Backstreaming of Impurity Gas Through a Leak in Pressurized Vessel

The presence of a leak in a vessel containing pure gas can induce the contamination by atmospheric gas diffusing into the vessel. In order to avoid this, a gas which has to be kept pure also in presen ce of a leak is usually pressurized, to reduce the flow of contaminating gas through the leak owing to the molecular drag by the outstreaming pure gas. In this paper, a simple model calculation of ba ckstreaming based on the solution of the diffusion + drag equation in cylindrical coordinates is presented. It is shown that both the pressure difference and the dimension of the leak are critical in determining the contaminating flow, a maximum in the backstreaming flow appearing when the drag velocity of the outstreaming gas equals the diffusion velocity

Governing Global Supply Chain Sustainability Through the Ethical Audit Regime

Over the past two decades multinational corporations have been expanding ‘ethical’ audit programs with the stated aim of reducing the risk of sourcing from suppliers with poor practices. A wave of government regulation—such as the California Transparency in Supply Chains Act (2012) and the UK Modern Slavery Act (2015)—has enhanced the legitimacy of auditing as a tool to govern labor and environmental standards in global supply chains, backed by a broad range of civil society actors championing audits as a way of promoting corporate accountability. The growing adoption of auditing as a governance tool is a puzzling trend, given two decades of evidence that audit programs generally fail to detect or correct labor and environmental problems in global supply chains. Drawing on original field research, this article shows that in spite of its growing legitimacy and traction among government and civil society actors, the audit regime continues to respond to and protect industry commercial interests. Conceptually, the article challenges prevailing characterizations of the audit regime as a technical, neutral, and benign tool of supply chain governance, and highlights its embeddedness in struggles over the legitimacy and effectiveness of the industry-led privatization of global governance

Technical Analysis and Statistics from Long Term Helium Cryoplant Operation with Experimental Superconducting Magnets at CERN

CERN regularly uses a large number of liquid helium cryoplants for cooling the superconducting magnets of large particle detectors. They are installed in the experimental areas of the electron-positron collider LEP and the proton (and heavy ion) accelerator SPS for the observation of high-energy interactions of elementary particles. The typical cold mass of a detector magnet ranges from 1 to 40 tons, and typical cryoplant cooling capacities are between 400 and 800 W/4.5 K entropy equivalent. Operation must be very flexible to meet the varying experimental requirements. We intend to present technical data of the system and statistics from over 180'000 running hours during the four years from 1992 to 1995. Operation includes phases of cool-down, steady-state cooling, recovery after magnet quench or other incidents and warm-up of the superconducting magnets. Emphasis will be laid on the analysis of fault conditions, multiple interaction between perturbations and consequences for the users of liquid helium supply interruption

Conclusions from 12 Years Operational Experience of the Cryoplants for the Superconducting Magnets of the LEP Experiments

The Large Electron Positron Collider (LEP) has ended its last physics run in November 2000, and it is at present being dismantled to liberate the tunnel for the Large Hadron Collider (LHC) project to be completed by end of 2005. The cryogenic systems for the superconducting solenoid and focusing quadrupoles for the two LEP experiments, ALEPH and DELPHI, each supplying a cooling power of 800 W/4.5 K entropy equivalent, have accumulated more then 100'000 hours of running time. The paper summarises the 12 years cryogenic experience in the various operating modes: cool-down, steady state, recovery after energy fast dump, utilities failures and warm-up of the superconducting magnets. The detailed operation statistics is presented and compared to the other CERN cryogenic systems. Emphasis is given to the technical analysis of the fault conditions and of their consequences on the helium refrigeration production time in view of the future operation of the LHC cryogenics

Cryogenic Facilities at 1.9 K for the Reception of the Superconducting Wires and Cables of the LHC Dipoles Magnets

CERN's LHC project has moved to an implementation phase. The fabrication of 1600 high-field superconducting magnets operating at 1.9 K will require about 6400 km of Nb-Ti cables. A cryogenic test facility has therefore been set up in order, on the one hand, to verify the quality of individual wires and, on the other hand, to control the critical current of the assembled cables. The facility is composed of a helium liquefier, a transfer line, a dewar and pumps. The paper describes the fully automatic operation of this installation and the different test cycles applied to these wires and cables

Refrigeration System for the ATLAS Experiment

The proposed ATLAS detector for the 27 km circumference LHC collider is of unprecedented size and complexity. The magnet configuration is based on an inner superconducting solenoid and large superconducting air-core toroids (barrel and two end-caps) each made of eight coils symmetrically arranged outside the calorimetry. The total cold mass approaches 600 tons and the stored energy is 1.7 GJ. The cryogenic infrastructure will include a 6 kW @ 4.5 K refrigerator, a precooling unit and distribution systems and permits flexible operation during cool-down, normal running and quench recovery. A dedicated LN2 refrigeration system is proposed for the three liquid argon calorimeters (84 m3 of LAr). Magnets and calorimeters will be individually tested prior to their definitive installation in a large scale cryogenic test area on the surface. The experiment is scheduled to be operational in 2005

New cryogenic facilities for testing superconducting equipments for the CERN Large Hadron Collider

CERN's major project, the Large Hadron Collider (LHC), has moved to an implementation phase with machine construction to be completed by 2005. To achieve the design proton-proton centre of mass energy of 14 TeV in the given 27 km circumference LEP tunnel, the LHC will make an extensive use of high-field superconducting magnets using Nb-Ti filament operated at 1.9 K. In order to test, on the one han d, the superconducting cables of the magnets and, on the other hand, the expected performance of several of these magnets assembled in a string representing the lattice period of the machine (107 m lo ng), CERN has installed new cryogenic test facilities. The paper briefly describes these new facilities with all their associated equipments

Electron gas polarization effect induced by heavy H-like ions of moderate velocities channeled in a silicon crystal

We report on the observation of a strong perturbation of the electron gas induced by 20 MeV/u U$^{91+}$ ions and 13 MeV/u Pb$^{81+}$ ions channeled in silicon crystals. This collective response (wake effect) in-duces a shift of the continuum energy level by more than 100 eV, which is observed by means of Radiative Electron Capture into the K and L-shells of the projectiles. We also observe an increase of the REC probability by 20-50% relative to the probability in a non-perturbed electron gas. The energy shift is in agreement with calculations using the linear response theory, whereas the local electron density enhancement is much smaller than predicted by the same model. This shows that, for the small values of the adiabaticity parameter achieved in our experiments, the density fluctuations are not strongly localized at the vicinity of the heavy ions