Creation of a control system for plasma delivery to increase automation and stability.

Abstract

Surface 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.