Fast figuring of large optics by reactive atom plasma

Abstract

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 λ/20 nm rms. These results highlight the possibility of figuring a metre-class mirror in about ten hours.

Estimation of the power absorbed by the surface of optical components processed by an inductively coupled plasma torch

“…In this work, the heat flux transferred by the bespoke ICP torch was managed by a motion controller that was used to implement raster scan toolpath algorithms [8]. The plasma torch was moved along the x axis whereas the substrate is moved along the y axis.…”

“…These plasma treatments are often based on dwell time methods and require a controlled exposure to obtain a uniformly treated surface [6][7]. One of these processes is called Plasma Figuring [8] and makes use of a bespoke inductively coupled plasma (ICP) torch [9]. The determination of the heat flux function of one type of these dedicated ICP torches is the motivation of this work.…”

Study of Atmospheric Pressure Plasma Temperature Based on Silicon Carbide Etching

“…However, because the ICP will generate a large amount of joule heat, it is difficult to cool down the plasma inside the generator to ensure the safety of the process. To solve this problem, Cranfield University tried to minimize the negative effects caused by high temperatures during processing and found that improving the temperature by changing the flow rate of the reaction gas had a very limited effect [ 16 ]. The scholar Renaud Jourdain built a simulation model and confirmed that the nozzles have a significant cooling effect on the plasma jet [ 17 ].…”