Section Of Arc Research Articles

Vacuum system for the 3-GeV RCS in J-PARC

The 3-GeV RCS (rapid cycling synchrotron) in the J-PARC (Japan Accelerator Research Complex) project accelerates a proton beam of 0.333 mA up to 3 GeV. At 25-Hz repetition rate, the RCS generates a high-power beam of 1 MW. In such an accelerated proton beam, neutrons as well as gamma rays are generated. The cumulative energy dose will be of the order of 100 MGy over 30 years of operation. So as to minimize the radiation exposure during maintenance, it is necessary to construct a vacuum system with reliable components which have a long life in such a high level of radiation. In addition, it is necessary to keep the operating pressure of the beam in ultra high vacuum to suppress pressure instability. Thus we should think not only of the outgassing mainly due to ion-induced desorption but also of the pumping efficiency. Based on the above, the vacuum system was designed as follows: The ring is divided by isolation valves into 6 sections (3 straight and 3 arc sections), which can be pumped down independently. To avoid any eddy current loss, ceramic ducts are used in the bending and focusing magnets. These ducts are connected to titanium ducts placing the Ti bellows between. Here, we adopt pure Ti as a material for the ducts and bellows because of its small residual radioactivity. The ring is evacuated with 20 ion pumps (0.7 m 3/s) and 24 turbomolecular pumps (TMPs) (1.3 m 3/s), which are attached to the Ti ducts. The TMPs are used not only for rough pumping but also for evacuation during the beam operation. Especially a collimator system for localizing beam losses in a restricted area is evacuated with the TMPs, because the outgassing from this region will probably be the greatest. In addition, the straight sections for beam injection and extraction are pumped down mainly by the TMPs. Each arc section is pumped by 4 ion pumps and 2 TMPs. To realize the above system, we developed components such as large aperture ceramic ducts and TMPs with high radioactive resistance, as well as several kinds of heat treatment to reduce the outgassing. Finally, we realized the UHV without baking in the RCS and the beam operation has been successful to date.

Geochemical Architecture of the Lower- to Middle-crustal Section of a Paleo-island Arc (Kohistan Complex, Jijal–Kamila Area, Northern Pakistan): Implications for the Evolution of an Oceanic Subduction Zone

The processes active in the deep crust above an oceanic subduction zone during its evolution have been constrained through a detailed geochemical study (major and trace elements and Sr, Nd and Pb isotopes) of representative samples through an ∼30 km thick exhumed crustal section of the Cretaceous Kohistan oceanic island arc (Northern Pakistan). The use of both trace elements and radiogenic isotopes reveals two distinct geochemical suites (suites A and B) within the Jijal–Patan–Kiru–Kamila (JPKK) complex. Suite A is characterized by a progressive enrichment in 207Pb and a decrease in 143Nd/144Nd with increasing LaN/SmN. Suite B has higher 207Pb/204Pb and lower 143Nd/144Nd ratios with approximately constant LaN/SmN. By combining trace elements with different partitioning behaviour it is demonstrated that there is an increasing contribution of the subduction component in the magmas with time. It is also possible to distinguish a slab component imprint carried by aqueous fluids from one corresponding to sediment melts. Intrusive granites are abundant in the upper levels of the JPKK section. All were generated at the arc root level (Jijal crustal section) during dehydration-melting of hornblende-rich plutonic rocks. A three-stage geodynamic model is proposed for the evolution of the arc over a period of ∼30 Myr. The first stage (∼117–105 Ma) starts with the onset of subduction, which was followed by the building of the volcanic arc. The second stage (∼105–99 Ma to ∼96–91 Ma) corresponds to a major igneous event, which was characterized by abundant magma underplating and granulite-facies metamorphism at the arc base. Recycling of the residual–cumulative lower crust into the convective asthenospheric mantle was efficient during this stage, and was probably related to thermo-mechanical erosion of the base of the crust. The last stage (∼95–85 Ma) corresponds to a period of low magmatic activity, which marked the end of the intra-oceanic subduction. This is related to the formation of a ‘cold blanket’ above the slab surface as a result of thermo-mechanical erosion of the cold walls of the subduction zone (i.e. the upper part of the slab and the base of the overriding plate), and corner flow dragging the cold material into the zone of melt generation. Ultimately, a voluminous magmatic pulse occurred around 85 Ma (forming the Chilas complex), before arc–continent collision.

