Soft X-ray Measurements Research Articles

Transition from quasiperiodicity to chaos just before sawtooth crash in the ASDEX Upgrade tokamak

In this paper we investigate the sawtooth crash in the ASDEX Upgrade tokamak and present evidence supporting the hypothesis of stochastization of magnetic field lines during the crash. We demonstrate on the basis of soft x-ray and electron cyclotron emission measurements that during the pre-crash phase the quasiperiodic transition to chaos occurs. Magnetohydrodynamic oscillations with two frequencies develop before the crash. Consistent with the most energetically favourable transition from quasiperiodicity to chaos, their frequency ratio is close to the conjugate golden ratio . We think that the sawtooth crash has a universal stochastic character and it would be worthwhile to search for these transition signatures in other tokamaks.

TAE internal structure through high-resolution soft x-ray measurements in ASDEX-Upgrade

The internal structure of toroidal Alfvén eigenmodes (TAEs) excited in ASDEX-Upgrade plasmas with ion cyclotron resonance heating (ICRH) has been measured through a high-resolution multichord soft x-ray diagnostic. The radial profiles of multiple TAEs are compared with predictions of the linear resistive magnetohydrodynamic code CASTOR and the nonperturbative gyrokinetic code LIGKA and reasonably good agreement is found. The evolution of the TAE radial profiles could be followed in time, both across sawteeth in pure ICRH discharges and in discharges with different types of transitions from L-mode to H-mode obtained with a combination of ICRH and neutral beam injection.

On magnetic interlayer coupling and proximity effect in a La0.67Ca0.33MnO3 (10nm)∕YBa2Cu3O7 (10nm) superlattice

The authors present a study of interlayer coupling and proximity effects in a La0.66Ca0.33MnO3 (10nm)∕YBa2Cu3O7 (10nm) superlattice. Using element-sensitive x-ray probes, the magnetic state of Mn can be probed without seeing the strong diamagnetism of the superconductor, which makes this approach ideal to study changes in the magnetic properties across the superconducting transition. By a combined experiment using in situ transport measurements during polarized soft x-ray measurements, they were able to see no noticeable influence of the superconducting state on the magnetic properties and no evidence for magnetic coupling across a 10nm YBa2Cu3O7 layer.

Temperature Diagnostics for Field-Reversed Configuration Plasmas on the Pulsed High Density (PHD) Experiment

The high flux source for the Pulsed High Density Experiment (PHDX) has been constructed, and field-reversed configuration (FRC) plasmas are being produced. To diagnose FRC plasma temperatures three kinds of diagnostic systems have been set up on PHDX; λ = 632.8 nm He-Ne laser interferometer system at the midplane for total temperature Ttotal, 16 channel spectrometer for ion temperature Ti and soft x-ray (SXR) measurement system for electron temperature Te viewed along z-axis, respectively. The SXR measurement system consists of five sets of collimators and AXUV100 photodiodes with directly deposited filters which have different bandpass regions. As a result of preliminary temperature diagnostics, the following plasma parameters are estimated during a relatively quiescent phase of FRCs; Ttotal ∼ 50 eV, Ti ∼ 50 eV and Te ∼ 25 eV. This result does not agree with the equilibrium relation of Ttotal ≈ Ti + Te, and it must be caused by plasma condition and our assumptions in the spectral analysis.

Stability of the induced magnetic V moment in Fe/V superlattices upon Hydrogen loading

We report on the stability of the V magnetic moment within Fe/V superlattices upon H-uptake. At the Fe/V interface V atoms acquire a magnetic moment, antiparallel aligned to the Fe moments. Upon hydrogen loading the saturation magnetization was found to increase. We utilized element specific X-ray resonant magnetic scattering to address the response of the induced V moments. An epitaxial [Fe(5ML)/V (16ML)]×30 superlattice was employed. The soft x-ray measurements were carried out using the ALICE diffractometer at BESSY II in Berlin, Germany. The data clearly show the stability of the V moment upon H-loading. No change of the magnetic asymmetry at the V edge could be recognized. Our measurements confirm ab-initio calculations predicting a stable V moment and an increase of the Fe moment in H loaded Fe/V superlattices.

