Slow-wave Region Research Articles

Nonlinear theory of a folded waveguide gyrotron traveling-wave tube amplifier.

We have developed a three-dimensional nonlinear theory for broadband gyrotron traveling-wave amplification in an H-plane-bend folded waveguide. The H-plane-bend configuration is formed by folding a rectangular waveguide so that the orientation of the magnetic field changes along the bend in contrast to the conventional E-plane bend, where the orientation of the electric field changes. Calculations show the feasibility of amplifier operation with large instantaneous bandwidth and moderate efficiency both in the slow wave region and the reduced dispersion fast wave region. Numerical results for a cold beam show an efficiency of 26% with 11% bandwidth in the slow wave region and an efficiency of 47% with 10% bandwidth in the fast wave region. For an electron beam with 2% velocity spread and 2% guiding center spread, the slow wave efficiency drops to 6% but the bandwidth increases to 16% whereas the efficiency and bandwidth of the fast waves decrease by a small amount to 42% and 9.5%, respectively. The efficiency is more sensitive to the axial beam velocity spread in the slow wave region due to larger Doppler shift. (c) 1995 The American Physical Society

Event-related potential and clinical correlates of neurodysfunction in obsessive-compulsive disorder

The current study confirmed with increased sample sizes our preliminary findings of event-related potential (ERP) abnormalities in obsessive-compulsive disorder (OCD) (Towey et al., 1990) and examined their relationship to symptom severity, treatment response, and neurological soft signs. Unmedicated patients ( n - 17) showed larger negativities in N200 and slow wave regions than normal control subjects ( n - 16) to correctly detected auditory “oddball” stimuli. N200 amplitude was larger over left than right hemispheres of OCD patients, but not normal control subjects. Greater N200 amplitude correlated with less severe obsessions, better response to subsequent treatment with serotonin reuptake blockers, and fewer neurological soft signs in OCD. With increased task difficulty, N200 and P300 latencies lengthened for normal subjects, but not for OCD patients. The abnormal ERP pattern in OCD supports hypotheses of cortical hyperarousal and overfocused attention.

Nonlinear theory of the slow-wave cyclotron amplifier.

We have developed a three-dimensional nonlinear theory for the slow-wave cyclotron instability in a dielectrically loaded waveguide. The efficiency, gain, and bandwidth of the amplifier are calculated from a self-consistent solution of a set of coupled nonlinear differential equations describing the growth of the electromagnetic field and the evolution of the electron trajectories. Calculations show that very broadband amplification with high efficiency is possible in the slow-wave region of the propagating waves. The instantaneous bandwidth (full width at half maximum) near saturation depends on the efficiency (\ensuremath{\eta}) of the amplifier. For a cold beam, the bandwidth \ensuremath{\Delta}\ensuremath{\omega}/\ensuremath{\omega}\ensuremath{\approxeq}6% at \ensuremath{\eta}\ensuremath{\approxeq}40%, but \ensuremath{\Delta}\ensuremath{\omega}/\ensuremath{\omega} increases to 30% if the amplifier is operated at an efficiency of 20%. The efficiency is very sensitive to the axial velocity spread of the beam since interaction occurs with waves having large propagation constants. The maximum efficiency drops from 48% to 10% as the velocity spread increases from 0% to 5% but the bandwidth shows a small increase.

Endogenous event-related potentials in obsessive-compulsive disorder

Brain event-related potentials (ERPs) were used to investigate central nervous system (CNS) correlates of cognitive function in obsessive-compulsive disorder (OCD). ERPs of 10 unmedicated OCD patients and 10 normal controls were measured in an auditory “oddball” task. Increasing task difficulty resulted in longer N200 and P300 latencies in normal subjects, but not in OCD patients. Moreover, OCD patients displayed shorter P300 latency than normal controls for the more difficult discrimination condition. This replicates prior findings of Beech et al. (1983) for a visual task. For both levels of task difficulty, OCD patients also showed greater negativity than normal controls in the N200 region, which extended into the subsequent slow wave region. Negativities in the N200 and slow wave regions were larger at sites over the left hemisphere than the right hemisphere. The enhanced negativities and reduced P300 latency in OCD patients are discussed in terms of current theories postulating cortical hyperarousal and left hemisphere involvement in OCD.

Spontaneous electrical activity of interstitial cells of Cajal isolated from canine proximal colon.

Interstitial cells of Cajal (ICC) have been suggested as pacemaker cells in the gastrointestinal tract. A method was developed to isolate ICC from the slow-wave pacemaker region of the canine proximal colon. These cells were identified under phase-contrast microscopy, and their identity was verified by comparing their ultrastructure with the morphology of ICC in situ. Patch-clamp experiments demonstrated that these cells are excitable; voltage-dependent inward and outward currents were elicited by depolarization. Inward current transients were identified as calcium currents. A portion of the outward current appears to be due to Ca2+-activated K channels commonly expressed in these cells. ICC were also spontaneously active, generating electrical depolarizations similar in waveform to slow-wave events of intact colonic muscles. These findings are consistent with the hypothesis that ICC initiate rhythmicity in the colon.

Excitatory effect of histamine on EEGs of the cortex and thalamus in rats

When low frequency electrical stimulations were repeatedly applied to the rat midbrain reticular formation, the EEG spectral power recorded at the cortex and thalamus increased markedly, especially in the low frequency bands (0-6 Hz). The intraventricular administration of histamine (Hi) inhibited this increase. Hi-induced inhibition was antagonized by simultaneous injection of an equimolar dose of either pyrilamine or diphenhydramine, but not by that of cimetidine. In accordance with this, 2-methylHi decreased the power in the slow wave region, while 4-methylHi was not effective. It was assumed that the EEG arousal effect of Hi is exerted via H1 receptors but not related to H2 receptors. Adverse effects of H1 blockers on the brain, such as drowsiness, may be produced as a consequence of this inhibitory action.

