Fast Field Changes Research Articles

Lightning generated electric field over land and sea at Northern Region of Peninsular Malaysia: Measuring Setup

Lightning is the transfer of significant charge between two charged object, it can appear between cloud-to-cloud, cloud-to-air and cloud-to-ground. All lightning activities are correlated with charge movement and therefore, it can be studied using measurement of electrostatic field (slow field changes), radiation field (fast field changes) and magnetic field affiliated with charge movement. The measuring equipment was a parallel flat plate and vertical whip antenna with an analog filter buffer circuit, connected to a digital high speed oscilloscope. In principle, both antenna have a similar measuring operation, with the exclusion of the antennas dimension, mode of transient recoder and the association of circuitry characteristic (decay time constant). This measurement setup are commonly used to perform electric field characterization related to various lightning events such as as preliminary breakdown, stepped leader, return stroke, subsequent return stroke, dart leader, dart-stepped leader, narrow bipolar pulses and cloud activity between strokes, such as regular pulse trains, K and M changes and chaotic pulse trains. The measurement systems are located very close to the sea, with respect in obtaining a perspicuous explaination of lightning propagation effects on sea instead on land. Noise-distorted effects in data captured usually introduce in a noisy environment or when the lightning strike far from the measurement station, thus the data gathered from the measurement need to be filter using MATLAB computer simulation before conducting further analysis. A promising wavelet 1-D technique, then, were used in signal denoising process rather using conventional filtration. In this works, the state of art in measuring vertical component of electric field focalized sea area during the thunderstorm days been demonstrate and explained.

On the direct measurement of the adiabatic temperature change of magnetocaloric materials

Direct measurement of an adiabatic temperature change in magnetocaloric materials is fundamental to design efficient and eco-friendly magnetocooling devices. This work reports an overview of the measurement principle and the main experimental issues that have to be considered to obtain a reliable characterization of materials. The effect of non-ideal adiabatic conditions, the role of the temperature sensor, and the influence of specific properties of the material are discussed on the basis of finite-difference thermal simulations and specially designed experiments. Two cases are considered in detail: the characterization of thin samples and the measurement of caloric response to fast field changes. Finally, the impact of different measurement protocols is discussed in the case of materials with first-order transitions.

Dynamics of the magnetoelastic phase transition and adiabatic temperature change in Mn1.3Fe0.7P0.5Si0.55

The adiabatic temperature change ΔTad of a Mn1.3Fe0.7P0.5Si0.55 Fe2P-type alloy was measured under different magnetic field-sweep rates from 0.93 Ts−1 to 2870 Ts−1. We find a field-sweep-rate independent magnetocaloric effect due to a partial alignment of magnetic moments in the paramagnetic region overlapping with the magnetocaloric effect of the first-order phase transition. Additionally, the first-order phase transition is not completed even in fields up to 20 T leading to a non-saturating behavior of ΔTad. Measurements in different pulsed fields reveal that the first-order phase transition cannot follow the fast field changes as previously assumed, resulting in a distinct field-dependent hysteresis in ΔTad.

Alternating-gradient canted cosine theta superconducting magnets for future compact proton gantries

We present a design of superconducting magnets, optimized for application in a gantry for proton therapy. We have introduced a new magnet design concept, called an alternating-gradient canted cosine theta (AG-CCT) concept, which is compatible with an achromatic layout. This layout allows a large momentum acceptance. The 15 cm radius of the bore aperture enables the application of pencil beam scanning in front of the SC-magnet. The optical and dynamic performance of a gantry based on these magnets has been analyzed using the fields derived (via Biot-Savart law) from the actual windings of the AG-CCT combined with the full equations of motion. The results show that with appropriate higher order correction, a large 3D volume can be rapidly scanned with little beam shape distortion. A very big advantage is that all this can be done while keeping the AG-CCT fields fixed. This reduces the need for fast field ramping of the superconducting magnets between the successive beam energies used for the scanning in depth and it is important for medical application since this reduces the technical risk (e.g., a quench) associated with fast field changes in superconducting magnets. For proton gantries the corresponding superconducting magnet system holds promise of dramatic reduction in weight. For heavier ion gantries there may furthermore be a significant reduction in size.

