@Lego laser tech

As you stack the amplifiers you eventually get to the point that the beam will break the amplifying medium due to its intensity (I have seen cracked crystals, not a cheap thing to do). So you need a larger beam, which makes building the amplifiers more difficult. You also get similar problems if your beam isn't good quality as you get hot spots which can break the amplifier.

There is the same problem with the optics, but that is easier as you don't need to make the electrical connections to them.

JET info

Just for comparison (on the computing front), JET used an IBM 3090 for data processing. It needed to get all the processing done in about half an hour as that is how frequently plasmas were run. There was often some preprocessing done on Solaris boxes nearer to the diagnostics but that couldn't have access to the other data (magnetic field strengths etc) that was necessary for full processing.

My memory says that in the 90s it was producing 30GB of data per pulse but don't rely on that data...

As for actually supplying energy. JET demonstrated "Breakeven" which means that the fusion reaction generated more energy that it took to create and sustain the plasma. But that is before the efficiencies of conversion. Then there is the "Lawson criterion" which means that you take into account the electrical generation efficiencies and the efficiency of the equipment supplying the power (microwaves or neutral beams). Finally there is ignition where you kick it off and the plasma generates enough power internally to sustain its own reaction. This means that anything you collect is usable and once you have gone beyond the amount you needed to create the plasma you are actually generating something.

ITER was originally designed to achieve ignition but I think this was later deemed impossible.

@Dave

With respect to the laser the difference may not be great. However the impact on the target is different. If the energy is delivered in a short pulse then the energy is concentrated in a smaller area (as there is no time for it to be conducted elsewhere in the target) and hence the damage is greater at that target point. If a single pulse is incapable of doing any damage then yes it makes no difference but you might hope to ablate some surface away or damage a sensor with less average power using a pulsed laser.

Continuous operation vs continuous beam

Pulsed power is typically preferred. Consider the difference between having a litre of water poured over your hand every second to having a 1kg weight dropped onto it each second (from the same height). Now imagine that it is poured/dropped onto a nail sitting on your hand. The water might not do too much damage, the weight would but they require the same energy input to maintain as a continuous system.

However the power of the dropped weight is much larger as the time to absorb the energy is much lower.