Risetime

At loading stresses between the HEL and the strong shock threshold, a two-wave structure is observed with an elastic precursor followed by a viscoplastic wave. The region between the two waves is in transition between the elastic and the viscoplastic states. The risetime of the trailing wave is strongly dependent on the loading stress amplitude [5]. 

The objective in these gauges is to measure the time-resolved material (particle) velocity in a specimen subjected to shock loading. In many cases, especially at lower impact pressures, the impact shock is unstable and breaks up into two or more shocks, or partially or wholly degrades into a longer risetime stress wave as opposed to a single shock wave. Time-resolved particle velocity gauges are one means by which the actual profile of the propagating wave front can be accurately measured. 

Figure 4.1. Profile of a steady shock wave, risetime imparting a particle velocity, e.g., Uj, pressure Pi, and internal energy density E, propagating with velocity U, into material that is at rest at density pQ and internal energy density Eq.

For example, a 10 GPa (total strain = 0.06) shock wave in copper has a maximum total strain rate 10 s [21] the risetime would thus be (eje) 0.6 ns. For uniaxial-strain compression, y averaged over the entire shock front. The resolution of the shock wave in a large-scale, multidimensional finite-difference code would be computationally expensive, but necessary to get the correct strength f behind the shock. An estimate of the error made in not resolving the shock wave can be obtained by calculating dt/dy)o with y 10 s (the actual plastic strain rate) and y 10 s (the plastic strain rate within the computed shock wave due to a time step of 0.06 qs). From (7.41) with y = 10 s (actual shock wave) and y = 10 s (computation) ... 

Grady and Asay [49] estimate the actual local heating that may occur in shocked 6061-T6 Al. In the work of Hayes and Grady [50], slip planes are assumed to be separated by the characteristic distance d. Plastic deformation in the shock front is assumed to dissipate heat (per unit area) at a constant rate S.QdJt, where AQ is the dissipative component of internal energy change and is the shock risetime. The local slip-band temperature behind the shock front, 7), is obtained as a solution to the heat conduction equation with y as the thermal diffusivity... 

Snubbers are passive networks that delay the risetime of the voltage waveform. Historieally, snubbers have been used to keep power deviees within their forward- and reverse-biased safe operating areas (FBSOA and RBSOA) or to eontrol RF emissions from the power supply. They are essentially lossy tank eireuits (L-C eireuits with R). Using them offered more of an advantage than the loss ineurred. Semieonduetor eomponents are more rugged today and the traditional need for the RFC snubber for proteetion has lessened, but oeea-sionally a snubber is still needed. 

Fig. 4.38 Unit step response of ship autopilot control system. RiseTime (to 95%) = 23 seconds ...

Thus, the region 2100-1830 cm 1 can be covered. This allows us to monitor CO(v,J) by resonance absorption and various M(CO)n [n = 3-6] as a result of near coincidences between the CO laser lines and the carbonyl stretching vibrations of these species. The temporal response of the detection system is ca. 100 ns and is limited by the risetime of the InSb detector. Detection limits are approximately 10 5 torr for CO and M(CO)n. The principal limitation of our instrumentation is associated with the use of a molecular, gas discharge laser as an infrared source. The CO laser is line tuneable laser lines have widths of ca. lO cm 1 and are spaced 3-4 cm 1 apart. Thus, spectra can only be recorded point-by-point, with an effective resolution of ca. 4 cm 1. As a result, band maxima (e.g. in the carbonyl stretching... 

To make a breakthrough in household appliances and other consumer product markets UV sensors have to become significantly cheaper while spectral selectivity as a major key feature must be guaranteed. Most of today s UV photodiodes are made from crystalline semiconductor materials. The cheaper materials (Si) lack spectral selectivity, and the wide band gap materials are very expensive. What they all have in common their top performance regarding sensitivity and speed. Crystalline photodiodes have risetimes of often below 1 s. However, the described processes to be sensed here are not faster than some milliseconds or even much slower. In order to obtain a reasonably-priced SiC or GaN photodiode, the photoactive area is often reduced to below 1 mm2 and barely fills the sensor housing. So far, the top sensitivity offered by the semiconductor has been sacrificed for a competitive... 

In the near future, UV photodiodes made from polycrystalline wide band-gap semiconductors may fill the gap in the market. Although they have a lower sensitivity (photocurrent per area) they promise to have a better merit-rating in terms of photocurrent per sensor costs. The other major drawback of polycrystalline photodiodes, the risetime of micro- to milliseconds, is not relevant for household applications. Fuji Xerox Laboratories in Japan are developing visible-blind UV photodiodes made from polycrystalline GaN [12], while twlux AG in Berlin, Germany is developing visible-blind UV photodiodes made from polycrystalline titanium dioxide [13]. A prototype is shown in Fig. 5.45. 

Fast time-response of typically 1.5 nsec risetime and 0.5 nsec FWHM jitter in single-photon transit time. 

Desc (i.e. Find the risetime of a step response curve with no Desc overshoot. If the signal has overshoot, use GenRiseQ.)... 

The name of the function is Risetime. It has 1 input argument. 1 Search forward level means search the first input forward and find a level. The level we are looking for is the 10% voltage level. 0% is defined as the minimum level of the trace 100% is defined as the maximum. The p means find the specified level when the trace has a positive-going slope. When the point is found, the text 11 designates its coordinates as xl and yl. Search forward level(90%, p) 2 means search the first input forward and find a point on the positive-going slope that is at the 90% level. When the point is found, the text 2 designates its coordinates as x2 and y2. The function returns x2-x1, which is the time difference between the two points. Since the x-axis is time in a Transient Analysis, x2-x1 is the time difference between the two points, or the rise time. A second function is ... 

This function is similar to the Risetime function, except that it finds the time between the 10% and 90% points when the points lie on the negative-going slope. 

Enter the text Risetime (V(VO)). Instead of typing, a quick way to do this is to click on the RISBtltnB(l) text in the right pane and then click on Vfl/Of in the left pane ... 

The screen shows all of the Performance Analysis traces available to us. Enter the trace Risetime (V(Vo))... 

The temporal widths of the IR pulses and the time resolution of this spectrometer are tested with the use of a Ge sample that, when exposed to the pump pulses, results in transient IR absorption at 2290 cm. Modeling the risetime of this absorption gives a cross-correlation width (full width at half-maximum, fwhm) of 1.8 ps. 

The transient absorption spectrum of DMABN-F4 in acetonitrile, recorded at 1-ps delay shows an absorption band near 360 nm (Fig. 3a). This band can be attributed to the CT state by comparison with that reported for DMABN in acetonitrile at lOOps (Fig. 3b). For the latter, both the LE state decay and CT state risetime were found to be 6 ps in time-resolved fluorescence measurements with a 4-ps time-resolution streak-camera [6]. From various studies, the CT formation time is now well known to be 4-6 ps [1] so that, at 100 ps, only the CT state is present. Fig. 3a shows that, for DMABN-F4, the CT state is populated in less than 1 ps in acetonitrile. We can thus conclude from the present observation that the access to the CT state for DMABN-F4 is significantly faster than for DMABN in acetonitrile. 

In n-hexane, a similar band with a maximum at around 384 nm was observed with a comparably fast risetime, so that one can conclude that the photoinduced charge-transfer process in this fluorinated derivative is a quasi-barrierless process in both polar and non-polar solvents. Preliminary DFT calculations indicate that in vacuum DMABN-F4 is nonplanar in the ground state in contrast to DMABN [7]. The fact that the observed CT state absorption spectrum is blue-shifted compared to that of DMABN and of the benzonitrile anion radical (Fig. 3) might be an indication that the equilibrium geometry of the CT state of DMABN-F4 is different from that of the TICT state of DMABN or might be due to the influence of the four fluorine atoms. 

The temporal resolution of both methods is limited by the risetime of the IR detectors and preamplifiers, rather than the delay generators (for CS work) or transient recorders (SS) used to acquire the data, and is typically a few hundred nanoseconds. For experiments at low total pressure the time between gas-kinetic collisions is considerably longer, for example, approximately 8 /is for self-collisions of HF at lOmTorr. Nascent rotational and vibrational distributions of excited fragments following photodissociation can thus be obtained from spectra taken at several microseconds delay, subject to adequate SNR at the low pressures used. For products of chemical reactions, the risetime of the IR emission will depend upon the rate constant, and even for a reaction that proceeds at the gas-kinetic rate the intensity may not reach its maximum for tens of microseconds. Although the products may only have suffered one or two collisions, and the vibrational distribution is still the initial one, rotational distributions may be partially relaxed. 

Experimental evidence for the distortion of a kinetic growth curve due to a closed ion source was reported by Martinez et al. [42]. In an FPTRMS investigation of the infrared laser multiphoton dissociation of CF2HC1, these authors found that the risetime of the HC1 molecular elimination product, which was expected to be formed on a microsecond time scale, was approximately 2 ms. This they attributed to holdup in the ion source, which was a partially enclosed box. Gas entered the ion source chamber through a hole at one end and exited through several holes at either end or at its sides. 

Because it can be efficient and selective, field ionization of Rydberg atoms has become a widely used tool.1 Often the field is applied as a pulse, with rise times of nanoseconds to microseconds,2"4 and to realize the potential of field ionization we need to understand what happens to the atoms as the pulsed field rises from zero to the ionizing field. In the previous chapter we discussed the ionization rates of Stark states in static fields. In this chapter we consider how atoms evolve from zero field states to the high field Stark states during the pulse. Since the evolution depends on the risetime of the pulse, it is impossible to describe all possible outcomes. Instead, we describe a few practically important limiting cases. 

When field ionization threshold curves such as the one shown in Fig. 7.4 are measured for many states, they can be plotted together to exhibit the n dependence of the ionization threshold field. In Fig. 7.5 we show a plot of the threshold fields (50% ionization) for the Na m = 0, 1, and 2 states obtained with a 0.5 / risetime field pulse similar to the one shown in Fig. 7.1(b).8 In Fig. 7.5 it is apparent that, while the threshold fields of the m = 0 states are described by Eq. 

In practice Eq. (7.5) means that n < 20 states with quantum defect differences of 10-3 satisfy the adiabaticity requirements for pulses with risetimes of 1 /us. 

On the other hand the high t states have quantum defect differences which are much smaller than 10-3 and they do not satisfy the adiabatic criterion of Eq. (7.5) for the same risetime. As a result, when the field is turned on they are projected diabatically onto the intermediate field states. From Eq. (7.5) it is clear that if the risetime is kept constant, the at which diabatic passage from zero field occurs becomes lower as n is increased. Since it is impossible to excite optically high t states, the statement that they pass diabatically from low field to the intermediate regime has not been tested, but it has been experimentally established that the optically accessible low ( states of n = 20 Na atoms do in fact pass adiabatically to the intermediate field regime for pulses with 1 /us risetimes. 

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