Relaxation Effects in the Transition Temperature of Superconducting HgBa2CuO4+delta

In previous studies on a number of under- and overdoped high temperature superconductors, including YBa_{2}Cu_{3}O_{7-y} and Tl_{2}Ba_{2}CuO_{6+\delta}, the transition temperature T_c has been found to change with time in a manner which depends on the sample's detailed temperature and pressure history. This relaxation behavior in T_c is believed to originate from rearrangements within the oxygen sublattice. In the present high-pressure studies on HgBa_{2}CuO_{4+\delta} to 0.8 GPa we find clear evidence for weak relaxation effects in strongly under- and overdoped samples ($T_c\simeq 40 - 50 K$) with an activation energy $E_{A}(1 bar) \simeq 0.8 - 0.9 eV$. For overdoped HgBa_{2}CuO_{4+\delta} E_{A} increases under pressure more rapidly than previously observed for YBa_{2}Cu_{3}O_{6.41}, yielding an activation volume of +11 \pm 5 cm^{3}; the dependence of T_c on pressure is markedly nonlinear, an anomalous result for high-T_c superconductors in the present pressure range, giving evidence for a change in the electronic and/or structural properties near 0.4 GPa

The Effect of Chemical Doping and Hydrostatic Pressure on Tc of Y1-yCayBa2Cu3Ox Single Crystals

We performed susceptibility measurements on Y1-yCayBa2Cu3Ox single crystals under high He pressure. For each Ca content various samples with different oxygen contents have been prepared to probe the influence of Ca on Tc(x), dTc/dp(x) and Tc,max. Starting from the parabolic Tc(nh) behavior we calculated nh values from Tc and Tc,max for each sample. It is shown that in the overdoped region dTc/dp can be described by a pressure induced charge transfer with dnh/dp = 3.7E-3 [1/GPa] and a dTc,max/dp value of 0.8 K/GPa, irrespective of the Ca content. In the underdoped region additional pressure effects lead to a peak in dTc/dp at approximately 0.11 holes/CuO2 plane. However, with increasing Ca content this peak is strongly depressed. This is explained in terms of an increasing disorder in the CuO chain system due to doping. Deviations in dTc/dp at very low nh values can be assigned to the ortho II ordering in the CuO chain system.Comment: 13 pages with 6 figures, accepted for publication in Physica

Advance in the conceptual design of the European DEMO magnet system

The European DEMO, i.e. the demonstration fusion power plant designed in the framework of the Roadmap to Fusion Electricity by the EUROfusion Consortium, is approaching the end of the pre-conceptual design phase, to be accomplished with a Gate Review in 2020, in which all DEMO subsystems will be reviewed by panels of independent experts. The latest 2018 DEMO baseline has major and minor radius of 9.1 m and 2.9 m, plasma current 17.9 MA, toroidal field on the plasma axis 5.2 T, and the peak field in the toroidal-field (TF) conductor 12.0 T. The 900 ton heavy TF coil is prepared in four lowerature-superconductor (LTS) variants, some of them differing slightly, other significantly, from the ITER TF coil design. Two variants of the CS coils are investigated - a purely LTS one resembling the ITER CS, and a hybrid coil, in which the innermost layers made of HTS allow the designers either to increase the magnetic flux, and thus the duration of the fusion pulse, or to reduce the outer radius of the CS coil. An issue presently investigated by mechanical analyzes is the fatigue load. Two variants of the poloidal field coils are being investigated. The magnet and conductor design studies are accompanied by the experimental tests on both LTS and HTS prototype samples, covering a broad range of DC and AC tests. Testing of quench behavior of the 15 kA HTS cables, with size and layout relevant for the fusion magnets and cooled by forced flow helium, is in preparation.</p

Towards a new image processing system at Wendelstein 7-X: From spatial calibration to characterization of thermal events

Wendelstein 7-X (W7-X) is the most advanced fusion experiment in the stellarator line and is aimed at proving that the stellarator concept is suitable for a fusion reactor. One of the most important issues for fusion reactors is the monitoring of plasma facing components when exposed to very high heat loads, through the use of visible and infrared (IR) cameras. In this paper, a new image processing system for the analysis of the strike lines on the inboard limiters from the first W7-X experimental campaign is presented. This system builds a model of the IR cameras through the use of spatial calibration techniques, helping to characterize the strike lines by using the information given by real spatial coordinates of each pixel. The characterization of the strike lines is made in terms of position, size, and shape, after projecting the camera image in a 2D grid which tries to preserve the curvilinear surface distances between points. The description of the strike-line shape is made by means of the Fourier Descriptors

