X-ray tube is the key and core component of CT machine, and its value is generally more than several hundred thousand yuan. 出线量减少，影响图象质量，甚至使其寿命缩短，造成不应有的经济损失。 Improper use will reduce the amount of X-ray tube output, affect image quality, and even shorten its life, resulting in undue economic loss. How to prolong the service life of the tube is an important task in front of the operator, especially when the tube is matched with the existing application, it needs to be comprehensively considered by the system.
Low kV / High mA emission
When a tube designed for high KV use is used at a lower KV (usually half or less of the maximum value), common mismatches can occur, and the filament must run at higher currents to overcome the limited emission . In a specific rotating anode tube, when lowered to 50kV and 300mA, operating at 125kV and 300mA, the filament must be operated at 16% of power to overcome the lower tube voltage. Since the filament is cooled by radiation, a 16% increase means a 3.8% increase in the filament temperature. Although this may seem small, the evaporation rate of tungsten at higher powers is about three times, resulting in a three-fold reduction in filament life in this case. If running at a higher tube current (> 300 mA in this case) at 50 kV, the filament current must be increased and even the filament life reduced.
Temperature / life
High temperature is the biggest factor affecting the life of X-ray tubes. The greater the input power, the shorter the tube life. However, without sufficient power, there may not be enough X-ray intensity to get the job done. Filament evaporation causes unwanted metal deposits that can eventually lead to fire. Operating at higher temperatures, the target will evaporate, and the radiation quality in terms of energy distribution and intensity will begin to change and will decrease due to microcracks.
There are thermomechanical stresses during tube operation. Glass-to-metal seals are stressed when heated. The more heat, the higher the temperature, resulting in increased stress. Eventually, tiny particles will break or the glass will form tiny cracks, which will increase as radiation passes through. Due to thermal cycling, there is always mechanical fatigue, the more cycles, the faster the fatigue. The higher the power, the higher the temperature and the accelerated fatigue. Operating the X-ray tube
at the lowest available power can extend its life.
One of the most critical characteristics of a power supply is its impedance. For a fixed anode tube operating at several hundred watts, the impedance can be very high, which means that it contains a lot of resistance. When the voltage supporting the arc decreases, the arc usually extinguishes. When the current in the arc passes through the high-voltage resistor, the voltage across the resistor increases, thereby reducing the voltage on the tube and other parts of the high-voltage circuit. If the pressure of the gas in the tube is high enough to sustain the arc, the impedance also protects the power supply and related electronics. When the gas level becomes too high, the tube cannot improve its performance.
Unfortunately, high impedance also means that if an arc discharge begins due to particle or field emission or light evaporation, there is usually not enough energy stored to clear or evaporate the cause and the arc can continue.
Rotary anode tubes work at higher power conditions, sometimes exceeding 100 kW or almost 1000 times that of fixed anodes. Here the power supply cannot have high impedance, otherwise it will not support the required power. In these cases, it is often necessary to limit the stored energy to typically less than 10 Joules. High-voltage cables and voltage doubler capacitors will store such energy and may cause arc tube damage. Ten joules is not a fixed value, it is just a guideline, because some tubes work satisfactorily and store more energy, while other electrodes do not run with less energy. Since energy is proportional to the square of the voltage, capacitance becomes more troublesome at higher voltages.