Shielding, pulsed lasers

Nonexpendable light sources, such as a Q-switched pulsed laser can be protected. from destructive forces encountered in the photography of explosive material by piping the light through fiber optics, to the experimental zone. Occasionally lens systems are used to relay the light from mirrors located near a protective barrier shielding the laser (Ref 16)... 

Since pulsed laser ionization produces well defined packets of ions and electrons, TOF analysers (which essentially are magnetically shielded, electric-field-free drift tubes with apertures and an electron multiplier) can readily be used. TOF resolution for slow electrons can approach 3 meV, and throughput is similar to that of electrostatic analysers operating without an extraction field (i.e. a detection efficiency < 1%). The kinetic energy is obtained from the flight time, which is proportional to the reciprocal velocity, (KE) /2, whereas the resolution varies as (KE)/2. Thus, the resolution for 1-5 eV electrons is comparable to that for electrostatic analysers, but degrades seriously for 5-10 eY electrons. 

A good shield for PZT against ambient noise and electrical pick-up . This is especially important when pulsed laser systems are used, where strong electromagnetic pulses are produced during the discharge. 

Safety Considerations. High-power lasers raise a number of safety issues. There are the flammability and the toxicity of dye solutions. Most importantly, the eye hazards of laser radiation require careful shielding of the beam, and interlocks that restrict access to the laser room and to the dome. The laser could also dazzle aircraft pilots if they look directly down the beam. It is therefore necessary to close a shutter in the beam when a plane comes too close, either manually by human spotters, or automatically by use of radar, thermal IR or CCD cameras. Care must also be taken to avoid hitting overhead satellites in the case of pulsed or high power laser systems. 

A resistor completed the circuit and signals were measured across the resistor. The laser was a flashlamp pumped tunable dye laser with a pulse duration of -1 ps and a peak power of several kW the bandwidth was 0.014 nm in the neighborhood of 589 nm. We used a stoichiometric H2-02 Ar flame of 1800 K, shielded with a mantle flame of identical composition. In the inner flame a 2500 pg/ml NaCl solution was nebulized. An extensive description of the experiment can be found elsewhere (7). 

Fig. 9.17 Range of soft ferrite components (i) TV scanning yoke (components kindly supplied by Philips Components Ltd.) (ii) UR core and TV line output transformer (iii) E core for switched mode power supply (iv) wide band transformer core (v) core giving good magnetic shielding (vi) high Q (adjustable) filter core (cf. Fig. 9.48) (vii) precision ferrite antenna for transponder (viii) multilayer EMI suppressors (ix) toroids for laser and radar pulse applications (x) typical EMI shields for cables, ((ii)—(x) Courtesy of Ferroxcube UK .)...

Deposition of the laser entrance window in the PLD chamber - reduced laser pulse energy - regular cleaning of the chamber entrance window after every growth run, shields between plume and window. 

The most popular lasers for ablation have nanosecond pulse lengths. Sufficiently high intensities for explosion-like ablation can be used and there is additional heating of the plasma gas. The intensities, the wavelength, the kind of gas and its pressure, must, however, also be chosen carefully in order to avoid plasma shielding of the sample. 

In this section we describe the details of the experimental setup, e.g., laser settings, pulse lengths, detection systems, etc. A picture of the experimental setup is shown in Fig. 1. In part (a) we see two cylindrical magnetic shields which each contain a paraffin coated vapour cell with cesium. The distance between the two cells is 35 cm. In part (b) of the figure we show schematically the timing of laser pulses and the detection system setup. 

The heat conductance through the sample and in the plasma is responsible for the fact that with the Nd YAG lasers available today, the crater diameters are still much wider than the values determined by the diffraction limitations. When using conventional lasers with pulses in the ns and ps range the plasma shields the radiation, whereas with the femtosecond lasers that are now available a free expanding plasma is obtained, where the heating of the plasma appears to be less supplemented by the laser radiation. This leads to less fractionated volatilization of the solid sample and differences in crater shape, which need to be investigated further [229]. 

FIGURE 14.1 Schematic top view of the crossed molecular beam apparatus. The two pulsed beam source chambers and the detector (electron impact + quadrupole mass filter) rotating chamber are visible. In the case of the CN radical beam source, the carbon rod holder and the incident laser beam are also sketched. The chopper wheel and the cold shield are also shown. 

Laser ablation with laser pulses in the femtosecond (fs) range yields unique advantages, that is, negligible heat affected zone, lower ablation threshold fluence, plasma shielding is not an issue, and the possibility to structure materials that are transparent at the irradiation wavelength. 

Provided that the pulse generator already exists, this is certainly the eheapest pi-eoseeond light source ever used. Of course, operating a laser diode this way is most likely not in eompliance with any laser safety regulations. In addition, RF emission can be a problem if the diode and the driving generator are not properly shielded. 

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