Browsing by Author "Jourdain, Renaud"
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Item Open Access Analysis of De-Laval nozzle designs employed for plasma figuring of surfaces(Springer, 2016-02-27) Yu, Nan; Jourdain, Renaud; Gourma, Mustapha; Shore, PaulPlasma figuring is a dwell time fabrication process that uses a locally delivered chemical reaction through means of an inductively coupled plasma (ICP) torch to correct surface figure errors. This paper presents two investigations for a high temperature jet (5000 K) that is used in the context of the plasma figuring process. Firstly, an investigation focuses on the aerodynamic properties of this jet that streamed through the plasma torch De-Laval nozzle and impinged optical surfaces. Secondly, the work highlights quantitatively the effects of changing the distance between the processed surface and nozzle outlet. In both investigations, results of numerical models and experiments were correlated. The authors’ modelling approach is based on computational fluid dynamics (CFD). The model is specifically created for this harsh environment. Designated areas of interests in the model domain are the nozzle convergent-divergent and the impinged substrate regions. Strong correlations are highlighted between the gas flow velocity near the surface and material removal footprint profiles. In conclusion, the CFD model supports the optimization of an ICP torch design to fulfil the demand for the correction of ultra-precision surfaces.Item Open Access Analysis of nozzle design used for the creation of advanced energy beam(American Society for Precision Engineering (ASPE), 2014-11-14) Yu, Nan; Jourdain, Renaud; Gourma, Mustapha; Shore, PaulA variety of scientific and industrial projects, such as segmented ground based telescopes, compact space based observers, short wavelength microlithography and high power laser systems, demand metre scale ultra-precise surfaces [1]. Cranfield University and Loxham Precision have been engaged in developing effective fabrication of medium to large optical surfaces for the aforementioned applications. A process chain of three sequential machining steps has been proposed (Figure 1). These steps are ultra-precision grinding, robot based polishing and plasma figuring. The fabrication target is to reach a 20 hours cycle time for each stage of surface generation for 1.5m size optics: equating to 1ft2 per hour [2-3].Item Open Access CFD analysis of an enhanced nozzle designed for plasma figuring of large optical surfaces(European Society for Precision Engineering and Nanotechnology, 2016-06-30) Yu, Nan; Jourdain, Renaud; Gourma, Mustapha; Shore, PaulFor addressing the correction of Mid Spatial Frequency (MSF) errors on metre scale optical surfaces induced by sub aperture figuring process, a new generation of non-contact plasma based surface figuring tools has been created at Cranfield University. In this context, this paper presents an investigation that focuses on novel enhanced nozzles that were created for a Radio Frequency (RF) Inductively Coupled Plasma (ICP) torch. The characteristics of plasma jet delivered by prototype nozzle and a selected enhanced nozzle are compared using an in-house created CFD model. The enhanced nozzle design is based on the results previously obtained throughout a numerical analysis that enabled to identify the key design aspects of these nozzles. This enhanced nozzle is predicted to provide 12.5% smaller footprint and 15.5% higher temperature.Item Open Access Creation of a control system for plasma delivery to increase automation and stability.(2016-09) Zhou, Hui; Endrino, José L.; Jourdain, RenaudSurface figuring of extremely large telescopes (ELT) addresses a highly challenging manufacture issue for the field of ultra precision. [1] High form accuracy and rapid fabrication are needed for ELT primary mirror surface figuring. In Cranfield University, Plasma Figuring (PF) [2] is used as a main method to correct ELT mirror surface figure error. The non-contact based material removal process brings PF to a high level of accuracy (under 1nm RMS level). Some other great features of PF are the capability to work at atmospheric pressure, the low-cost of consumables. Other figuring methods make use of vacuum chamber (ion beam Figuring) which are expensive. On the other hand magnetorheological finishing requires expensive consumables. Although PF is dominant for the surface correction of metre scale surfaces, challenges still exist to improve the automation and stabilization of the plasma source. In the context of ever-increasing dimensions of optical components, there is a need for improving the robustness and securing the performance of the unique Plasma Delivery System (PDS) available in Cranfield. The current PDS is based on an inductive output L-type radio frequency (RF) circuit, Inductively Coupled Plasma (ICP) torch and computer numerically controlled (CNC) motion system. The combination of optical component surface dimensions and the material removal rate of the plasma jet lead to significant processing duration. Based on the existing PDS for our unique