Emitter Temperature

In practice, moderate heating of the emitter at constant current serves to reduce adsorption to its surface during FI measurements. Heating at a constant rate (1-8 mA min ) is frequently employed to enforce desorption of analytes from the emitter in FD-MS. To avoid electric discharges resulting from too massive ion de- 

Emitters can be heated by applying an electric current via the emitter holders. It is somewhat difficult to establish a precise calibration of emitter temperature versus emitter heating current (EHC) [56,57]. The actual temperature not only depends on the emitter material, but also on diameter and length of the emitter as well as on length and area density of the whiskers. A useful estimate for tungsten emitters with carbon whiskers is given below (Fig. 8.7). 

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According to KirehhofPs law, the emissivity and absorptivity of a surface in surroundings at its own temperature are the same for both monochromatic and total radiation. When the temperatures of the surface and its surroundings differ, the total emissivity and absorptivity of the surface often are found to be different, but, because absorptivity is substantially independent of irradiation density, the monochromatic emissivity and absorptivity of surfaces are for all practical purposes the same. The difference between total emissivity and absorptivity depends on the variation, with wavelength, of Zx and on the difference between the emitter temperature and the effective source temperature. 

Ktimmler, D. Schulten, H.-R. Correlation Between Emitter Heating Current and Emitter Temperature in Field Desorption Mass Spectrometry. Org. Mass Spectrom. 1975,10, 813-816. 

Gross, J.H. Weidner, S.M. Inflnence of Electric Field Strength and Emitter Temperature on Dehydrogenation and C-C Cleavage in Field Desorption Mass Spectrometry of Polyethylene Oligomers. Eur. J. Mass Spectrom. 2000, 6, 11-17. 

The spectrum of sphingomyelin (Figure 11) taken at higher emitter temperatures (25-28 ma) shows a transition from the pattern described in Scheme A to one dominated by ions at m/is 834 and 916 and 918. These correspond to addition of cholTne (mass 104) to the major molecular species CigiO (MW 730), C24 1 (MW 812) and C24 0 (MW 814). This assignment of (M + choline)+ has been confirmed by determination of the exact mass of the m/e 832 ion as m/e 834.7106 (calculated for C46Hg707N3P 834.7060J. 

It has been found in a number of studies that changes in emitter temperature significantly affect the rates of decomposition observed... 

The dependence of rates of ionic decomposition upon emitter temperature can be attributed to changes in the internal energy of the molecular ion, i.e. vibrational excitation is temperature dependent. The temperature of the sample molecules is governed, to some extent, by the emitter temperature (vide infra) and increasing the vibrational energy of the neutral molecule results in increased vibrational energy within the molecular ion [519]. 

In the above studies [41, 490, 491, 519], the emitter has been heated by passing an electric current through it. The source and sample inlet temperatures were not necessarily the same as that of the emitter. It has been shown that under these circumstances the sample molecules may not accommodate fully to the emitter temperature prior to ionization [122]. The broad significance of this suggestion is that the studies [41, 490, 491, 519] would have underestimated the effects of temperature upon rates of decompositions. 

The total absorptance exhibits a decrease with an increase in temperature of the radiation source similar to the decrease in emittance with an increase in the emitter temperature. 

Assuming no space charge, electron scattering between the electrodes, or back emission from the collector, the current flow from the emitter to the collector J, will be determined by the emitter temperature, Tg, the potential barrier the electrons must overcome, (f), and the Rlchardson-Dushman equation ... 

Values are also specified for the emitter temperature Tg, the collector temperature Tg, the reservoir temperature Tg, the cesiated emitter work function ( )g, the collector work function ( ) Q, and the interelectrode spacing d. 

Calculated Power and Efficiency. The simplified analytical models of thermionic characteristics have been used to project the converter efficiency and power density with the barrier index as a parameter. These projections are shown in Figures 8 and 9 as functions of the emitter temperature. The dashed lines in these two figures are for a constant current density of 10 A/cm. If the current density is adjusted to maximize the efficiency at each temperature, the calculated performance is represented by the solid lines. Typical present generation themionlc converters operate with Vg near 2.0. Ignited mode converters in laboratory experiments have demonstrated practical operation with 1.85 < Vg < 1.90. Other laboratory devices with auxiliary sources of ions and/or special electrode surfaces have achieved Vj < 1.5, but usually not under practical operating conditions. 

