Magnetic focusing

In a simple electron microscope, a primary beam of electrons is produced using a conventional electron gun, where a heated cathode, maintained at ground potential, emits electrons which are drawn out by a positive potential (typically 30 kV) to form a high energy electron beam. This beam is easily electrostatically and/or magnetically focused (since electrons are charged particles) to a few microns across, and can be directed to any point on the sample by a series of magnetic lenses. The system must be evacuated to reduce attenuation and scatter of the electron beam. When an electron beam strikes the sample, a number of processes take place (Fig. 5.6 Pollard and Heron 1996 51). 

Fig. 14.13 Experimental arrangement for velocity selecting and focusing the atomic beam. The rotating slotted disc and the pulsed laser beam select atoms in a velocity group, and the hexapole magnet focuses them where they cross the laser beam (from ref. 18).

Figure 1. Schematic representation of the ion optical configuration of commercial magnetically focusing mass spectrometers...

Traveling-wave tubes (frequency range 0.3 to 50 GHz, invented by Kompfner,47 usually part of a traveling-wave amplifier, Fig. 10.13) are low-Q amplifiers for microwaves, with typically a four-decade frequency range the tube is a long vacuum tube, in which the magnet focuses an axial electron beam, while the helix, fed externally by a small microwave beam, acts as a delay line whose electric field bunches the electrons this induces even more electrons to travel along the helix the amplification is as much as 70 dB. 

Many other diatomic molecules with1X ground states have been studied by molecular beam magnetic resonance. Where magnetic nuclei are present, magnetic focusing is based upon the nuclear Zeeman effects. This is the case with 15N2 for which the... 

Lawrence then built an accelerator that could produce one million volts by using an 11-in. magnet to accelerate protons, rather than positively charged hydrogen ions. Livingston perfected the magnetic focusing of the partide beams. 

FIGURE 10 The first free-electron user facility was established at the University of California, Santa Barbara. This device operates in the far infrared and employs an electrostatic accelerator, as shown in the background. A magnetically focused system of electron-beam optics is used to transport the beam from the accelerator through the wiggler (in foreground) and back to the accelerator. (Photo courtesy of Luis Elia.)... 

The injector magnet is 90° dipole magnet. It separates the particular ion from flux of ions coming from the ion-source. The injector magnet focuses the ion beam on to the slit through the beam profile monitor. A slit and a Faraday cup are provided just behind the injector magnet to separate the unwanted ions and to measure the beam flux to be injected into the accelerator. 

Fig. 4.99. Single-stage image intensifier with magnetic focusing...

MS diode gun Plmnbicon A diode gun Plumbicon tube with magnetic focus and electrostatic deflection. 

Busch s theorem Angular velocity of an electron in a magnetically focused electron beam is proportional to the difference of magnetic flux at the electron beam cross section, where the electron first encounters the focusing magnetic field, and at the cross section that includes the observation point. It is inversely proportional to the beam cross-sectional area at the observation point. The proportionaHty constant is 0.879 x 10" C/kg. 

Once ions of the sample have been formed by one of the methods above, they are separated and detected by the mass analyzer component of the spectrometer. Several common ways exist to accomplish mass analysis. We shall first describe the classic method of magnetic focusing, and then briefly mention several other important approaches. 

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