Helium Leak Test Research Articles

Key technologies of manufacturing and testing for the ITER blanket shield block

The SB is one of the main in-vessel components manufactured with SS316L(N)-IG to play a major role in neutron shielding, to support the FW panels and to supply the FW panel with cooling water. Cooling channels are located inside of the SB to remove the neutron heat deposition while keeping the structure temperature to acceptable levels. The water coolant is routed first through the plasma-facing first wall panel which is attached to the SB and then through the SB. A number of deep slits are introduced into the SB to disrupt eddy current loops and reduce the Electro-Magnetic (EM) loads on the support system and VV. Water headers are machined on the side of the module with 7–10 mm welded cover plates. A Procurement Arrangements (PAs) for the manufacturing and testing of the SBs have been signed with Korea (50 %) and China (50 %).As part of the PA in Korean Domestic Agency (KODA), Process Qualification (PQ) to ensure the manufacturing and testing ability of the SB was completed successfully through manufacture of a Full Scale Prototype (FSP) as an essential step before starting series production. This paper reports key technologies, such as machining, welding, NDE and hot helium leak testing, of manufacturing and testing for the ITER SB series production in Korea toward successful completion of the missions.

Optimization of Wall-Conditioning Techniques on the MT-II Tokamak

The MT-II is a small spherical tokamak that is currently under construction at the Pakistan Tokamak Plasma Research Institute. Wall conditioning of the MT-II vacuum vessel (VV) is an essential step to achieve a good quality vacuum for plasma experiments. This study presents an overview of the wall-conditioning techniques implemented on the MT-II VV, including baking and glow discharge cleaning (GDC). Prior to wall conditioning, the system is checked via a helium leak test machine and residual gas analyzer (RGA) to identify and remove leaks. The VV walls are baked at ~180 °C to get rapid desorption of water vapors and other impurities. After the baking process, the partial pressure of most of the carbon- and oxygen-containing impurities is reduced. In particular, the partial pressure of water vapors is reduced by 93%. Consequently, the total leak and outgassing rate is significantly reduced. To further improve the vacuum condition in the vessel, hydrogen GDC is carried out. The fill hydrogen pressure and anode voltage are optimized to get a stable glow discharge. The RGA scan shows that GDC reduces the partial pressure of H2O, O2, and CO2 by 57%, 63%, and 51%, respectively. The results signify that baking and GDC are effective techniques for wall conditioning of the MT-II VV.

Construction of hot helium leak test facility for ITER blanket shield block

The ITER Blanket Shield Block (SB) is one of the main in-vessel components that provides nuclear shielding and removes nuclear heating for the Vacuum Vessel (VV) and external vessel components. The SB, which contains internal cooling channels, is manufactured from 316L(N)-IG stainless steel, and the internal channels are designed to provide the required thermal-hydraulic performance while aiming for a water/steel ratio that results in good nuclear shielding with headers and welded cover plates. Therefore, the soundness of all SB products' welded joints must be confirmed to eliminate the risk of leakage during manufacturing and the Factory Acceptance Test (FAT). However, a test facility capable of testing large components at high temperatures has never been constructed anywhere. The ITER Korea Domestic Agency (KODA) has therefore successfully completed the construction and commissioning of two Hot Helium Leak Test (HHLT) facilities, and all SB series products are being entirely tested under RT and an elevated temperature of 250 °C with a maximum operational pressure of 5.0 MPa. This paper provides information on the design features of the two HHLT facilities and lessons learned from the commissioning test.

China's progress on hot helium leak test of ITER shield blocks

China Domestic Agency (DA) will provide ITER with 220 shield blocks (SB) which are important, large, and actively water-cooled components in ITER vacuum vessel, their leakage rate shall not exceed 2.69×10−10Pa·m3/s in hot helium leak tests (HHLT) at 250±20 °C. The HHLT work started with developing a HHLT facility and manufacturing a SB full-scale prototype (FSP). The result showed the FSP could satisfy ITER's requirement at 235 °C. H2 outgassing from stainless steel surfaces is the main factor that impacts both the background leakage rate and sensitivity of the testing system in hot testing cases. Hydrogen-removal measures were taken to improve the facility performance, and a new facility was built. A better facility performance came out at 250 °C rather than 235 °C, the background leakage rate decreased by one order of magnitude while the system sensitivity increased by two orders of magnitude. The background is at 10−10 Pa.m3/s and 10−9 Pa.m3/s level at room temperature (RT) and 250 °C, respectively, while the system sensitivity is at 10−12 Pa.m3/s and 10−11 Pa.m3/s level at the two temperatures. With the two facilities, series HHLTs have been carried out on 50 SBs covering two types, with all results complying with ITER requirements.

