RF electric field

For an RF electric field sensor, bandwidth (speed) is another important requirement. The speed of a resonator is determined by the cavity ring down time r = QX/c. As in any devices using resonating structures, a trade off between sensitivity (which increases with Q) and speed (which decreases when Q increases) has to be made. At the wavelength of 1.55 pm, Q factors of 104 and 106 translates to a sensor bandwidths of 20 GHz and 200 MHz, respectively. 

Fig. 2.17 The output of the RF electric field sensor at 550 MHz and 10 dBm RF input. Reprinted from Ref. 15 with permission. 2008 Institute of Electrical and Electronics Engineers...

Fig. 4.49. Motion of positive ions in a uniform magnetic field B. (a) The radius is a function of ion velocity, but the frequencyof circulation is not. (b) Excitation of the ions by an RF electric field oscillating at their cyclotron resonance frequency. Adapted from Ref. [196] by permission. John Wiley Sons, 1986.

A second spectrum is then taken for which an rf electric field at the frequency appropriate for the mass of the reaction intermediate is applied continuously throughout the same reaction delay as that employed in the first spectrum, taken without this rf electric field. 

Other types of accelerators use various forms of rf electric fields, at relatively low voltage, which are applied many times in a given direction... 

In a quadrupole device, not as accurate and precise as double-focusing instruments but fast, a quadrupolar electrical field comprising radio-frequency (RF) and direct-current components is used to separate ions. Quadrupole instruments as mass analyzers are used together with ESI as the ion source the configuration employing a three-dimensional quadrupolar RF electric field (Wolfgang Paul, University of Bonn, 1989 Nobel prize for physics) is referred to as an ion trap analyzer (see below). 

Thus, by appropriate scaling of the rf electric field magnitude at the ICR frequencies of ions of various m/z-ratios, we can suppress, excite and then detect, or eject ions at each m/z -ratio. 

Figure 1. Fundamentals of ICR excitation. The applied magnetic field direction is perpendicular to the page, and a sinusoidally oscillating radiofrequency electric field is applied to two opposed plates (see upper diagrams). Ions with cyclotron frequency equal to ("resonant" with) that of the applied rf electric field will be excited spirally outward (top right), whereas "off-resonant" ions of other mass-to-charge ratio (and thus other cyclotron frequencies) are excited non-coherently and are left with almost no net displacement after many cycles (top left). After the excitation period (lower diagrams), the final ICR orbital radius is proportional to the amplitude of the rf electric field during the excitation period, to leave ions undetected (A), excited to a detectable orbital radius (B), or ejected (C).

The specific carrier-wave amplitudes (field intensities) which have been found to be effective in producing Ca ion efflux are discussed in terms of tissue properties and relevant mechanisms. The brain tissue is hypothesized to be electrically nonlinear at specific field intensities this nonlinearity demodulates the carrier and releases a 16 Hz signal within ljie tissue. The 16 Hz signal is selectively coupled to the Ca ions by some mechanism, perhaps a dipolar-typ +(Maxwell-Wagner) relaxation, which enhances the efflux of Ca ions. The hypothesis that brain tissue exhibits a slight nonlinearity for certain values of applied RF electric field intensity is not testable by conventional measurements of e because changes... 

The signals are defined as the fractional decrease in the Lyman-o photocurrent N produced by a fixed amplitude rf electric field in the SOF region through the... 

Power monitoring diodes were used to set the rf power so the rf electric field did not vary as a function of frequency. The diodes were calibrated using a Hewlett Packard 432A Power Meter with a 8478B thermistor power sensor and calibrated attenuators. The power measurement was used in conjunction with slotted line studies of the rf interaction regions to determine the rf electric field at each frequency. 

The method of symmetric points was used to determine the center of the interference curve. Extensive calculations showed that the line profile should be symmetric about the center frequency. The line center was then corrected for the second order Doppler shift, The Bloch-Siegert and rf Stark shifts, coupling between the rf plates, the residual F=1 hyperfine component, and distortion due to off axis electric fields. A small residual asymmetry in the average quench curve was attributed to a residual variation of the rf electric field across the line and corrected for on the assumption this was the correct explanation. Table 1 shows the measured interval and the corrections for one of the 8 data sets used to determine the final result. 

Thompson, B.E. and Sawin, H.H. (1986) Comparison of Measured and Calculated S F, Breakdown in rf Electric-Fields. 

The modification of cotton cellulose by treatment with low-temperature, low-pressure ammonia plasma created by passing ammonia gas through a radiofrequency (rf) electric field of 13.56 MHz has been reported (1). Earlier reports (2,3,4) were on the effects of rf plasma of argon, nitrogen or air on a group of polysaccharides that included cotton and purified cellulose. 

A second possible cause of spurious ringing which is limited only to some solids is piezoelectric resonance of the sample. (Of course the cause need not be limited to a sample. More on this later.) A piezoelectric crystal has the property that a mechanical deformation of the material takes place in the presence of an electric field. Therefore, an acoustic ringing can be induced in a piezoelectric material located within an NMR probe by the rf electric field associated with the rf magnetic field. Such a ringing due to piezoelectricity has the distinguishing property that it is independent of the applied magnetic field intensity in contrast with the magnetic field induced effect described in the first part of this section. 

The RF quadrupole ion trap mass spectrometer (ITMS) is a close relative of the QMF and ideally can be thought of as a three-dimensional quadrupole (see Fig. 17.8). The close relationship of these two devices is evident by the fact that ion motion in the two devices is governed by essentially the same mathematical equations. As with the QMF, the ITMS uses DC and RF electric fields and the operation of the IT is described by solutions to the Mathieu equation. Unlike the QMF, ITMS analyzers trap ions within the mass analyzer. Ions are trapped, ejected to select the mass of interest, and then ejected in a controlled manner for detection. 

The first idea was to build a trap from the linear quadrupole mass filter structure but, rapidly, the properties of multi-pole potentials were exploited also. For a quadrupole linear trap, the RF electric field [19] is transverse to the z-axis of the ion trap near this axis, the time potential, ( ), in the x- and y-directions can be expressed by... 

Theoretically, in a pure quadrupole ion trap, the RF electric field increases linearly in radial and axial directions. The axial and radial direction motions of ions are decoupled in such a pure quadrupole ion trap. In reality, it is impossible to create a pure quadrupole ion trap due to the finite sizes of the trap electrodes and orifices in the electrodes. 

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