Intra-arc extension in Central America: Links between plate motions, tectonics, volcanism, and geochemistry

This study revisits the kinematics and tectonics of Central America subduction, synthesizing observations of marine bathymetry, high-resolution land topography, current plate motions, and the recent seismotectonic and magmatic history in this region. The inferred tectonic history implies that the Guatemala–El Salvador and Nicaraguan segments of this volcanic arc have been a region of significant arc tectonic extension; extension arising from the interplay between subduction roll-back of the Cocos Plate and the ~ 10–15mm/yr slower westward drift of the Caribbean plate relative to the North American Plate. The ages of belts of magmatic rocks paralleling both sides of the current Nicaraguan arc are consistent with long-term arc-normal extension in Nicaragua at the rate of ~ 5–10mm/yr, in agreement with rates predicted by plate kinematics. Significant arc-normal extension can ‘hide’ a very large intrusive arc-magma flux; we suggest that Nicaragua is, in fact, the most magmatically robust section of the Central American arc, and that the volume of intrusive volcanism here has been previously greatly underestimated. Yet, this flux is hidden by the persistent extension and sediment infill of the rifting basin in which the current arc sits. Observed geochemical differences between the Nicaraguan arc and its neighbors which suggest that Nicaragua has a higher rate of arc-magmatism are consistent with this interpretation. Smaller-amplitude, but similar systematic geochemical correlations between arc-chemistry and arc-extension in Guatemala show the same pattern as the even larger variations between the Nicaragua arc and its neighbors.

Research of Grinding Revolving Curved Surface with Arc Envelope Grinding Method

Revolving curved surface of mould should have low roughness as well as high accuracy of geometry profile. And it is an essential method to meet the accuracy request with grinding process. In order to solve the problem of grinding such revolving curved surface, an Arc Envelope Grinding Method (AEGM) is presented in this paper, which uses a wheel with arc section. By analyzing the movement of the grinding point, the relationship between the length of trajectory that the grinding point moves along and radius of the arc, curvature of workpiece is explained. Meanwhile, the relationship between the radius of arc, the curvature and the grinding area is studied.

Vacuum system design of the third generation synchrotron radiation source PETRA III

Within the next two years the 2.3 km long storage ring PETRA will be rebuild into one of the most brilliant x-ray sources worldwide (PETRA III). The large bending radius and the use of damping wigglers allow to achieve small beam emittances and extremely brilliant x-ray beams. In this paper we describe the design and the expected performance of the vacuum system for the storage ring. It consists of standard arc sections, an experimental octant which is equipped with undulators and several straight sections that include the damping wigglers. Because of the long length of the storage ring a cost effective solution had to be found. Besides the requirement to quickly provide acceptable residual gas pressures the technical challenges of the system include the provision of high thermal stability with respect to magnets and BPM's, and the design of thin walled insertion device chambers as well as high power synchrotron radiation absorbers.

Reconstruction of the Talkeetna intraoceanic arc of Alaska through thermobarometry

The Talkeetna arc is one of two intraoceanic arcs where much of the section from the upper mantle through the volcanic carapace is well exposed. We reconstruct the vertical section of the Talkeetna arc by determining the (re)crystallization pressures at various structural levels. The thermobarometry shows that the tonalites and quartz diorites intruded at ∼5–9 km into a volcanic section estimated from stratigraphy to be 7 km thick. The shallowest, Tazlina and Barnette, gabbros crystallized at ∼17–24 km; the Klanelneechena Klippe crystallized at ∼24–26 km; and the base of the arc crystallized at ∼35 km depth. The arc had a volcanic:plutonic ratio of ∼1:3–1:4. However, many or most of the felsic plutonic rocks may represent crystallized liquids rather than cumulates so that the liquid:cumulate ratio might be 1:2 or larger. The current 5‐ to 7‐km structural thickness of the plutonic section of the arc is ∼15–30% of the original 23‐ to 28‐km thickness. The bulk composition of the original Talkeetna arc section was ∼51–58 wt % SiO2.