Experimental Research and Numerical Simulation of Laser Welding of 303Cu/440C-Nb Stainless-Steel Thin-Walled Natural-Gas Injector for Vehicles.

This paper presents the results of research on laser lap welding technology of heterogeneous materials and a laser post-heat treatment method to enhance welding performance. The purpose of this study is to reveal the welding principle of austenitic/martensitic dissimilar stainless-steel materials (3030Cu/440C-Nb) and to further obtain welded joints with good mechanical and sealing properties. A natural-gas injector valve is taken as the study case where its valve pipe (303Cu) and valve seat (440C-Nb) are welded. Experiments and numerical simulations were conducted where the welded joints' temperature and stress fields, microstructure, element distribution, and microhardness were studied. The results showed that the residual equivalent stresses and uneven fusion zone tend to concentrate at the joint of two materials within the welded joint. The hardness of the 303Cu side (181.8 HV) is less than the 440C-Nb side (266 HV) in the center of the welded joint. The laser post-heat treatment can reduce the residual equivalent stress in the welded joint and improve the mechanical and sealing properties. The results of the press-off force test and the helium leakage test showed that the press-off force increased from 9640 N to 10,046 N and the helium leakage rate decreased from 3.34 × 10-4 to 3.96 × 10-6.

Overview of recent ITER TBM Port Plug R&D activities

The objective of ITER project is the demonstration of the scientific and technological feasibility of fusion energy. Tritium fuel is required for the fusion reaction in DEMOnstration reactors and future commercial reactors but it is not available in nature because of its instability. To investigate tritium breeding self-sufficiency, two equatorial ports in ITER are dedicated to operating test blanket modules (TBMs) using the TBM Port Plug (TBM-PP). The TBM-PP design should be investigated with the physical mock-ups to validate the leak tightness in the primary vacuum boundary and to ensure TBM replacement and refurbishment compatible with Remote Handling (RH) operations in the ITER Hot Cell Facility during the ITER lifetime. This paper summarizes the achievements on the two most relevant R&D topics as described herewith. Several TBM-PP components (i.e., back parts of the TBM-Frame, the Dummy-TBMs, the TBM-Shields, and the TBM feeding pipes) are part of the vacuum boundary implying that these components are Vacuum Quality Classification (VQC) 1A. To test and develop high vacuum sealing for the TBM-Frame flange and the TBM-Set or Dummy-TBM flange, the ITER Large Seal Test Rig (LSTR), and metallic gasket seals for the TBM application have been manufactured and installed in the ITER site. The helium leak tests were performed at different operating temperatures to investigate the vacuum performance of metallic gasket seals with VQC 1A. The TBM-PP design shall guarantee the feasibility of a rapid replacement and refurbishment compatible with ITER Remote Handling (RH) operations by using RH tools in the Hot Cell Facility during the ITER's lifetime. Therefore, an experimental program has been performed to demonstrate the feasibility of the critical RH refurbishment tasks including the insertion and removal of TBM-Set/Dummy-TBM and the RH bolting.

Research on improving the system sensitivity for Hot Helium Leak Test

Hot Helium Leak Test (HHLT) can realize more reliable leak tightness assessment than the conventional cold helium leak testing for components that run at elevated temperatures. To ensure successful operation of the International Thermonuclear Experimental Reactor (ITER), the in-vessel components are required to ensure high vacuum sealing performance by HHLT. The biggest challenge for the HHLT is that the background leak rate and sensitivity of the test system can hardly meet the requirements of high accuracy at high temperature. Through years of research, it is found the main factor affecting the rate and the sensitivity of HHLT facility is the hydrogen released from the vacuum-facing materials at high temperature. By adopting a series of measures, the background leakage rate was successfully reduced by one and a half order of magnitude to 1.17×10−10 Pa·m3/s at 253℃, and the system sensitivity was improved by more than one order of magnitude to reach the order of 10−12 Pa·m3/s, even down to 9.85×10−13 Pa·m3/s at the temperature.

