Multiple Arcs Research Articles

32. Linac based VMAT radiosurgery for multiple brain lesions: comparison between a conventional multi-isocenter approach and a new dedicated mono-isocenter 4π technique (HyperArc™)

Purpose Linac based VMAT radiosurgery (SRS) of multiple brain lesions (MBL) is typically performed by a multiple-isocenter approach, i.e.one isocenter per lesion.Here we present the worldwide first experience with a new mono-isocenter technique with multiple non-coplanar arcs (HyperArc, Varian Inc.)in terms of a plan comparison with a multiple-isocenter approach. Methods From August to October 2017, 20 patients with MBL (mean 5, range 2–10) have been treated by HyperArc: prescribed doses ( D p ) were 18–25 Gy in single-fraction, and 21–27 Gy in three-fractions.Total PTV, defined by a 2 mm isotropic margin from each lesion, had mean size of 9.6 cm3 (range 0.5–27.9 cm3).HyperArc VMAT plans (HA), with 5 non-coplanar 180°-arcs (couch at 0°, ±45°, ±90°), and multiple-isocenter VMAT plans (RA), with 2 coplanar 360°-arcs per isocenter, were generated.100% D p at 98%PTV was prescribed, and D 2 % (PTV) D p was accepted.Constraints for 1(3) -fraction to the brainstem, D 0.5cc D 0.5cc V 12 and mean dose to the brain-minus-PTV, MU and overall treatment time (OTT) per fraction.OTT was measured for HA treatments, whereas estimated (3 min.for patient setup plus 5 min.for each CBCT per isocenter) for RA plans. Results As detailed in Table 1 , HA plans resulted in significantly improved CI ( p V 12 to the brain-minus-PTV (p = .023), MU (p D p isodose shells around the targets for the HA plans is depicted for an example patient.By contrast, no significant difference in terms of mean doses to the brain-minus-PTV resulted (p = .31). Conclusions For linac based VMAT-SRS of MBL, HyperArc plans assured a higher CI and a lower GI, a lower V 12 to the brain-minus-PTV, and a lower OTT than standard multiple-isocenter VMAT plans. Finally, all HyperArc treatments were completed within a typical 20 min. time slot.

Analytical investigation of exergetic performance on jet impingement solar air heater with multiple arc protrusion obstacles

In this present work, an exergetic efficiency of the solar air heater (SAH) with jet impingement on multiple arc protrusion roughened absorber plate is analytically studied. It is examined out for roughness parameters of relative width ratio (WP/WAP) of 1–5, relative height ratio (eP/dP) of 0.5–2, relative pitch ratio (PP/eP) of 8–12 and angle of arc (αa) of 35°–75°. Performance of SAH based on the second law has been effectively assessed by enumerating the useful exergy gain, irreversibility and losses occurred due to the inefficiency of the system components. Results indicate that the exergetic efficiency has been enhanced by 56.8%. Effects of roughness parameters on exergetic efficiency have been presented based on Reynolds number and temperature rise parameter. The optimized values of roughness parameters are identified, and the maximum exergetic efficiency is found to be 10.5%. Maximum exergetic destruction caused due to temperature difference between absorber plate and Sun is 65.2% at optimized conditions. In order to identify the optimum values of roughness parameters to attain the required temperature rise with maximum exergetic efficiency, design plots have been developed.

Comparison of Two Mono-Isocenter Techniques with Multiple Non-Coplanar Arcs for Linac Based SRS of Multiple Brain Lesions

Comparison of Two Mono-Isocenter Techniques with Multiple Non-Coplanar Arcs for Linac Based SRS of Multiple Brain Lesions

A Dosimetric Analysis of Dorsal Nerve Root Ablation Therapy using a Virtual Cone Technique