Generation of 0.1 THz coherent synchrotron radiation with compact S-band linac at AIST

The 0.1THz coherent synchrotron radiation (CSR) was successfully generated in the 90° bending magnet of the compact S-band linac with the achromatic arc section using the ultra-short electron bunch which has the energy of 40MeV, the bunch charge of about 1nc and the bunch length less than 1ps (rms). The electron bunch compression of 1 nC electron bunch was achieved less than 1ps (rms) by controlling the Q-magnets in the achromatic arc section as the bunch length was measured by the rms bunch length monitor.

Recent studies on photoelectron and secondary electron yields of TiN and NEG coatings using the KEKB positron ring

In order to obtain a method to suppress electron-cloud instability (ECI), the photoelectron and the secondary electron yields (PEY and SEY) of a TiN coating and an NEG (Ti–Zr–V) coating on copper have been studied so far by using the KEK B-factory (KEKB) positron ring. Recently, test chambers with these coatings were installed at a straight section of the ring where the irradiated photon density was considerably smaller than that at the arc section of a previous experiment. The number of electrons around beams was measured by an electron current monitor; this measurement was performed up to a stored beam current of approximately 1700 mA (1389 bunches). For the entire range of the beam current, the electron currents of the NEG-coated and the TiN-coated chambers were clearly smaller as compared to those of the uncoated copper chamber by the factors of 2–3 and 3–4, respectively. The small photon density, that is, the weak effect of photoelectrons, elucidated the differences in the SEYs of these coatings when compared to the measurements at the arc section. By assuming almost the same PEY ( η e) values obtained in the previous study, the maximum SEY ( δ max) for the TiN and NEG coatings and the copper chamber was again estimated based on a previously developed simulation. The evaluated δ max values for these three surfaces were in the ranges of 0.8–1.0, 1.0–1.15, and 1.1–1.25, respectively. These values were consistent with the values obtained so far. As an application of the simulation, the effective η e, η e−eff (which included the geometrical effect of the antechamber) and δ max values were also estimated for copper chambers with one or two antechambers. These chambers were installed in an arc section and a wiggler section, respectively. The evaluated η e−eff and δ max values were approximately 0.008 and 1.2, and 0.04 and 1.2, respectively, where η e=0.28 was assumed on the side wall. As expected, the η e−eff values were considerably smaller than those obtained in the case of a simple circular chamber ( η e=0.28–0.3). Further, the δ max values were consistent with those obtained so far. With regard to the uncertainty in the simulation, the effect of the SEY spectrum on the estimation of δ max values is briefly discussed. As the next step in our study, we plan to combine beam ducts with antechambers and TiN coatings; this combination is the most promising solution to ECI at present.

Lattice and beam optics design for suppression of CSR-induced emittance growth at the KEK-ERL test facility

The lattice and beam optics of the arc section of the KEK-ERL test facility, having an energy of 200 MeV, were optimized to efficiently suppress emittance growth based on a simulation using a particle-tracking method taking coherent synchrotron radiation effects into account. The lattice optimization in the arc section was performed under two conditions: a high-current mode with a bunch charge of 76.9 pC without bunch compression, and a short-bunch mode with bunch compression, producing a final bunch length of around 0.1 ps. The simulation results showed that, in the high-current mode, emittance growth was efficiently suppressed by keeping a root-mean-square (rms) bunch length of 1 ps at a bunch charge of 76.9 pC, and in the short-bunch mode, emittance growth was kept within permissible limits with a maximum allowable bunch charge of 23.1 pC at an rms bunch length of 0.1 ps.

