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Microwave generated plasma figuring for ultra precision engineering of optics for aerospace & defence

New optical technologies increase the demands on the engineering specifications of optical surfaces, with manufacturing specifications of up to 1nm RMS form accuracy and 0.1nm RMS surface finish. To achieve these fabrication requirements novel ultra-precision methods must be developed. The proposed solution is microwave generated activate plasma figuring.

Student

Adam Bennett

Supervisor

Dr Renaud Jourdain

Project presentation

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Executive summary

An exhaustive and comprehensive literature review has been conducted on the different aspects of microwave plasma torch designs. Theory detailing how the microwaves are generated, propagate through air, interact with gas atoms and create plasma is available in this report. Different torch designs are grouped together according to forwarded power operation, the type of plasma generated and industrial applications.

Examples of microwave plasma characterisation by Optical Emission Spectroscopy is sourced from literature and used as a support to validate the experimental results obtained through the Adtec microwave plasma torch.

The Adtec torch was tested under a rigorous design of experiment procedure. The main torch parameters – microwave power, gas flow rate, type of gas, quartz tube design, and nozzle design – were systematically altered and their respective effects on the emission spectra of the plasma discharge were recorded. Mapped displays of the 14 areas of interest were chosen and presented.

Through this experimental work it was found that when the microwave power was increased from 13W to 15W, the intensity of the emission spectra within the plasma discharge increased.

Furthermore, when the gas flow rate was decreased from 2L/min to 1L/min, the power density within the plasma discharge also increased. The predominant factor that affected the power density was the microwave forward power.

The effect of the different quartz tube designs was found to incur no influence on the emission spectra within the plasma discharge. However the new Cranfield designed quartz tube, Cranfield Quartz Tube Design II, dissipates the thermal energy more efficiently – as is evidenced from the reduction in discolouration – and this increases the lifetime of the quartz tube.

The plasma plume emission from the Cranfield Nozzle has an effective discharge length of 3mm and a radius of 1mm, over which the power density is over 90% of that of the maximum power density. The discharge from the Adtec nozzle is almost entirely confined within the Adtec nozzle.

Literature states that helium has a higher thermal conductivity compared to argon. This will yield a plasma plume diameter that is smaller and therefore a smaller tool function width. Plasma inhomogeneity varies inversely proportionally to the thermal conductivity of the gas. Hence if helium is employed as a carrier gas, compared to heavier elements such as argon, then highly stable plasma may be maintained for long periods of time.

This gas characteristic concurs with the experimental observations. The 3D maps of the helium plasma discharges exhibit lower power densities relative to that of the argon plasma discharges and the distribution of the thermal energy within the helium plasmas was more uniform than that of the argon plasmas where the energy was more concentrated in the centre of the plasma plume.

Vector Network Analysis was conducted on the three stub tuning cavity of the microwave plasma torch. The results showed that the Adtec torch is designed to operate efficiently when transmitting forward microwave powers over an extremely tight bandwidth around 2.45GHz.


Research focus

This research was conducted to further the understanding of microwave generated micro plasmas to enable the design of a new and novel microwave plasma torch, which will be targeted at enhanced optical fabrication of crystal quartz for applications at Gooch & Housego.

Firstly an exhaustive and comprehensive literature review has been conducted to explain the different aspects of microwave plasma theory and secondly detailed experimental methods are explained with their results. The Adtec microwave plasma torch was characterised by Optical Emission Spectroscopy (OES). The microwave forwarded power, gas flow rate, type of gas, quartz tube design, and nozzle design were systematically investigated and their respective effects on the plasma plume emissions were mapped.


Purpose

This work has been conducted for Adtec Europe by the Engineering & Physical Sciences Research Council (EPSRC) Centre for Innovative Manufacturing in Ultra Precision (CIMUP). The EPSRC CIMUP is a partnership between Cranfield University and the University of Cambridge.

This research has been undertaken by the Precision Engineering Institute (PEI) that is located at Cranfield University. This work was carried out using the RAP300 plasma figuring machine shown below. This technology currently employs an Inductively Coupled Plasma torch, which is designed to discharge argon and reactive plasmas.


Experimental setup

The Adtec microwave generator was connected to a coaxial cable, which was in turn connected to the Adtec plasma torch. The Adtec plasma torch was installed into the RAP300, in a fixed position. Existing within the RAP300 was a precision motion stage, which could be moved to a place in 3D space within ±1μm. Upon the precision motion stage was placed a camera for observation, a Magic Stick for ignition and the Ocean Optics HR4000 Interferometer for Optical Emission Spectroscopy characterisation of the plasma plume.

The Adtec torch consists of a tuning cavity which has an antenna support attached to the end. Within the antenna support sits an antenna enclosed by a quartz tube. This arrangement is housed within an aluminium cavity room, which is in turn connected to a nozzle.


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