Plasma Figuring machine named Helios1200, we designed an enhanced PDS version. The novel design was given the capability to detect phases and automatically tune the impedance of the plasma. The novel control capability is aiming at secure the process determinism, assisting the machine operator by tuning key electrical components of the RF network and monitoring crucial processing parameters. Furthermore, specific assistances were provided during the three identified processing phases (ignition phase, regular operation and critical circumstance) of the plasma processing. Our design addressed particular functions on each phases to ensure an optimum performance during the Plasma Figuring process.Item Open Access Design of a motorised plasma delivery system for ultra-precision large optical fabrication(IOP, 2020-09-02) Zhou, Hui; Bennett, Adam; Castelli, Marco; Jourdain, Renaud; Guo, Jiang; Yu, NanA unique plasma figuring (PF) process was created and demonstrated at Cranfield University for manufacturing extremely large telescopes. The atmospheric pressure processing is faster and more cost-effective than other finishing processes; thus, providing an important alternative for large optical surfaces. The industrial scale manufacturing of thousands of ultra-precision metre-scale optics requires a robust PF machine: this requirement is achieved by making the plasma delivery system (PDS) performance repeatable. In this study, a dedicated PDS for large optical manufacturing was proposed to meet the industrial requirement. The PDS is based on an L-type radiofrequency (RF) network, a power supply, and an inductively coupled plasma torch. However, the complexities of these technologies require an in depth understanding of the integrated components that from the PDS. A smart control system for the modified PDS was created. This novel control system aims to make the characterization process deterministic: by automating the tuning of critical electrical components in the RF network, which is achieved by the use of in-line metrology. This paper describes the main design aspects. The PDS was tested with a good correlation between capacitance and RF frequencies. The robust PDS design enables a stable discharge of plasma with a low deviation of RF signals during the total 15 hours' tesItem Open Access Estimation of the power absorbed by the surface of optical components processed by an inductively coupled plasma torch(Elsevier, 2016-08-06) Jourdain, Renaud; Castelli, Marco; Yu, Nan; Gourma, Mustapha; Shore, PaulThe focus of this work is the determination of the heat flux function -thermal footprint- of a plasma jet generated by an inductively coupled plasma (ICP) torch. The parameters of the heat flux function were determined through the correlation of modelling and experimental results. One surface of substrates was exposed to an impinging jet while the temperature changes of the unexposed surface was recorded, analysed and used to derive the parameters of the heat flux function. From a modelling viewpoint, a series of finite element analyses (FEA) were carried out to predict temperatures of substrate surfaces. From an experimental viewpoint, the plasma torch was powered by a 1 kW radio frequency signal generator tuned at 39 MHz. The ICP torch equipped with a De-Laval nozzle impinged the surfaces of selected substrates at atmospheric pressure. Three sets of experiments -static, single pass and multi passes- were carried out to determine and validate the numerical description of the plasma jet. Also this work enabled to determine the maximum intensity of the heat flux distribution and the total power absorbed by substrate surfaces. Finally, the most advanced numerical model was used to assess the effect of a bi-directional raster scanning strategy that was used for the processing of large optical components.Item Open Access Fast figuring of large optics by reactive atom plasma(2012-09-13T00:00:00Z) Castelli, Marco; Jourdain, Renaud; Morantz, Paul; Shore, Paul; Ramón, Navarro; Colin, R. Cunningham and Eric Prieto.The next generation of ground-based astronomical observatories will require fabrication and maintenance of extremely large segmented mirrors tens of meters in diameter. At present, the large production of segments required by projects like E-ELT and TMT poses time frames and costs feasibility questions. This is principally due to a bottleneck stage in the optical fabrication chain: the final figuring step. State-of-the-art figure correction techniques, so far, have failed to meet the needs of the astronomical community for mass production of large, ultra-precise optical surfaces. In this context, Reactive Atom Plasma (RAP) is proposed as a candidate figuring process that combines nanometer level accuracy with high material removal rates. RAP is a form of plasma enhanced chemical etching at atmospheric pressure based on Inductively Coupled Plasma technology. The rapid figuring capability of the RAP process has already been proven on medium sized optical surfaces made of silicon based materials. In this paper, the figure correction of a 3 meters radius of curvature, 