On the short-term relationship between solar soft X-ray irradiances and equatorial total electron content (TEC)

[1] The relationship between total electron content (TEC) and the solar soft X-ray irradiances is presented. Three bands (2–7 nm, 6–19 nm, and 17–20 nm) of solar soft X-ray measurements from the Student Nitric Oxide Explorer (SNOE) satellite are examined and all show a similar relationship with TEC. The TEC data are from a GPS receiver near Ancon, Peru (� 11.78 degrees latitude, � 77.15 degrees longitude) from 11 March 1998 to 23 August 1999 and 2 October 1999 to 10 June 2000. During these periods the average TEC measurement was calculated from all observations whose ionospheric pierce point occurred within � 12 ± 2 degrees latitude and � 77 ± 2 degrees longitude and within the hour selected. TEC shows a more significant correlation with soft X-ray irradiances than with F10.7. The X rays lead the TEC by approximately 0.8–1.3 days, which is consistent with the neutral density affecting the TEC. The magnitude of these short term (27 days or less) changes is approximately 0.18 of the total TEC. During the period examined geomagnetic activity, as represented by Ap, could account for half as much variation in TEC (0.1 of the total TEC) as the solar irradiance.

Proton Enhancements and Their Relation to the X-Ray Flares During the Three Last Solar Cycles

Energetic proton measurements obtained from the GOES and IMP-8 satellites as well as from ground-based neutron monitors are compared with the GOES soft X-ray measurements of the associated solar flares for the period 1975-2003. The present study investigates a broad range of phenomenology relating proton events to flares (with some references to related interplanetary disturbances), including correlations of occurrence, intensities, durations and timing of both the particle event and the flare as well as the role of the heliographic location of the designated active region. 1144 proton events of >10 MeV energy were selected from this 28-year period. Owing primarily to the low particle flux threshold employed more than half of this number was found to be reliably connected with an X-ray flare. The statistical analysis indicates that the probability and magnitude of the near-Earth proton enhancement depends critically on the flare's importance and its heliolongitude. In this study all flares of X-ray importance >X5 and located in the most propitious heliolongitude range, 15 ◦ Wt o 75 ◦ W, were succeeded by a detectable proton enhancement. It was also found that the heliolongitude frequently determines the character of the proton event time profile. In addition to intensity, duration and timing, proton events were found to be related to the other flare properties such as lower temperatures and longer loop lengths.

Updated Expressions for Determining Temperatures and Emission Measures from Goes Soft X-Ray Measurements

{We investigate the conversion of the 0.5–4 and 1–8 A soft X-ray flux measurements made by detectors on the Geostationary Operational Environmental Satellites (GOES) into temperature and emission measures of coronal plasma using modern spectral models and modern understanding of coronal abundances. In particular, the original analysis by Thomas, Starr and Crannell (1985) is updated to take into account the realization that coronal abundances may be quite different from photospheric abundances. An important result of this analysis is that the derived temperatures and emission measures depend strongly on the assumed abundances even at high temperatures where continuum rather than spectral lines dominates the Sun’s X-ray spectrum. This occurs because the higher coronal abundances mean that most of the continuum is due to free–bound emission processes, not free–free emission, and thus is abundance-dependent. We find significant differences between modern calculations of the temperature response of the flux measurements and the versions currently in use: for a typical flare, emission measures may be up to a factor of 4 smaller than the current software suggests. Derived temperatures are similar for both photospheric and coronal abundances for cool flares (e.g., 15 MK), but for hot flares (e.g., 35 MK) coronal abundances can lead to significantly (~25%) lower temperatures being derived.

Flat-field grating spectrometer for high-resolution soft x-ray and extreme ultraviolet measurements on an electron beam ion trap

A R=44.3 m grazing-incidence grating spectrometer has been implemented on the Livermore electron beam ion traps for high-resolution measurements in the soft x-ray and extreme ultraviolet region spanning from below 10 up to 50 Å. The instrument uses a grating with variable line spacing (about 2400 l/mm for a flat field of view. Spectra are recorded with a back-illuminated charge-coupled device detector. The new instrument greatly improves upon the resolution achieved with existing grating spectrometers and complements crystal spectrometers at the shorter wavelengths both in terms of wavelength coverage and polarization independent reflectivity response.

