Hot Fusion Plasmas Research Articles

Rugged superconducting detector for monitoring infrared energy sources in harshenvironments

Broadband electromagnetic characterization of hot plasmas, such as in nuclearfusion reactors and related experiments, requires detecting systems that mustwithstand high flux of particles and electromagnetic radiations. We propose arugged layout of a high temperature superconducting detector aimed at 3 THzcollective Thomson scattering (CTS) spectroscopy in hot fusion plasma. TheYBa2Cu3O7 − x superconducting film is patterned by standard photolithography and the sensing area of thedevice is created by means of high-energy heavy ion irradiation, in order to modify thecrystal structure both of the superconducting film and of the substrate. Thismethod diminishes process costs and resulting device fragility due to membrane orair-bridge structures that are commonly needed for MIR and FIR radiation detection.Moreover the sensing area of the device is wired by the same superconductingmaterial and thus excellent mechanical strength is exhibited by the whole device,due to the oxide substrate. Continuous wave operation of prototype devices isdemonstrated at liquid nitrogen temperature, for selected infrared spectra of broadbandthermal energy sources. Several solutions, which exploit the advantages comingfrom the robustness of this layout in terms of intrinsic radiation hardness of thesuperconducting material and of the needed optical components, are analysed withreference to applications of infrared electromagnetic detectors in a tokamak machineenvironment.

Emission of Visible Light by Hot Dense Metals

Author(s): More, R.M. | Abstract: We consider the emission of visible light by hot metal surfaces having uniform and non-uniform temperature distributions and by small droplets of liquid metal. The calculations employ a nonlocal transport theory for light emission, using the Kubo formula to relate microscopic current fluctuations to the dielectric function of the material. We describe a related algorithm for calculating radiation emission in particle simulation of hot fusion plasmas.

Charging and E×B rotation of ablation clouds surrounding refueling pellets in hot fusion plasmas

The finite resistivity magnetohydrodynamic code FRONTIER-MHD [R. Samulyak et al., Nucl. Fusion 47, 103 (2007)] is used to simulate the ablation rate of refueling pellets, including the novel effect of electrostatically induced E×B rotation of the ablation cloud about its symmetry axis parallel to the magnetic field. The key finding is that the centrifugal force of cloud rotation pushes the cloud density radially outwards, creating a more “transparent” ablation channel. With reduced shielding, the steady state ablation rate of a deuterium pellet significantly increases from ∼35% to 100%, depending on the B-field strength. This new effect brings the ablation rate into better accord with a known theoretical scaling law, which agrees with most current experiments.

Freestanding diffractive optical elements as light extractors for burning plasma experiments

Optical diagnostics will be critical for the operation and performance assessment of burning plasma experiments, such as ITER. At the same time, extracting light for these diagnostics with reflective mirrors becomes difficult in the burning plasma environment due to the deleterious effects of the prolonged exposure on plasma and nuclear radiations. As an alternative, we explore the possibility to use freestanding diffractive optical elements, such as transmission gratings and zone plates, as light extractors. Since in diffractive systems, light is deflected by periodic slits rather than by a surface, these may withstand plasma exposure with less degradation of their optical properties. To investigate this possibility, we developed freestanding transmission gratings for the visible range and exposed them to conditions resembling (or even exceeding) those expected for the ITER “first mirrors.” The results of this study indicate that the gratings can withstand high heat fluxes and plasma and energetic radiation bombardment. Additionally, in contrast to the reflective elements, the extraction efficiency of diffractive elements may even improve with plasma exposure, which is possibly due to the shaping and thinning of the grating bars by plasma erosion. Moreover, in tightly collimated configurations, even very thin gratings can be used to extract light from hot fusion plasmas, as demonstrated by our tests of an extreme ultraviolet extractor at the National Spherical Torus Experiment.