Virtual cone radiosurgery has been established as an effective treatment for trigeminal neuralgia. This delivery technique employs multiple static-field arcs with HDMLC-defined apertures to deliver high doses to small target volumes (<1cc) with sharp dose gradients. As a replacement to a physical cone, virtual cone delivery has the advantage of reduced treatment times and superior intra-fractional motion monitoring capability, while eliminating the need for Co-60 radiosurgery units. There is currently clinical interest in employing virtual cone techniques in the ablation of dorsal nerve root ganglia as a treatment for chronic thoraco-abdominal pain. We hypothesize that a standard template for single-fraction virtual cone treatment may be established to achieve ablative 90Gy point doses to the ganglia while adhering to conventional OAR dose constraints. This would enable the first such application of virtual cone treatment outside of cranial cases. Dosimetric analysis of virtual cone plans was performed within our institution’s treatment planning system using a linear Boltzman transport equation model on a 1mm grid. 10MV flattening filter-free beams were used at a 2400 MU/minute dose rate. Calculation accuracy was assessed using orthogonal film measurements in an anatomical phantom. Film-based verification indicated that dose discrepancies relative to the calculation model were <4%. Having established the accuracy of our calculations, various arc arrangements, HDMLC-defined field sizes, and collimator angles were investigated. The optimal field arrangement involved 10 equally weighted arcs at 0, ±7.5, and ±15 degree couch angles. For left-nerve root treatment, arcs ran from 10-170 degrees, with a 60-120 degree skip arc (with mirrored arcs for right-nerve root treatment). Delivery time was 25 minutes. The four innermost HDMLC leaves formed a 10mm x 4mm aperture, with jaws at 11mm x 5mm. ±5 degree collimator angles were used in repeated arcs. Doses were normalized to a 90Gy point dose at the target center. In both a thorax phantom and a test patient plan, this arrangement produced a 60Gy isodose surface approximately 5.5mm x 7.5mm x 7.5mm along the lateral, A-P, and S-I axes, respectively. The maximum dose to the spinal cord was 4.14Gy, and the maximum dose to the esophagus was 12.08Gy, with a V12 of <0.001cc. Results of this dosimetric study indicate that ablative doses can be delivered to dorsal nerve root ganglia while respecting normal tissue constrains. Furthermore, due to the robustly conformal dose profile achieved and the elimination of inverse-planning, it is probable that this virtual cone technique can be safely employed as a standardized template treatment for most patients (subject to dose constraint verification), eliminating the need for time-consuming optimization and allow for the expedited treatment of patients with intractable pain.

Dosimetric Analysis of Multiple Breath-Hold Segmented Volumetric Modulated Arc Radiation Therapy Plans with and without Fiducial Markers for Pancreatic Cancer