Spatially resolved bolometric measurement and electron temperature measurement using diode arrays

In this article, the measurement system for the total radiation and electron temperature profiles to be installed in a reversed-field pinch machine, toroidal pinch experiment, RX [TPE-RX, R/a=1.72/0.45 m, Ip<MA (designed)], is presented. We plan to measure the plasma radiation and electron temperature profiles using three sets of diode arrays. Each array can measure radiation along 20 lines of sight and the radial profile of the radiation. One set of the arrays is used for the bolometric measurement in the range from visible light to soft x-ray. Two sets of the arrays are used for the soft-x ray and electron temperature measurements employing a double-filter method. We will use this system to investigate the plasma-wall interaction, radiation loss, and confinement properties in the core plasma region. We will extend the use of this system for tomographic analysis of electron temperature, a concept of which is also presented.

Soft x-ray tomography system for the toroidal pinch experiment-RX reversed-field pinch

A soft x-ray (SXR) measurement system for tomography analysis on a reversed-field pinch machine. torodial pinch experiment, RX [TPE-RX, R/a=1.72/0.45 m, Ip<1 MA (designed)], is presented. The soft x-ray imaging system consists of two surface barrier detector (SBD) arrays that are vertical and horizontal. Thirteen SBDs are installed on the vertical ports and used for the measurement along vertical lines of sight. Eleven SBDs are installed on the horizontal port and used for the measurement along a fan-shaped line of sight. These detectors have 15-μm-thick Be foil with sensitivity in the soft x-ray range. This system is installed in order to study the structure of the SXR emission from the plasma core and to know the relation between global performance and magnetohydrodynamics dynamics. This system has been used under several operating conditions in addition to those of standard operation. The first results of these experiments are reported.

Resistive wall mode identification by contrast enhancing technique of soft x-ray measurements on DIII-D

A contrast enhancing technique (CET) for soft x-ray (SXR) measurements has been developed and tested for the early identification of the low amplitude resistive wall mode (RWM) on the DIII-D tokamak. The technique is simple and fast. It utilizes the chord-by-chord difference of low-pass digitally filtered time derivatives of the signals from the twelve-chord fan-shape soft x-ray arrays located at toroidal angles of 195° and 45°. The two arrays allow a demonstration of the CET method principle, although they cannot completely resolve the RWM structure. The time derivative of the x-ray signal amplifies the effect of the temperature perturbation convected by the RWM, while naturally incorporating the equilibrium evolution effect. The correlation with the parameters measured by other diagnostics, such as the radial magnetic field δBr from the magnetic probes, the radial profiles of plasma current density j, pressure p, and safety factor q from the motional Stark effect, the radial profile of the temperature perturbation ΔTe from electron cyclotron emission, the normalized beta parameter βN=βT(I/aB)−1, and the Dα light emission, confirm the early detection of the low-amplitude RWMs by the soft x-ray CET. Upon validation the SXR CET could be implemented into a multisensor scheme using other diagnostics for the real-time identification of RWM.

Controlled release of rhBMP-2 from collagen minipellet and the relationship between release profile and ectopic bone formation

The purpose of this study was to examine the effects of various additives on the profiles of rhBMP-2 release from minipellet, which is a sustained release formulation for protein drugs using collagen as a carrier, and to examine the influence of varying release profiles on ectopic bone formation. When the amount of rhBMP-2 remaining in the preparation after subcutaneous implantation to mice was examined, it was found that the addition of sucrose, glucose, PEG4000, alanine (Ala) or acacia in a concentration of 20% (w/w) to the minipellet with 5% (w/w) of rhBMP-2 did not accelerate the drug release in a noticeable manner, while the addition of sodium chondroitin sulfate, glutamic acid (Glu) or citric acid accelerated the release of rhBMP-2 markedly. When two types of minipellets (a fast release type added with 20% Glu and 20% Ala and a slow release type without additives) containing varying amounts of rhBMP-2 were implanted subcutaneously to mice, the soft X-ray observation, histological examination and measurement of calcium formation 3 weeks after implantation revealed extensive ectopic bone formation in mice implanted with the fast release type preparation. Ectopic bone formation was dose-dependent. The result of this study exhibited that the effects of controlled release formulation of rhBMP-2 on bone formation vary depending on their release profiles, and suggested that combination of initial burst and sustained release was effective for bone formation. It was also shown that minipellet is useful as a controlled release formulation which can release rhBMP-2 to areas around the implanted site with various release profiles.

