Electric heating – Metal heating – By arc
Reexamination Certificate
1999-12-13
2001-05-01
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121540, C315S111210, C606S029000, C606S030000, C606S032000
Reexamination Certificate
active
06225593
ABSTRACT:
This invention relates to apparatus for generating an ionised gas plasma flame for use in medicine, particularly for cauterisation. A known form of apparatus for generating an ionised gas plasma flame for use in cauterisation is disclosed in WO 95/26686 and provides a corona-type flame issuing from a nozzle. The corona flame has a high electron temperature but a low molecular temperature, typically about 20° C. When the nozzle is brought close to (within 5 mm of) a surface that is connected to electrical earth, either directly or by stray capacitance, the corona-type flame changes to an arc-discharge flame (which has a high molecular temperature typically of the order of 800° C). The flame takes place in the plasma provided by the flowing gas which, being inert, minimises oxidation occurring at the earthed surface.
Accordingly, when the surface is part of a human or animal body the plasma can be used to stop flow of blood from damaged tissue by cauterisation. Alternatively, the heat from the arc-discharge flame can be used to remove layers of tissue.
Some surgical techniques require very low operating powers, for example less than 50 W. However, the prior art cauterising apparatus requires complex control methods to produce operating powers of less than 50 W. Typically, these complex control methods apply variable-length bursts of radio-frequency (in the region of 1 MHz) energy to the plasma.
Operating at power levels of less than 5 W requires special consideration because the voltage required to initiate a discharge at these low operating powers (5 W) is the same as the voltage required to initiate a discharge at higher operating powers (greater than 150 W). However, using a high voltage increases the power supplied to the plasma. The prior art apparatus uses a high output-impedance circuit to limit current flow when operating at power levels below 5 W, however, these high output-impedance circuits are very complex.
It is an object of the present invention to provide an improved circuit for controlling cauterising apparatus at low powers.
According to a first aspect of the present invention there is provided apparatus for generating an ionised gas plasma flame for use in medicine, particularly for cauterisation, comprising:
an electrical power source providing an alternating voltage at a fixed frequency in the Kilohertz region and with a magnitude below 100 V,
a resonant circuit approximately resonant at the fixed frequency connected to the electrical power source,
a single insulated electrical conductor leading from the output of the resonant circuit and terminating with an insulation-free tip,
conduit means surrounding the electrical conductor along at least part of its length for delivering a supply of substantially inert gas to the insulation-free tip,
an end piece terminating the conduit means and surrounding the insulation-free tip of the conductor,
wherein the end-piece at its free end defines a nozzle through which the gas-flow can emerge,
whereby the resonant circuit includes an in-series inductance to limit plasma current flow and the resonant circuit provides a magnified voltage so that the gas-flow may be ionised to form a plasma by the magnified voltage supplied to the tip.
By virtue of the present invention, a low voltage fixed frequency power supply may be used to initiate and maintain a low power plasma discharge. This is achieved by using a reactive circuit that is resonant at this fixed frequency so that when the plasma is formed the voltage is reduced because of the impedance provided by the plasma relative to the resonant circuit, thereby reducing the current flow and the power delivered to the plasma.
Preferably, a step-up transformer is used to increase the voltage from the power source.
Preferably the power source operates at a fixed frequency in the range 30-90 KHz and at a voltage in the range 2-50 V with a power level less than 30 watts.
Preferably, the gas has a low breakdown potential, suitable gases include He, Ar, Ne, H
2
or N
2
or a combination thereof. The gas may additionally include a small amount (e.g. less than 25%) of O
2
.
Preferably, a voltage detector continuously monitors the voltage at the tip so that the amplitude of the output voltage from the power source is reduced when the voltage at the tip is above a predetermined value.
According to a second aspect of the present invention there is provided a method of controlling cauterising apparatus, where the method comprises the steps of
measuring the capacitance of a reactive element in the cauterising apparatus,
selecting a fixed frequency in the kilohertz region for a sinusoidal generator which provides an alternating voltage at the said fixed frequency, the alternating voltage having a magnitude less than 100 V,
and selecting a value of a resonating reactive element such that the reactive element and said resonating reactive element are approximately at resonance at the said fixed frequency,
so that voltage magnification is provided whereby the voltage has sufficient amplitude to initiate and maintain a plasma discharge.
It will be understood that the reactive element may be implemented by parasitic effects such as stray capacitance. It will also be understood that the term “at resonance” refers to the circuit being sufficiently close to resonance for voltage magnification effects to be important.
Preferably, the method includes the step of using a resistive element to limit the voltage magnification produced by the reactive element and the resonating reactive element.
REFERENCES:
patent: 5086205 (1992-02-01), Thommes
patent: 5376768 (1994-12-01), Pasquini et al.
patent: 5747935 (1998-05-01), Porter et al.
patent: 0353178 (1990-01-01), None
patent: 95/26686 (1995-10-01), None
Cain Peter John
Howieson Maurice
Fleshner & Kim LLP
Helica Instruments Limited
Paschall Mark
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