Apparatus for controlling a plasma

Claim 1

Apparatus for preventing damage to a plasma torch comprising: a plasma torch which includes a quartz tube through which an inert gas flows and a helical RF coil disposed on the outside diameter of said quartz tube substantially at one end thereof; said RF coil being energizable so as to ignite said inert gas emanating from said one end of said quartz tube, the ignited inert gas forming a plasma discharge; light gathering means, directed at a predetermined volume above said RF coil, for gathering a portion of the light from the plasma discharge; detector means, remote from said light gathering means, for producing a detector signal indicative of the light intensity of the plasma discharge; light transmitting means, coupled between said light gathering means and said detector means, to transmit light from said light gathering means to said detector means; reference signal means for supplying two reference signals comprising a static reference signal, having a predetermined value, and a dynamic reference signal proportional to the instantaneous power to said RF coil; comparing means for comparing the detector signal with said two reference signals; and de-energizing means to de-energize said RF coil whenever the detector signal is below either of said two reference signals.