Tracing ultrafast dynamics of strong fields at plasma-vacuum interfaces with longitudinal proton probing

If regions of localized strong fields at plasma-vacuum interfaces are probed longitudinally with laser accelerated proton beams their velocity distribution changes sensitively and very fast. Its measured variations provide indirectly a higher temporal resolution as deduced from deflection geometries which rely on the explicit temporal resolution of the proton beam at the position of the object to probe. With help of reasonable models and comparative measurements changes of proton velocity can trace the field dynamics even at femtosecond time scale. In longitudinal probing, the very low longitudinal emittance together with a broad band kinetic energy distribution of laser accelerated protons is the essential prerequisite of the method. With a combination of energy and one-dimensional spatial resolution, we resolve fast field changes down to 100 fs. The used pump probe setup extends previous schemes and allows discriminating simultaneously between electric and magnetic fields in their temporal evolution.

Demise of the rapid-field-change hypothesis at Steens Mountain: The crucial role of continuous thermal demagnetization

Rapid continuous thermal demagnetization in a Triaxe vibrating sample magnetometer disproves an earlier hypothesis that an episode of extraordinarily fast field change is recorded in the remanent magnetization of a single lava flow that cooled during a geomagnetic polarity reversal. The original evidence for this hypothesis was the distinctive pattern of variation in direction with vertical position after stepwise demagnetization to 500 °C, but continuous thermal demagnetization now proves that this flow actually carries consistent high-temperature remanence parallel to that of the flow immediately below. Why does continuous thermal demagnetization succeed when conventional thermal and alternating-field demagnetization fail to isolate this direction? Detailed investigation of the demagnetization trajectories and underlying rock-magnetic properties suggest that normal-polarity secondary remanent magnetization, acquired both at room temperature in today's polarity chron and during modest reheating in a normal field during cooling of the overlying flow, is responsible for the misleading pattern of directions. Alteration during ordinary thermal demagnetization evidently promotes the overprint to higher blocking temperatures so that it masks the primary component, whereas the rapid heating (∼40 °C/min) during continuous demagnetization is fast enough to demagnetize the normal overprint before this masking can happen.

Statistical Analysis of Lightning Electric Field Measured Under Equatorial Region Condition

Abstract Lightning can be described as transient high current electric discharges usually within few tens of kilometers resulting from the atmospheric climate condition. Lightning are due to the electrostatic field (slow field changes), radiated field (fast field changes) and magnetic fields associated with the motion of charges. The various lightning parameters such as electric current, charge transfer, as required for analysis of atmospheric electricity research can be extracted from the measurement of lightning field components. Lightning characteristics and their statistical information are the foundation for the design of lightning protection system as well as for the calculation of lightning radiated fields. In this paper, the characteristics of electric fields generated by negative cloud-to-ground lightning discharges in south of Malaysia are analyzed. A total of 220 cloud-to-ground lightning discharge from 5 different thunderstorm events (each event lasts for about 4-5 hours) were investigated. The findings of the statistical results showed that 7% of the flashes were single stroke flashes, and the maximum number of strokes recorded was 14 multiple strokes per flash. A preliminary breakdown signature in more than 95% of the flashes can be identified. The maximum and minimum of preliminary breakdown time duration of the observed lightning signals were 38 ms and 490 us, respectively.

Transitional geomagnetic impulse hypothesis: Geomagnetic fact or rock‐magnetic artifact?

A striking feature of the Steens Mountain (Oregon) geomagnetic polarity reversal is the two (maybe three) extremely rapid field directional changes (6 degrees per day) proposed to account for unusual behavior in direction of remanent magnetization in a single lava flow. Each of these very fast field changes, or impulses, is associated with a large directional gap (some 90°) in the record. In order to check the spatial reproducibility of the paleomagnetic signal over distances up to several kilometers, we have carried out a paleomagnetic investigation of two new sections (B and F) in the Steens summit region which cover the second and the third directional gap. The main result is the description of two new directions, which are located between the pre second and post second impulse directions. These findings weigh against the hypothesis that the geomagnetic field cause the unusual intraflow fluctuations, which now appears to be more ad hoc as an explanation of the paleomagnetic data. However, the alternative baking hypothesis remains also ad hoc since we have to assume variable rock magnetic properties that we have not yet been able to detect within the flows at the original section Steens A and D 1.5 km to the north. In addition, new results for 22 transitional and normal lava flows in section B are presented that correlate well with earlier results from section A.