Observations of the Sun at Vacuum- Ultraviolet Wavelengths from Space. Part I: Concepts and Instrumentation

Studies of the high-temperature solar atmosphere are to a large extent based on spectroscopic observations of emission lines and continuum radiation in the vacuum-ultraviolet (VUV) wavelength range of the electromagnetic spectrum. In addition, important contributions stem from soft X-ray measurements. Most of the VUV radiation is produced by transitions of atoms and ions. The resulting atomic and ionic spectral lines have formation temperatures between 10 000 K and 20 MK, representative of the chromosphere, transition region, corona, and solar flares. Some molecular lines and the continua originate in cooler regions of the Sun, around and below the temperature minimum between the photosphere and the chromosphere. Radiation at VUV wavelengths is strongly absorbed by the Earth's atmosphere. Consequently, it can only be detected with instruments on sounding rockets and spacecraft operating above the atmosphere. The progress in this field of research, in particular over the last 25 years, will be presented in the first part of this review by describing the concepts and instrumentation of modern spectrographs and imaging telescopes. This presentation is accompanied by some examples of high-resolution solar images and a discussion of radiometric-calibration aspects and wavelength measurements. A second part will follow in the near future, summarizing important results obtained on the plasma conditions in the solar atmosphere.

Present and perspective roles of soft X-ray tomography in tokamak plasma position measurements

This paper shows the importance and feasibility of real-time tomography in fusion experiments for the example of soft X-ray (SXR) position measurements. The requirement of non-magnetic real-time diagnostics in low frequencies for ITER is discussed. This is illustrated by recent results of rapid tomographic inversion of SXR measurements on tokamak TCV. Comparison with the magnetic reconstruction data has not only shown the valuable resolution capabilities of both techniques, but also revealed a slight dependence of magnetic measurements on toroidal magnetic field and an unnoticed drift of plasma position observer. A feasibility study using current hardware capacities for programmable real-time tomographic system with plasma position feedback output was carried out. A compact solution is found to be tractable opening wide perspectives for development.

Signal processing techniques based on singular value decomposition applied to modulated ECH experiments

The analysis and interpretation of perturbative modulated electron cyclotron heating (MECH) discharges for power deposition localization using different diagnostics, such as electron cyclotron emission (ECE) and soft x-ray (SXR) measurements, are presented. In particular, the experimental determination of the MECH power deposition is investigated, which is a fundamental requirement, for example, for heat transport studies. The most important problem is related to the coupling between the MECH and the sawtooth activity of the plasma, which disturbs both ECE and SXR measurements. Several techniques have been adopted to circumvent this difficulty. In particular, the singular value decomposition (SVD) and the generalized singular value decomposition (GSVD) have been tested. However, both methods are incapable of treating the problem correctly, which leads to potential misinterpretation of the results. A new method based on system identification using the SVD (SI-SVD) is developed and applied. This method, within reasonable limits induced by the assumption of linearity, is capable of simultaneously separating the MECH from the sawtooth contributions to both ECE electron temperature measurements and SXR emission measurements. Such a method is in particular applied to a neutral beam injection (NBI) heated ASDEX Upgrade discharge in which MECH is added in order to analyse electron heat transport in a mostly ion-heated plasma. Since the NBI heating is also partly modulated with short pulses which coincide with the sawtooth crashes to improve their stability, both the MECH and the NBI deposition profiles are determined. Moreover, treating the signals with the SI-SVD procedure enables a study of the plasma dynamic response also at higher MECH harmonic numbers.

