The electron gun is the starting point of the electron emission and is used in many areas. For example it is used in radar, microwave, RF devices as well as traveling wave tubes (TWT amplifiers) and electron accelerators for medical and industrial applications.
The characteristic function of the electron gun is to produce a continuous flow of electrons. Only through the interaction between the electron beam with an electromagnetic wave a desired functionality can be achieved. The technical progress of the development also benefits from the simulation software advancement. The progress of 3D visualization software enables a constantly optimization of the design of electron guns.
The main steps in the development of an electron gun can be divided as follows:
The most common used cathodes are thermionic cathodes which overcome the work function for electron emission with the help of a heating element. Another option is called field emission. Thereby, the electron emission is achieved by applying high electric field strength (in the order of 109V/m).
Figure 2: Schematic representation of the electron emission process
The thermionic cathodes can be categorized depending on the choice of material or manufacturing method. Mainly th cathodes are used in which the material is either impregnated or provided as a reservoir. Figure 2 shows the electron emission process schematically.
In order to get an electron emission you have to overcome the work function of the source material at the emission surface. This results in a space charge cloud in front of the cathode. The maximum achievable current density for thermionic cathode is dependent on the absolute temperature and the work function. In general a thermionic cathode is not operating in saturation mode because the saturation current is temperature-dependent and the lifetime of the cathode is shortened by that. A temperature-independent operating point will be chosen which provides a smaller space charge current.
For decades the thermionic cathode production has been known as a sophisticated technology. By continuous improvement of the manufacturing process vendors can guarantee an increasing lifetime. The most widely used cathodes are reservoir cathodes and oxide cathodes. Another way to generate an emission current density is called field emission, which is characterized by applying a high electric field strength.
Oxide cathodes can be realized by coating a nickel cathode body with a mixture of barium, strontium and calcium carbonate. By heating the mixture under vacuum condition the required barium, strontium, calcium oxide will be created by disposal of carbon dioxide. Higher operating temperatures of the cathode can be achieved by using barium and tungsten. The advantage of low operating temperature is that oxide cathodes can have a larger size. This allows quick and inexpensive ways to generate many electrons for research projects or for other purposes. But the disadvantage of oxide cathodes is that only an average current density of approx. 1 A/cm2 is achieved. During pulsed emission operation mode current densities of more than 20 A/cm2 can easily be achieved. A disadvantage of the oxide coated nickel is that it is only designed for single use. If the oxide-coated nickel cathode is in contact with air then the cathode must be replaced. This is particularly a problem of modular systems, since the system configuration have to be changed occasionally due to optimization studies.
The structure of dispenser cathodes consists basically of a porous tungsten emission disc, the heating element and the barium reservoir. Dispenser cathodes can be implemented as an impregnated version or as a reservoir version that have a porous tungsten matrix. The operating principle is that at operating temperature the barium supply diffuses slowly to the surface of the tungsten-emission disc. The production of the emission disc is done by compressing and sintering of tungsten powder. The porosity of the disc depends on the grain size of the used material.
The barium of impregnated cathodes is stored in the pores of the matrix. The barium-tungsten version has a low work function (about 2eV) which can be the further reduced by addition of osmium. It is usually possible to get a current density of 4-5 A/cm2 from an osmium impregnated cathode at 980° Celsius. This can be increased to over 20 A/cm2 by increasing the temperature. These cathodes have a longer lifetime than oxide-coated cathodes. They can be operated more than 60,000 hours at 4 A/cm2.
The barium supply of reservoir cathodes is located behind the porous tungsten emission disc. By heating up the barium, it diffuses to the surface of the emission disc. Reservoir cathodes can even achieve higher current densities (up to 100 A/cm2). Moreover, these cathodes have a very long lifetime, more than 100,000 hours.
To improve the lifetime of the cathode, the operating temperature or work function must be lowered. This allows influencing the evaporation rate of barium at the surface of the cathode.
Another cathode version consists of lanthanum hexaboride which operates at about 1450°C. It is often used in scanning electron microscopes, lithography systems for semiconductor and systems that need to be opened. The characteristic properties do not deteriorate despite the lack of vacuum environment.
Finally, there is the pure tungsten wire cathode which operates at 2100°C. If the surface is coated with thorium, this temperature can be lowered by 100 -200°C.