400 mm diameter spherical ULE mirror is presented. This work demonstrates the large scale figuring capability of the Reactive Atom Plasma process. The figuring is carried out by applying an in-house developed procedure that promotes rapid convergence. A 2.3 μm p-v initial figure error is removed within three iterations, for a total processing time of 2.5 hours. The same surface is then re-polished and the residual error corrected again down to& lambda;/20 nm rms. These results highlight the possibility of figuring a metre-class mirror in about ten hours.Item Open Access Heat transfer properties of laser assisted plasma processing(Cranfield University, 2012-08) Parkins, Jonathon; Jourdain, Renaud; Shore, PaulLaser assisted plasma processing (LAPP) is a novel extension to already established Reactive Atom Plasma (RAP) processing for atmospheric pressure dry chemical etching of silicon based materials; this is mainly applied for optical uses. The development of the new technology involves the implementation of an additional laser energy beam to tune the thermal footprint of the hybrid tool. This will influence the temperature-dependent etching reaction. The aim of the project was to develop a model to predict the temperature footprint of components of the LAPP tool and assess the suitability of this model in simulating the thermal effects during an actual process. This was undertaken via two routes: model development and experimental temperature investigation. The two materials investigated were Corning Ultra Low Expansion glass (ULE) and also Silicon Carbide (SiC). The model was developed using Matlab from an established analytical method and evolved for LAPP use. An analytical method based on a Green’s function solution to the heat equation for a moving Gaussian heat source on a surface was chosen as it would be an adaptable and rapid alternative to costly experimental measurements for LAPP. Experimental temperature measurements were investigated using pyrometers, resistance temperature detectors and thermocouples. Typical LAPP process parameters were investigated for both a laser source and a RAP torch, and the temperature was measured. Additionally, surface reflectivity was measured for appropriate wavelengths for LAPP applications using a Fourier transform infrared spectrometer to determine the absorbed portion of laser energy. The experimental work conducted using RTDs found strong correlation with the modelling, with 63% of results matching within experimental error. The pyrometer measurements were less successfully replicated, the reason for which is expected to be the cooling of the substrate from its upper surface not being accounted for in the model. Overall trends of temperature rise decreasing with increasing feed speed or decreasing power were observed. Thermocouple characterisation of the RAP torch was approximated using the radiative model.Item Open Access Investigation of power dissipation in a collimated energy beam(Trans Tech Publications, 2015-08-20) Yu, Nan; Jourdain, Renaud; Gourma, Mustapha; Shore, PaulTo satisfy the worldwide demand for large ultra-precision optical surfaces, a fast process chain - grinding, polishing and plasma figuring- has been established by the Precision Engineering Institute at Cranfield University. The focus of Cranfield Plasma Figuring team is the creation of next generation of highly collimated energy beam for plasma figuring. Currently, plasma figuring has the capability to shorten processing duration for the correction of metre-scale optical surfaces. High form accuracy can be achieved (e.g. 2.5 hours and 31 nm RMS for 400mm diameter surface). However, it is known that Mid Spatial Frequency (MSF) surface errors are induced when the plasma figuring process is carried out. The work discussed in this paper deals with the characterisation of highly collimated plasma jets delivered by the Inductively Coupled Plasma (ICP) torches. Also a computational fluid dynamics (CFD) model is introduced. This model is used to assess the behaviour of the plasma jet within the best known processing condition. Finally temperature measurement experiments were performed to determine the energy dissipated values that characterise best the ICP torch coil and its De-Laval nozzle.Item Open Access Microwaves enable activated plasma figuring for ultra-precision fabrication of optics(European Society for Precision Engineering and Nanotechnology, 2016-06-30) Bennett, Adam; Jourdain, Renaud; Kirby, Paul; MacKay, P.; Shore, Paul; Nicholls, John R.; Morantz, PaulActivated plasma figuring using microwaves aims at providing highly efficient activated energy beams for rapid fabrication of optics. The chemical nature of this type of energy beam leads to targeting silicon-based materials. Furthermore this technology is proposed to address the needs of ultra-precision optical components. In this paper, we present a novel ADTEC microwavegenerated plasma torch design which is operated at atmospheric pressure. In this study, the plasma torch is fed with either argon or helium carrier gas. However this novel design for Plasma Figuring is targeted at