Volcanogenic Tsunamis in Lakes: Examples from Nicaragua and General Implications

This paper emphasizes the fact that tsunamis can occur in continental lakes and focuses on tsunami triggering by processes related to volcanic eruptions and instability of volcanic edifices. The two large lakes of Nicaragua, Lake Managua and Lake Nicaragua, host a section of the Central American Volcanic Arc including several active volcanoes. One case of a tsunami in Lake Managua triggered by an explosive volcanic eruption is documented in the geologic record. However, a number of events occurred in the past at both lakes which were probably tsunamigenic. These include massive intrusion of pyroclastic flows from Apoyo volcano as well as of flank-collapse avalanches from Mombacho volcano into Lake Nicaragua. Maar-forming phreatomagmatic eruptions, which repeatedly occurred in Lake Managua, are highly explosive phenomena able to create hugh water waves as was observed elsewhere. The shallow water depth of the Nicaraguan lakes is discussed as the major limiting factor of tsunami amplitude and propagation speed. The very low-profile shores facilitate substantial in-land flooding even of relatively small waves. Implications for conceiving a possible warning system are also discussed.

A New Grinding Method for Large-Scale Surface of Revolution and Error Analyzing of Its Wheel Dressing

Large-scale surface such as mould should have low roughness as well as high accuracy of geometry profile. And it is an essential method to reach the accuracy request with grinding process. In this research, due to the variety of geometry profiles, an Arc Envelope Grinding Method (AEGM) is presented, the difficult problem of grinding large-scale revolution surface can be solved. Meanwhile, the grinding wheel dressing and its shape precision are the keys to the precision of revolution surface in the grinding with AEGM. In this paper, the influences of setting error of diamond point dresser, which dress the grinding wheel into an arc section by using numerical control function of NC grinding machine, upon the shape accuracy of plate-shaped grinding are analyzed. It is shown that shape error of the grinding wheel increases with the increases of dresser setting error in diameter direction, radius of tip arc on the grinding wheel, while it reduces with the increase of radius of the grinding wheel, and shape error distributes no uniformly but symmetrically along axial direction.

Subduction-related magmatism at the southern tip of the North Fiji backarc basin

The North Fiji backarc basin (NFB) began opening ~12 Ma in response to subduction reversal along the Vanuatu section of the Proto-Vitiaz arc. The triangular shape of the basin reflects ongoing (since ~5 Ma) southward propagation of the backarc spreading centre in response to westward roll-back of the Vanuatu trench and eastward roll-back of the Tonga trench. During this time (5-3 Ma), a short-lived intraoceanic arc, the Hunter Ridge, formed along the southeast margin of the NFB. This arc magmatism occurred in response to a short period of NNW-directed subduction of the South Fiji basin crust under the NFB. The southern tip of the North Fiji backarc basin (22°S, 174°E) is a volcanically active submarine triple junction between the backarc basin spreading centre, Vanuatu trench and Hunter Fracture zone. This area has been mapped and sampled during the SS10/2004 cruise of R/V ``Southern Surveyor'. Volcanic rocks erupted within 100 km cover the entire spectrum of subduction-related magmas from backarc basin basalts to boninites, arc tholeiites, calc-alkaline basalts, high-Mg andesites and adakites. Subduction-related lavas in the area are dominated by olivine-clinopyroxene-phyric basalts with high-Mg phenocrysts (Mg# to 94). We will present detailed geology, geochemistry and mineralogy (including melt inclusion compositions) of all volcanic rock from this region and discuss their origin.

Stability of arc lower crust: Insights from the Talkeetna arc section, south central Alaska, and the seismic structure of modern arcs

One process for the formation of continental crust is the accretion of arc terranes at continental margins. A longstanding problem with this model is that although the composition of the continental crust is andesitic, the majority of arc lavas are basaltic. Moreover, those arc lavas that are andesitic tend to be evolved (lower Mg #) compared to the continental crust. Continental crust can be produced through mixing of basaltic and silicic arc lava compositions, assuming that mafic cumulates formed during generation of the silicic component are removed. If these cumulates are denser than the underlying mantle, removal can occur via foundering of lower arc crust. Indeed, field observations of the Talkeetna arc section in south central Alaska, combined with modeling of fractionation in primitive arc magmas, suggest that large amounts of primitive gabbronorite and pyroxenite are missing from the lower crust. Using rock compositions from the Talkeetna section and the free energy minimization program Perple_X, we calculated equilibrium mineral assemblages for a range of gabbroic and ultramafic compositions at P, T, oxygen fugacity (fO2), and H2O contents appropriate for arc lower crust. The quartz‐olivine‐garnet‐free mineral assemblage found in the Talkeetna gabbronorites (and in the similar Kohistan section in Pakistan) defines a narrow range of fO2 centered on NNO+2 (±1 log unit). Predicted mineral assemblages calculated under these conditions were used to estimate the density and seismic structure of the arc lower crust. We find that the missing gabbroic and ultramafic rocks from the Talkeetna section were likely denser than the underlying mantle, while the gabbronorites that remain are either neutrally or slightly positively buoyant. Generalizing, we show that lower crustal Vp > 7.4 km/s in modern arcs is indicative of lower crust that is convectively unstable relative to the underlying mantle. However, most lower crust in modern arcs is observed to have Vp < 7.4 km/s, implying that gravitationally unstable material must founder rapidly on geologic timescales, or high Vp plutonic rocks crystallize beneath the Moho.