Angle-resolved soft x-ray spectroscopy study of the electronic states of single-crystalMgB2

Angle-resolved soft x-ray measurements made at the boron K edge in single-crystal ${\mathrm{MgB}}_{2}$ provide insights into the B 2p local partial density of both unoccupied and occupied band states. The strong variation of absorption with incident angle of exciting x rays permits the clear separation of contributions from $\ensuremath{\sigma}$ states in the boron plane and $\ensuremath{\pi}$ states normal to the plane. A careful comparison with theory accurately determines the energy of selected critical k points in the conduction band. Resonant inelastic x-ray emission at an incident angle of 15\ifmmode^\circ\else\textdegree\fi{} shows a large enhancement of the emission spectra within about 0.5 eV of the Fermi level that is absent at 45\ifmmode^\circ\else\textdegree\fi{} and is much reduced at 60\ifmmode^\circ\else\textdegree\fi{}. We conclude that momentum transferred from the resonant inelastic x-ray scattering process couples empty and filled states across the Fermi level.

Investigation of the consistency of magnetic and soft x-ray plasma position measurements on TCV by means of a rapid tomographic inversion algorithm

A rapid algorithm for tomographic inversion is presented that allows post-discharge reconstruction of the soft x-ray (SXR) emissivity evolution in the Tokamak à configuration variable (TCV). Simultaneously, it determines the centre of gravity of the emissivity that may serve as a reliable measurement of the plasma position, that is independent of magnetic measurements. Tests of the algorithm were performed using shaped phantom datasets and all past SXR measurements were processed to obtain plasma position statistics. Multiple linear regression is applied in matching the results with the magnetic axis data. Systematic functionalities including magnetic field dependence and drift of the magnetic axis are observed. The joint resolution limit of magnetic and SXR position measurements at TCV is derived from the residual discrepancies and equals 2.2 and 3.1 mm in the radial and vertical directions, respectively.

Past and recent observations of the solar upper atmosphere at vacuum-ultraviolet wavelengths

Our understanding of the high-temperature solar atmosphere is to a large extent based on spectroscopic observations of emission lines and continuum radiation in the vacuum-ultraviolet (VUV) wavelength range of the electromagnetic spectrum. In addition, important contributions stem from soft X-ray measurements. The VUV radiation is produced by transitions of atoms and ions, or to some extent, of molecules. The atomic and ionic emission lines have formation temperatures between 10 000 K and several million kelvin, representative of the chromosphere, the transition region and the corona. The molecular lines and the continua originate in cooler regions of the Sun. Radiation at VUV wavelengths is strongly absorbed by the Earth's atmosphere leading to important geophysical processes at high altitudes. In our context it means that this radiation can only be detected with instruments on sounding rockets and spacecraft above the atmosphere. Detailed studies of the spectral radiances together with atomic physics data furnish information on the electron density and temperature of the solar atmosphere, as well as on elemental abundances, whereas Doppler line-shift measurements show bulk plasma motions, turbulence, and ion temperatures. Highlights of the research in this field will be presented from the first solar ultraviolet spectra to Skylab observations and the modern measurements in the SOHO and TRACE era, and their relevance to solar physics investigations will be outlined. Special emphasis will be put on imaging telescopes and spectrographs, which provide insight both into the morphology and the dynamics of the solar plasma structures.

A microcapillary target as a metastable hollow ion source

Interaction of highly charged ions (HCIs) with solid surfaces has been studied with microcapillary metallic foils, which have many straight holes of ∼100 nm in diameter and ∼1 μm in thickness distributed regularly over the foil. It was found that some considerable fraction of ions can pass through the small capillary without suffering violent collisions with the capillary wall but at the same time capturing multiple electrons in their highly excited Rydberg states, i.e. hollow atoms (ions) are effectively extracted in vacuum. The study of ion–capillary interaction now gets even more promising because of new developments of nano-lithographic techniques, which allow preparing highly ordered microcapillaries. The interaction of HCIs with microcapillaries has been studied through (1) visible light measurements, which provide information on the very beginning of the charge transfer processes, (2) soft X-ray measurements, which reveal the electronic configuration at the moment of inner shell filling processes in a later stage and also (3) angular distribution measurements of each charge-changed component, which provide insight on the position of the ion from the inner wall when the charge transfer takes place. It was found that the ion trajectory is not very much deformed during electron transfers, i.e. one can prepare a well-collimated monoenergetic beam of metastable hollow ions (atoms), which is a unique probe carrying high potential energy at relatively low charge states.