Generally speaking, before the new accelerating tube leaving the factory, voltage of electron gun filament has been completed calibrated, adjusted and installed. An electron gun (also called electron emitter) is an electrical component in some vacuum tubes that produces a narrow, collimated electron beam that has a precise kinetic energy. Under the long working time, due to aging the emission instinct of the electron gun would gradually waken, which would lead to drift and decline of accelerator measurement rate. Then it is time to do the voltage optimization of tungsten filament of electron gun, which plays a very important role for ensuring the stable and exact dosage.
If the voltage is too low, the number of electrons injected into the accelerating waveguide would be too less, resulting INJI become unstable, so voltage value would be too high, finally influence the life time of the electron gun.
1. Prepare 6MV X line and enter maintenance mode, shielding the related chain of dosage.
2. Connect the oscilloscope to monitor the waveform of INJI, and then set the INJI value into 100mA.
3. Close RF dirver, K1-CB1 and RF modulator.
4. Turn the body frame into 270℃.
5. Ensure that the injector voltage is turned off, and then turn off the high-pressure start switch.
6. Connect multimeter to X12 and X13, and turn R11 to reduce voltage of filament into 4.0V, if the voltage cannot be reduced to 4.0V, set R11 into the lowest voltage.
7. Disconnect DMM.
8. Turn on high-voltage.
9. Start to beam.
10. Close rays until the injector access safely. Connect multimeter with X12 and X13. Move clockwise R11, increase 0.1V each time, then disconnect the multimeter, meanwhile write down INJI current and electron gun filament voltage. Repeat this process until the total number of INJI would not changed any more.