Given the proximity of the target volume to critical structures, radiation therapy for pancreatic cancer is challenging. The present study aimed to estimate planning target volume (PTV) margins of pancreatic tumors using daily image guided radiation therapy (IGRT) and to compare the dosimetric outcomes with and without fiducial markers (with-FM and without-FM) during multiple breath-hold segmented volumetric modulated arc therapy (MBHS-VMAT). Based on re-planned VMAT technique, two types of PTV margins, with-FM or without-FM, were retrospectively estimated in 10 patients with unresected pancreatic cancer. Daily IGRT shifts based on MBHS-cone-beam volume CT obtained prior to daily fractionated MBHS-VMAT and cone-beam motion CT under fractionated MBHS-VMAT were recorded and evaluated to estimate the two PTV margin types. Accordingly, 10 patients, 270 3D images, and 1080 2D images were considered. Appropriate margins were estimated using the equation M = 2.5Σ + 0.7σ. Regarding the treatment plans, the organs at risk (OAR) included the kidneys, liver, stomach, duodenum, jejunum, and spinal cord. The prescribed dose was 55 Gy in 25 fractions and was set to encompass 95% of the PTV. For both the plans, the planning objectives used in the optimizer were constant and were based on NCCN guidelines. Homogeneity and conformity indices, doses to OARs with attention to the digestive tract (stomach, duodenum, and jejunum), and PTV doses were evaluated between the two margin types. The respective PTV margins in the lateral, vertical, and longitudinal directions were 4, 4, and 5 mm with-FM and 7, 6, and 10 mm without-FM. For each patient, treatment planning implied two PTV margin types. For the digestive tract, a consistent trend indicated a significantly (p <0.05) lower V45, V50, and V55 using the with-FM MBHS-VMAT technique; these respective values decreased by 4.9%, 3.8% and 2.0% using with-FM MBHS-VMAT plans. There were no differences (p >0.05) in digestive tract (mean), PTV, liver dose (mean), and total kidney (V18) between the two margin types. Conformity and homogeneity indexes were also similar. The pancreas carries the potential for significant motion. In the absence of fiducial markers, our data suggest that the PTV margin will increase from 5 to 10 mm. These margins would surely overlap the surrounding bowel, duodenum, and/or stomach, suggesting that the dose coverage of margin-based PTVs is not be feasible. Our comparison shows that with-FM plans provide superior OAR-sparing abilities (particularly for the digestive tract) without decreasing target coverage.Abstract TU_41_3298; Table 1Comparison of various parameters for the two margin typesWith-FMWithout-FMp-valueDigestive tractV458.4 %13.3 %< 0.05V504.2 %8.0 %< 0.05V551.1 %3.1 %< 0.05Dmean, digestive tract (Gy)22.125.60.118PTVD2 (Gy)59.959.80.543D98 (Gy)53.853.40.342D50 (Gy)58.558.30.384LiverDmean, liver (Gy)8.410.30.190KidneysV1814.6 %16.6 %0.668 Open table in a new tab

Arc Appearance and Cathode Spot Distribution in a Long-Gap High-Current Vacuum Arc Controlled by an External Axial Magnetic Field

An experimental study of a high-current vacuum arc generated between two static CuCr25 contacts spaced 20 or 30 mm apart was conducted to characterize the arc appearance and the cathode spot (CS) distribution. The arc was ignited from the lateral surface of the cathode and controlled using an external axial magnetic field. Under the investigated experimental conditions, three distinct arc modes have been observed: multiple arc, diffuse, and diffuse columnar modes. The latter appeared at low $B_{\mathrm {AMF}}/I_{\mathrm {arc}}$ values, typically below 4 mT/kA. The temporal evolutions of the spot spatial distribution and the associated distribution of the current density were analyzed by processing high-speed video images of the cathode. Various types of distribution depending mainly on current were identified. At low currents (up to 13 kA), the CS distribution covered only a fraction of the cathode surface. At intermediate currents (in 17.2–31.1-kA range), CSs were present on the whole circumference of the cathode and the CS distribution included a closed region without spot. The latter was progressively filled by CSs, yet it totally disappeared before the end of arcing only when its location was off-centered with respect to the cathode axis. At high currents (up to 36.3 kA), the whole cathode surface was occupied by CSs at current peak. The azimuthally averaged radial distribution of the current density was found to be relatively uniform in the regions occupied by CSs. An average spot current of 36.5 ± 2.5 A has been estimated.

Thermal Cycles and Peculiar Properties of Austenite Decomposition when Laser-Hybrid Welding Steel of K60 Strength Class

Welding is an important technology of joint in engineering structures. In addition to a residual stress influence to a quality of joint, an obstacle in welding analysis are complex phenomena including phase transformation, thermal cycle and microstructure kinetics. The influence appears in microstructure development, formation of defects and in transformation of metallurgy in a welded zone. This article experimentally defines thermal cycles and shows the results in study of peculiar properties of austenite decomposition kinetics when using the technology of laser-hybrid welding in combination with multiple arc automatic sub-merged welding. There defined the rates of cooling influencing the change of properties of welded joints from tube assortment steel of K60 strength class. We found that the result of impact of laser-hybrid welding process in a heat-affected zone is that the austenite decomposition in studied steels flows in martensite area generally. The hardness of seam metal and heat-affected zone of the studied steels is of range 350-360 HV, which increases a probability of crack-like, defects formation. We revealed the normative value of hardness, which can be provided if a metal cooling rate in laser-hybrid welding is about 20 °C/s.

