What is a Klystron Tube?

A klystron is a type of microwave tube that uses periodic modulation of the electron beam velocity to achieve oscillation or amplification. It first speed-modulates the electron beam in the input cavity, and then converts it into density modulation after drifting. Then the clustered electron blocks exchange energy with the microwave field in the gap of the output cavity. The electrons give kinetic energy to the microwave field to complete oscillation or amplification . ^[1]

In the klystron, the signal electric field input to the cavity slit modulates the speed of the electrons. After the drift, a density modulation is formed in the electron beam. The density-modulated electron beam and the microwave field outputting the cavity gap perform energy conversion. Microwave field, complete the function of amplification or oscillation. ^[2]

In 1937, American physicist Varian, RH and SF Varian produced a dual-cavity klystron oscillator. The reflective klystron was developed in 1940 by Soviet engineers Jevac, Daniel Gemini, Buskonov and Kovalenko. ^[2]

According to the trajectory of electronic travel, klystrons are divided into two types: direct klystrons and reflective klystrons. Usually, direct klystrons are simply referred to as klystrons.

Direct shot klystron

The direct firing klystron includes the following parts in structure: electron gun, resonant cavity, chirping system, drift tube between each cavity, energy coupler, collector and focusing system. Klystrons with two resonant cavities are called dual-cavity klystrons; those with more than two resonant cavities are called multi-cavity klystrons.

Double cavity klystron

A dual-cavity klystron has only two resonant cavities, namely an input cavity and an output cavity. The electron beam generated by the electron gun first reaches the input cavity slot. The input microwave signal is sent into the input cavity through the energy coupler, and a microwave signal voltage is formed outside the resonance cavity gap. Here, the electron beam is modulated by the speed of the microwave field and then enters the field-free drift tube. During the drifting process, electrons cluster and form a density modulation in the electron beam. The density-modulated electron beam exchanges energy with the microwave field outputting the slot of the cavity, and the electrons pass the energy to the microwave field to complete the function of amplification or oscillation.

The microwave power is sent to the load through the energy coupler.

The gain of the double cavity klystron is only about 10 dB. In order to increase the gain, one or more intermediate cavities may be provided between the input cavity and the output cavity to form a cascade amplifier. This type of klystron is called a multi-cavity klystron). The introduction of intermediate cavities can also improve efficiency; if the frequencies of the cavities are slightly different, the frequency band can also be broadened. The characteristics of multi-cavity klystron are high gain, high efficiency, good stability, large output power, and the disadvantage is that the frequency band is narrow. The multi-cavity klystron has a stable gain of 80 dB, an efficiency of up to 75%, a pulse power of 60 MW, and a continuous wave power of 1 MW. The frequency band is generally only 1% to 2%, and individual high-power pulse klystrons can reach 10% to 12%.

Electron clustering After the electrons emerge from the cathode, they are accelerated by a high voltage, and the speed of all electrons is the same when they reach the input cavity. The microwave signal to be amplified enters the input cavity, and a microwave signal voltage is established on the gap. The voltage across the slot changes sinusoidally with time. Electrons that reach the gap at different times are affected by different instantaneous voltages.

Electron guns commonly used in klystrons are vagina-controlled guns, sun-controlled guns, grid-controlled guns, non-intercepted grid-controlled electron guns, and magnetron-injected hollow injection electron guns (see

First, the ion pump current is too large

After the transmitter is turned on, before the high voltage is applied, watch the value of the ion pump ammeter. Because only the ion pump voltage and filament voltage are available at this time, other factors can be excluded to cause inaccurate readings. If the ion pump current indication is large and does not fall back, the tube should be replaced at this time. But some of the new tube ion pump current is greater than 20uA, then there is a magnet at the cathode, remove the magnet. If it continues to grow (about 10gA), it can be judged that the pipe is broken. Due to aging, the pipe leaks gas, which causes the pipe to be filled with gas, which disrupts the normal working conditions of the pipe.

Dirty high-voltage wires will also make the ion pump current meter head larger than 10g ^. If it is still large after wiping, it means that the tube is leaking.