The future trend in X-ray imaging systems is towards higher-throughput, low radiation dose, better image quality, lower false alarm rate and low operating and maintenance costs. Therefore, the X-ray imaging technology will have to deal with the problems of static computer tomography systems, which are equipped with a high number of X-ray emitters. These emitters have to be accurately controlled by a sophisticated and robust multi-channel electronic. A possible configuration is the multi-focus X-ray tube.
Different application areas of a multi-channel electronics are:
Applications are mainly computer tomography equipment in medical technology and X-ray scanners for security-related institutions. Industrial applications are particularly material investigations of critical components (micro-cracks, voids etc.).
In order to operate the emitter cathode in a so-called scanning mode, the control electronics have to be able to control each emitter cathode in its optimum operating point. Moreover, the timing of a predefined sequence has to be ensured. This applies particularly to the controlled release of a required X-ray dose per pulse, which can be adjusted according to system specifications.
The task of the control electronic is the setting of the operating point of each emitter with a high voltage source and a subsequent suitable switching matrix. Because of manufacturing variations, the operating points of the emitter and the required high voltage signal can vary and also serves to control and adjust the pulse on/off time of the channels.
The predefined switching-on/off time is of enormous importance for the application. The main system components are on the one hand, the high voltage source and on the other hand, the microcontroller/FPGA which is responsible for selecting the active channel by controlling the MOSFET switches.
A fundamental problem during the switching-on/off time of the high voltage source is the parasitic effects. This leads to an undesirable overshoot behavior. During this time period the system cannot make any measurement. Therefore the duration of the transient effect has to be minimized by circuitry means.
The background for the gain of a traveling wave tube amplifier is the interaction between an electromagnetic wave and an electron beam. To achieve an amplification of the electromagnetic wave the velocities of the electron beam and the electromagnetic wave have to be approximately equal. That means the electromagnetic wave must be slowed down. The principle of the gain based on the fact that the electron delivers a portion of its kinetic energy and thus increases the electrical signal.
The electrons are first set free by a cathode and accelerated by a high voltage field. A magnetic field prevents the spreading of the electron beam as it passes through the helix. A portion of the kinetic energy of the electron beam is transferred to the electrical signal within the helix when the speed of the electron is slightly higher than the phase velocity of the electrical signal. At the end of the tube, the electrons are slowed down and collected by the collector. Here, the kinetic energy of the electrons is converted into heat losses. In order to minimize the cooling problem and to maximize the overall efficiency, the collector potential should adapt to the potential of the delay line.
Traveling wave tubes (TWT-Amplifiers) are low-noise broadband microwave amplifiers with a large gain. There are usually gain values up to 40 dB at a bandwidth of more than one octave. Traveling wave tubes are built for frequencies from 300 MHz up to frequencies of more than 50 GHz. The traveling wave tube is primarily a voltage amplifier. The high bandwidth and high gain factor make the traveling wave tube to a widely used component in radar technology.
The figure above shows the basic structure of a traveling wave tube. Starting from the cathode the electron beam is focused by the Wehnelt cylinder and an external permanent magnetic field and guided within the helix all the way to the collector. Otherwise the electron beam would expand due to mutual repulsion of the electrons. The electromagnetic wave coupled into the RF-input reaches the wire helix and propagates in the direction of the RF-output. As the electromagnetic wave has to cover a longer path (helix) the higher speed of the wave must be adjusted to the lower velocity of the electron beam so that both move with almost same speed to the RF-output. The wave with its resulting electric field on the helix locally influences the speed of the electrons by partly accelerating and slowing down the electrons (velocity modulation). The velocity modulation is then transformed into a density modulation and forms electron packages which results in amplification of the coherent RF signal due to electric induction.
The figure below shows the longitudinal electric field inside the helix generated by the wave.
The density modulation begins at the beginning of the helix and reaches its highest expression at the end of the helix. The frequency of the density modulation is equal to the frequency of the coupled-wave. The results of the density modulation are electron bunches which affect the wave in return by withdrawing and feeding of energy. Constructional measures ensure that the wave is fed significantly with more energy and so a considerably amplified RF signal is present at the RF-output.
The achievable gain mainly depends on the following factors:
From the picture above it can be observed that there is a linear behavior for small input power and therefore a constant power gain of about 26 dB. Increasing the input power does not increase the output power which means the gain decreases. This limiting effect prevents an overload of the subsequent stage (e.g. mixer stage) if there is a very strong input signal.