The Polo coil, a prototype tokamak poloidal field coil, design features and test results

A prototype tokamak poloidal field coil of 3 m diameter was successfully tested. The coil was designed to withstand typical field transients like fast ramping, plasma controlling and plasma disruptions. In order to achieve these requirements, a low loss conductor was developed. The coil with its components was designed according to rules of high voltage technology. A realistic test of the magnet requires at 23 kV a current of 15 kA, that means a peak power of 345 MW. Therefore, a fast switching circuit was used combined with an additional current lead at the middle of the magnet winding. This allowed the performance of fast field changes with increasing or decreasing field. In one operation mode, the coil was discharged into a dump resistor. In another operation mode, only one coil half is discharged which generated a field increase in the second short circuited half of the coil. Discharging time constants of 1 ms and local field sweep rates of about 240 T/s were obtained without quench. A reproducible and sharp quench boundary was found. The results showed clearly that ramp rate limitations are only given by the expected stability limit or the load line boundary.

Radio interferometric observations of cloud‐to‐ground lightning phenomena in Florida

Radio frequency observations of cloud‐to‐ground lightning in Florida have been analyzed to document a number of features of the lightning. The observations have been made using an interferometer system which determined the direction to the lightning radiation as a function of time during close lightning discharges. Observations are presented for about 50 radiating events during five multiple‐stroke, normal‐polarity flashes to ground. The results confirm and extend results of a similar study of New Mexico lightning by Rhodes et al. (1994). Dart leaders, in‐cloud “K” events, and attempted leaders are found to be the same phenomena, namely, a negative‐polarity streamer that propagates horizontally and/or vertically to ground at estimated speeds of about 107 ms−1 down to 106 ms−1. The step or impulsive fast electric field change produced by some K events is found to be caused by positive breakdown back at the starting point of the negative streamer, after the streamer has progressed over some or all of its distance. This breakdown appears to initiate a “forward” stroke along the streamer channel which can renew the breakdown at the front end of the channel. K breakdown which happens to occur during a continuing current discharge to ground produces an M event (channel brightening) when it connects with the conducting channel to ground. The step fast field change of the M events is found to be produced at the time of connection inside the cloud; no fast field change is detected when this breakdown reaches ground that would be associated with a return stroke. M events can also be initiated by fast (107 ms−1) positive streamers which often propagate away from the leader source region within a few milliseconds of the arrival of a return stroke back in the source region. These streamers often generate an even faster negative recoil event back along their extent which propagates down the channel to ground, as the M event. In all cases the current increase or channel brightening produced by K and M events is predicted to occur in a forward direction, that is, toward or down the channel to ground, in the same direction as the initiating negative streamer. Positive breakdown is detected only immediately following return strokes or during some K streamers. In both cases the electric stresses are expected to be very large, suggesting that positive breakdown which produces a conducting channel is difficult to initiate. Results for one flash suggest that negative charge was left at low altitude by the initial stroke to ground; this forced the subsequent leaders to develop a new path to ground, first as series of attempted leaders and then as a dart‐stepped leader.

Observations of lightning phenomena using radio interferometry

A radio interferometer system is described which utilizes multiple baselines to determine the direction of lightning radiation sources with an angular resolution of a few degrees and with microsecond time resolution. An interactive graphics analysis procedure is used to remove fringe ambiguities from the data and to reveal the structure and development of lightning discharges inside the storm. Radiation source directions and electric field waveforms have been analyzed for different types of breakdown events for two lightning flashes. These include the initial breakdown and K type events of in‐cloud activity, the leaders of initial and subsequent strokes to ground, and activity during and following return strokes. Radiation during the initial breakdown of one flash was found to consist of intermittent, localized bursts of radiation that were slow moving. Source motion within a given burst was unresolved by the interferometer but was detected from burst to burst, with negative charge being transported in the direction of the breakdown progression. Radiation during initial leaders to ground was similar but more intense and continuous and had a characteristic intensity waveform. Radiation from in‐cloud K type events is essentially the same as for dart leaders; in both cases it is produced at the leading edge of a fast‐moving negative streamer that propagates along a well‐defined, often extensive, path. K type events are sometimes terminated by a fast field change that appears analogous to the field change of a return stroke. Dart leaders are sometimes observed to die out before reaching ground; these are termed “attempted leaders” and, except for their greater extent, are no different than K type events. Several modes of breakdown during and after return strokes have been documented and analyzed. One mode corresponds to the launching of a positive streamer away from the upper end of the leader channel, apparently as the return stroke reaches the leader start point. In another mode, the quenching of the dart leader radiation upon reaching ground reveals concurrent breakdown in the vicinity of the source region for the leader. In both instances the breakdown appears to establish channel extensions or branches that are followed by later activity of the flash. Finally, a new type of breakdown event has been identified whose electric field change and source development resemble those of an initial negative leader but which progresses horizontally through the storm. An example is shown which spawned a dart leader to ground.

