Electric heating – Metal heating – By arc
Patent
1993-05-26
1995-05-09
Paschall, Mark H.
Electric heating
Metal heating
By arc
51912152, 51912145, 51912148, 519696, 31511121, 219690, B23K 1000
Patent
active
054142350
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a gas plasma generating system for use, for example, in a welding application.
It is known that very high frequency electric power can be transmitted via hollow conductors (commonly known as waveguides). The source of such high frequency includes a resonant cavity device such as a magnetron, klystron or free electron laser. Attempts have been made in the past to utilise this very high frequency power to create gas plasmas. In one arrangement, gas flows along a conduit across which high frequency power is passed. (High Power Microwave Plasma Beam as a Heat Source--Application to Cutting. Arata et al, Transactions of JWRI, Vol 4, No. 2 (1975) pp1-6). Although this produces a plasma, the plasma itself forms a load on the power transmission line and it is necessary to blow the gas at high rate through the conduit to extract the energy. Typical flow rates are in the range 250-400 liters/minute.
SUMMARY OF INVENTION
In accordance with the present invention, a gas plasma generating system comprises a resonant cavity for connection to a source of very high frequency power; a plasma cavity defined by an electrically non-conductive material positioned within the resonant cavity for containing an ionisable gas such that in use a plasma is formed in the plasma cavity, the cavity having an exit opening to enable plasma to exit from the system, wherein the plasma cavity comprises a tubular member extending through opposed walls of the resonant cavity, the tubular member receiving at one end a plasma gas, in use, and plasma exiting from the other end; and a movable tuning member whose position can be adjusted to achieve the desired tuning condition, the tubular member defining the plasma cavity extending through the tuning member.
High frequency discharges at random are known from power supplies operating at very high frequency, where random ionisation has occurred of the surrounding atmosphere or where there has been inadequate contact between one component and another carrying the very high frequency current. These discharges are uncontrolled and indeed are unwanted since in general they result in significant power loss in a transmission of the very high frequency current.
We have found that it is possible to harness these previously undesireable discharges so that the very high frequency power can be used to create a gas plasma.
By utilising a non-electrically conductive material to define the plasma cavity, electrical shorting across the resonant cavity is prevented since the ionising gas is restrained within the plasma cavity. Typically, the plasma cavity is confined by a ceramic wall.
The space within the resonant cavity surrounding the plasma cavity is filled with an insulating gas which is preferably air since this is particularly good for cooling.
It has been found with the invention that gas flows as low as of one liter/minute are achievable.
In this context, by very high frequency we mean generally frequencies in excess of 100 MHz and preferably in excess of 1 GHz, even in excess of 10 GHz. In this latter range, the tuned cavity dimensions are of the order of tens of millimeters while the exiting plasma can be used for heating, surface treatment, welding or cutting as are known at comparatively low frequencies or with DC in the field of welding technology.
The tuning member will typically comprise a tuning stub. In some cases an additional fine tuning member will also be provided.
The source of very high frequency power could be tunable or indeed both the source and resonant cavity could be tunable. By providing at least one tunable component it is possible to optimize both the striking and the running of the discharge. It should be noted that before the discharge is established, the resonant cavity is in effect open-circuited. During use, retuning of the source cavity is preferably carried out so that a high current flows through the plasma discharge to provide heating and ionization of the gases forming the discharge.
Any conventional source of high frequency power could be used s
REFERENCES:
patent: 4049940 (1977-09-01), Moisan et al.
patent: 4780642 (1988-10-01), Jacquot
patent: 4883570 (1989-11-01), Efthimion et al.
patent: 5021919 (1991-06-01), Engemann
patent: 5225740 (1993-07-01), Ohkawa
Journal of Vacuum Science and Technology: Part B. vol. 4, No. 1, Jan. 1986, New York US pp. 295-298; Roppel et al.: `Low temperature oxidation of silicon using a microwave plasma disk source`.
Lucas James
Lucas William
Novack Martin
Paschall Mark H.
The Welding Institute
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