[en] As part of the European Power Plant Conceptual Study, two different divertor designs were proposed, based on previous work on HETS (High Efficiency Thermal Shield) performed at FZK and ENEA. The coolant is helium gas at pressures in the range 10-14 MPa and the inlet temperatures are in the range of 500-800 deg. C. The geometrical complexity of the designs made prediction of heat transfer coefficients, needed for conducting thermal and structural analysis, difficult, and the calculated values from empirical correlations uncertain. This paper presents and summarises results of thermal-fluid calculations performed on both divertor concepts and gives estimates of effective values of heat transfer coefficients based on the local flow conditions and temperature distributions. The agreement of calculations with experimental values for similar conditions, inspires confidence in results from such calculations, and demonstrates that computational fluid dynamic finite element codes can accurately predict behaviour, and can be used to optimise the designs

[en] Studies of the safety and environmental impacts of fusion, both of future power plants and of ITER, have shown that a good performance can be achieved. Although it is difficult to anticipate the regulatory regime in which future fusion power stations will be licensed, the areas of public and occupational safety and short and long-term environmental impact are likely to remain important. In each of these areas, the outcome of various studies have been reviewed, leading to a list of issues which should be given attention to facilitate eventual licensing of a fusion power plant. Many of these relate to reducing conservatisms and uncertainties in the analyses, but also included are improved understanding of tritium retention and of dust generation, and development of materials to provide long component lifetimes. A full appraisal is also recommended of the viability of recycling of active materials after end of plant life

[en] The world fusion programs have had a long goal that fusion power stations should produce only low level waste and thus not pose a burden for the future generations. However, the environmental impact of waste material is determined not only by the level of activation, but also the total volume of activated material. Since a tokamak power plant is large, the potential to generate a correspondingly large volume of activated material exists. The adoption of low activation materials, while important for reducing the radiotoxicity of the most active components, should be done as part of a strategy that also minimizes the volume of waste material that might be categorized as radioactive, even if lower in level. In this paper we examine different fusion blanket and shield designs in terms of their ability to limit the activation of the large vessel/ex-vessel components (e.g. vacuum vessel, magnets) and we identify the trends that allow improved in-vessel shielding to result in reduced vessel/ex-vessel activation. Recycling and clearance are options for reducing the volume of radioactive waste in a fusion power plant. Thus, the performance of typical fusion power plant designs with respect to recycling and clearance criteria are also assessed, to show the potential for improvement in waste volume reduction by careful selection of materials' combinations. We discuss the impact of these results on fusion waste strategies and on the development of fusion power in the future

[en] The development of novel techniques is critical for maintaining a state-of-the-art core competency in atomic physics and readiness for evolving programmatic needs. We have carried out a three-year effort to develop novel spectroscopic instrumentation that added new dimensions to our capabilities for measuring energy levels, radiative transition probabilities, and electron-ion excitation processes. The new capabilities created were in areas that heretofore had been inaccessible to scientific scrutiny and included high-resolution spectroscopy of hard x rays, femtosecond lifetime measurements, measurements of transition probabilities of long-lived metastable levels, polarization spectroscopy, ultra-precise determinations of energy levels, and the establishment of absolute wavelength standards in x-ray spectroscopy. Instrumentation developed during the period included a transmission-type crystal spectrometer, a flat-field EUV spectrometer, and the development and deployment of absolutely calibrated monolithic crystals. The new capabilities enabled very sensitive tests of atomic wave functions, of calculations of magnetic sublevel populations, and of fundamental theories in uncharted regimes, and provided the basis for developing new diagnostic techniques of high-density plasmas

[en] We present H-like Fe XXVI and He-like Fe XXV charge-exchange spectra resulting from collisions of highly charged iron with N₂ gas at an energy of ∼10 eV amu^-1 in an electron beam ion trap. Although high-n emission lines are not resolved in our measurements, we observe that the most likely level for Fe²⁵⁺ → Fe²⁴⁺ electron capture is n_max ∼ 9, in line with expectations, while the most likely value for Fe²⁶⁺ → Fe²⁵⁺ charge exchange is significantly higher. In the Fe XXV spectrum, the Kα emission feature dominates, whether produced via charge exchange or collisional excitation. The Kα energy centroid is lower in the former case than the latter (6666 versus 6685 eV, respectively), as expected because of the strong enhancement of emission from the forbidden and intercombination lines, relative to the resonance line, in charge-exchange spectra. In contrast, the Fe XXVI high-n Lyman lines have a summed intensity greater than that of Lyα, and are substantially stronger than predicted from theoretical calculations of charge exchange with atomic H. A discussion is presented of the relevance of our results to studies of diffuse Fe emission in the Galactic Center and Galactic Ridge, particularly with ASTRO-E2

