Pole piece

An example of a serial-recording EEL spectrometer is shown in Eig. 2.33 it features a magnetic prism system which was constructed for a TEM/STEM of the type JEOL JEM lOOS [2.199, 2.200]. Its second-order aberrations are corrected by curved pole-piece boundaries, an additional field clamp, and two extra hexapoles acting as stig-mators. The electron beam can be adjusted relative to the optical axis by use of several deflection coils. A magnetic round lens is positioned just in front of the prism to... 

Pol-papier, -reagenipapler, n. Elec.) pole paper, -platte, /, Elec.) pole plate, pole piece. -Starke,/, pole strength. 

The coil (wound on a light metal former) can be suspended by a fine strip of phosphor bronze between the pole pieces. Attached to this suspension is a small mirror which reflects on to a scale a beam of light which is focused upon it. An instrument of this kind is known as a D Arsonval galvanometer and is used in potentiometer circuits and various methods of measurement of resistance. 

For electron spin resonance (ESR) measurements, the sample is placed in a resonant microwave cavity between the pole pieces of an electromagnet. The magnetic field is gradually increased, which induces a Zeeman splitting of the excila-... 

A four-ounce Alnico permanent magnet was mounted in a brass yoke. Two wedge-shaped pole pieces of soft iron, each with a slot sawed part way through it, were also attached, and the dumbbell assembly was inserted with the ends of the silica fiber in the slots and the spheres of the dumbbell between the pole pieces, one sphere being a little in front and the other a little behind the plane of the pole pieces, as shown in Figure 1. 

Fig. 3.—Diagram of instrument, showing dumbbell assembly mounted between pole pieces of two small permanent magnets, source of light, and scale.

The permanent magnets are now made of Alnico V, which has a higher retentiv-ity than Alnico II, which was first used. It has been customary to use a pair of magnets weighing five ounces each, arranged as shown in Fig. 3. Pole pieces of various designs have been found to be satisfactory examples are illustrated in Figs. 3, 4, and 5. 

Fig. 6.—Large magnet with test chamber enclosing pole pieces and dumbbell assembly windowed cover and gasket have been removed from test chamber.

Fig. 2.3.1 A schematic diagram of GARField magnet pole pieces and the field pattern they produce together with a magnified sketch of the sample and sensor mounting showing the relative field, gradient and profile [/(r)] orientations.

These systems work by placing a sample between the pole pieces of a magnet (electromagnet or permanent), surrounded by a coil of wire. Radio frequency (r.f.) is fed into the wire at a swept set of frequencies. Alternatively, the magnet may have extra coils built onto the pole pieces which can be used to sweep the field with a fixed r.f. When the combination of field and frequency match the resonant frequency of each nucleus r.f. is emitted and captured by a receiver coil perpendicular to the transmitter... 

The complications of windows can be avoided by substituting small apertures above and below the sample to restrict the diffusion of gas molecules while allowing penetration of the electron beam. Typically, pairs of apertures are added above and below the sample, with differential pumping lines attached between them. In the early in situ experimentation, an ECELL system (69) could be inserted inside the EM column vacuum between the objective lens pole pieces. 

The gas reaction chamber and the objective aperture assembly occupied the gap between the upper and lower objective pole pieces, leading to a gas reservoir around the sample. Such ECELL systems were a major step forward in scientific capability, being used by Gai et al. (3,73-78), Doole et al. (79), Crozier et al. (80), and Goringe et al. (81) to characterize catalysis. Other developments for catalytic studies include an ex situ reaction chamber attached externally to the column of a TEM, for example, by Parkinson and White (82) and Colloso-Davila et al. (83). Reactions were carried out in the ex situ chamber (and not in situ), and the sample was cooled to room temperature and inserted into the column of the TEM (without exposure to the atmosphere) under vacuum. Baker et al. (84) used ETEM at gas pressures of a few mbar with limited resolution, and, in these experiments, representative higher gas pressures were not employed. 

Fig. 14. Schematic of the basic geometry of the aperture system and objective lens pole pieces incorporating radial holes for differential pumping system in the novel atomic resolution-ETEM design of Gai and Boyes (85-90) to probe catalysis at the atomic level.

Fig. 4.20. Flight tube passing through the gap of the magnetic sector of a JEOL JMS-700 instrument seen from the ESA side. The shapes of the pole pieces of the yoke and the additional blocks around the tube are designed to minimize fringing fields. In addition the pole faces are rotated to increase the mass range.

Another problem is due to fringing fields. To reduce these defocusing effects at the entrance and exit of the field, the pole pieces of the yoke of the electromagnet generally have specially shaped edges (Fig. 4.20). 

Compounds crystallized directly onto the carbon grid or with a defined orientation, due to other preparation methods, normally exhibit a suitable initial zone close to 0°. Samples from insoluble compounds are almost statistically oriented only biased by the particle shape. In this case, it is difficult to find a single crystalline part of appropriate thickness oriented with a suitable zone parallel to the surface. The best flexibility, and therefore the best possibility to orient a zone correctly, is given by a recently developed rotation-double tilt holder (Gatan Inc.). Through the combination of rotation and additional tilt (beta tilt) it is possible to orient the tilt axis exactly even if the crystal is not sitting flat on the support film (see Fig. 4). The tilt range, dependent on the pole piece distance of the objective lens, should be at least 40°. 

Such a cap, manufd by the Germans during WWII, is described in Vol 5, p E55-L, including its cut. It contains a primer mixt of LA LDNResorcinate, a pole piece, insulating cap, Pb/Sn foil washer (attached by shellac to an insulating material washer) and a contact ring. The cap required 120 to 160 volts to fire it. 

Forced sinusoidal nonresonant shear directly applied by pole pieces of electromagnet to sample disk Forced sinusoidal shear strain imposed by mechanical drive of clamped annular plate of propellant... 

By using non-normal beam incidence and curved pole pieces, the effective radius of curvature of the magnetic sector field is extended, which results in an increase in dispersion and an improvement in the abundance sensitivity of the mass spectrometric system.4... 

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