector allowed to estimate the average value of ion energy of tungsten as 4.6 keV. Basing on this value the electron temperature corresponding to the initial stage of the plasma formation was estimated to be about 320 eV. Optical microscope investigation showed that laser irradiation caused structural changes on the surface of the target. [ABSTRACT FROM AUTHOR]

The low-Z impurities of the magnetic confined fusion plasmas can provide important information about the wall condition and plasma--wall interactions. In order to accomplish this aim, a special spectrometer called "C/O Monitor" was designed for the W7-X experiment. This system is dedicated to measure Lyman-α transitions of four low-Z impurities: carbon (3.4 nm), oxygen (1.9 nm), nitrogen (2.5 nm) and boron (4.9 nm). It is a high throughput and high time resolution spectrometer which allows to measure the line intensities evolution of indicated elements including information of the background (continuum). The designed spectrometer consists of two vacuum chambers positioned nearly horizontally -- each chamber containing two dispersive elements. Their lines of sight cross at the main magnetic axis of the W7-X plasma. The spectrometer's installation is planned prior to the next Operational Phase (OP 2) which is to be performed in 2021. The "C/O Monitor" system has entered the executive stage. In the "C/O Monitor" diagnostic system, it is important to determine the incoming photon flux and to estimate the detector's response and the topmost time resolution. Moreover, this might be relevant for a quantitative interpretation of the experimental data as well as for an attempt at the calibration of this system. As a consequence, the incoming photon flux from the Lyman-α transitions of selected ions should be analyzed. In this paper, the estimation of the photon flux of the oxygen Lyman-α line and the whole methodology are described. [ABSTRACT FROM AUTHOR]

Abstract: Paper presents experimental results of laser-removal of deposits from mixed materials sample (mixes of C, W and Al on aluminum substrate) and investigation of the effects on morphology and microstructure of deposits and substrates. Two kinds of laser were applied – high power density Nd:YAG laser and high average power, but lower power density Yb:fiber laser. A constant repetition rate of 100kHz fiber laser was used with variable pulse energy (0.5–1mJ) to examine the phenomena in dependence of power density. To verify differences in the removal process, pulses of energy of 500mJ in 3.5ns of the Nd:YAG laser were used. The surface of examined sample was characterized by SEM (Scanning Electron Microscopy) in order to monitor substrate/deposit morphology changes introduced by both lasers. In contrary to the Nd:YAG, the Yb:fiber laser does not cause substrate damages unless it operates at the power density above 106 W/cm2 and does not produce macroscopic dust. [Copyright &y& Elsevier]

The self-broadened absorption profile of Zn (213.8 nm) and Cd (228.8 nm) lines are measured and carefully analysed. The interaction potentials for the electronic states 0+u and 1u dissociating into (41P1 + 41S0) and (51P1 + 51S0) for Zn2 and Cd2, respectively, are derived in the long-range region of (11–25) Å. It is found that although the resonance dipole–dipole interactions, Cm3/R3, dominate in this region, the higher order van der Waals terms, Cm6/R6 are also not negligible. The coefficients Cm3 and the differences ΔCm6 between Cm6 for each of the 0+u and 1u excited states and Cg6 for the ground state of both molecules are determined. For the energy in eV and R in Å it is found that C03 = −23.74 ± 0.40, ΔC06 = −483 ± 30 and C13 = 11.87 ± 0.20, ΔC16 = −227 ± 20 for Zn2, as well as C03 = −24.54 ± 0.10, ΔC06 = −280 ± 15 and C13 = 12.27 ± 0.05, ΔC16 = −215 ± 10 for Cd2. A problem of the Cg6 coefficient for the considered systems is discussed accepting finally the following values: −178±10 and −291±15 eVÅ6 for Zn2 and Cd2, respectively. Thus, for the excited states involved, one obtains C06 = −661 ± 40 and C16 = −405 ± 30 for Zn2 as well as C06 = −571 ± 30 and C16 = −506 ± 25 for Cd2. The C6m coefficients have never been determined experimentally in any of these cases and only for Zn2 were they calculated by Ellingsen et al (2001 J. Phys. B: At. Mol. Opt. Phys.34 2313) by means of a time-dependent gauge-invariant method—a variation-perturbation approach. Our experimental results for this case agree very well with theoretical predictions. [ABSTRACT FROM AUTHOR]

