Abstract
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’ test.
Highlights
The fixed match RF network on Helios 1200 was chosen to reduce the weight of the plasma torch assembly in the large optical fabrication, so the axis of the computer numerically controlled (CNC) system could drive the plasma torch assembly with better dynamic performances, and achieve a higher capability to correct complex surfaces
In order to enhance the processing capability of plasma delivery systems used in CNC machine tools and created for high-end optical fabrication, a highly reliable and deterministic plasma delivery system (PDS) was designed, manufactured, and tested to satisfy the requirements of ultra-precision applications
The existing L type fixed match RF circuit was modified with motorized capacitors
Summary
Several world-class scientific facilities (e.g. extremely large telescopes [1], EUV lithography [2], and laser fusion plants [3]) require thousands of meter-scale ultraprecision optics. As opposed to the RAP300, which is a small-scale prototype facility [14], the processing capacity of Helios1200 was conceived for the rapid PF process of large optics up to 1.2 m diameter at atmospheric pressure [15] These machines combine plasma technology operating at atmospheric pressure and computer numerically controlled (CNC) motion systems. The fixed match RF network on Helios 1200 was chosen to reduce the weight of the plasma torch assembly in the large optical fabrication, so the axis of the CNC system could drive the plasma torch assembly with better dynamic performances, and achieve a higher capability to correct complex surfaces This configuration of the axes lost the advantage of the matching network, which could reduce the reflected power to zero when environmental conditions change sharply (generator frequency > ±20%) [17]. The purpose of this research focuses on developing a hybrid PDS for supporting a deterministic PF process, assisting the machine operator by tuning key electrical components in the RF network, and monitoring important processing parameters
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