Efficiency of a thermodynamic motor at maximum power

Several recent theories address the efficiency of a macroscopic thermodynamic motor at maximum power and question the so-called "Curzon-Ahlborn (CA) efficiency." Considering the entropy exchanges and productions in an n-sources motor, we study the maximization of its power and show that the controversies are partly due to some imprecision in the maximization variables. When power is maximized with respect to the system temperatures, these temperatures are proportional to the square root of the corresponding source temperatures, which leads to the CA formula for a bi-thermal motor. On the other hand, when power is maximized with respect to the transitions durations, the Carnot efficiency of a bi-thermal motor admits the CA efficiency as a lower bound, which is attained if the duration of the adiabatic transitions can be neglected. Additionally, we compute the energetic efficiency, or "sustainable efficiency," which can be defined for n sources, and we show that it has no other universal upper bound than 1, but that in certain situations, favorable for power production, it does not exceed 1/2

Design, manufacturing and performance of a pair of superconducting solenoids for a neutron spin-echo spectrometer at the SNS

A Neutron spin-echo (NSE) spectrometer of the next generation is under construction at the Spallation Neutron Source (SNS) in Oak Ridge, USA. A NSE spectrometer measures small velocity changes of the neutrons encoded by the neutrons spin clock at a sample while the Neutron spin precesses in large magnetic fields following Bloch's equation. This instrument will be the best of its class both with respect to resolution and dynamic range. In order to reach this ambitious goal, a large magnetic precision field integral before and after the sample is required which directly scales linearly with the resolution of the instrument. Therefore superconducting technology will be used to allow for a higher magnetic field integral. Here, we present the design, manufacturing and test performance of the solenoids which are the result of a collaboration between Julich Research Center (FZJ) and Babcock Noell GmbH (BNG). The solenoids generate an integrated magnetic flux density of 1.8 T*m and use active shielding to reduce the fringe field. The operation temperature of about 4 K is reached by means of pulse-tube coolers in order to minimize vibrations. A special feature of this magnet system is the very accurate measurement of the cold mass-position after cool-down and during operation. The accuracy of this measurement system is in the order of micrometers. This information is required for the adjustment of so called Fresnel coils outside the cryostat. Using these correction elements, a field integral homogeneity better than 10(-6) T*m for different Neutron paths through one of the solenoids can be achieved

Design of a pair of superconducting solenoids for a neutron spin-echo spectrometer at the SNS

A Neutron spin-echo spectrometer (NSE) of the next generation will be build for the Spallation Neutron Source (SNS) in Oak Ridge, USA. A NSE spectrometer measures tiny velocity changes of the neutrons encoded by the neutrons spin clock at a sample while the Neutron spin precesses in large magnetic fields following Bloch's equation. This instrument will be the best of its class both with respect to resolution and dynamic range. In order to reach this ambitious goal, a large magnetic precision field integral before and after,the sample is required Which directly scales linearly with the resolution of the instrument. Therefore superconducting technology will be used to allow for a higher magnetic field integral. Here, we present the first basic design of the solenoids which has been developed in a joint project study by Julich Research Center (FZJ) and Babcock Noell GmbH (BNG). The solenoids will generate an integrated magnetic flux density of 1.8 Tm. To reduce the fringe field, active shielding is foreseen. To minimize vibrations modern pulse-tube cryocoolers cool the NbTi windings below Tc. A special feature of the magnet system is the position measurement with an accuracy in the order of micrometers for the windings in the cryostat during operation. This information is required for the adjustment of so called Fresnel coils outside the cryostat. Together with the setup of these correction elements, a field integral homogeneity better than 10(-6) for different Neutron paths through one of the solenoids will be achieved. In addition special care has been taken in the selection procedure for the SC strands. This is necessary for the minimization of its hysteresis values and subsequently for full determination and control of the magnetic field with an accuracy of about 10(-5)

Optimierung eines Quenchdetektionssystems fuer supraleitende Magnetspulen

Gegenstand dieses Berichts ist die Quenchdetektion bei supraleitenden Magnetspulen. Supraleitende Magnete beduerfen zum sicheren Betrieb aufgrund ihres hohen Stroms und der dadurch gespeicherten Energie einer Menge technischer Vorkehrungen. Eines der wichtigsten Schutzsysteme ist das Quenchdetektionssystem, das den Magneten auf den Erhalt der Supraleitung hin ueberwacht und bei Bedarf dessen Sicherheitsentladung ausloest. Kommt es zu einem Zusammenbruch der Supraleitung (Quench), muss der Magnet moeglichst schnell entladen werden, damit keine Gefahr fuer den Magneten oder gar seine Umgebung auftritt. Zunaechst soll dem weniger fachkundigen Leser ein einleitender Ueberblick ueber die Thematik gegeben werden. Anschliessend werden verschiedene Methoden zur Quenchdetektion, zum Teil anhand ausgefuehrter Beispiele aus der Praxis, vorgestellt, sowie deren Eignung in den unterschiedlichen Betriebsweisen supraleitender Magnete aufgezeigt. Die einzelnen Quench-Detektions-Varianten werden zusammenfassend miteinander verglichen, wobei fuer die Bewertung unter anderem die im Anhang beschriebenen Versuchsreihen herangezogen werden. Als Anwendungsbeispiel wird ein Vorschlag fuer das Quenchdetektionssystem der in Bau befindlichen Anlage Wendelstein 7-X erarbeitet.Subject of this report is the detection of a quench in a superconducting magnet. For the safe operation of superconducting magnets one of the most important issues is the quench detection system which controls the superconducting state of the magnet and triggers a safety discharge if necessary. If it comes to a breakdown of the superconductivity (quench), the magnet has to be discharged very quickly to avoid any damage or danger for the magnet or its environment. First an introducing overview is given. Next different methods of quench detection will be presented, partially on the basis of existing quench detection systems and the applicability of these methods in different states of the magnet operation will be shown. The different quench detection methods are compared and evaluated partially by using test experiments described in the appendix. As an application example this report contains a proposal for the quench detection system for the Wendelstein 7-X facility, actually built by the Institute for Plasma Physics, Garching.SIGLEAvailable from TIB Hannover: ZA5141(7076) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman