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Sorry if this seems a stupid question to some, but I am having trouble finding a consistent answer....

The European XFEL hard X-ray headline proudly states that it is currently the fastest of its kind in the world, for example, at '27,000 pulses per second'. That would be about 37 milliseconds per second for each 'pulse'...

Does that mean the XFEL is actually 'on' for for half or even a third of that time...

Confused here....

P.S.: The Nobel for Physics was just given for 'attosecond' lasers, so this question is very timely...

Kurt Hikes
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The pulse rate probably tells you how often a pulse starts. So if the pulse rate is 1,000 per second (1 kHz) then a new pulse starts every millisecond (1 ms). How long each pulse is depends on the duty cycle of the laser pulsing or just the pulse length. The pulse length must be of course be shorter than the inverse of the pulse rate, but other than that there are no restrictions. A laser pulse may have a duration of a few femtoseconds (fs) but a repetition period of a microsecond (us) or millisecond (ms).

The pulse repetition period and the pulse duration (or pulse duty cycle) are, in theory, independently adjustable parameters.

duty cycle = pulse duration / pulse repetition period

Ed V gave an example in the commments: Ed V had an old eximer laser with a 100 Hz (10 ms repetition period) with 5 ns pulse duration. This is a duty cylce of $$ 5\times 10^{-7} = \frac{5 \text{ ns}}{10 \text{ ms}} $$

Jagerber48
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