local surface correction of crystal quartz which is a material of great interest for optical systems, such as acousto-optic devices. Also this novel design is targeted at reducing midspatial frequency errors such as waviness, ripple errors and residual sub-aperture tool footprints. These are responsible for the scattering of light at small angles, resulting in optical hazing effects, photonic energy loss and pixel cross-talk. Also the results of a preliminary investigation using Optical Emission Spectroscopy (OES) are reported and discussed. These results show the operat ing range when the main processing parameters are changed: microwave forward power values, gas flow rates and the types of gasses.Item Open Access Numerical and experimental modal analysis applied to an optical test system designed for the form measurements of metre-scale optics(Hindawi Publishing, 2018-11-04) Golano, P. G.; Zanotti Fragonara, Luca; Morantz, Paul; Jourdain, RenaudThe work focuses on the structural design and performances of a unique optical test system (OTS) used for measuring metre-scale optical surfaces. The investigation was carried out through a modal analysis. Two sets of results are presented. Both modal analysis of the entire OTS and transmissibility function related to its use as an optical system are carried out and analysed. The OTS is used for the measurements of the form accuracy at nanometre level of metre-scale concave surfaces. The OTS is a four and half-metre-tall mechanical structure made of bolted aluminium profiles, two structural platens, two dedicated precision positioning supports, a test piece, and a state-of-the-art laser interferometer. The OTS was numerically modelled and fully instrumented with triaxial accelerometers. The results of the modal analysis highlight the natural modes of the entire OTS. Both numerical and experimental methods are designed. The investigation methods are iterative. Indeed, a preliminary numerical model is created using finite element analysis (FEA). FEA results enable the determination of the dynamic range and suitable locations of accelerometers that are mounted onto the OTS for the experimental validation of the FEA model and further to carry out the transmissibility study. Natural frequencies, damping ratios, and mode shape values are obtained and scrutinized. These results are used for refining the FEA model. In fact, the lack of symmetry and the use of feet are identified as the key design feature that affects the OTS. The correlation between experimental and numerical results is within five percent for the first four modes. The results of the transmissibility study highlight the specific natural modes that influence the OTS measurement capability. Overall, the study enables to guide engineers and researchers towards a robust design using a validated and methodical approach.Item Open Access Power dissipation of an inductively coupled plasma torch under E mode dominated regime(MDPI, 2021-07-18) Yu, Nan; Jourdain, Renaud; Gourma, Mustapha; Xu, Fangda; Bennett, Adam; Fang, FengzhouThis paper focuses on the power dissipation of a plasma torch used for an optical surface fabrication process. The process utilizes an inductively coupled plasma (ICP) torch that is equipped with a De-Laval nozzle for the delivery of a highly collimated plasma jet. The plasma torch makes use of a self-igniting coil and an intermediate co-axial tube made of alumina. The torch has a distinctive thermal and electrical response compared to regular ICP torches. In this study, the results of the power dissipation investigation reveal the true efficiency of the torch and discern its electrical response. By systematically measuring the coolant parameters (temperature change and flow rate), the power dissipation is extrapolated. The radio frequency power supply is set to 800 W, E mode, throughout the research presented in this study. The analytical results of power dissipation, derived from the experiments, show that 15.4% and 33.3% are dissipated by the nozzle and coil coolant channels, respectively. The experiments also enable the determination of the thermal time constant of the plasma torch for the entire range of RF power.Item Open Access Process characterisation and key tasks for cost-effective 3D figuring of specular surfaces using RAP(2010-06-01T00:00:00Z) Jourdain, Renaud; Castelli, Marco; Shore, Paul; Henny, SpaanRecently established the Helios 1200 (RaptTM ) is a unique facility designed for the figuring of large optical surfaces [1]. It combines a CNC machine tool with a reactive atom plasma (RAP) process. This provides a unique rapid surface figuring capability with tool size and tool path motion flexibility. RAP is a proven technology for processing silicon based optical materials [2]. The aim with this technique is to achieve figuring correction of metre size optical components in 10 hours - a much reduced process time compared to the 100 hours currently needed. This paper focuses on key technical tasks to achieve a cost-effective figuring method using RAP. Classically a figuring process is carried out iteratively by analyzing surface