Characteristics of Changes of Transportation Performance for Pairs of Critical Edges

When the robustness of a public service system design is tested, we can often use scenarios where possible random failures can occur and they can influence the time the service is accessible which is provided for system users. The construction of a suitable scenario is based on the choice of links of the transportation network which influence the system performance in a substantial way. In such scenarios one or multiple arcs can be affected by this failure. In our contribution we present characteristics of pairs of critical arcs that can be used to develop an algorithm for the creation of critical scenarios.

Spiral arm crossings inferred from ridges in Gaia stellar velocity distributions

The solar neighborhood contains disc stars that have recently crossed spiral arms in the Galaxy. We propose that boundaries in local velocity distributions separate stars that have recently crossed or been perturbed by a particular arm from those that haven't. Ridges in the stellar velocity distributions constructed from the second Gaia data release trace orbits that could have touched nearby spiral arms at apocentre or pericentre. The multiple ridges and arcs seen in local velocity distributions are consistent with the presence of multiple spiral features and different pattern speeds and imply that the outer Galaxy is flocculent rather than grand design.

Wind turbine wake characterization in complex terrain via integrated Doppler lidar data from the Perdigão experiment

During the intensive period (May-June 2017) of the Perdigäo experiment, three sets of Doppler lidar were operated to scan the wake of the wind turbine (WT) on the southwest ridge. CU operated a Doppler scanning lidar in the valley bottom approximately 1 km northeast of the WT and conducted multiple arc scans and two RHI scans every 10-minutes centred on the WT. DTU used a dual Doppler lidar system scanning almost horizontally from the northeast ridge. Two of the three DLR lidars were in-plane with the WT for the main wind direction, one in the valley and one on the distant mountain ridge. The third DLR lidar was on the southwest ridge. All three systems (CU, DTU and DLR) were operated such that in data processing vertical and/or horizontal profiles of the wake can be derived at different distances from the WT. The paper describes the strategies used to scan the wake by the three groups and compares wake characteristics derived from the different systems.

On the relationship between the skew-rank of an oriented graph and the rank of its underlying graph

An oriented graph Gσ is a digraph without loops and multiple arcs, where G is the underlying graph of Gσ. Let S(Gσ) denote the skew-adjacency matrix of Gσ, and A(G) be the adjacency matrix of G. The rank (resp. skew-rank) of G (resp. Gσ), written as r(G) (resp. sr(Gσ)), refers to the rank of A(G) (resp. S(Gσ)). It is natural and interesting to study the relationship between sr(Gσ) and r(G). Wong, Ma and Tian (European J. Combin. 54 (2016) 76–86) [30] determined the sharp upper bound on sr(Gσ)−r(G). As a continuance of it, in this paper, a sharp lower bound on sr(Gσ)−r(G) is determined; as well a sharp upper bound on sr(Gσ)/r(G) is determined. All the corresponding extremal oriented graphs Gσ are characterized, respectively.

Experimental Analysis of Double Flow Solar Air Heater by using Multiple Arc Shape Baffle as Artificial Roughness- A Review

Experimental Analysis of Double Flow Solar Air Heater by using Multiple Arc Shape Baffle as Artificial Roughness- A Review

Optimizing radiosurgery with photons for ocular melanoma.