The Lubrication characteristics of the vane tip under pressure boundary condition of oil hydraulic vane pump

The Lubrication Mode of line contacts between the vane and the camring in an oil hydraulic vane pump has been investigated. First, the variations of the radial force of a vane were calculated from previous measurements of dynamic internal pressure in four chambers surrounding a vane. Next, the lubrication modes were distinguished with Hooke’s chart, which is an improvement over Johnson’s chart. Finally, the influence of the boundary conditions in the lubrication region on the fluid film lubrication was examined by calculating the film pressure distributions. The results showed that the lubrication mode of the vane tip exists in the rigidvariable-viscosity region, and that discharge pressure higher than 7 MPa greatly affects the oil film pressure in the small and the large arc section because of the Piezo-viscous effect.

Is there a compositional gap at Nisyros volcano? A Comment on: Magma generation at the easternmost section of the Hellenic arc: Hf, Nd, Pb and Sr isotope geochemistry of Nisyros and Yali volcanoes (Greece) [Lithos 83 (2005) 29–46

Is there a compositional gap at Nisyros volcano? A Comment on: Magma generation at the easternmost section of the Hellenic arc: Hf, Nd, Pb and Sr isotope geochemistry of Nisyros and Yali volcanoes (Greece) [Lithos 83 (2005) 29–46

Early Cretaceous extensional exhumation of the lower crust of a magmatic arc: Evidence from the Mount Irene Shear Zone, Fiordland, New Zealand

The Mount Irene Shear Zone (MISZ) in Fiordland, New Zealand, is a low‐angle extensional mylonitic structure that has contributed to exhumation of a deep section of a Cretaceous magmatic arc. The dominantly metasedimentary hanging wall last equilibrated at ∼5.9 kbar, 603°C, whereas the metadioritic footwall, correlated with the Western Fiordland Orthogneiss (WFO), equilibrated at ∼8.5 kbar, 722°C, during retrogression associated with the MISZ. Displacement across the MISZ is also implied from differences in deformational episodes, metamorphic fabrics, and the presence/absence of partial melt products. A late stage synkinematic granitoid dike swarm intrudes the MISZ, but the dikes are invariably truncated and sheared within the basal calc‐silicate mylonites of the hanging wall. U/Pb dates from one such dike demonstrate that juxtaposition of the footwall against the hanging wall was effectively complete by ∼108 Ma. This date makes the MISZ the earliest Cretaceous extensional shear zone yet established in Fiordland and indicates that the switch from a convergent to an extensional tectonic regime on the paleo‐Pacific arc margin of Gondwana occurred between 111 and 108 Ma. Therefore there was only 8 Myr between final intrusion and high‐pressure metamorphism of the WFO and the initiation of extensional exhumation of the lower crust of this magmatic arc.

