Browsing by Author "Castelli, Marco"

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    Advances in optical surface figuring by reactive atom plasma (RAP)
    (Cranfield University, 2012-10) Castelli, Marco; Shore, Paul; Morantz, Paul
    In this thesis, the research and development of a novel rapid figuring procedure for large ultra-precise optics by Reactive Atom Plasma technology is reported. The hypothesis proved in this research is that a metre scale surface with a form accuracy of ~1 μm PV can be figure corrected to 20 – 30 nm RMS in ten hours. This reduces the processing time by a factor ten with respect to state-of-the-art techniques like Ion Beam Figuring. The need for large scale ultra-precise optics has seen enormous growth in the last decade due to large scale international research programmes. A bottleneck in production is seen in the final figure correction stage. State-of-the-art processes capable of compliance with requisites of form accuracy of one part in 108 (CNC polishing, Magneto-Rheological Finishing and Ion Beam Figuring) have failed to meet the time and cost frame targets of the new optics market. Reactive Atom Plasma (RAP) is a means of plasma chemical etching that makes use of a Radio Frequency Inductively Coupled Plasma (ICP) torch operating at atmospheric pressure. It constitutes an ideal figuring alternative, combining the advantages of a non-contact tool with very high material removal rates and nanometre level repeatability. Despite the rapid figuring potential of this process, research preceding the work presented in this manuscript had made little progress towards design and implementation of a procedure for metre-class optics. The experimental work performed in this PhD project was conducted on Helios 1200, a unique large-scale RAP figuring facility at Cranfield University. Characterisation experiments were carried out on ULE and fused silica surfaces to determine optimum process parameters. Here, the influence of power, surface distance, tool speed and surface temperature was investigated. Subsequently, raster-scanning tests were performed to build an understanding on spaced multiple passes ... [cont.].
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    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, Nan
    A 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' tes
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    Effect of plasma processing parameters on the surface modification of fibre reinforced composites by atmospheric pressure plasma treatment
    (euspen, 2021-06-10) Frumosu, Lydia; Bishop, Alex; Castelli, Marco; Bennett, Adam; Nicholls, John R.; Huang, Zhaorong
    This report presents a study on the effect of microwave plasma sources on the surface modification of carbon fibre reinforced composites. The adhesion property of polymer composites is an important consideration in manufacturing processes. A key challenge in polymer adhesion is the need for adhesive pre-treatment to improve the wettability properties of the surface. Often three sequential steps are needed: contaminant removal, physically induced surface modification and chemical treatment. Atmospheric plasma treatment (APT) has been shown to either achieve, or eliminate the need for, one or more of these steps. The effects of APT on the surface properties of thermoplastics and thermosetting polymers have been reported recently, but the report on the effect of different plasma sources on the APT of polymers is limited. 50% carbon fibre epoxy samples were subjected to a microwave plasma source with varied flow rate, power, and samples’ distance from source, under atmospheric conditions. The surface energy of the composites was determined using liquid contact angle analysis. A coherence scanning interferometer was used to characterise the surface changes. The underlying mechanisms for the observed change of surface properties will be discussed.
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    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, Paul
    The 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.
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    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.
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    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, Spaan
    Recently 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).
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    Reactive Atom Plasma (RAP) figuring machine for meter class optical surfaces
    (Springer, 2013-04-11) Jourdain, Renaud; Castelli, Marco; Shore, Paul; Sommer, P.; Proscia, David
    A 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.