Forward modeling of collective Thomson scattering for Wendelstein 7-X plasmas: Electrostatic approximation

In this paper, we present a method for numerical computation of collective Thomson scattering (CTS). We developed a forward model, eCTS, in the electrostatic approximation and benchmarked it against a full electromagnetic model. Differences between the electrostatic and the electromagnetic models are discussed. The sensitivity of the results to the ion temperature and the plasma composition is demonstrated. We integrated the model into the Bayesian data analysis framework Minerva and used it for the analysis of noisy synthetic data sets produced by a full electromagnetic model. It is shown that eCTS can be used for the inference of the bulk ion temperature. The model has been used to infer the bulk ion temperature from the first CTS measurements on Wendelstein 7-X

Study of the transient voltage behaviour of the present ITER TF coil design for determination of the test voltages and procedures

Studie über das transiente elektrische Verhalten der aktuellen ITER Spulenkonstruktion zur Bestimmung von Testspannung und Testverfahren ITER ist ein experimenteller Tokamak Fusionsreaktor, in dem die kontrollierte Kernfusion gezündet und aufrechterhalten wird. Einschluß und Regelung des Plasmas erfolgen hierbei mit supraleitenden Magneten. Vom "ITER International Team" wird als wahrscheinlichste Ursache für einen Ausfall eines Großmagneten ein Isolationsfehler angesehen. Aufgrund der schwierigen Austauschbedingungen für Toroidalfeldspulen (TF) im ITER Magnetsystem und der verschiedenen Probleme, die sich während der Tests der ITER Modellspulen gezeigt hatten, sind für die ITER Magnete Verbesserungen in unterschiedlichen Bereichen der Hochspannungstechnik erforderlich. Besonders wichtig ist die Berücksichtigung des transienten elektrischen Verhaltens, da schnelle Spannungsänderungen (z. B. Blitz- und Schaltstoßspannungen) nichtlineare Spannungsverteilungen und Resonanzen in Spulen erzeugen können. Derartige Hochspannungsbeanspruchungen können örtliche Überlastungen und Zerstörungen am Isolationssystem verursachen. Dieser Bericht zeigt für die Schnellentladung und zwei Fehlerfälle die Berechnungen der Spannungen an den Spulenenden im ITER TF System sowie die Spannungsbelastungen der 3 Arten elektrischer Isolierung (Erd-, Radialplatten- und Leiterisolation) innerhalb einer ITER TF Einzelspule. Für eine ITER TF Einzelspule wird das elektrische Ersatzschaltbild erstellt. Hierzu werden die internen Induktivitäten und Kapazitäten sowie die Erdkapazitäten bestimmt. Mit FEM-Berechnungen werden Skin- und Proximity-Effekt sowie die von Wirbelströmen in den Radialplatten verursachten Dämpfungen ermittelt. Ein weiteres Netzwerkmodell wird für das aus 18 TF Einzelspulen und 9 Schnellentladeeinheiten bestehende ITER TF System erstellt. Das Ergebnis einer niedrigeren Resonanzfrequenz für die TF Spule als für die TF Modellspule kann auf die größeren Abmessungen der TF Spule zurückgeführt werden. Für die Schnellentladung ergibt sich für alle 3 Isolierungsarten eine nichtlineare Spannungsbelastung, die im Falle eines Erdschlusses verstärkt wird. Zur Sicherstellung eines zuverlässigen Betriebes für die vorgesehene Einsatzdauer von ITER muß die Festlegung der Testspannungen somit unter Berücksichtigung von Spannungsbelastungen erfolgen, die bei Schnellentladungen und realistischen Fehlerfällen auftreten können. Ausgehend von den berechneten Spannungsbelastungen und den Erfahrungen, die während dem ITER Modellspulentest gemacht wurden, werden Vorschläge für Testverfahren diskutiert und verschiedene Abnahmekriterien vorgeschlagen. Als Ergänzung werden Sicherheitsempfehlungen vorgestellt, die der Verhinderung unzulässiger Belastungen während der Tests dienen