In this paper the laser ablation of tokamak carbon codeposits in vacuum and different gases will be presented. A strong deuterium emission decrease has been observed from the third laser pulse at the same location, which suggests the annealing of a part of the remaining codeposit (up to 0.5–0.7 μm) to temperatures of 400–600 °C. In addition, at the third pulse in oxygen deuterium emission is close to the noise level, indicating its earlier elimination by thermo-oxidation from the remaining film on the laser spot. Moreover, the suppression of the redeposited film surrounding the laser impact has also been observed together with CO + emission. Both observations indicate a fast reaction of the laser-activated oxygen molecules with the released hydrocarbon molecules and probably also with the hot, emitted dust particles. On the other hand, dust particles ejection has been analyzed by fast camera. Large particles with almost the initial deuterium concentration have been observed to be produced in the first laser pulse. Finally a brief future-work study with ITER metallic—W and Be—codeposits with a few % of trapped deuterium has been made. In that case the produced dust particles are expected to have part of the initial deuterium due to the large annealing temperatures required for its elimination. This deuterium concentration will probably be reduced during laser ablation in an oxygen atmosphere. [ABSTRACT FROM AUTHOR]

Abstract: In order to apply Laser Induced Breakdown Spectroscopy (LIBS) for diagnostics of fuel retention and removal and wall composition in ITER it is necessary to validate the method in terms of repeatability and quality of its results for mixed, especially ITER relevant materials and its sensitivity to thin layers which contain small amounts of retained fuel. The experiments, which are within the scope of this paper were carried out at the IPPLM Warsaw to assess the method for the laser treatment of ASDEX Upgrade (AUG) strike-point tiles and calibrated C:W:Al samples prepared by the Institute of Electronic Materials Technology (IEMT). The samples under investigation contained hydrogen isotopes in the ITER-relevant mixed material layer (as the Beryllium analogue, Al was used). An electrostatic ion energy analyzer was used as the auxiliary diagnostics. The results proved that optical spectroscopy, although not straightforward in use, can give reliable results even for small amounts of fuel present in the mixed-material layer, although more effort should be put into investigating calibrated samples, to optimize the method itself and to yield a quantitative measure of fuel retention. [Copyright &y& Elsevier]

The results of experimental and numerical studies of high-intensity proton beam generation driven by a short laser pulse of relativistic intensity are reported. In the experiment, a 350 fs laser pulse of 1.06 or 0.53 μm wavelength and intensity up to 2×1019 Wcm-2 irradiated a thin (0.6-2 μm) plastic (PS) or Au/PS (plastic covered by 0.2 μm Au front layer) target along the target normal. The effect of laser intensity, the target structure and the laser wavelength on the proton beam parameters and laser-protons energy conversion efficiency were examined. Both the measurements and one-dimensional particle-in-cell simulations showed that MeV proton beams of intensity ∼1018 Wcm-2 and current density ∼1012 Acm-2 at the source can be produced when the laser intensity-wavelength squared product ILλ2 is ∼1019 Wcm-2 μm2 and the laser-target interaction conditions approach the skin-layer ponderomotive acceleration (SLPA) requirements. The simulations also proved that at ILλ2≥slant 5×1019 Wcm-2 μm2 and λ≤slant 0.53 μm, SLPA clearly prevails over other acceleration mechanisms and it can produce multi-MeV proton beams of extremely high intensities above 1020 Wcm-2. [ABSTRACT FROM AUTHOR]

In this paper, the plasma volume averaged impurity confinement of selected charge states and impurity species has been characterized for the Stellarator Wendelstein 7-X (W7-X), covering a wide range of atomic charges (Z = 12–44) and atomic masses (M = 28–184). A comparison of the experimental findings to theoretical neoclassical and turbulent transport expectations suggests, aside from/in addition to the neoclassical transport, an additional significant anomalous transport mechanism, which is not inconsistent with the predictions of a turbulence dominated impurity transport and is in agreement with the experimental results from recent transport studies based on the direct measurements of impurity diffusion profiles, performed at W7-X. [ABSTRACT FROM AUTHOR]