Excitatory effect of histamine on the arousal system and its inhibition by H 1 blockers

To clarify whether the sedative effect of H 1 blockers is exerted in relation to H 1 receptors in the brain, EEG activity recorded from the cortex and thalamus of rats was studied by power spectral analysis. EEG processing was performed by the FFT method and displayed as compressed spectral arrays. When a train of low frequency electrical stimulation was applied to the midbrain reticular formation of conscious rats, there was an increase in spectral power recorded at the cortex and thalamus, especially in the low frequency bands (0–6 Hz). The intraventricular administration of histamine suppressed the increase in power; this inhibition was antagonized by simultaneous administration of pyrilamine or diphenhydramine, though not in combination with cimetidine or ranitidine. As in the case of histamine, the administration of 2-methylhistamine decreased power in the slow wave region, while administration of 4-methylhistamine did not. It was assumed that the arousal effect of histamine is exerted via H 1 and not related to H 2 receptors. Adverse effects of H 1 blockers, such as drowsiness, may be caused by their inhibition of histamine's arousal effect.

Propagation in corrugated waveguides

It is possible to determine the nature of the modes which propagate in corrugated, cylindrical waveguides, by imposing a nonisotropic, surface-reactance, boundary condition at the corrugated walls. Such a condition, in ignoring periodic effects, implies corrugated gaps much less than a wavelength separated by even smaller fins, and leads to simplified expressions for the fields in the waveguide.Two types of propagation occur: fast waves, characterised by low electric-field intensity at the walls, and slow waves by strong electric field at the walls. Both types are hybrid in nature, and degenerate to combinations of TE and TM modes when the corrugation slot depths are equivalent to multiples of half a wavelength. The dominant mode is a slow wave, with a cutoff equal to that of the dominant mode in a similar, uncorrugated waveguide. Mode charts, which distinguish the fast- and slow-wave regions as a function of normalised frequency and slot depths, may be used to design waveguides suitable for either fast or slow waves, and for matching such guides to uncorrugated launching sections.

A crossed-field gain mechanism employing discontinuities

An extension of an earlier analysis by the author gives results which predict a gain enhancement in a crossed-field amplifier via discontinuities. The discontinuities are introduced into the slowwave structure as short drift-tube sections. The resultant gain is larger than could be obtained without the interspersed drift-tube sections, i.e., larger than would be obtained with an equivalent length slow-wave structure. It can be shown that the mechanism is neither a velocity jump nor a diocotron phenomenon. The discontinuity must be located where the three waves - attenuating, growing, and constant amplitude - are comparable in magnitude. If the discontinuity is introduced where the growing wave is dominant, the usual result is obtained, i.e., the growing wave just beyond the discontinuity suffers a loss when compared with its value immediately prior to the discontinuity. At the beam exit from the drift-tube region all three waves are excited even if the growing wave was dominant prior to the discontinuity. It is then possible to insert a series of alternating drift-tube discontinuities and slow-wave structures and utilize the gain mechanism. The discontinuities must be followed, ultimately, by a long slow-wave structure region where the growing wave will become dominant. If this is not done, the total circuit voltage, which is represented by the sumof the voltages at the circuit of the three waves, will be small and the advantages of boosting the growing wave are lost.

Propagation behaviour of slotted inhomogeneous circular waveguides

By employing an approximate transverse-equivalent-network representation, the propagation behaviour of a slotted circular waveguide containing an axial dielectric rod is investigated. The complex transverseresonance equation is solved for E and Htype modes possessing an azimuthal dependence of zero and unity. The behaviour of both fast-wave and slow-wave regions is considered, and it is shown that, whereas the E mode propagation coefficient is a continuous function of frequency when the normalised phase-change coefficient passes through unity, the H mode propagation coefficient is discontinuous. The theoretical results for the slotted cylinder also demonstrate the form of the transition between the slow-wave behaviour of completely closed and completely open dielectric-rod structures. Theoretical phase-change coefficients in the fast-wave region are compared with those previously obtained from measurements of leaky-wave radiation patterns. Although good agreement is obtained, it is concluded that the perturbation caused by the introduction of a slot can be quite large in certain cases. A method is proposed whereby the perturbation might be reduced for modes with an azimuthal dependence of unity. it is suggested that the technique could prove useful in determining the fast-wave propagation characteristics of those asymmetric inhomogeneous-waveguide structures that are unamenable to analysis.

A small-signal field theory analysis of crossed-field amplifiers applicable to thick beams

Various authors have presented analyses of crossed-field amplifiers. Generally, these analyses have dealt with very thin electron beams in the presence of crossed-fields or with beams in which the bulk ac charge density within the beam has been neglected. It is the purpose of this paper to develop a field-theory analysis which is applicable to both thick or thin beams and which includes the effect of space charge. In place of the usual approximate match at the input boundary plane, a rigorous variational technique is presented, which considers the effect of the cutoff waves on the amplitudes of the propagating waves. This enables one to solve the input boundary problem for the first time insofar as the amplitudes of the propagating waves are concerned, without determining the individual amplitudes of the cutoff waves implicitly. As opposed to the usual approximate match, which is only valid for a thin, unmodulated entering electron stream, this variational method is valid for both thick and thin beams, as well as unmodulated or modulated beams entering a crossed-field slow-wave interaction region or entering a crossed-field drift region. The paper presents propagation constants obtained for thick beams based on the author's model. The variational procedure, which is a valid technique for other models as well as other types of beam devices, has been applied to one numerical case involving a thin beam and the results are compared with the results of the usual approximate match. A modification of the complex amplitudes of the excited waves, even in the case of thin beams, is noted.