I want to convert pulses of long duration obtained from a comparator to pulses of short duration so that I drive a thyristor which I will use as a half wave rectifier. Please help. It's urgent.
you can use quantisers or shift registers
set up a linear (op) amp as an integrator. You can change the feedback capacitor to adjust the pulse length at the output.
It's not entirely clear what you are trying to do. The above answers are all valid for different interpretations of what you've written however there is yet another one which implies you want a stream of short pulses when the comparator pulse is "true". This would be achieved using a counter which outputs a series of short pulses when enabled by the long comparator pulse.
If you just want a half wave rectifier, use a rectifier.
I suspect you want to trigger or not trigger the thyristor with the output of the comparator? Why not apply the output of the comparator directly to the thyristor? Assuming it has the voltage and current required. thyristors can be DC triggered, you don't need a pulse
Use the comparator output to gate an oscillator that generates a stream of pulses to ensure triggering, or trigger a monostable to generate a single pulse. This monostable could be as simple as an RC high pass filter (differentiator) and a buffer following it (sometimes known as a half monostable).
The reason for using pulses is only so that a transformer can be used for isolation of the thyristor gate circuit from the rest of the circuit. Otherwise DC will work.