A process of irradiating and ablating solid-state targets with hot plasma and fast ion streams in two Dense Plasma Focus (DPF) devices – PF-6 and PF-1000 was examined by applying a number of diagnostics of nanosecond time resolution. Materials perspective for use in chambers of the mainstream nuclear fusion facilities (mainly with inertial plasma confinement like NIF and Z-machine), intended both for the first wall and for constructions, have been irradiated in these simulators. Optical microscopy, SEM, Atomic Emission Spectroscopy, images in secondary electrons and in characteristic X-ray luminescence of different elements, and X-ray elemental analysis, gave results on damageability for a number of materials including low-activated ferritic and austenitic stainless steels, β -alloy of Ti, as well as two types of W and a composite on its base. With an increase of the number of shots irradiating the surface, its morphology changes from weakly pronounced wave-like structures or ridges to strongly developed ones. At later stages, due to the action of the secondary plasma produced near the target materials they melted, yielding both blisters and a fracturing pattern: first along the grain and then “in-between” the grains creating an intergranular net of microcracks. At the highest values of power flux densities multiple bubbles appeared. Furthermore, in this last case the cracks were developed because of microstresses at the solidification of melt. Presence of deuterium within the irradiated ferritic steel surface nanolayers is explained by capture of deuterons in lattice defects of the types of impurity atoms, pores and oxycarbonitride particles existed in the material. [ABSTRACT FROM AUTHOR]

Analysis and understanding of wall erosion, material transport and fuel retention are among the most important tasks for ITER and future devices, since these questions determine largely the lifetime and availability of the fusion reactor. These data are also of extreme value to improve the understanding and validate the models of the in vessel build-up of the T inventory in ITER and future D-T devices. So far, research in these areas is largely supported by post-mortem analysis of wall tiles. However, access to samples will be very much restricted in the next-generation devices (such as ITER, JT-60SA, W7-X, etc) with actively cooled plasma-facing components (PFC) and increasing duty cycle. This has motivated the development of methods to measure the deposition of material and retention of plasma fuel on the walls of fusion devices in situ, without removal of PFC samples. For this purpose, laser-based methods are the most promising candidates. Their feasibility has been assessed in a cooperative undertaking in various European associations under EFDA coordination. Different laser techniques have been explored both under laboratory and tokamak conditions with the emphasis to develop a conceptual design for a laser-based wall diagnostic which is integrated into an ITER port plug, aiming to characterize in situ relevant parts of the inner wall, the upper region of the inner divertor, part of the dome and the upper X-point region. [ABSTRACT FROM AUTHOR]

The paper reports on experimental studies of plasma generated by the irradiation of pure titanium (Ti) and deuterium-saturated (Ti+D 2) targets with intense laser beams from a Nd:YAG system operated at 1063 nm (0.5 J) or at 355 nm (0.1 J). The FWHM of laser pulses was 3 ns and the laser power density on the target surface amounted to (0.7- 3)×1010 W/cm2. Spectroscopic studies of plasma plumes were performed with a Mechelle®900 spectrometer in the wavelength range of 300-1100 nm, at a variable exposition time. The recorded spectra showed numerous lines originating from different Ti-ions and D+ desorbed from the (Ti+D 2) target. Estimates of electron temperatures during the plasma expansion gave Te=1.4-2 eV for 0.1 J pulses and 2.3-3 eV for 0.5 J ones. In the Ti+D 2 experiment, the electron density, as estimated from the Dα line, was Ne=(1.8- 4.6)×1016 cm-3 and (1.9- 2.1)×1017 cm-3 for 0.1 and 0.5 J pulses, respectively. For the given Te, it was estimated that the Ti-I component density was about one order lower. The emission of D+ and various Ti-ions (ranging from Ti+ to Ti+14) was confirmed by corpuscular measurements, which gave higher values of energy 〈 ETi〉=420-1200 eV and electron temperature Te=12-34 eV for the initial hot-plasma phase. [ABSTRACT FROM AUTHOR]