Hermetic and Bioresorbable Packaging Materials for MEMS Implantable Pressure Sensors: A Review

Pressure sensors based on microelectromechanical system (MEMS) technology have been employed for measuring the pressure inside the human body due to their small size, lightweight, and low energy consumption. Hermetic packages are necessary to protect the implants from body fluid corrosion. Materials such as metal and glass are usually considered to be hermetic and can ensure several years of lifetime. Materials with low permeability, such as liquid crystal polymers and parylene, are also supposed to be short-term hermetic when the expected lifetime is not too long (less than a year). Water penetration and interface adhesion determine whether the package fails. Helium leakage tests and water concentration tests can be used to predict when the package fails. In addition, for more short-term implantable devices (several days or weeks), bioabsorbable packages can be helpful. The main advantage is that the self-degradation of the packaging material can avoid tissue damage caused by long-term implantation or infection caused by secondary removal surgery.

Friction surfacing and cold spray deposition for surface crack repair in austenitic stainless steels

Two technologies, friction surfacing (FS) and cold spray deposition (CSD), have been investigated for field repair of cracks in 304 L austenitic stainless steel. Both technologies are low-heat-input, solid-state technologies that do not introduce the detrimental effects of solidification, such as compositionally segregated microstructures and shrinkage stresses. Friction surfacing was performed using a 304 L stainless steel rod over the substrate of the same material. The process was performed at a high spindle speed of 20,000 RPM allowing for process forces below 500 N to be used effectively. Spherical gas-atomized 304 L stainless steel powder, in size range of 25 µm - 44 µm, was used as feedstock powder for the cold spray process for deposition on 304 L substrates. Both technologies showed leak-tight crack repair capabilities on ~ 30-50 µm wide simulated through-crack specimens based on helium leak tests.

Evaluation of the functional acceptability of the ITER vacuum vessel

The International Thermonuclear Experimental Reactor (ITER) vacuum vessel (VV) is one of the critical components of the ITER tokamak fusion reactor. The first sector of the ITER VV was delivered to ITER Organization in 2020, and it is ready to assemble into the tokamak system. After manufacturing the ITER VV, an evaluation should ensure that the components are designed and manufactured to meet the functional requirements, such as vacuum leak tightness and structural integrity. The factory acceptance test (FAT) is essential for confirming acceptance in engineering and manufacturing. This paper introduces the engineering process and technical method of the FAT, which is applied explicitly to the first-of-a-kind ITER VV. We establish a visual inspection, pre-pumping assessment, pressure test, helium (vacuum) leak test, and final dimensional inspection for the FAT. The visual inspection revealed no blockages in the cooling channels of the double walls. The pre-pumping assessment conducted to check the vacuum level and residual gas condition, concluded that the inside of the VV was flawless and thus met the leak test requirements of 1 × 10−8 Pa m3 s−1. We confirmed no leakage or deformation through the pressure test under reduced pressure. The helium leak test demonstrated engineering soundness with leak tightness of 6.08 × 10−9 Pa m3 s−1, which is more stringent than the allowable limit. Furthermore, three-dimensional metrology was utilized to determine the as-built dimensions of the manufactured sector. Due to unavoidable weld deformation and tight tolerances, the as-built result does not perfectly meet the assigned tolerance level. Nevertheless, it can be considered as advanced information for assembly with in-vessel components and other sectors. Based on the conformance and suitability of the suggested FAT for the first ITER VV sector, we will determine the acceptability of the upcoming VV sectors, which will be manufactured and delivered by Korea shortly.

Fabrication and Performance Tests for ODS Cu/CLF-1 Clad Plate as Divertor Heat Sink via Explosive Welding

In order to realize the effective connection of oxide dispersion strengthened copper (ODS Cu) alloy and Chinese low-activation ferritic/martensitic steel (CLF-1) manufactured for the divertor heat sink of the future fusion reactor, explosive welding is adopted for its exceptional advantages. The ultrasonic testing (UT) result reveals a soundness ODS Cu/CLF-1 bonding interface with no detectable defects or cracks. The bonding interface presents a typical wave interface. Welding hardening occurs in the interface zone and reaches a maximum at the interface. The tensile strength of the five sample joints ranges from 360 to 366 MPa and all failed on the ODS Cu side. The cyclic helium leak testing results reveal that the ODS Cu/CLF-1 joint has favorable gas tightness and no deformation. The results demonstrate the reliable joining of explosive welded ODS Cu/CLF-1 plates can meet the application requirements involving the use of the divertor heat sinks of the future fusion reactor.