Complex Spectra in Fusion Plasmas

The need for quantitative evaluation of complex line emission spectra as observed in hot fusion plasmas initiated a challenging development of sophisticated interpretation tools based on integrating advanced atomic modelling with detailed treatment of the plasma environment. The successful merging of the two worlds has led to routine diagnostic procedures which have contributed enormously to the understanding of underlying plasma processes and also to a wide acceptance of spectroscopy as a reliable diagnostic method.In this paper three characteristic types of spectra of current and continuing interest are presented. The first is that of medium/heavy species with many ionisation stages revealed in survey VUV and XUV spectra. Such species occur as control gases, as wall materials, as ablated heavy species and possible as layered wall dopants for monitoring erosion. The spectra are complex with line-like and quasi-continuum regions and are amenable to advanced `pattern recognition' methods.The second type is of few electron, highly ionised systems observed as line-of-sight integrated passive emission spectra in the soft x-ray region. They are analysed successfully in terms of plasma parameters through matching of observation with predicted synthetic spectra. Examples used here include highly resolved helium-like emission spectra of argon, iron and titanium observed on the tokamaks TEXTOR and Tore Supra.The third type, and the emphasis of this work, comprises spectra linked to active beam spectroscopy, that is, charge exchange recombination spectroscopy (CXRS) and beam emission spectroscopy (BES). In this case, a complex spectrum is again composed of a (usually) dominating active spectrum and an underlying passive emission spectrum. Its analysis requires modelling of both active and passive features. Examples used here are from the CXRS diagnostic at JET and TEXTOR. They display characteristic features of the main light impurity ions (C+6, He+2, N+7, Ne+10 and Ar+18), as well as that of the bulk plasma ions, H+, D+ and T+.A main conclusion is that spectral complexity is not necessarily negative, but that `complex structures' can provide a rich source of information on the plasma and its parameters—provided it is matched with integrated analysis—and that the methods can have universal applicability. In the present preparatory phase of the next generation fusion experiment ITER (International Thermonuclear Experimental Reactor) the concepts and expectations of complex spectra and integrated data analysis play an important role in the design and optimisation procedure of the ITER diagnostic assembly.

Fusion and the cosmos

In the following investigation we pay special attention to the role of selforganization in fusion plasma physics and in the cosmos. We present a new approach to the expansion of the universe. Formally the technique developed relies on our experience from treating hot fusion plasmas. We account for the possibility that the universe, as it seems, could have a finite life-time (even if it is counted in billions of years), and combine this assumption with the experimental observation that the velocity of separation of distant galaxies is proportional to the distance between the galaxies (the Hubble law). By analysis of a NL PDE (nonlinear partial differential equation) we succed in proving that the crucial value of an exponent has a simple linear relationship with the Hubble constant. It is recognized that the scale-length that we use as a measure of the expansion is equivalent to the Einstein radius of curvature. The final results suggest that the Hubble law should be extended by a factor, which could have an explosive tendency of growth in time (open universe), or a decaying character (closed universe). The possibility of reversed expansion or an oscillating universe “cosmic pendulum” is also discussed.

An integrated system to measure the effective charge of fusion plasmas in the ASDEX Upgrade tokamak

At the ASDEX Upgrade tokamak a new approach has been developed to determine the profiles of the effective charge (Zeff) from bremsstrahlung emission in hot fusion plasmas. The signals of all diagnostics measuring bremsstrahlung have been incorporated into one evaluation algorithm, resulting in the possibility to check whether or not signals are disturbed during each discharge. Especially helpful to this aim is a recently installed diagnostic measuring the bremsstrahlung spectrally resolved by using a spectrometer and a charge-coupled-device (CCD) camera as a detector. The experimental results are verified qualitatively and quantitatively using a discharge with silicon laser blow-off. Modeling of the bremsstrahlung as measured with the CCD camera using the routines and data of the ADAS package reveals that the background of the spectra is dominated by bremsstrahlung for most sight lines. Only those passing mainly outside the last closed flux surface measure a considerable amount of line and molecular band radiation. This edge emission can be considered as a radiative mantle. Subtracting it from all other signals results in reduced statistical errors of the deduced Zeff profiles while not changing the profile shape inside a normalized minor radius of 0.8.