On the assumption of the Earth's surface as a perfect conductor in atmospheric electricity

The assumption of considering the Earth's surface as a plane perfect conductor is examined. The electric field produced due to a point charge or a model thundercloud in the atmosphere and the field changes produced due to a leader in a lightning flash have been calculated if the Earth's surface, instead of a perfect conductor, is considered as a dielectric of finite dielectric constant and zero conductivity. It is shown that when the dielectric constant of the surface is <10, the horizontal and vertical components of electric field may be comparable to each other at comparatively large distances from the charge. The net electric field in such a case may be inclined from the vertical by as much as 45° or even more. At different distances, the field changes caused due to the leader in a lightning flash change not only its magnitude but direction as well. Certain igneous and metamorphic rocks with low moisture contents, fine‐grained sand when it is dry or has a moisture content of <1% or is at low temperatures, and ice at very low temperatures of <−50°C or oriented such that its c axis is perpendicular to the direction of electric field have dielectric constant values < 10 and conductivity < 10−9 S/m. Therefore, for some regions, such as polar regions, high‐latitude regions covered with sea ice or dry sand, hot and dry regions covered with sand in the midst of vast deserts, and regions covered with hot and dry igneous and metamorphic rocks, the average representative values of dielectric constant and conductivity of the surface may be such that the Earth's surface at those places cannot be assumed as a perfect conductor. Consequently, the assumption of treating the Earth's surface as an insulating surface generally taken for fast field changes during a stepped leader in a lightning flash, may be extended for much slower field changes occurring even in fair weather. It needs to be mentioned, however, that thunderstorm activity is generally minimal in most of the regions mentioned above. It is proposed that instead of measuring only the vertical component of electric field, as is normally done, a measurement of the electric field vector will give a fifil description of the intensity of the electric field at some places.

On the assumption of the Earth's surface as a perfect conductor in atmospheric electricity

The assumption of considering the Earth's surface as a plane perfect conductor is examined. The electric field produced due to a point charge or a model thundercloud in the atmosphere and the field changes produced due to a leader in a lightning flash have been calculated if the Earth's surface, instead of a perfect conductor, is considered as a dielectric of finite dielectric constant and zero conductivity. It is shown that when the dielectric constant of the surface is <10, the horizontal and vertical components of electric field may be comparable to each other at comparatively large distances from the charge. The net electric field in such a case may be inclined from the vertical by as much as 45° or even more. At different distances, the field changes caused due to the leader in a lightning flash change not only its magnitude but direction as well. Certain igneous and metamorphic rocks with low moisture contents, fine-grained sand when it is dry or has a moisture content of <1% or is at low temperatures, and ice at very low temperatures of <−50°C or oriented such that its c axis is perpendicular to the direction of electric field have dielectric constant values < 10 and conductivity < 10−9 S/m. Therefore, for some regions, such as polar regions, high-latitude regions covered with sea ice or dry sand, hot and dry regions covered with sand in the midst of vast deserts, and regions covered with hot and dry igneous and metamorphic rocks, the average representative values of dielectric constant and conductivity of the surface may be such that the Earth's surface at those places cannot be assumed as a perfect conductor. Consequently, the assumption of treating the Earth's surface as an insulating surface generally taken for fast field changes during a stepped leader in a lightning flash, may be extended for much slower field changes occurring even in fair weather. It needs to be mentioned, however, that thunderstorm activity is generally minimal in most of the regions mentioned above. It is proposed that instead of measuring only the vertical component of electric field, as is normally done, a measurement of the electric field vector will give a fifil description of the intensity of the electric field at some places.

Radiation from lightning return strokes over a finitely conducting Earth

The effects of the conductivity of the earth on radiation from lightning return strokes are examined theoretically, using a piecewise linear transmission line model for the return stroke. First, calculations are made of the electric field radiated during the return stroke, and then this electric field is used to compute the response of conventional AM radio receivers and electric field change systems during the return stroke. The calculations apply to the entire transient waveform (they are not restricted to the initial portions of the return stroke) and yield fast field changes and RF radiation in agreement with measurements made during real lightning. This research was motivated by measurements indicating that a time delay exists between the time of arrival of the fast electric field change and the RF radiation from first return strokes. The time delay is on the order of 20 μs for frequencies in the HF‐UHF range for lightning in Florida. The time delay is obtained theoretically in this paper. It occurs when both the effects of attenuation due to the conductivity of the earth and the finite velocity of propagation of the current pulse up the return stroke channel are taken into account in the model.