Second, the body current is too large

Generally, the body current of the klystron is less than 50mA. When the tube leaks or defocuss, the body current reaches about 100mA. Under normal circumstances, the protection circuit will operate. There are several reasons for the excessive body current:

(1) Focusing current failure causes the electron beam to defocus; (2) Excessive excitation causes the klystron to work in a saturated state; (3) Excessive current causes the electron beam to become thick; (4) Improper transportation or the installation position is not in Level, the klystron is deformed or not straight; (5) the excitation coil is not concentric; (6) the collector or the device connected to it touches the ground, which will cause excessive body current. It can be seen from the above that excessive body current does not necessarily mean that the tube is broken. You should first check the excitation power, focusing and other peripheral circuits to reduce excitation. The collector-to-ground resistance should be 5Q-7Q. Zero or too large is not suitable. Sometimes the fault can be eliminated as long as you replace it with a suitable resistor. There is no need to change the tube. The body current is greater than 100mA, and the gain of the tube is very low. If the protection circuit fails and loses its protective effect, defocused electrons bombard somewhere on the wall of the tube, causing its temperature to rise and the tube to have burn marks.

Third, the injection current is small

The bet current is relatively small at normal control voltage. It was bigger at first, and then smaller and smaller. There are two cases in the analysis: (1) The filament voltage is too low, and the injection current is small. Because the electrons are forcibly pulled out from the cathode, it is easy to break the cathode, and the injection current of the tube is reduced; (2) the service life of the tube has exceeded the manufacturer's nominal value (using proper maintenance, the life can reach tens of thousands of hours), the cathode The transmitting ability is reduced, and the gain is low, which cannot meet the broadcasting requirements.

Fourth, a cavity gain is low

The middle of the frequency response curve of the camera is sunken. How to adjust is not good, and the gain is reduced. The outline of the image is blurred and the colors are not bright. The sound machine is similar, and the ideal frequency response curve cannot be adjusted. After the end of the tube's service life, performance will normally decline, sometimes as a decrease in one-cavity gain. However, the filament voltage is too high for a long time, and the cathode material evaporates too fast, which will cause the insulating layer of the one-cavity ceramic to become a conductive oxide, pollute the ceramic wall, and cause the one-cavity Q value to drop. The latter situation can be avoided. The voltage of the Klystron filament just used is 5V, and after 600 hours of operation, it drops to 4. Stare again for long-term use. Because the sponge cathode barium can give higher radioactivity after activation. At this time, although the filament voltage is reduced, the radiation current is not reduced, and the service life of the klystron can be extended.

Five or three cavity resonance frequencies are tuned into the band too much

It is required that the resonance frequency of the three-cavity must be maintained at the high end of the passband. If it is adjusted beyond the requirements in the band, the klystron will self-ignite and crack and leak.

Six or four cavity coupling is too light

The four-chamber coupling cannot be less than 50 degrees. If the coupling is too light, the back EMF will be strong, which will cause the electrons to re-bounce back and the four-cavity to ignite. The fire time is long, the ceramic is easy to crack and leak, and the pipe is damaged.

Seven, high integral voltage resistance fracture

The high integral resistance divider breaks, the modulation anode potential becomes low, the injection current increases sharply, and the cavity overheats. If the protection circuit does not work, the pipe will be overheated and damaged.

Eight, uneven placement

In some stations, the four wheels of the pipe cart were placed unevenly and were not on the same horizontal line. They were deformed for a long time and caused cracks in the pipe.

Nine, cooling is not in place

Klystrons are less efficient and require a lot of heat to be dissipated during work. The cooling water flow and air volume should meet the equipment requirements. If the cooling system fails, the water temperature is too high, the air volume is insufficient, and the surface temperature difference between the metal and ceramic seals of the electron gun exceeds the limit, which will also cause the tube to burst.

The above are just a few aspects of judging the end of the life of the klystron, and some are not summarized. However, there are two points that must be paid attention to: First, the small box of the protection circuit must be tested regularly to ensure that the protection circuit works normally, so that many failures can be avoided and the pipe will not be damaged; second, the correct use and reasonable maintenance can greatly extend the speed adjustment Tube life. ^[3]