Thermal Properties and Joinability Investigation of BaO–SrO–SiO2–B2O3 Glasses for Oxygen Transport Membrane Application

Three new BaO–SrO–SiO2–B2O3 (BS) glasses with different SrO contents (6–25 mol%) are developed for oxygen transport membrane (OTM) joining application. The content of strontium is investigated first in terms of its effect on the glass‐forming tendency, thermal expansion coefficient, crystallization, shrinkage behavior, and viscous flow properties. Differential scanning calorimetry (DSC) is carried out. Dilatometric tests are performed to obtain coefficients of thermal expansion (CTEs) of BS glasses. The crystallization behavior of the BS glasses is investigated by X‐ray powder diffraction (XRD). Sinking dilatometric measurements simulate the joining procedure and observe the shrinkage behavior of the BS glasses. The viscous flow behavior of the BS glasses is examined via hot stage microscopy. The glass with 15 mol% SrO (BS15) glass shows the best glass‐forming tendency, most matching CTE (11.9 × 10−6 K−1), densest microstructure, highest shrinkage rate (24%), and good viscous behavior at high joining temperatures compared with other BS glasses. BS15 glass is chosen for helium leak test and assembly test joining with Aluchrom and SrTi0.75Fe0.25O3−δ membrane (STF25). The sandwiched sample with two Aluchrom plates sealed by BS15 glass at 1075 °C for 5 min achieves good gas‐tightness with low helium leakage rate <10−9 mbar·l s−1.

Low-Temperature Sandwich Structure Wafer-Level Hermetic Packaging via Three-Layer Simultaneous Bonding for 3-D Microsystems

A sandwich structure, which is stacked by a wafer-level copper–copper (Cu–Cu) thermocompression bonding and a silicon–silicon (Si–Si) direct bonding, has been investigated for potential application as a hermetic seal in 3-D microsystem packaging. First, the bottom and middle layers are prebonded by hydrophilic Si direct bonding. Oxygen plasma is employed for surface activation. Then, the prebonded wafers and the top wafer are bonded via Cu thermocompression bonding at a bonding temperature of 300 °C and a bonding force of 8000 mbar for a duration of 1 h. The entire bonding process of the three layers has borne only one heating procedure, which could effectively reduce the residual stress after bonding. Cavities are etched to a volume of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$21.4\times 10^{-3}$ </tex-math></inline-formula> cm3 following the MIL-STD-883E standard prescribed for microelectronics packaging. The helium leak test demonstrates that 96.8% of the samples present a superior leak rate smaller than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.7\times 10^{-8}$ </tex-math></inline-formula> atm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula> cm3/s. Moreover, the interfacial adhesion energy between the Cu–Cu bonding interface and the Si–Si bonding interface was projected at 16.4 ± 4.6 J/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{2}$ </tex-math></inline-formula> and Si breakage energy. The results hold great promise for 3-D microsystem wafer-level packaging.

Performance testing of the liquid nitrogen cooled sorption cryopump for application in SST-1 Tokamak

SST-1 is a Superconducting Tokamak for a proposed plasma discharge of long duration. To have plasma discharges, ultra-high vacuum is maintained in the vacuum vessel of this plasma device before filling discharge gases [1]. To attain a targeted pressure of ∼2.0E-8 mbar in the vacuum vessel, the plasma-facing components are baked to a temperature of 250 °C and the SS vacuum vessel is baked around 150 °C to remove impurities adsorbed/absorbed, from the bulk of the materials [2]. During baking, mainly the gas load of water vapour is increased significantly. Therefore, to cater the need of pumping speed of water vapour during baking, a customized liquid nitrogen based sorption pump was designed [3] and fabricated for the installation on the radial port of the machine. In this report, the performance testing of the pump will be discussed for the helium leak test, operating temperature test, pressure test, pumping speed test for water vapour and the regeneration process of the pumped gases. This paper also describes the assembly of the pump and the pumping speed of the gases up to the temperature around 150 °C, which is close to the baking conditions of the vacuum vessel of the SST-1. The pump demonstrates a pumping speed of ∼ 26,000 l/s for water vapour at a dosing rate of ∼ 2.6E-2 mbar-l/s at room temperature.

