Wehnelt electrode

The general structure of an electron gun is composed of three main parts cathode or electron source, Wehnelt electrode and anode, as illustrated in Figure 3.2. Electrons are emitted from the surface of the cathode and accelerated by an electric field toward the cathode. The Wehnelt electrode is placed between the cathode and the anode. It is biased a few hundred volts negative with respect to the cathode in order to stabilize the electron beam against voltage fluctuation by... 

The electron chamber has a demountable triode electron gun with a very compact metal-ceramic insulator profile that can support voltages up to 100 kV. Cathode replacement is a routine bench operation with adjustments for concentricity and axial spacing relative to the Wehnelt electrode for optimum AVcw The electron gun is mounted on a three-axis traverse for alignment with the first anode and for adjustment in the extraction gap to provide optimum AV a over the energy range 0 to 100 to keV. 

Thermionic cathodes consist of a directly heated tungsten hairpin cathode at = 2500 -3000 K, or an indirectly heated pointed rod of lanthanum or cerium hexaboride (LaB, CeB(,) at 1400 - 2000 K. The electrons must overcome the work function of 4.5 eV (W) or 2.7 eV (LaBfe) by thermal activation (Fig. 78, curve a). Between the cathode at the potential -V and the grounded anode, a negatively biased Wehnelt electrode forms a crossover of diameter 20-50 pm (W) or 10-20 pm (LaBe) as an effective electron source. The emitted electrons show an energy spread A = 1 - 2 eV (W) or 0.5- 1 eV (LaBft). A measure of the quality of an electron gun is the axial gun brightness [i ... 

The electron gun consists of a spiral-shaped tungsten cathode and a Wehnelt cylinder. These two components not only constitute the electrodes of the acceleration gap, but also form the optical assembly to control and shape the electron beam. Current signals are linear and have a repetition frequency... 

The electron gun consists of a spiral-shaped tungsten cathode and a Wehnelt cylinder. These two components not only constitute the electrodes of the acceleration gap, but also form the optical assembly to control and shape the electron beam. Current signals are linear and have repetition frequency about 800 Hz. They are used to deflect the electron beam horizontally and vertically over the exit window plane. The scanner can be equipped by two cathodes for maximum output. Then, the width of the exit window is more than double that of a standard unit with a single cathode. The exit window containing the 12-15 prn-thick titanium foil is relatively large to assure an effective cooling of the foil. 

Figure 2.2 Experimental apparatus used by Wehnelt for his studies on electrochemical discharges [119]. After a first series of experiments (left), he improved the set-up by enclosing the active electrode c in a glass tube d (right). 

Figure 2.4 The Wehnelt interrupter commercialised in 1899 by the German company Ferdinand Ernecke (left). The length of the platinum electrode e could be adjusted. The electrical contact was made with the help of the cooper rod / [119]. On the right is shown a model commercialised in France by Armagnat-Carpentier [10].

Once the device is connected to a constant voltage U, the current in the Wehnelt interrupter grows (the switch in the equivalent circuit is closed). An increasing volume of gas is produced until a gas film is built, isolating the active electrode from the electrolyte. The switch in the equivalent circuit is now open. The unstable gas film is quickly removed from the active electrode and the process starts again. From the equivalent circuit, the evolution of the current is given by ... 

Equation (2.3) is able to explain the operation of the Wehnelt interrupter. The number of interruptions is inversely proportional to the inductance L. As Rk= R + Rv and Rvactive electrode length (cylindrical geometry), it follows that n will increase inversely proportional to l, as first experimentally observed by Ludewig [83]. 

According to this model, interruption numbers n virtually as large as desired can be obtained with the Wehnelt interrupter. This is obviously not correct. In reality, for very short electrical time constants r, the time needed to build up the gas film is limited by the time needed for gas bubbles to grow on the active electrode surface. Relation (2.3) will overestimate n for small r. As explained later, the typical growth times for gas bubbles are a few milliseconds. This timescale gives the limit of validity of Equation (2.3). The proportionality between the electrical time constant r and the interruption period T, as well as the importance of the gas film formation time, was first recognised by Compton [26] in his master s thesis in 1910. 

The cathode emits electrons that are accelerated towards the anode with a defined voltage, typically 50-30,000 V. There are basically two types of electrodes thermionic cathodes (tungsten or LaBs (lanthanum hexaboride)) and field emission cathodes. The Wehnelt cylinder controls the current density and brightness of the electron beam. Brightness is defined as current per unit area normal to the given direction, per unit solid angle, and a criterion for beam quality. 

The size, shape, and location of the electrodes, namely the Wehnelt (grid) and the first anode (extractor), relate to the diameter of the Wehnelt aperture, the diameter of the first anode aperture, and to the spacing between them. The brightness of the electron source is also determined by the ratio dy h, where h is the... 

An electron gun is placed on top of this column. This gun usually consists of a thermionic cathode made of tungsten or LaBg in a triode configuration, i.e., an additional electrode (Wehnelt) between the filament and anode. The pressure in the specimen chamber is 10 -10 Pa. This pressure is much... 

The simplest and still the most widely used electron gun is the triode gun, consisting of a heated filament or cathode, an anode held at a high positive potential relative to the cathode, and, between the two, a control electrode known as the wehnelt. The latter is held at a small negative potential relative to the cathode and serves to define the area of the cathode from which electrons are emitted. The electrons converge to a waist, known as the crossover, which is frequently within the gun itself (Fig. 9). If jc is the current density at the center of this crossover and 0 s is the angular spread (defined in Fig. 9), then... 

As described in Sect. 12.2, the electron beam in electronic microscopy is generated in a cathode, heated by electric current. As shown, the electron beam is accelerated by an electrode system, the electron gun. The three cannon components are the filament (cathode), the cylinder Wehnelt, and the anode. 

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