| 摘要 |
An ionizable gas is accommodated in an ionization chamber having a first end and a second end which is spaced from the first end and is an outlet of the chamber. A first electrode is disposed at the first end and a second electrode in the form of a fine grating or grid is disposed at the second end. The electrodes are connected to a source of radio frequency current which is capable of establishing a potential difference of such magnitude between the electrodes as to ionize the gas. The gas is at a low pressure so that ionization of the gas produces a low-pressure plasma. The area of the first electrode is selected to be so much larger than the area of the second electrode that the magnitude of the voltage drop between the plasma and the first electrode is negligible compared to the voltage drop between the plasma and the second electrode. The positive ions resulting from ionization of the gas are accelerated towards the second electrode and travel through the same towrds an object to be bombarded. Prior to exiting the ionization chamber, the stream of positive ions is combined with a stream of electrons to produce an electrically neutral stream of plasma. Magnetic coils disposed adjacent to the outlet of the chamber generate magnetic fields which shape the streams so that the electrically neutral plasma issuing from the chamber is in the form of a well-defined and homogeneous jet or beam. |