Background and purposePhoton radiotherapy has been established for the treatment of ocular melanoma (OM). Here we investigate the planning qualities of two different planning approaches, a combination of dynamic conformal arcs (DCA) complemented with multiple non-coplanar static intensity-modulated (IMRT) fields (DCA-IMRT), and volumetric modulated arc therapy (VMAT) in combination with automated planning (AP).Materials and methodsThirteen consecutive patients treated for ocular melanoma with curative intent on a Linac-based radiosurgery system were analyzed. Fractionated stereotactic radiosurgery (fSRS) was applied using 50 Gy in 5 fractions using the combination of DCA-IMRT. Plans were reviewed and the thirteen cases were compared to plans obtained with optimized automated VMAT based on a set of 28 distinct patients treated with DCA-IMRT who were selected to generate the AP model for the prediction of dose volume constraints.ResultsOverall, plan quality of DCA-IMRT was superior to AP with VMAT. PTV coverage did not exceed 107% in any case treated with DCA-IMRT, compared to seven patients with VMAT. The median PTV covered by >95% was 98.3% (91.9%–99.7%) with DCA-IMRT, compared to 95.1% (91.5%–97.9%) (p < 0.01) with VMAT. The median mean dose delivered to the treated eye was 22.4 Gy (12.3 Gy–33.3 Gy) with DCA-IMRT compared to 27.2 Gy (15.5 Gy–33.7 Gy) (p < 0.01). Dose to the ipsilateral lacrimal gland and the ipsilateral optic nerve were comparable for DCA-IMRT and VMAT, however, the dose to the lens was lower with DCA-IMRT compared to VMAT.ConclusionsThe combination of multiple arcs complemented with multiple IMRT fields sets the gold standard for fSRS of ocular melanoma for photon therapy.

Relation between the skew-rank of an oriented graph and the independence number of its underlying graph

An oriented graph $$G^\sigma $$ is a digraph without loops or multiple arcs whose underlying graph is G. Let $$S\left( G^\sigma \right) $$ be the skew-adjacency matrix of $$G^\sigma $$ and $$\alpha (G)$$ be the independence number of G. The rank of $$S(G^\sigma )$$ is called the skew-rank of $$G^\sigma $$ , denoted by $$sr(G^\sigma )$$ . Wong et al. (Eur J Comb 54:76–86, 2016) studied the relationship between the skew-rank of an oriented graph and the rank of its underlying graph. In this paper, the correlation involving the skew-rank, the independence number, and some other parameters are considered. First we show that $$sr(G^\sigma )+2\alpha (G)\geqslant 2|V_G|-2d(G)$$ , where $$|V_G|$$ is the order of G and d(G) is the dimension of cycle space of G. We also obtain sharp lower bounds for $$sr(G^\sigma )+\alpha (G),\, sr(G^\sigma )-\alpha (G)$$ , $$sr(G^\sigma )/\alpha (G)$$ and characterize all corresponding extremal graphs.

Is volumetric modulated arc therapy with constant dose rate a valid option in radiation therapy for head and neck cancer patients?

Intensity-modulated radiotherapy (IMRT) improves dose distribution in head and neck (HN) radiation therapy. Volumetric-modulated arc therapy (VMAT), a new form of IMRT, delivers radiation in single or multiple arcs, varying dose rates (VDR-VMAT) and gantry speeds, has gained considerable attention. Constant dose rate VMAT (CDR-VMAT) associated with a fixed gantry speed does not require a dedicated linear accelerator like VDR-VMAT. The present study explored the feasibility, efficiency and delivery accuracy of CDR-VMAT, by comparing it with IMRT and VDR-VMAT in treatment planning for HN cancer. Step and shoot IMRT (SS-IMRT), CDR-VMAT and VDR-VMAT plans were created for 15 HN cancer patients and were generated by Pinnacle3 TPS (v 9.8) using 6MV photon energy. Three PTVs were defined to receive respectively prescribed doses of 66Gy, 60Gy and 54Gy, in 30 fractions. Organs at risk (OARs) included the mandible, spinal cord, brain stem, parotids, salivary glands, esophagus, larynx and thyroid. SS-IMRT plans were based on 7 co-planar beams at fixed gantry angles. CDR-VMAT and VDR-VMAT plans, generated by the SmartArc module, used a 2-arc technique: one clockwise from 182° to 178° and the other one anti-clockwise from 178° to 182°. Comparison parameters included dose distribution to PTVs (Dmean, D2%, D50%, D95%, D98% and Homogeneity Index), maximum or mean doses to OARs, specific dose-volume data, the monitor units and treatment delivery times. Compared with SS-IMRT, CDR-VMAT significantly reduced the maximum doses to PTV1 and PTV2 and significantly improved all PTV3 parameters, except D98% and D95%. It significantly spared parotid and submandibular glands and was associated with a lower Dmean to the larynx. Compared with VDR-VMAT, CDR-VMAT was linked to a significantly better Dmean, to the PTV3 but results were worse for the parotids, left submandibular gland, esophagus and mandible. Furthermore, the Dmean to the larynx was also worse. Compared with SS-IMRT and VDR-VMAT, CDR-VMAT was associated with higher average monitor unit values and significantly shorter average delivery times. CDR-VMAT appeared to be a valid option in Radiation Therapy Centers that lack a dedicated linear accelerator for volumetric arc therapy with variable dose-rates and gantry velocities, and are unwilling or unable to sanction major expenditure at present but want to adopt volumetric techniques.

