How to Change Electron Gun Filament of Precise Accelerator?

Under a long term using of Precise accelerator, the filament will gradually aging, resulting in decline of its measurement rate. Therefore, it is time to increase the filament voltage and adjust the transmit power. If the measurement rate is too low to be measured, electron gun filament should be changed, after changing, operators should adjust the automatic frequency control (AFC) and gun servo.

At the beginning of changing the electron gun tungsten filament, rotating gantry should be adjust to 108 °, and then cut off the ion pump power supply.  Plastic bag should be aerated with dry and clean nitrogen gas and tied it at the interface of high vacuum repair valve. On the basis of direction of the mark on the valve body, open high vacuum valve slowly, and then the nitrogen would be inhaled gradually. When the pressure of nitrogen of system is balance, turn off the high vacuum valve repair. Remove the outer casing housing of gun filament (pay attention to make red dot be directed at 0) and remove its subassembly. The process of the replacement of the gun filament and seals should be in a clean and dry air. Engineers should wear gloves and masks. In addition, tools can be used after cleaning by absolute ethyl alcohol in order to avoid the dust into the high vacuum system to influence beam stability and life of the filament. Finally, reinstall the gun filament subassembly and outer casing housing.

The next step is to adjust the AFC. When the 6MV X-ray light is stable and sends light, set the Gun Man / Auto as manual and set the Phase Auto / Man as manual. Input the Tuner ctrl. Part4 value to Part1, adjust tuner ctrl so that its output is maximum, adjust Lp phase ctrl to make Comp Phase value is 0, set the value Lp phase posn Lp as phase ctrl.

Finally, adjust the servo gun. Before doing it, operators should ensure that the automatic frequency control has been completed adjusted, on this basis, and then adjust the X-ray and each file of electron beam individually. When adjusting the X-ray gun servo, under the dosage rate of 400MU / min, set the i181 Gun auto as manual. adjust i327 Gun I ctrl to make its output become maximum, change the i181 gun auto to automatism, adjusting i186 hump gain to make the part 4 of i327 Gun I ctrl the same as part1. Replacement of the electron gun filament is complicated and fussy, engineers must grasp the replacement process in every detail to ensure the stable operation of the accelerator.

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