[en] We report laboratory studies of the role played by multiple-electron capture (MEC) in solar wind induced cometary X-ray emission. Collisions of Ne¹⁰⁺ with He, Ne, Ar, CO, and CO₂ have been investigated by means of the traditional singles X-ray spectroscopy in addition to the triple-coincidence measurements of X-rays, scattered projectile, and target recoil ions for the atomic targets. The coincidence measurements enable the reduction of the singles X-ray spectra into partial spectra originating in single-electron capture (SEC) and MEC collisions. The measurements provide unequivocal evidence for a significant role played by MEC, and strongly suggest that models based solely on SEC are bound to yield erroneous conclusions on the solar wind composition and velocities and on cometary atmospheres. The experimental relative importance of MEC collisions is compared with molecular classical-over-the-barrier model (MCBM), classical trajectory Monte Carlo (CTMC), and multi-channel Landau-Zener (MCLZ), calculations which can qualitatively reproduce the experimental trends

[en] K -shell x-ray emission following low-energy charge exchange collisions (≤11 eV/amu ) has been measured for ten bare and hydrogenic ions up to U⁹¹⁺ . The data resolve capture into angular momentum states with ell=1 and provide a stringent test of theory. The assumption of Stark mixing of angular momentum states is inappropriate at such energies. Results from detailed calculations give a better description of the data, but significant discrepancies are noted. The measurements show that the hardness ratio of charge-exchange induced K -shell x-ray emission represents a diagnostic of the dynamics of ion-atom interactions in situations where the energy is below 1keV/amu, e.g., in solar wind collisions

[en] Recent results from x-pinches at the NTF provide experimental evidence for the existence of strong electron beams in x-pinch plasmas and motivate the development of a new diagnostic, x-ray spectropolarimetry, for investigating the anisotropy of such plasmas. This diagnostic is based on theoretical modeling of polarization-dependent spectra measured simultaneously by spectrometers with different sensitivity to polarization. Results of the first polarization-sensitive experiments at the NTF are presented. K-shell emission from Ti x-pinches is recorded simultaneously by two identical spectrometers with the dispersion plane perpendicular and parallel to the discharge axis. The spectroscopic analysis of more than eight Ti x-pinch shots show how spectropolarimetry complements the usual diagnostics of a z-pinch plasma. The polarization-sensitive spectra, generated by a Maxwellian electron beam at LLNL EBIT have been collected and analyzed. These data make an important contribution to the plasma polarization spectroscopy program at the NTF. In particular, the study of multiply-charged Ti ion spectra help in the interpretation of the polarization-sensitive spectra from Ti x-pinches at the NTF

[en] An experimental apparatus has been developed at UNR to study few-body dynamics in target atoms, molecules, and projectile ions. For example, ionization-excitation cross sections will be studied at intermediate incident projectile velocities. Projectile ions are extracted from a small accelerator, with an energy range of 20 to 150 keV. Ions then collide with gaseous atoms or molecules in a differentially pumped gas cell, and are finally collected in a Faraday cup. A grazing incidence monochromater is used to measure the EUV radiation produced by the collision event. Cross sections are derived by normalizing this data to the ion beam current. An electron gun can be mounted in the experimental chamber to provide data for calibration of the EUV monochromatic. Calibration data and preliminary cross section measurements for ionization-excitation of He by proton impact will be reported

[en] The results from the last six years of x-ray spectroscopy and spectropolarimetry of high energy density Z-pinch plasmas complemented by experiments with the electron beam ion trap (EBIT) at the Lawrence Livermore National Laboratory (LLNL) are presented. The two topics discussed are the development of M-shell x-ray W spectroscopic diagnostics and K-shell Ti spectropolarimetry of Z-pinch plasmas. The main focus is on radiation from a specific load configuration called an 'X-pinch'. X-pinches are excellent sources for testing new spectral diagnostics and for atomic modelling because of the high density and temperature of the pinch plasmas, which scale from a few (micro)m to several mm in size. They offer a variety of load configurations, which differ in wire connections, number of wires, and wire materials. In this work the study of X-pinches with tungsten wires combined with wires from other, lower-Z materials is reported. Utilizing data produced with the LLNL EBIT at different energies of the electron beam the theoretical prediction of line positions and intensity of M-shell W spectra were tested and calibrated. Polarization-sensitive X-pinch experiments at the University of Nevada, Reno (UNR) provide experimental evidence for the existence of strong electron beams in Ti and Mo X-pinch plasmas and motivate the development of x-ray spectropolarimetry of Z-pinch plasmas. This diagnostic is based on the measurement of spectra recorded simultaneously by two spectrometers with different sensitivity to the linear polarization of the observed lines and compared with theoretical models of polarization-dependent spectra. Polarization-dependent K-shell spectra from Ti X-pinches are presented and compared with model calculations and with spectra generated by a quasi-Maxwellian electron beam at the LLNL EBIT-II electron beam ion trap.