Linac-based VMAT radiosurgery for multiple brain lesions: comparison between a conventional multi-isocenter approach and a new dedicated mono-isocenter technique

BackgroundLinac-based stereotactic radiosurgery or fractionated stereotactic radiotherapy (SRS/FSRT) of multiple brain lesions using volumetric modulated arc therapy (VMAT) is typically performed by a multiple-isocenter approach, i.e. one isocenter per lesion, which is time-demanding for the need of independent setup verifications of each isocenter. Here, we present our initial experience with a new dedicated mono-isocenter technique with multiple non-coplanar arcs (HyperArc™, Varian Inc.) in terms of a plan comparison with a multiple-isocenter VMAT approach.MethodsFrom August 2017 to October 2017, 20 patients with multiple brain metastases (mean 5, range 2–10) have been treated by HyperArc in 1–3 fractions. The prescribed doses (Dp) were 18–25 Gy in single-fraction, and 21–27 Gy in three-fractions. Planning Target Volume (PTV), defined by a 2 mm isotropic margin from each lesion, had mean dimension of 9.6 cm3 (range 0.5–27.9 cm3). Mono-isocenter HyperArc VMAT plans (HA) with 5 non-coplanar 180°-arcs (couch at 0°, ±45°, ±90°) were generated and compared to multiple-isocenter VMAT plans (RA) with 2 coplanar 360°-arcs per isocenter. A dose normalization of 100%Dp at 98%PTV was adopted, while D2%(PTV) < 150%Dp was accepted. All plans had to respect the constraints on maximum dose to the brainstem (D0.5cm3 < 18 Gy) as well as to the optical nerves/chiasm, eyes and lenses (D0.5cm3 < 15 Gy). HA and RA plans were compared in terms of dose-volume metrics, by Paddick conformity (CI) and gradient (GI) index and by V12 and mean dose to the brain-minus-PTV, and in terms of MU and overall treatment time (OTT) per fraction. OTT was measured for HA treatments, whereas for RA plans OTT was estimated by assuming 3 min. For initial patient setup plus 5 min. For each CBCT-guided setup correction per isocenter.ResultsSignificant variations in favour of HA plans were computed for both target dose indexes, CI (p < .01) and GI (p < .01). The lower GI in HA plans was the likely cause of the significant reduction in V12 to the brain-minus-PTV (p = .023). Although at low doses, below 2–5 Gy, the sparing of the brain-minus-PTV was in favour of RA plans, no significant difference in terms of mean doses to the brain-minus-PTV was observed between the two groups (p = .31). Finally, both MU (p < .01) and OTT (p < .01) were significantly reduced by HyperArc plans.ConclusionsFor linac-based SRS/FSRT of multiple brain lesions, HyperArc plans assured a higher CI and a lower GI than standard multiple-isocenter VMAT plans. This is consistent with the computed reduction in V12 to the brain-minus-PTV. Finally, HyperArc treatments were completed within a typical 20 min. time slot, with a significant time reduction with respect to the expected duration of multiple-isocenters VMAT.