The record holder for converter life is LC-9, a converter built for NASA by General Atomic as part of the in-core nuclear space reactor program. LC-9 operated with perfectly stable performance for over five years at an emitter temperature of 1970 K. As shown in Figure 12 ( ), LC-9 had an electrode efficiency of 17%, and generated 8 W/cm of output power (80 KW /m ). The converter was still performing stably when tests were terminated for programmatic reasons. This test illustrates well the long life capability of the thermionic converter process. 

Recent converter hardware development has concentrated on flame heated devices for terrestrial topping cycle applications. Significant progress has been made with chemical vapor deposited silicon carbide as the oxidation protection hot shell. Flame heated converters have now been operated by Thermo Electron Corporation with emitter temperatures over 1700 K for 7000+ hours ( ). 

In thermophotovoltaic batteries the heat emitted by the radionuclides is converted to electric energy by means of infrared-sensitive photoelements (e.g. Ge diodes), which must be cooled effectively because the efficiency decreases drastically as the temperature rises. With respect to high emitter temperatures, thermophotovoltaic conversion is of interest for power levels between about 10 W and 1 kW, but the efficiency is relatively low (up to about 5%). 

Parametric studies have shown that, in order to achieve high efficiencies and power densities at realistic spacings and emitter temperatures, it is necessary to have a high emitter bare work function 5>gQ at or above 5.2eV and a barrier index Vg at or below 2 eV. 

Emitter temperature Vmax I Voltage at the maximum output power Collector temperature Jmax I Current at the maximum output power Reservoir temperature Pmax I Maximum output power... 

The output current operated by the unignited mode increases by 10 50 times due to the illumination for the emitter temperature Tg less than 900 K and Tg from 300 to 450 K. But the difference of the output currents with and without illumination decreases as Te increases. 

As is well known, the thermionic electron emission from the tungsten hot plate in the cesium gas has an excellent nature that the thermionic electron current from the tungsten hot plate becomes large at the lower hot plate temperature than 1000 K, although it depends on the hot plate temperature and cesium gas pressure. This is because the hot plate is covered by cesium thin layer, which reduces the work function of the hot plate to around 2.0 eV at low hot plate temperature. This nature seems to be very attractive from the view point of the development of thermionic energy converter because it enables to the operation of thermionic energy converter at the low emitter temperature. 

However, the operation of low emitter temperature causes the large negative space potential near the emitter since the number of cesium ion near the emitter produced by... 

The characteristics operated by the ignited mode are observed at the higher emitter and cesium gas temperature, as shown in Fig.6(a) to 6(d). The output voltage needed to induce the ignited mode operation depends on the illumination, in addition to the emitter and cesium gas temperatures. It should be noted that the ignited mode appears even in the positive output voltage at the emitter temperature of 1280 K and cesium gas temperature of 443 K as shown in Fig.7. It is found that the output current becomes sensitive to the external magnetic field under this condition. 

Figure 5. The relation between the emitter temperature and the short circuit current with and without illumination and external magnetic field.

The increase of output current in the unignited mode operation due to the illumination can be explained by the impovement of the space charge neutrality a. As mentioned above, a is usually smaller than unity at the low emitter temperature so that most electrons emitted from the emitter cannot reach the collector. However, the illumination creates so many electrons and ions by photoionization that a approaches to unity [2]. Though the remarkable effect of illumination on the output current is observed at the operation of the low emitter temperature, the increase of a does not contribute to the increase of the output current at the higher emitter tern-... 

As for the ignited mode operation, the increased electrons and ions in the space between both electrodes contibute to induce the breakdown because the probability of collisional ionization is proportional to the electron and cesium atom density. Therefore, the larger output current will be obtained when the ignited mode operation will take place by increasing the cesium gas pressure and emitter temperature and by illuminating the intense light on the thermionic energy converter. 

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