Response of chromium-doped alumina screens to soft x rays using synchrotron radiation

We have measured the response of chromium-doped alumina screens to soft x-ray radiation and derived quantum efficiency curves for the energy range from 2.5 to 4.5 keV. Persistent luminescence (or afterglow) from this material is observed for several minutes following the removal of the x-ray source. It is also observed that the luminescence output rises gradually for several minutes before maximizing during x-ray irradiation. In the article we discuss possible sources of this delayed luminescence and its consequences for application of these screens as soft x-ray detectors. Such screens have found application as narrow-band radiation detectors for a hot fusion plasma diagnostic.

Characterization of the response of chromium-doped alumina screens in the vacuum ultraviolet using synchrotron radiation

We have measured the response of chromium-doped alumina screens to vacuum ultraviolet radiation and derived quantum efficiency curves for the energy range from 30 to 300 eV. A model is presented to explain the structure in this curve. In addition, the radiation hardness of such screens, which have found application as narrow-band radiation detectors for a hot fusion plasma diagnostic, is reported here for MeV electrons. Finally, a simple model is constructed to obtain the carrier diffusion length and the bulk efficiency of this material.

A generalized information theoretical approach to tomographic reconstruction

A generalized maximum-entropy-based approach to noisy inverse problems such as the Abel problem, tomography or deconvolution is presented. In this generalized method, instead of employing a regularization parameter, each unknown parameter is redefined as a proper probability distribution within a certain pre-specified support. Then, the joint entropies of both, the noise and signal probabilities, are maximized subject to the observed data. After developing the method, information measures, basic statistics and the covariance structure are developed as well. This method is contrasted with other approaches and includes the classical maximum-entropy formulation as a special case. The method is then applied to the tomographic reconstruction of the soft x-ray emissivity of the hot fusion plasma.

Possibility of Profile Control using Compact Toroid Injection on Large Helical Device

Compact toroid (CT) injection is an attractive method for central fueling into hot fusion plasmas. Induction of plasma rotation is expected by momentum input along the three-dimensional trajectories of the injected CT. The CT trajectories in the magnetic field of a large helical device (LHD) are calculated while changing the initial injection velocity and injection point, to determine the optimum conditions for CT injection into LHD. The possibility of central fueling is confirmed with a model CT with a diameter of 20 cm, 1022 m-3 electron density, and 300 km/s initial velocity, in the case of CT injection into an LHD magnetic field of 1.5 T. The input profiles of the density and the momentum obtained assuming a simple CT decay model show the potential of CT injection as a profile control method.

Analytical approximation of cross-section effects on charge exchange spectra observed in hot fusion plasmas

An analytical procedure is presented which enables a fast estimate of collision-energy-dependent cross-section effects on thermal charge exchange spectra. The model is based both on experimental evidence and numerical simulations showing that the observed charge exchange (CX) spectra are essentially Gaussian in their shape. The collision-energy-dependent emission rate leads effectively to a lineshift (apparent velocity), usually to a reduction in linewidth (apparent temperature), and to a change in the effective emission rate averaged over the entire thermal velocity distribution function. It is demonstrated that the cross-section effect can be treated analytically introducing an approximated emission rate factor which retains the characteristics of a Maxwellian velocity distribution function using an exponential expression with only linear and quadratic velocity terms in its exponent. An algebraic deconvolution procedure is described, which enables the reconstruction of true temperature, velocity and intensities from measured CX spectra. Examples taken from a recent JET experimental campaign are used to illustrate the cross-section effects on low-Z impurity CX spectra for a comprehensive variety of neutral beams (deuterium, tritium or helium), target densities, temperatures and toroidal rotation speeds. An overview is given of representative correction factors established for high-power, high-temperature plasmas, as well as for plasmas with combined neutral beam and radiofrequency heating, and for the case of locked modes.