Scheduling in A Single-Stage, Multi-Item Compatible Process Using Multiple ARC Network Model and Excel Solver

Scheduling in A Single-Stage, Multi-Item Compatible Process Using Multiple ARC Network Model and Excel Solver

Highly porous micro-roughened structures developed on aluminum surface using the jet of rotating arc discharges at atmospheric pressure

Aluminum 6061 samples were exposed to the jet of an atmospheric pressure rotating arc discharge operated in either nitrogen or air. After multiple passes of treatment with an air-based plasma jet at very short source-to-substrate distances, scanning electron microscopy combined with x-ray photoelectron spectroscopy revealed a highly porous micro-roughened alumina-based structure on the surface of aluminum. Based on optical emission spectroscopy and high-speed optical imaging of the jet interacting with aluminum samples, it was found that the process is mainly driven by the energy transfer from the plasma source to the surface through transient plasma-transferred arcs. The occurrence of multiple arc discharges over very short time scales can induce rapid phase transformations of aluminum with characteristics similar to the ones usually observed during laser ablation of materials with femto- to nanosecond laser pulses or during the formation of cathode spots on the surface of metals.

Detrital Zircon Record of a Mesozoic Collisional Forearc Basin in South Central Alaska: The Tectonic Transition From an Oceanic to Continental Arc

AbstractThe collision of oceanic arcs with continental margins is an important mechanism for the growth of continents. Ancient forearc basin strata in collisional orogens provide a record of the upper crustal response to this tectonic process. In south central Alaska, Mesozoic forearc basin strata are exposed in a complete crustal section. U‐Pb detrital zircon geochronology from the forearc basin strata was analyzed within a ~107 Ma stratigraphic framework. The Jurassic strata contain unimodal detrital zircon populations that become progressively younger upsection and range from 175 to 151 Ma. These strata are derived from the active oceanic Talkeetna arc. The Cretaceous strata were deposited above multiple unconformities that collectively represent as much as ~30 Ma of nondeposition and/or erosion in the forearc basin. Erosion in the forearc basin and a general absence of detrital zircon ages between 140 and 120 Ma are interpreted as a hiatus of magmatism triggered by collision of the oceanic arc with the former continental margin. The Cretaceous strata have two main detrital zircon populations: a Cretaceous population ranging from 90 to 68 Ma that becomes progressively younger upsection and a Jurassic population with a broad range of peak ages from 194 to 144 Ma. The Cretaceous population marks the establishment of an active Cretaceous continental arc following the collisional event, and the older population reflects continued erosion of the remnant Jurassic oceanic arc plutons. Our results show that detrital zircon geochronology provides a powerful approach for delineating stages of forearc basin collision and the erosion of multiple magmatic arcs.

Experimental investigation on the initial expansion stage of vacuum arc on cup-shaped TMF contacts

Arc behavior and measures to control it directly affect the properties of vacuum circuit breakers. Nowadays, transverse magnetic field (TMF) contacts are widely used for medium voltages. A magnetic field perpendicular to the current direction between the TMF contacts makes the arc move, transmitting its energy to the whole contact and avoiding excessive local ablation. Previous research on TMF arc behavior concentrated mainly on the arc movement and less on the initial stage (from arc ignition to an unstable arc column). A significant amount of experiment results suggest that there is a short period of arc stagnation after ignition. The duration of this arc stagnation and the arc characteristics during this stage affect the subsequent arc motion and even the breaking property of interrupters. The present study is of the arc characteristics in the initial stage. Experiments were carried out in a demountable vacuum chamber with cup-shaped TMF contacts. Using a high-speed camera, both single-point arc ignition mode and multiple-point arc ignition (MPAI) mode were observed. The experimental data show that the probability of MPAI mode occurring is related to the arc current. The influences of arc-ignition mode, arc current, and contact diameter on the initial expansion process were investigated. In addition, simulations were performed to analyze the multiple arc expansion process mechanically. Based on the experimental phenomena and simulation results, the mechanism of the arc expansion motion was analyzed.