Thermionic diodes

With thermionic diodes sensitive and accurate measurements of atomic and molecular Rydberg levels have been performed [6.72-74]. With a special arrangement of the electrodes a nearly field-free excitation zone can be realized which allows the measurement of Rydberg states up to the principal quantum numbers n = 300 [6.74] without noticeable Stark shifts. 

A more detailed representation of ionization spectroscopy and its various applications to sensitive detection of atoms and molecules can be found in [6.58-60,64-66]. 

The current magnification factor M = A/ion/Atei can reach values of up to A/ = 10.  

Sensitive and accurate measurements of atomic and molecular Rydberg levels have been performed [112-114] with thermionic diodes. With a special arrangement 

1 Doppler-Limited Absorption and Fluorescence Spectroscopy with Lasers 

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Fig. 12.1 Thermionic diode and cw dye laser used by Weber and Niemax to measure Cs self...

Figure 3. Adsorption cell showing details of the thermionic diode employed to detect photoionization (5).

Thermionic converters are high temperature devices which utilize electron emission and collection with two electrodes at different temperatures to convert heat into electric power directly with no moving parts. Most thermionic converters operate with a plasma of positive ions in the interelectrode space to neutralize space charge and permit electron current flow. Both the plasma characteristics and the surface properties of the electrodes are controlled by the use of cesium vapor in thermionic diodes. 

Figure 1. Scheme of thermionic diode for directly converting heat into electricity. 

A simple analytical model of thermionic converter performance must be made before the impact of converter performance on system behavior can be studied. Fortunately, a very simple model of converter performance has been found to be sufficiently accurate for this purpose. The ideal thermionic diode serves as the basis for this model. Motive diagrams and converter current voltage characteristics for an ideal diode are shown in Figure 2. 

In the so-called thermionic diode, a heated collector electrode is simply brought into the vicinity of the laser beam and isolated towards the vessel walls. No extraction voltage is applied and the ion current flowing as a result of the decreasing negative space charge around the collector, as a result of the ionization by the laser,... 

Niemax K. (1985) Spectroscopy using thermionic diode detectors, Appl Phys, Part B 38 147-157. 

Fig. 2.7. Very high Rydberg states of the Ba atom recorded by laser spectroscopy using thermionic diode detection (after J.-P. Connerade et at. [27]).

By virtue of their enormous size and comparatively high stability against radiative decay, high Rydberg states are very susceptible to collisions. Indeed, some of the detectors which are used to observe them, such as the thermionic diode (see section 8.16) depend for their operation on the presence of collisions. Collisions between excited species affect ionisation and recombination rates in such diverse environments as gaseous nebulae [41], laboratory plasmas [42] and flames [43] their study is therefore of some considerable intrinsic interest. 

Instead of measuring the attenuation of a beam, one may also count the ions produced with very high efficiency by the use of channelplates or a hot-wire detector [387], an approach which has mainly been applied in laser spectroscopy, where high sensitivity can be achieved by space charge amplification. The principle of the thermionic diode is that the atomic vapour under study is formed within the detector, and a current limited by the space charge is obtained by appropriately biasing a diode, consisting of an external anode (often the outer wall of the vacuum system, formed by a metal tube) and a heated cathode made of a suitable material to emit many electrons (thoriated W is suitable in many cases). A sketch of... 

Fig. 8.2. Diagram of a typical thermionic diode arrangement to observe interacting autoionising resonances (after W.G. Kaenders et al. [389]).

Although most suitable for use with lasers, Thermionic diodes have also been successfully applied to synchrotron radiation studies by using wiggler magnets to enhance the intensity of the beam [390]. Last but not least, one should mention the important category of atomic beam experiments, complemented by the techniques of photoelectron and photoion spectroscopy. All these techniques are suitable for the experimental study of interacting resonances. We turn now to their theoretical description, which will be illustrated by experimental examples. 

FIGURE 11 Uranium oxide-fueled thermionic diode. 

Fig. 1.41 Thermionic diode (a) level scheme (b) schematic arrangement and (c) field-free excitation scheme, where the laser beam passes through the field-free central region in a symmetric arrangement of cathode wires...

The spectroscopic methods described above have the disadvantage that not all excited atoms contribute to the analytical signal at the same time, but only those of a confined velocity group. This degrades the limit of detection however, this can then be improved if efficient methods for ion detection are applied, such as LEIS combined with the thermionic diode. In the thermionic diode the ions are detected by means of the diode current, which is induced by charge effects with internal amplification by a factor of 10 -10. ... 

If the discharge cell has windows of optical quality, it can be placed inside the laser resonator to take advantage of the -fold laser intensity (Sect. 6.2.2). With such an intracavity arrangement. Doppler-free saturation spectroscopy can also be performed with the optogalvanic technique (Sect. 7.2 and [6.101]). An increased sensitivity can be achieved by optogalvanic spectroscopy in thermionic diodes under space-charge-limited conditions (Sect. 6.4.5). Here... 

Thermionic Diode Vacuum tube that permits current to flow in only one direction, also called a vacuum tube diode. 

As the electronics and the radio communication industries developed, it became apparent that there would be a need for human-made diodes to replace the natural crystals that were used in a trial-and-error manner. Two development paths were followed solid-state diodes and vacuum tube diodes. By the middle of the twentieth century, inexpensive germanium-based diodes had been developed as solid-state devices. The problem with solid-state diodes was that they lacked the ability to handle large currents, so for high-current applications, vacuum tube diodes, or thermionic diodes, were developed. In the twenty-first century, most diodes are semiconductor devices, with thermionic diodes existing only for the rare very high-power applications. 

Solid-State Diodes. Thermionic diodes, or vacuum tube diodes, tend to be large and consume a lot of electricity. However, paralleling the development of... 

Thermionic diodes are used in only a few industries, such as the power industry and radio and television broadcasting, where the extremely high-power applications would burn out semiconductor diodes. Semiconductor diodes are used in all other applications, since they are more efficient to operate and less expensive to produce. 

Thermionic Diode Grid Tubes Vacuum Tube Specifications Biasing Methods to Establish a DC Operating Point Power Amphfier Classes Power-Output Calculation... 

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