3.2.1 Principle of operation
In the synchroscan operation of a streak image tube, a sinusoidal waveform voltage signal at a high frequency (e.g. > 100 MHz) is applied to the deflection system synchronously with the repetitive input light signal on the photocathode. When the deflection signal frequency is equal to or multiple of the repetition frequency of the input light signal and is correctly phased, the streak images are superimposed and become integrated in intensity on the phosphor screen. A schematic of the operating principle is indicated below in Fig.3.2.1.
SINUSOIDAL DEFLECTION SIGNAL
SYNCHRONOUS INCIDENT LIGHT SIGNAL Fig. 3.2.1 Synchroscan operation principle
It is seen that if the time duration of the streaked images does not exceed the time window for the linear sweep of the deflection signal, the instrumental time or spatial resolution should be the limiting resolution of the streak image tube itself.
Chapters
3.2.2 Linearity of the sinusoidal deflection
The duration of the linear time window depends on the requirement of linearity in particular applications. The nonlinearity can be defined as
(Vt-Vt) 1 sin(cùt) (3.2.1)
t mt
The meaning of the variables are shown in Fig. 3.2.2a. Nonlinearity values at given time windows (i.e. photoelectron transit time in the deflectors) and deflection signal frequencies can be calculated by using expression (3.2.1) and are plotted in Fig.3.2.2b. It is seen clearly that higher deflection frequencies imply shorter time windows for a required linearity.
120 V(t) 500 MHz 80 - 6 0- 300 MHz 200 MHz 20 - 150 MHz t 0.0 0.2 0.4 0.6 0.8 1.0 1.2 TIME WINDOW (ns) (a) (b)
Fig.3.2.2 The nonlinearity of synchroscan deflection
For example, to ensure less than 5.0 % nonlinearity of deflection speed, the time window is
required to be no longer than 833 ps for 2(K) MHz and to be only 333 ps for 500 MHz. For
high deflection frequency operation, a shorter deflector length is necessary to ensure adequate linearity of the deflection speed and under no circumstances should the transit time of the photoelectrons exceed a half period of the deflection signal. There is, therefore, a trade-off between deflection sensitivity and repetitive frequency of operation which implies that an increase of deflection sweep speed can not be reached solely by increasing the operating frequency.
3.2.3 Dynamic range of synchroscan streak cameras
Since the resultant output image in synchroscan streak cameras is achieved by superimposing successive streak traces at a frequency of up to several hundred megahertz, the effective system gain for a single pulse can be as high as 10^ to 10^ whereas in single-shot cameras the use of intensifier can only provide 10^ to 10^. Thus the minimum acceptable current density for synchroscan streak cameras even without using an intensifier can be a factor
of 1Q2 to 1Q5 less than that for single-shot cameras and the use of an intensifier would further
extend its lower limit of detectable current density. Also the photocurrent limit associated with space-charge induced pulse broadening is no longer the limiting factor in the dynamic range of synchroscan streak cameras. Instead it is parameters such as phosphor screen saturation, damage thresholds and the dynamic range of the readout system chosen that establish the dynamic range of a synchroscan camera system. Therefore, the dynamic range of such systems is expected to be several orders of magnitude higher than that of single-shot cameras^»^.