Rapid Current Rise Research Articles

Terahertz electron cyclotron maser interactions with an axis-encircling electron beam

To generate terahertz radiation via the electron cyclotron maser instability, harmonic interactions are essential in order to reduce the required magnetic field to a practical value. Also, high-order mode operation is required to avoid excessive Ohmic losses. The weaker harmonic interaction and mode competition associated with an over-moded structure present challenging problems to overcome. The axis-encircling electron beam is a well-known recipe for both problems. It strengthens the harmonic interaction, as well as minimizing the competing modes. Here, we examine these advantages through a broad data base obtained for a low-power, step-tunable, gyrotron oscillator. Linear results indicate far more higher-harmonic modes can be excited with an axis-encircling electron beam than with an off-axis electron beam. However, multi-mode, time-dependent simulations reveal an intrinsic tendency for a higher-harmonic mode to switch over to a lower-harmonic mode at a high beam current or upon a rapid current rise. Methods are presented to identify the narrow windows in the parameter space for stable harmonic interactions.

The highest postglacial shore levels and glacio‐isostatic uplift pattern in northern sweden

The detailed clay varve chronology and an extensive knowledge of the highest postglacial shore level elevation (HS) in northern Sweden, along the Bothnian western coast, provides opportunities for determining the pattern of isostatic rise and centre of uplift from the early Holocene. The shore level of c.10-100 cal-yr bp (10-ka) for this area is determined by subtracting, from the metachronous HS elevations, the fall in relative sea level between local deglaciation time and the chosen reference time. The area of highest uplift since 10-ka is situated somewhat north of the location with the world‐record HS (Skuleberget in Ångermanland), but south of the area with most rapid current rise. Wave erosion marks in the studied area are seen to be more consistent indicators of HS than glaciofluvial delta levels. The gradients of shore marks at 10-ka are generally small within the investigation area. The regional 10-ka shore level pattern shows considerable irregularity compared to the current uplift. Central Sweden and western Finland show 10-ka gradients that indicate isostatic response to late (13–10-ka bp) glacial unloading. Indications of a secondary uplift centre west of the present investigation area are reported in previous work; this also suggests rapid isostatic response to unloading. Finally, the possibility of identifying errors in the varve‐dated deglaciation chronology via the 10-ka shore level pattern is illustrated.

Synthesis and photochemistry of a photolabile precursor of N-methyl-D-aspartate (NMDA) that is photolyzed in the microsecond time region and is suitable for chemical kinetic investigations of the NMDA receptor.

The amino acid L-glutamate is a major neurotransmitter at excitatory synapses within the central nervous system. Neuronal responses to glutamate are mediated by at least three receptor types, one of which is the NMDA subtype, named for its specific ligand N-methyl-D-aspartic acid. Neurotransmitter receptors are transmembrane proteins that can form ion channels upon binding a specific ligand and are involved in many physiological activities of the brain and in some neurological disorders. Elucidating the mechanisms of the formation of transmembrane receptor-channels and of receptor regulation and inhibition is necessary for understanding nervous system function and for designing potential therapeutic agents. This has been hampered by the lack of rapid reaction techniques suitable for investigating protein-mediated reactions on cell surfaces. Recently a laser-pulse photolysis technique was developed to study the chemical reactions of channel-forming receptor proteins in the microsecond-to-millisecond time region. To apply the technique to NMDA1 receptors a photolabile NMDA precursor (beta-DNB NMDA) was synthesized. In this precursor the side chain carboxylate was protected as a photosensitive 2,2'-dinitrobenzhydryl ester. Photolysis with 308 nm laser light generated free NMDA with a time constant of 4.2 +/- 0.1 microseconds at pH 7 and a photolysis quantum yield of 0.18 +/- 0.05. In rat hippocampal neurons the beta-DNB NMDA (250 microM) neither activated endogenously expressed receptors nor potentiated or inhibited the NMDA response. Equilibration of hippocampal neurons in the whole-cell current recording mode with 250 microM caged precursor followed by a pulse of 333 nm laser light resulted in a rapid current rise with a rate constant of 100 s-1 due to opening of NMDA-activated receptor-channels. The caged NMDA precursor described here now makes it possible to investigate the mechanism of NMDA receptors in the micro- to millisecond time region.