The Beam-Handling Magnet System for the J-PARC Neutrino Beam Line

The facility of the long baseline neutrino oscillation experiment using the J-PARC's 50 GeV-0.75 MW proton beam is now under way. The primary proton beam line consists of three sections, i.e. the first preparation (PP) section with normal conducting magnets, the second arc (ARC) section with superconducting magnets and the third final focus (FF) section with normal conducting magnets. In the PP section, we have to clean the primary proton beam extracted from the 50 GeV-PS and transport halo-less pure beam only to the ARC section. In the FF section the magnets have to be placed very close to the pion production target and horns. Therefore the normal conducting magnets have to work in the very high radioactive environment. The R&D works on the radiation resistant magnets for handling a high-intensity proton beam have already been continued at KEK as reported in . Another important point regarding high-intensity beam handling is to realize easy maintenance of the beam line. Any magnet experiencing trouble can be easily removed from beam line and repaired remotely. For this purpose, we developed new tools for the magnet maintenance. These are automatic sling apparatus, quick alignment and installation guide, and the quick disconnect devices of cooling water and electric power. In this paper, we will report the beam line maintenance scheme developed for the neutrino beam line, as well as the design of normal conducting magnet sections

A Detailed Geochemical Study of Island Arc Crust: the Talkeetna Arc Section, South–Central Alaska

Author Posting. © The Author, 2006. This is the author's version of the work. It is posted here by permission of Oxford University Press for personal use, not for redistribution. The definitive version was published in Journal of Petrology 47 (2006): 1051-1093, doi:10.1093/petrology/egl002.

Calculation of water activation for the LHC

The management of activated water in the Large Hadron Collider (LHC) at CERN is a key concern for radiation protection. For this reason, the induced radioactivity of the different water circuits is calculated using the Monte-Carlo (MC) code FLUKA. The results lead to the definition of procedures to be taken into account during the repair and maintenance of the machine, as well as to measures being necessary for a release of water into the environment. In order to assess the validity of the applied methods, a benchmark experiment was carried out at the CERN-EU High Energy Reference Field (CERF) facility, where a hadron beam (120 GeV) is impinging on a copper target. Four samples of water, as used at the LHC, and different in their chemical compositions, were irradiated near the copper target. In addition to the tritium activity measured with a liquid scintillation counter, the samples were also analyzed using gamma spectroscopy in order to determine the activity of the gamma emitting isotopes such as Be 7 and Na 24 . While for the latter an excellent agreement between simulation and measurement was found, for the calculation of tritium a correction factor is derived to be applied for future LHC calculations in which the activity is calculated by direct scoring of produced nuclei. A simplified geometry representing the LHC Arc sections is then used to evaluate the different calculation methods with FLUKA. By comparing these methods and by taking into account the benchmark results, a strategy for the environmental calculations can be defined.

Sedimentary Processes in Modern and Ancient Oceanic Arc Settings: Evidence from the Jurassic Talkeetna Formation of Alaska and the Mariana and Tonga Arcs, Western Pacific

Abstract Sediment deposited around oceanic volcanic ares potentially provides the most complete record of the tectonic and geochemical evolution of active margins. The use of such tectonic and geochemical records requires an accurate understanding of sedimentary dynamics in an arc setting: processes of deposition and reworking that affect the degree to which sediments represent the contemporaneous volcanism at the time of their deposition. We review evidence from the modern Mariana and Tonga arcs and the ancient arc crustal section in the Lower Jurassic Talkeetna Formation of south-central Alaska, and introduce new data from the Mariana Arc, to produce a conceptual model of volcaniclastic sedimentation processes in oceanic arc settings. All three arcs are interpreted to have formed in tectonically erosive margin settings, resulting in long-term extension and subsidence. Debris aprons composed of turbidites and debris flow deposits occur in the immediate vicinity of arc volcanoes, forming relatively continuous mass-wasted volcaniclastic records in abundant accommodation space. There is little erosion or reworking of old volcanic materials near the arc volcanic front. Tectonically generated topography in the forearc effectively blocks sediment flow from the volcanic front to the trench; although some canyons deliver sediment to the trench slope, most volcaniclastic sedimentation is limited to the area immediately around volcanic centers. Arc sedimentary sections in erosive plate margins can provide comprehensive records of volcanism and tectonism spanning < 10 My. The chemical evolution of a limited section of an oceanic arc may be best reconstructed from sediments of the debris aprons for intervals up to ~ 20 My but no longer, because subduction erosion causes migration of the forearc basin crust and its sedimentary cover toward the trench, where there is little volcaniclastic sedimentation and where older sediments are dissected and reworked along the trench slope.