Influence of Porosity on Ultra-High Vacuum Gas-Tightness in Cold-Sprayed Aluminum Coatings

Vacuum chambers used in high-energy particle accelerator experiments are conventionally made of bulk beryllium, which shows significant drawbacks due to cost and toxicity. An alternative solution could be to develop chambers made of polymer-based composites. Since these materials exhibit high outgassing not compatible with an ultra-high vacuum environment, a suitable gas-tight coating is required. Cold spray deposition of aluminum can be a solution, provided that the coating behaves as a perfect vacuum barrier. Porosity, especially percolating porous networks, is key to coating gas tightness issues. This work addresses the relationship between porosity and gas-tightness in cold spray coatings. To do so, coatings with different porosity were achieved playing with powder morphology, composition, and process parameters. Their gas tightness was evaluated by helium leak tests. Classical microscopy, being essentially a 2D analysis, is strongly limited when dealing with 3D properties as porosity percolation. For this reason, 3D X-ray microtomography images of coatings were obtained and treated by image analysis methods: pores were compared in terms of size and shape. Overall porosity properties, including percolation and a homogeneity criterion, were also investigated. Percolating porosity was highlighted for several samples which showed poor gas-tightness properties. The permeability of percolating pore structures was then numerically computed by a fast Fourier transform-based method, to quantify the mass flow through the coating. Results of those computations were finally compared to experimental coating leak rate measurements, in an effort to elucidate the link between gas tightness and morphology of the pore space.

Containment Integrity and Sealing Assessment for HTR-PM600 Fresh Fuel Transport Package Under Impact Loading

Reliable transportation of fuel elements is vital for High Temperature Reactor (HTR) power plant. Therefore, the safety performance of the containment boundary in HTR-PM600 fresh fuel transport package under impact loading should be assessed. Specifically, factors such as the structural integrity, bolt safety, and sealing performance of the flange are considered. However, it is not convenient to assess the sealing performance using the helium leakage test without a prototypical experiment. Therefore, to accurately assess the safety performance in the design stage, we propose a detection technique combing with the finite element method to measure the leak channel in the flange for HTR-PM600 transport package. The results show that the boundary containment has no failure, and there is no complete leak channel in the flange of HTR-PM600 package. Hence, this package has a great safety performance under impact loading. This work also provides references for design of fresh fuel transport packages in pebble-bed reactors.

Fabrication and performance examination on explosively welded ODS-Cu / 316 L clad plate for CFETR divertor heat sink

Copper alloys are among the most promising candidate materials for divertor heat sinks owing to their remarkable advantages. However, it is challenging to use CuCrZr alloys as heat sink materials because of the harsh loading environment expected for plasma-facing components (PFCs) in future fusion reactors. Explosive welding is an effective method for manufacturing high-quality oxide dispersion strengthened copper (ODS-Cu) / 316 L stainless steel (SS) clad plates. Non-destructive testing results revealed a soundness bonding interface between ODS-Cu and 316 L without any detectable defects or cracks. In this study, optical microscopy and scanning electron microscope analysis revealed that the bonding interface did not contain any cavities and a typical wave-bonding morphology was observed. Moreover, the tensile stress of the bonding interface was higher than that of the ODS-Cu parent metal, and there was an increment in hardness on both sides near the bonding interface. The helium leak testing results indicate that the welded ODS-Cu / 316 L exhibits soundness without deformation or leakage at cyclic room and high temperatures. The results prove that the explosive welding method is a practical method for manufacturing high-quality ODS-Cu / 316 L clad plate that meet the design requirements and can provide critical references for the engineering design of the China Fusion Engineering Test Reactor (CFETR) divertor heat sink.