The paper presents studies of the application of ion diagnostics and optical spectroscopy for on-line measurement of the amount and characteristics of co-deposits from the laser-ablated surface of the plasma-facing components (e.g. graphite tiles). For removal of the co-deposit layer a repetitive Nd:YAG laser was used. Determination of the characteristics of ions emitted from the laser-illuminated targets was performed using ion collectors (on the basis of a time-of-flight method) and an optical spectrometer. The main ion stream parameters and spectral lines of deuterium and carbon or tungsten ions were measured depending on laser pulse parameters. The research proved that optical spectroscopy could be a convenient method for on-line observation of the co-deposited layer removal by means of laser ablation. In combination with the investigation of collected co-deposit dust, the performed study made it possible to state that laser-induced breakdown spectroscopy can be useful as a diagnostic method for the ablative co-deposited layer removal and the wall conditioning. The properties of modified surfaces of samples and collected dust (evaporated co-deposit) were determined using different measuring methods. [ABSTRACT FROM AUTHOR]

Abstract: The paper reports on applications of solid-state nuclear track detectors (SSNTDs) of the CR-39/PM-355, manufactured by Pershore Moulding Ltd., for measurements of fast protons emitted from laser-produced plasma. The experiment was performed at LULI, Ecole Politechnique, on the 100 Tera-Watt laser facility. A 1.05μm laser pulse of 350fs duration and intensity up to 2×1019 W/cm2 irradiated a thin (1–3μm) polystyrene (PS) or Au/PS target (PS foil covered with a 0.1–0.2μm Au layer) along the target normal. The measurements revealed that very intense MeV proton beams can be produced under specially chosen laser-target irradiation conditions. The proton beam characteristics were measured using the TOF method (ion collectors), SSNTDs, and radiochromic films (RCFs). The SSNTDs appeared to be especially useful for the experimental analysis of more energetic protons (Ep >3MeV). Using in the same laser shots many detector samples covered with Al foils of different thicknesses (from 15μm up to 400μm) it was possible to estimate energy spectra of protons emitted under various irradiation conditions. The most energetic protons (of energy up to 10MeV) and the most intense proton beams were generated from a double-layer Au/PS target. [Copyright &y& Elsevier]

Abstract: The paper reports on experiments, which concerned studies of plasma produced from a tungsten (W) target bombarded by powerful (ca. 5μs, 1–5MW/cm2) plasma-ion streams in RPI-IBIS plasma accelerator, and a similar target irradiated with intense Nd:YAG laser pulses (0.5J, 3ns, ca. 5.3×109 W/cm2) in another vacuum chamber. In both experiments optical measurements were performed with a Mechelle®900 spectrometer, which enabled the spectrum from 300nm to 1100nm to be recorded, and different WI- and WII-lines to be identified. From space- and time-resolved measurements of those lines, basic W-plasma parameters were estimated. During W-plasma expansion the electron temperature was found to be 0.8–1eV and electron concentration (2–8)×1016 cm−3. The emission of higher-ionized W-ions (up to W+6) was confirmed by measurements with an ion-energy analyzer. Structural changes in the irradiated targets were investigated with an optical microscope and SEM. [Copyright &y& Elsevier]

An overview of the diagnostics which are essential for the first operational phase of Wendelstein 7-X and the set of diagnostics expected to be ready for operation at this time are presented. The ongoing investigations of how to cope with high levels of stray Electron Cyclotron Resonance Heating (ECRH) radiation in the ultraviolet (UV)/visible/infrared (IR) optical diagnostics are described. [ABSTRACT FROM AUTHOR]

Abstract: Laser Induced Breakdown Spectroscopy (LIBS) is a potential candidate to monitor the layer composition and fuel retention during and after plasma shots on specific locations of the main chamber and divertor of ITER. This method is being investigated in a cooperative research programme on plasma devices such as TEXTOR, FTU, MAGNUM-PSI and in other various laboratorial experiments. In this paper LIBS results from targets of D–H-rich carbon films and mixed W–Al–C deposits on bulk tungsten substrates are reported (simulating ITER-like deposits with Al as proxy for Be). Two independent methods, one to determine the relative elemental composition and the other the absolute contents of the target based on the experimental LIBS signals are proposed. The results show that LIBS has the capability to provide the relative concentrations of the elements on the deposited layer when the experimental conditions on the targets surface are identical to the calibration samples. [Copyright &y& Elsevier]