Free-running generator

Q. Explain the difference between a free-running and a crystal-controlled radiofrequency generator 4.4.3 Sample Introduction... 

Lasers have been used to initiate deton in RDX. Three types of initiation mechanisms have been described (Ref 102) (1) instantaneous deton caused by a shock wave in a thin metallic film (deposited on the expl) with the shock wave generated by a Q-switched laser pulse (2) instantaneous deton by direct interaction of a Q switched laser pulse and the test expl and (3) DDT produced by free-running laser pulses. Coarse RDX cannot be initiated, but milled RDX (particle size less than 40 microns) is readily initiated at various packing densities. The threshold fluences for the initiation of 1.18g/cc l,52g/cc milled RDX via mechanism (1) are 45,3J/cm2 and 127.9J/cm2, respectively. Detons are either essentially instantaneous or the sample bums without deton. For direct initiation [mechanism (2)], the threshold laser energy for 1.18g/cc RDX was 0.8J, or the same as in thin film initiation. However, deton was no longer instantaneous but required about 2 microsec for build-up. The 1.52g/cc RDX was initiated directly without delay (laser energy not given)... 

Low energy (less than 10 J) free-running mode lasers will not generate intense shock waves in films or expls. They can be used to initiate expls via the DDT route. RDX columns... 

The beam was then made divergent with a lens. The wafer holder was positioned to ensure a uniform exposure of the resist. In line with the wafer holder was a Scientech 365 Power and Energy Meter equipped with a 2.5 cm diameter thermopile detector. Power/ pulse/cm was obtained while the laser was in a free running mode. The number of pulses needed for a particular dose was calculated. The laser was then triggered and controlled with a Hewlett Packard Model 3314A Pulse Generator. 

The free-running mode has an internal clock generator that periodically triggers the flash lamp. For long-duration degradation experiments, it is important that the irradiation lamp intensity reaching the measurement cell, MC, remains constant. 

TCSPC needs a timing reference signal from the light source. This is no problem for picosecond diode lasers, which deliver a trigger output pulse from the laser diode driver. For free-running solid-state lasers or jet-stream dye lasers, a suitable synchronisation signal can be generated by a photodiode. A simple solution is to use a fast PIN photodiode in one of the circuits shown in Fig. 7.42. 

While 50 O generators are generally on the order of 20% more expensive than the free-running oscillator type, they have some major advantages in certain circumstances ... 

In all free-running synchronous systems there is quick decay of synchrony. In the case of Tetrahymena this pertains to synchrony obtained with multiple heat shocks, with repetitive shocks, and by collection of normal cells in the division stage. In all cases the rate of decay of synchrony is about the same, which immediately tells us that no cure for the special weaknesses in the system obtained with multiple heat shocks (p. 134) can be expected to improve the synchrony greatly. Full synchronization would always be resisted by the existence of a natural spread in cell generation times in the population... 

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