Beam emission spectroscopy as a comprehensive plasma diagnostic tool

A beam of injected fast atomic hydrogen presents a superb probe for hot fusion plasmas. The neutral particles experience excitation and ionization by collisions with electrons and ions as they penetrate into a plasma. The emitted characteristic line radiation is Doppler shifted and the spectral lines are split due to motional Stark fields. Measurements of wavelength, intensity and polarization of the Balmer- alpha emission reveal information about the neutral beam, such as beam attenuation, beam-geometry, beam-divergence and species mix. Local pitch angles and toroidal fields can be derived from the simultaneous measurement of the polarization pattern and the wavelength separation of the Stark multiplet. The implementation and application of beam emission spectroscopy as a quantitative diagnostic tool on the Joint European Torus (JET) experiment is reviewed.

Development of a transient internal probe diagnostic

The transient internal probe (TIP) diagnostic is a novel method for probing the interior of hot magnetic fusion plasmas. In the TIP scheme, a probe is fired, using a two-stage light gas gun, through a hot plasma at velocities up to 5 km/s, and makes direct, local measurements of the internal magnetic field structure. The data are relayed to the laboratory optical detection system using an incident laser that is directed through a Faraday rotator payload acting as a magneto-optic sensor. Ablative effects are avoided by minimizing the probe size, limiting the time that the probe is in the hot plasma, and encasing the probe with a diamond cladding. The degree to which the diamond probe cladding is susceptible to ablative effects will determine the plasma density and temperature regime in which the TIP diagnostic can be used. If the TIP suffers significant ablation it is an indication that the diagnostic is not usable on this hot and dense of a plasma (or that greater velocity must be imparted to the probe to further minimize the time that it is in the plasma). A quantitative experimental study of the ablation rates of diamond is planned as part of the TIP development. The integrated TIP system will be functional in 1992 and installed on the Helicity Injected Torus (HIT) [T. R. Jarboe, Fusion Tech. 15, 9 (1989)] at the University of Washington.

Can a Penning ionization discharge simulate the tokamak scrape-off plasma conditions?

The tokamak scrape-off (the region between the vacuum vessel wall and the magnetically confined fusion plasma edge), represents a source/sink for the hot fusion plasma. The electron densities and temperatures are in the ranges 1011-1013 cm−3 and 1-40eV respectively (depending on the size, magnetic field intensity and configuration, plasma current, etc.). Relatively low charge states of various impurities are formed here in a variety of atomic and molecular processes. Since it is very difficult to study these processes in situ, we looked for a way to simulate them under conditions similar to those of the tokamak scrape-off layer. In the work reported, the electron temperature and density have been estimated in a Penning Ionization Discharge by comparing its spectroscopic emission in the VUV with that predicted by a collisional radiative model. An attempt to directly compare this emission with that of the tokamak edge is briefly described.

Self-generation and nonlinear diffusion – an analytic approach

The problem of simultaneous diffusion and creation processes, in particular self-generating mechanisms, is analysed with special regard to localization, dynamic confluence, collapse and anti-collapse. Exact solutions and central expansions for one- and multidimensional systems are developed. The analysis comprises a study of the case, where the coefficient of the self-generation source term is spatially inhomogeneous.It also introduces the concept of a dynamic equilibrium width, which may be approached in time by several interacting distributions, starting out with different initial conditions. The analysis considers various powers of nonlinearity of the diffusion coefficient as well as of the dynamic variable in the source term. Comparisons between various approaches to the problem are presented. Applications are considered in the physics of hot fusion plasmas, and occur quite generally in heat conduction with self-generating sources.

Influence of damage on the thermal release of deuterium implants in graphite

Trapping and release of keV deuterium in pyrolytic graphite has been the subject of several investigations because of its relevance to the problem of interaction of a hot fusion plasma with the solid wall. In the present work the influence of He-induced damage on implantation profiles for deuterium in pyrolytic graphite is investigated for either a smaller (9 keV D +, 3 keV He +) or larger He range (1.5 keV D +, 13 keV He +). During annealing, the implanted D is found to be retained more strongly at depths where He-damage has been introduced. The results show that the trapping sites of deuterium in pyrolytic graphite are at least partly connected with radiation damage.