Design of a motorised plasma delivery system for ultra-precision large optical fabrication

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 total15 hours’test.

Investigation of a plasma delivery system for optical figuring process

Plasma figuring process enables the correction of metre-scale optical surfaces,which has advantages such as rapid convergence to ultra-precision accuracy level,no subsurface damage,ease of upscale thanks to the conditions of operation at atmospheric.Such attributes make plasma figuring a highly competitive technology.To maintain a stable discharge of plasma for long period,a robust Plasma Delivery System(PDS)is required for large optics manufacturing.A dedicated PDS,mainly based on an L-type radio frequency(RF)network and an inductively coupled plasma(ICP)torch,had been previously proposed and designed.This study addresses the requirement for improving the robustness and securing the performance of the plasma etching.Due to the complexity of interactions of PDS parameters,single variable optimization is conducted for adjusting the impedance of the plasma torch.The PDS optimization is analysed based on ignition power demand,test repeatability and reflected power values.The results show that reflected power values are controllable within 10 W,when the RF power is supplied from 200 W up to 800 W.Finally,plasma processing is carried out on fused silica glass,using the optimum parameters of the PDS.