Since the amplification effect of the traveling wave tube is based on the interaction between the electron beam and the traveling wave on a delay line, the achievable bandwidth in the first place depends on the frequency response of the helix. A frequency-independent field distribution on a line will only be achieved if this line is operated in a matched mode. Of course this matching can be maintained only over a limited frequency band, but can be over a range of several GHz.
The most important parameter for the use of traveling wave tube amplifiers in radar units is the noise figure. The sensitivity of the entire receiver is determined by the noise figure and thus the range of the radar station. The noise figure of modern traveling wave tubes is about 3 ... 10 dB.
Main causes of noise are:
The noise figure depends on the size of most supply voltages of the traveling wave tube. If the voltages at the electrodes are for example 5% lower than the optimum values, the noise figure will be doubled.
As a matching of the traveling wave tube cannot be ensured over the entire frequency band, this can lead to reflections between the input and output of the traveling wave tube. To reduce the risk of self-excitation (oscillation), the helix is additionally attenuated.
This attenuation is realized either as:
The Ring-Loop TWT uses a special form of delay line and achieves a slightly better performance. Due to its parasitic capacitances, it has a much lower upper limit frequency of only 18 GHz compared to traveling wave tubes with helix.
The gain is about 40…60 dB which is comparable to the Helix-TWT. But the bandwidth is only about 5…15%. In the X-Band 8 kW peak power and about 400 W continuous wave power were generated.
The Ring-Bar TWT has similar characteristics as the Ring-Loop TWT. But the delay line can be manufactured more easily (laser cutting of a tube) and is also more stable against vibrations.
The coupled-cavity traveling wave tube also uses a special form of the delay line. Tuned cavity resonators are used, which allows the passing of the electron beam and also allows the coupling of the electromagnetic energy through the slots.
The RF-path runs through the coupling slots in the resonators in a zig-zag way, and thus constantly crosses the electron beam.
The high quality of the individual resonators results in a substantial increase in performance and a better upper limit frequency. The disadvantage of frequency-dependent resonators is the very low bandwidth.
The application area of a particle accelerator is located mainly in the particle physics and medical technology and is used for acceleration of charged particles, mostly electrons and protons.
Particle accelerators are often related to high-energy physics. About 50% of the worldwide installed systems are used for medical applications (radiation therapy, production of medical radioisotopes, biomedical research).
Particle accelerators play an important role in improving the technical characteristics of medical diagnostics.
The fact is that the use of radio nuclides in sophisticated medical imaging is constantly growing. This is valid both for conventional radiography (CT and MRI) and for SPECT and PET imaging in nuclear medicine.
Example of a particle accelerator application:
The importance of particle accelerators for medical applications is constantly increasing. The synergy between the disciplines of medicine (radio therapy, radiology, nuclear medicine, oncology) and physics (nuclear and accelerator physics) is also increasing and will be a challenge to achieve a better quality of life in the future. Already different studies were accomplished to determine the number of potential patients for the hadron therapy (particle therapy), but with different results. It is estimated that approximately 30% of the patients treated with conventional radiation therapy will benefit from the treatment with particle therapy.
By utilization of the particle accelerator technology the efficiency of the medical imaging will increase significantly. As a result, many diseases are diagnosed and treated at an early stage. This applies particularly to the early detection of tumors.
Food safety is a global problem. This affects hundreds of millions of people suffering from diseases caused by contaminated food. The World Health Organization calls it "one of the most widespread health problems and a major cause of reduced economic productivity".
By e-beam and X-ray irradiation, food is protected from decay over a long period of time. No chemical solution is used.
It was noticed that the irradiation is the only method to preserve food without changing the quality, taste, appearance and texture.
X-rays has long been used because of its radiographic characteristics for non-destructive testing purposes. The primary interest is the determination of the contour of hidden objects. Because these methods are very time consuming and provide little information on the material properties of the objects, only a small portion of cross-border freight shipments was examined.
Latest national security initiatives now aim to have 100% control of cross-border freight shipments at ports, airports and land borders. The new targets are a high throughput, automatic detection, possibility of determining the atomic number of the hidden objects and a much higher false alarm rate. Thus hidden explosives and drugs could be discovered and should allow a distinction between hazardous and nonhazardous materials.
Radiography of thick-walled shut-off valves
The production of heavy and large castings is always associated with an in-house material testing used for the final inspection.
The photo shows a 7,000kg heavy valve casting that was 100% radiographically examined with an 8.5MeV linear accelerator.
Other applications are for example: