Pulse width determination

For beam energies less than 20 MeV a gas Cerenkov cell would have to be pressurized to meet the criterion of /3n 1. Instead, it is more practical to use a thin fused silica plate as a Cerenkov radiator. The optical collection system should be aligned with the Cerenkov cone angle (6[, 47°) and the plate should be normal to the detection axis, at an angle to the electron beam, to minimize internal reflection problems. 

Transition radiation (TR) is generated when an energetic charged particle passes across an interface between regions of substantially different dielectric 

Transition radiation is considerably weaker than Cerenkov radiation, however since it is a surface phenomenon it avoids problems with radiator thickness and reflections inherent to Cerenkov-generating silica plates. Optical TR can be measured using a streak camera. An optical TR system has been used to time-resolve the energy spread of an electron macropulse in a free-electron laser facility [10]. Interferometry of coherent, far-infrared TR has been used to measure picosecond electron pulse widths and detect satellite pulses at the UCLA Satumus photoinjector, using charges on the order of 100 pC [11], 

Comparison of microwave power at two frequencies, for example 20 and 40 GHz, permits the determination of the pulse width. This method has been demonstrated by measuring the power induced in a pair of 25- and 36-GHz cavities attached to the beam line as pick-ups [12], and also by measurement of the power spectrum using a sweep oscillator [13], This method is extremely attractive because it does not intercept the beam it can therefore be used in real time to monitor pulse width during experiments. 

Other non-destructive pulse length diagnostic techniques which have been proposed are based on diffraction radiation, which is generated when a beam passes through an aperture [14], and on coherent Smith-Purcell radiation, which is induced by passing the beam over a conductive grating with a period comparable to the bunch length [15]. 

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In TOF-SIMS, the source of primary ions is pulsed at a rate of a few kHz. The pulse width is on the order of 1 ns. Secondary ions ejected from the sample surface are accelerated through a potential V and then drift through a field-free TOF analyzer with different velocities, depending on their masses. The drift velocity of an ion with charge-to-mass ratio zjm can be determined from the expression ... 

The apparatus for the PFAM film coating on the slider surface is shown in Fig. 1 (a). The film thickness was measured by the TOF-SIMS as shown in Fig. 1 (b). It used a pulsed primary Ga+ ion beam to impact the surface of the PFAM film with an inset energy of 15 keV, an extractor current of 2 fj,A, beam current of 600 pA, a pulse width of 17.5 ns, and a frequency of 10 kHz, respectively. The positive TOF-SIMS spectra on the slider surface is shown in Fig. 2 where the peaks at m/z 31, 50, 69, 100, and 131 in Fig. 2(a) correspond to the positive secondary ion fragments of CF+, CFj, C2F4, and C3F5, respectively. The peak at m/z 469 apparent in Fig. 2(b) corresponds to the ion C12H7F 15O2H+ which is the characteristic ion of PFAM molecules. Therefore, the positive TOF-SIMS spectra demonstrates the existence of PFAM film [24,25]. The thickness of the PFAM film can be determined... 

To carry out a spectroscopy, that is the structural and dynamical determination, of elementary processes in real time at a molecular level necessitates the application of laser pulses with durations of tens, or at most hundreds, of femtoseconds to resolve in time the molecular motions. Sub-100 fs laser pulses were realised for the first time from a colliding-pulse mode-locked dye laser in the early 1980s at AT T Bell Laboratories by Shank and coworkers by 1987 these researchers had succeeded in producing record-breaking pulses as short as 6fs by optical pulse compression of the output of mode-locked dye laser. In the decade since 1987 there has only been a slight improvement in the minimum possible pulse width, but there have been truly major developments in the ease of generating and characterising ultrashort laser pulses. 

A series of measurements in which the pump wavelength is varied reveal that at some energies the oscillations predominate for times beyond lOps, whilst at others the decay of population by curve-crossing wins out within 400 fs or so. The time resolution of the experiment is in this example is determined by the convolution of the two laser pulse widths, here about 125fs. 

It is important to avoid saturation of the signal during pulse width calibration. The Bloch equations predict that a delay of 5 1] will be required for complete restoration to the equilibrium state. It is therefore advisable to determine the 1] values an approximate determination may be made quickly by using the inversion-recovery sequence (see next paragraph). The protons of the sample on which the pulse widths are being determined should have relaxation times of less than a second, to avoid unnecessary delays in pulse width calibration. If the sample has protons with longer relaxation times, then it may be advisable to add a small quantity of a relaxation reagent, such as Cr(acac) or Gkl(FOD)3, to induce the nuclei to relax more quickly. 

The whole sequence of successive pulses is repeated n times, with the computer executing the pulses and adjusting automatically the values of the variable delays between the 180° and 90° pulses as well as the fixed relaxation delays between successive pulses. The intensities of the resulting signals are then plotted as a function of the pulse width. A series of stacked plots are obtained (Fig. 1.40), and the point at which the signals of any particular proton pass from negative amplitude to positive is determined. This zero transition time To will vary for different protons in a molecule. 

In practice it is usually unnecessary to determine exact pulse widths for each sample we can use approximate values determined for each probe-head, except in certain 2D experiments in which the accuracy of pulse widths employed is critical for a successful outcome. Proper tuning of the probehead is advisable, since pulse widths will normally not vary beyond 10% with well-tuned probeheads. 

Since there is a slight delay between when a pulse is switched on and when it reaches full power, an error may be introduced when measuring 90° or smaller pulses directly. If the 90° pulse width is required with an accuracy of better than 0.5 fi, then it may be determined more accurately by using self-compensating pulse dusters that produce accurate flip angles even when there are small (<10%) errors in the setting of pulse widths. 

The spin-echo is used to suppress the production of spurious signals due to field inhomogeneities or to eliminate errors in the setting of pulse widths. It is also possible to use the spin-echo to follow the decay of transverse magnetization and to determine the transverse relaxation time (7 2). How might we do this in practice ... 

Pulse width The time duration for which a pulse is applied determines the extent to which the magnetization vector is bent. 

OL behavior is assessed simply by monitoring the transmission of a (usually solution) sample as a function of the incoming laser fluence measured in joules per square centimeter (rather than intensity in watts per square centimeter).22,23 Limiting thresholds Fth, defined as the incident fluence at which the actual transmittance falls to 50% of the corresponding linear transmittance, are then commonly quoted. Since excited-state absorption processes generally determine the OL properties of molecules, the excited-state structure and dynamics are often studied in detail. The laser pulse width is an important consideration in the study of OL effects. Compounds (1-5)58-62 are representative non-metal-containing compounds with especially large NLO and/or OL... 

The determination of electron concentration by the frequency shift method is limited to time resolution greater than a few hundred nanoseconds and is therefore not applicable to liquids. The microwave absorption method can be used virtually down to the pulse width resolution. Under conditions of low dose and no electron loss, and assuming Maxwellian distribution at all times, Warman and deHaas (1975) show that the fractional power loss is related to the mean electron energy (E) by... 

To determine optical damage in bulk benzil crystals a Q-switched Nd YAG laser with 1KW peak power, pulse width of 0.1 ps and pulse repetition rate of 500Hz was used. The laser power was attenuated using a set of neutral density filters and focussed onto a bulk benzil crystal using a x10 microscope objective. No optical damage was observed with optical intensities of upto 100MW/cm - Also, no optical damage was observed in benzil cored fibres with similar optical intensities. 

Spectra were determined using a pulse width of 4 yseconds, which corresponds to a flip angle of 18° and a 1 second pulse delay time. The 4000 Hz spectrum was described using 8192 data points. 

Sinusoidal excitation provides only one harmonic at the modulation frequency. In contrast, pulsed light provides a large number of harmonics of the excitation repetition frequency. The harmonic content, the number of harmonics and their amplitude, is determined by the pulse width and shape.(25) For example, a train of infinitely short pulses provides an infinite number of harmonics all with equal amplitude. A square wave provides only three modulation frequencies with sufficient amplitude to be usable. Equation (9.74) gives the harmonic content of a train of rectangular pulses R(t) of D duty cycle (pulse width divided by period) and RP peak value ... 

Note that pins 14 and 75 on the 74123 are not modeled in the simulation. The pins are available for connection, but the circuit connected to the pins is not used in the simulation. Thus, Cl and R4 do not determine the pulse timing in the simulation. They are included for completeness and could be omitted if only a simulation is required. If you were constructing a PC board, you would need to include them. The pulse width is determined by one of the attributes of the 74123. 

Notice the attribute PUL5E=Un. This attribute determines the pulse width rather than the time constant of R4 and Cl. The results are shown on the following screen capture. To plot traces on different plots, add the first trace, select Plot and then Add Plot to Window to add a new plot add the second trace, select Plot and then Add Plot to Window to add a new plot and then add the third trace. Digital traces are automatically plotted on a separate section of the screen. 

This is a warning message and not an error message. The warning is that pin CEXT of U2A (our 74123) is connected to other components, and that this pin is not modeled. In a real 74xxl23 IC, R4 and Cl would determine the pulse width. In the model used by PSpice, pins 14 and 15 are not used and the pulse width is set by the property Pulse. This error message warns us that we might have made a mistake because we used these pins and they have no effect on the simulation. Pins 14 and 15 are provided in the event you want to create a PC board and will have components connected to pins 14 and 15. 

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