| 摘要 |
PURPOSE:To almost completely compensate for the increase and decrease in the magnetic flux of a focusing magnet due to the change in temperature, by disposing a temperature sensor near the focusing magnet, and controlling a direct current applied to an auxiliary focusing coil. CONSTITUTION:When the temperature of a diode D1, which serves as a temperature sensor, has gone up together with that of a magnet, an electrical current flowing through the diode increases and an electrical current applied to one input terminal of a differential amplifier OP1 decreases, so that the output of the amplifier rises. As a result, the output of a differential amplifier OP2 provided with a negative feedback circuit falls so that an electrical current i applied to an auxiliary focusing coil LM1 decreases. For that reason, a focusing magnetic flux can be kept almost constant within a practical temperature range from 20 deg.C to 70 deg.C if the initial value of the direct current applied to the auxiliary focusing coil LM1 and variable resistances VR1, VR2 are preset.
|