figure error and removing material using an optimum tool path algorithm. In this work, the material removal is achieved by decomposing the compound SF6 in a plasma jet to obtain free fluorine radicals which etch away silicon based material. In this paper, measurements of the specific material removal rate and footprint of the plume over a range of substrate temperatures are presented. Then the authors present a base-line process for the neutral removal of material over a large area. Various tool path algorithms are investigated some of which include time-dwell adaptation based on substrate temperature. Finally, the issue of heat transfer is discussed, and both experimental and finite element analysis results are presented. The processed surfaces are analyzed using coherence probe and phase-shifting interferometers for surface morphology and 3D surface form respectively. Surface roughness (Sq) is reported within the 2-3 nanometre range on fused silica and surface flatness is within the +/-50 nanometre range after 0.5 micrometre deep material neutral removal (Typical processed area: 70x200millimetre).Item Open Access Reactive Atom Plasma (RAP) figuring machine for meter class optical surfaces(Springer, 2013-04-11) Jourdain, Renaud; Castelli, Marco; Shore, Paul; Sommer, P.; Proscia, DavidA new surface figuring machine called Helios 1200 is presented in this paper. It is designed for the figuring of meter sized optical surfaces with form accuracy correction capability better than 20 nm rms within a reduced number of iterations. Unlike other large figuring facilities using energy beams, Helios 1200 operates a plasma torch at atmospheric pressure, offers a high material removal rate, and a relatively low running cost. This facility is ideal to process large optical components, lightweight optics, silicon based and difficult to machine materials, aspheric, and free form surfaces. Also, the surfaces processed by the reactive atom plasma (RAP) are easy to fine polish through hand conventional sub-aperture polishing techniques. These unique combined features lead to a new capability for the fabrication of optical components opening up novel design possibilities for optical engineers. The key technical features of this large RAP machine are fast figuring capabilities, non-contact material removal tool, the use of a near Gaussian footprint energy beam, and a proven tool path strategy for the management of the heat transfer. Helios 1200 complies with the European machine safety standard and can be used with different types of reactive gases using either fluorine or chlorine compounds. In this paper, first the need for large optical component is discussed. Then, the RAP facility is described: radio frequency R.F generator, plasma torch, and 3 axis computer numerically controlled motion system. Both the machine design and the performance of the RAP tool is assessed under specific production conditions and in the context of meter class mirror and lens fabrication.Item Open Access Thermal analysis of energy beam using de-laval nozzle in plasma figuring process(Cranfield University, 2016-10) Yu, Nan; Jourdain, Renaud; Gourma, Mustapha; Shore, PaulIn 2012, plasma figuring was proven to be an alternative solution for the fabrication of large scale ultra-precise optical surfaces. Indeed, plasma figuring was successfully demonstrated on a metre class glass surface. The process was exceptionally rapid but residual errors were observed. This thesis addresses this issue by proposing an enhanced tool that provides a highly collimated plasma jet. The enhanced tool is characterized by a targeted material removal footprint in the range 1 to 5 mm FWHM. The energy beam is provided by an Inductively Coupled Plasma (ICP) torch equipped with a De-Laval nozzle. This thesis focuses on characterization and optimisation of the bespoke plasma torch and its plasma jet. Two research investigations were carried out using both numerical and experimental approaches. A novel CFD model was created to analyse and understand the behaviour of high temperature gas in the De-Laval nozzle. The numerical approach, that was based on appropriate profiles of temperature and velocity applied to the nozzle inlet, led to a significant reduction of computational resources. This model enabled to investigate the aerodynamic phenomena observed from the nozzle inlet up to the processed surface. Design rules and the effect of changing nozzle parameters were identified. Sensitivity analysis highlighted that the throat diameter is the most critical parameter. A challenging power dissipation analysis of the plasma torch was carried out. Temperature and flow rate in key components of the torch were measured. Experimental results enabled to calculate the power dissipation values for RF power up to 800 W and for the entire series of designed nozzles. This work enabled to scientifically understand the power dissipation mechanism in the bespoke ICP torches. In addition, by comparing the intensity of the power dissipation values, one nozzle was clearly identified as being more capable to provide a highly efficient plasma jet.