Heating of a current filament and formation of constrictions in a pulsed vacuum discharge

It is shown that a rapid current rise in a pulsed vacuum discharge is accompanied by enhanced compression of the current filament by its self-induced magnetic field. As a result, a constriction forms at a distance L≃1 mm from the cathode and the electron temperature increases to 102–103 eV at currents of order 1 kA. This behavior explains the observed increase in the degree of ion charge and the appearance of x-rays as the current pulse length decreases. The criterion for a rapid rise is the condition τ<L/V≃10−27 s, where τ is the characteristic current amplification time and V≃106 is the velocity of the cathode plasma.

Ion channels based on bis-macrocyclic bolaamphiphiles: effects of hydrophobic substitutions

Four new bis-macrocyclic bolaamphiphiles were prepared to explore the effects of hydrophobic substitutions on ion transport. In bilayer vesicles the new compounds were remarkably similar to more hydrophilic derivatives prepared previously. Planar bilayer conductance experiments showed the new compounds induced an unique current-time signal consisting of a rapid rise time, followed by a slower decay time. Signal shape was cation dependent and was related to a modest selectivity between cations. Cation-anion selectivity was very high, approaching an ideal cation selectivity. One compound also showed voltage dependence of the signal shape and duration. Qualitative changes in signal shape, duration, and voltage dependence were provoked by variation in the electrolyte pH and by masking the head-group electrostatic interactions with low levels of barium ions. A model for the signal shape is proposed, involving a rapid current rise due to aggregate restructuring, followed by slower decay due to development of the local Donnan potential that results from the high cation-anion selectivity.Key words: ion channel, synthesis, bilayer membrane, bilayer clamp, mechanism.

Virtual cathode formation due to electromagnetic transients

The process of virtual cathode formation in a gap is critically examined via particle simulations. It is found that the limiting current obtained from the electrostatic approximation is valid only in the deeply nonrelativistic regime. For injection energy as low as 30 keV, the transients in the injected current may produce an inductive voltage that can significantly lower the limiting current from the classical, electrostatic value. Although the source of the virtual cathode is transient in nature, the virtual cathode formed due to this inductive voltage persists long after the injected current has reached a steady-state value. The self-magnetic field is unimportant, however, at this low energy. This result has implications for scaling devices with rapid current rise times to weakly nonrelativistic energies as well as for Particle-In-Cell (PIC) codes which use field emission models with electric field thresholds.

Observation of improved confinement at the center of a FT-2 tokamak plasma with rapid current rise and lower-hybrid heating

The problem of optimizing the conditions for central lower-hybrid heating has stimulated experiments with improved confinement at the center of the discharge in analogy to discharges with an inverted shear of the q profile. To this end, a current pulse rising rapidly from 22 to 30 kA over 0.5 ms was used in the FT-2 tokamak during lower-hybrid heating. In these experiments a substantial increase in the lifetime of the energy in the ionic component was observed. A decrease of the fluxes of high-energy charge-exchange atoms and a suppression of the microoscillations of the plasma determined in the central regions of the discharge are observed. These and other data were used together with the computer simulation to clarify the mechanism leading to the improvement of energy and particle confinement at the center of the discharge. The influence of variations in both the q profile and the electric fields, which accompanied the rapid current rise, on the transport processes is studied.

Spatial and temporal distribution of potential in the pseudospark switch

The understanding of emission and transport in the pseudospark is complex because these are related to a transition between two fundamentally different modes of electron emission. In this paper probe measurements of plasma parameters are presented providing evidence for oscillatory behavior of the current and potential during this transition. It is shown that these fluctuations are related to a glow-to-arc transition wherein smooth current growth is interrupted by cathode spot formation inside of the hollow cathode resulting in very rapid rise in current. These results support a view that quenching observed in the pseudospark is a double-layer phenomenon that occurs when emission cannot exceed secondary emission produced by ion bombardment. >

Processes in the prebreakdown stage of a low-pressure discharge and the mechanism of discharge initiation in pseudospark switches

A time-dependent model for the rapid current rise during initiation of the discharge in a pseudospark switch is presented. The following assumptions are made: (1) the gas is ionized only by electrons that originate at the cathode; (2) the cathode emissivity is infinite; (3) the average ionization cross section is constant; (4) the ion charge density is independent of gap position. The results are in qualitative agreement with a previous experimental study of a nitrogen-filled switch operating between 6 and 20 kV for a range of gas pressures. The model provides a straightforward approach for estimating pseudospark breakdown times.

The Control of Luminous Color of the Discharge Lamps.

