| 摘要 |
PURPOSE:To stabilize the current by inserting a limiter circuit limiting the difference output in the feedback loop of the control system of the beam current. CONSTITUTION:A differential amplifier 11 receives the beam current preset value from a voltage dividing circuit 11a and the beam current measured value from a current detector and amplifies and outputs the difference. This output is applied to an adder amplifier 31 via a limiter circuit 30, the difference from the amplifier 11 is added, and it is fed to a V/F converter 13 as the control signal. The output of the converter 13 is fed to an extracting voltage generating circuit as the control signal, and the beam current emitted from an emitter can be controlled at a constant value. The circuit 30 outputs its input signal as it is under the normal operating condition. A display circuit 33 monitors whether the output of the amplifier 11 is at the level not limited by the circuit 30, and a variable resistor 32 is adjusted so that the circuit 30 does not perform the limiting operation under the normal condition. Accordingly, the ion current of the gas ion beam can be stabilized without applying the overvoltage.
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