Thermionic cathodes

Each cavity contributes ca 7 MeV to the injected electrons. The electron beam ejected from thermionic cathode in the electron gun, pass thru injector lens into accelerator cavities and from there is focused upon a 3-mm diam aperture in a thick Be collimator. After emerging from it, the beam passes thru a 0.5 mm thick Be window and falls on a 1-mm thick tungsten target, which is located ca 10 meters outside of reinforced concrete housing, which provides protection for PHERMEX from blast or shrapnel... 

On the Osaka University thermionic cathode L-band linac, a time resolution of two picoseconds was achieved using magnetic pulse compression and time jitter compensation systems (Fig. 13). The time jitter between the Cerenkov light from the electron beam and the laser pulse was measured shot-by-shot with a femtosecond streak camera to accurately determine the relative time of each measurement in the kinetic trace. In this way, the time jitter that would otherwise degrade the time resolution was corrected, and the remaining factor dominating the rise time was the electron-light velocity difference over the 2-mm sample depth. 

Trains of -120 fs FWHM electron pulses have even been generated by an electron gun with a (continuous) thermionic cathode by using an alpha magnet and the excellent phase space control of electron guns to compress the accelerated bunches see Kung, P. Lihn, H. Wiedemann, H. Bocek, D. Phys Rev. Lett. 1994, 73, 967-970. 

As mentioned above, a 3.5-cell RF photocathode gun is in operation as the accelerator for the Brookhaven National Laboratory Laser-Electron Accelerator Facility. Recently, 1.6-cell RF photocathode guns have replaced thermionic cathode systems as injectors for 30 MeV linear accelerators at Osaka University and the Nuclear Engineering Research Laboratory in Tokai-mura, Japan [6]. Another RF photocathode gun accelerator is under construction at the ELYSE facility at the Universite de Paris-Sud at Orsay, France. A magnesium cathode is in use at LEAF, copper is used at NERL, while the Orsay accelerator will use Cs Te. 

Sommerfeld Formula for Thermionic Emission. Using the Sommerfeld formula (2-100), calculate the saturation current densities of thermionic emission for a tungsten cathode at 2500 K. Compare the calculated value of cathode current density with a typical value for the hot thermionic cathodes presented in Table 2-12. 

Electron beams are generated by applying an accelerating voltage (150 kV) to a thermionic cathode. An electron beam (ca. 6 mm diameter) is spread out into a curtain beam by a beam splitter. The electrons leave the beam distribution housing through a very thin metal sheet. When these electron beams strike binder monomers, they initiate polymerization in the paint film. Polymerization occurs in a fraction of a second and must be performed in a vacuum or in an inert gas atmosphere. The equipment must be screened to protect the operators. 

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. 

In NEA Si cold cathode devices, a planar diode is biased to inject electrons from an n-type substrate to a p-type NEA surface layer, where current is drawn off by an electric field. (Other cold cathodes of different design have been demonstrated in III-V structures [5.130-132].) This efficient emission could be useful in replacing hot (thermionic) cathodes in IR sensitive devices where a low luminence source with small electron energy spread is desirable, but requirements of zero contamination along with electron emission-density restrictions and long-term instability problems have so far prevented practical use. 

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... 

Electron gun An electronic structure that contains a thermionic cathode, an accelerator, and a focusing electrode to emit an electron beam. 

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 ... 

LaBe and CeBe are well known for being excellent field emitters, and have actually been commercialized as thermionic cathode materials. With low work functions around 2.6 eV, they can provide greater brightness and lower operation temperatures (longer service life) than tungsten cathodes, for example. A simple method to grow high-quality physical vapour deposition (PVD) films of CeBe was recently reported. ... 

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