It is qualitatively verified by the energy conservation equation that high electron temperature is attained by the pulsed discharge with rapid current rise. In the glow discharge of the tube filled with mercury vapor and neon gas, luminous color of the positive column varies from the color close to red emitted by neon to blue of mercury by the square-wave pulsed discharge. Brightness of blue light is hundreds nits but that of red light is 100 nts at most. In the arc discharge, red light with strong brightness is easily obtained by square-wave pulses. On the other hand, blue light may be obtained at high frequency but the discharge becomes unstable at that frequency. Therefore, the current waveform alteration method is effective for luminous color control of the arc discharge.

Plasma current startup by lower hybrid waves in the JIPP T-IIU tokamak

The paper describes the characteristic behaviour of lower hybrid current startup in JIPP T-IIU. The current startup is carried out by the injection of 800 MHz lower hybrid waves into cold and low density plasmas (Te = 10 − 20 eV, n̄e = (1−2) × 1012 cm−3 produced by electron cyclotron resonance or lower hybrid waves (LHW) only. The plasma current rises with a characteristic rise-time of τr (≳ 30-50 ms) and approaches a quasi-steady state value, Ipm (= 5-20 kA), whereupon 10-50 kW LHW power is injected into the torus, controlling the vertical field. The rise-time is inversely proportional to the bulk electron density, ne, and is comparable to the collision time of current carrying high energy electrons with the bulk plasmas. On the other hand, the current drive efficiency in the quasi-steady state is almost independent of n̄e, i.e. Ipm/PLH = 0.4−0.7 A·W−1 for n̄e = (0.8−4) × 1012 cm−3. The conversion efficiency of RF energy injected into the torus is typically 5% during the current rise phase and 10% in the most efficienct case. The effects of the initial injection of ECH power and the observed parametric instabilities on the current startup are investigated from the viewpoint of seed current generation. During the rapid current rise when an appreciably negative loop voltage is observed, the bulk electrons are heated up to 150 eV. Various heating mechanisms responsible for the bulk electron heating are discussed.

Early expansion in exploding multiple wire arrays

A simple model is developed for the initial stage (blow-off and the onset of pinching) of a multiple exploded wire array. With obvious modifications the model can also be applied to single wires or other self-pinched plasma columns. Each wire in the array is modeled as a plasma cylinder undergoing self-similar radial motion, and the whole array is coupled self-consistently to an external circuit. The plasma expands initially; the onset of self-pinch is shown to be coincident with the plasma conductivity reaching a value large enough for skin effect to occur. The model is run for a variety of system parameters, and the choice of machine and wire parameters to attain particular objectives (e.g., rapid current rise, limited wire expansion, etc.) is discussed.

Field emission from p-type GaAs and GaP crystals

In order to define field electron emission from p-type GaAs and GaP crystals as the first stage of the applications of a field emission microscope to the surfaces, the emission current-voltage characteristics for the clean surfaces have been investigated as well as the effects of temperature, illumination, anneal, and gold and oxygen adsorption on them. The emission from the clean surfaces has been classified into three groups by the emission c-v characteristics; the linear, the characteristic constructed of two or three straight line segments, and the non linear. It was quite sensitive to temperature and light. The threshold for the light sensitivity was located at the little longer wavelength than that expected from the band gap energy. The emission with nonlinear emission c-v characteristics exhibited a hysteresis, occurring a rapid current rise. High temperature anneal at 1080 K caused the precipitates of dissociated Ga and or inner impurity Au onto a tip surface, which made the emission c-v characteristic shift to a lower voltage side in addition to the decrease of its slope. The same behavior was also obtained by Au adsorption, while oxygen adsorption made the emission c-v characteristic shift to a higher voltage side.

A High-Field Emission Ionization Detector for Gas Chromatography.

A field-emission type ionization detector working at gas is described. It is designed so that the ionization current is dependent on the concentration of vapor molecules in the electric field and that the too rapid current rise due to secondary processes at the cathode would be avoided. (L.N.N.)

Atmospherics from lightning flashes with multiple strokes

The atmospherics resulting from the individual return strokes of a multiple lightning discharge to earth are considered. Special attention is paid to the differences between the source spectra for first and subsequent strokes. It is shown that the change in the velocity of ascent of the return stroke, between a first and a following stroke, implies a shift of the spectrum toward the low-frequency end. On the other hand, the more rapid current rise associated with subsequent strokes entails an intensification of the higher frequencies. A combination of these effects suggests that the source spectrum is effectively constant for all strokes at frequencies exceeding 10 kc/s, and that the main differences occur below this frequency. A straightforward experimental method of checking these conclusions is outlined.