Assembly and interconnection technology for high-temperature bulk acoustic wave resonators

Abstract. A sensor based on a piezoelectric single crystal enables operation even under harsh environmental conditions. In addition to the sensor element, the packaging technology is crucial for sensor performance. In this paper, a suitable assembly and interconnection technology concept of Ca3TaGa3Si2O14 (CTGS) resonators for high-temperature applications is presented as a platform for future sensor assemblies. The concept described here has already been functionally tested as a temperature sensor (Schulz et al., 2021). The concept includes a sapphire base plate, a housing lid, and a spacer made from aluminium oxide (Al2O3). The substrate is metallised with platinum manufactured into thin film and thick film technology. The ceramic components are fused with glass solder. The connection of the resonator to the conductive tracks is realised by thermosonic bonding with 25 µm platinum wire. Initially, the stability of the metallisation must be investigated before subsequent electrical testing under high temperature. Diffusion processes play a major role in this temperature range, and the stability of the layer is a necessary condition for subsequent investigations. A suitable set of bonding parameters and the strength of the platinum bonds prior to and after thermal load is analysed. Shear tests are used to evaluate the quality of the ceramic materials fused with glass solder after thermal ageing. The dielectrical properties of sapphire and glass solder such as the isolation resistance, the relative permittivity, and the loss factor at high temperatures are evaluated using interdigital structures. The loss factor is measured on both bare interdigital structures and the samples coated with glass solder to make an estimation about the conductive behaviour up to 1000 ∘C. A ceramic lid for the sensor housing is attached by a high-temperature stable glass solder. Since platinum conductors are fed through this glass solder connection, the electrical conductivity of the glass solder is characterised at high temperature. Furthermore, the hermeticity of the assemblies is verified by means of helium leakage tests. These investigations are the basis for the implementation of an assembly and interconnection technology that is suitable for reliable operation under extreme temperature conditions. The packaging technology also offers further possibilities for pressure or chemical sensors that can withstand high-temperature loads.

Development of automated fumigation chamber for treatment of grapes with SO2 and CO2

AbstractThe export of grapes from one country to another is dependent on the grape's quality that should pass the strict phytosanitary requirements of the importing country. There is a requirement of fumigation of grapes with CO2 and SO2 at specified concentrations for a certain period besides the other pack‐house protocols and export standards. This study was focused to develop and establish an electronically controlled model fumigation chamber for the treatment of grapes in the grapes‐producing region. The gas injection and evacuation systems are electronically operated, and the fumigant concentration can be varied according to the requirement. Heaters of 2 kW are fitted inside the chamber to maintain the treatment temperature above 16°C for grapes fumigation. The fumigation chamber was found leak‐proof during pressurized helium leak testing. The CO2 and SO2 gas concentrations can be maintained between 600 and 60,000 ± 100 ppmv and 10 and 10,000 ± 2 ppmv, respectively, for 1–90 min (1% is equivalent to 10,000 ppmv). The fumigation of grapes with 6% CO2 for 30 min followed by fumigation with 1% SO2 for 30 min ensured 100% mortality of the adult fruit fly (Drosophilla melanogaster). This chamber may also be used for the fumigation treatment of other fruits, vegetables, nuts, and so on, as per the time and concentration of gases required for the 100% mortality of target pest. Nevertheless, the protocol for such treatments should be developed for other fruits, and the effect of the gases on the quality of the products should be studied.Practical ApplicationsThe developed automated fumigation chamber for treatment of grapes with CO2 and SO2 is an effective measure for disinfesting the grapes from invasive pests like Drosophilla suzukii, spiders, and so on. This would help in overcoming phytosanitary measures put in place by many grapes importing countries like New Zealand, Australia, and so on. These countries offer huge market potential for export of Indian grapes, and if the trade barriers are overcome, it would lead to earning huge foreign exchange. Furthermore, the developed automated fumigation chamber can be used as an integral part of horticultural pack houses both for domestic as well as international trade as it would eliminate most of the adult field pests at preliminary stages before pack‐house operation.

Glass welding with ultra-short laser pulses and long focal lengths A process model

The welding of glasses with ultra-short laser pulses and high-NA optics has matured to a well-established process. The short focal length requires relatively thin work pieces in the order of 1-2 mm, and the short Rayleigh length requires a very precise adjustment of the laser focus relative to the interface to be welded.To make welding of thick samples possible with relaxed alignment requirements, we welded optically contacted fused silica with a focal length of 65 mm. The resulting welding seams can be as deep as 2 mm and more. It turned out, that they grow from the focal point towards the laser source with increasing pulse energy and irradiation time. A thermal model for the growth of the molten zones will be presented and compared to the process with short focal lengths. Furthermore, the results of tensile tests and helium-leakage tests of welded fused silica samples will be presented.