What I'd like to know is, what is the temperature at the tower, where all beams converge?
No matter what the target is made out of, without that circulation to withdraw heat, it wouldn't get much hotter before it vaporized and there was no target. No doubt it'd be possible to get a much higher temperature if that was the goal.
I've always wondered what they do if the conditions don't provide a high enough temperature... seems they'd have to drain it entirely of it's salt before it solidifies, or they'd never be able to get it restarted. That is, unless they have some form of heater all the way through the plumbing to melt the salt so it'll flow.
I first read about using salt this way back in (Popular Science? in) the early/mid-'80s with a parabolic reflector heating the salt to be pumped to a Stirling engine. They didn't address that question then either.
Molten salt is used for two reasons.
It is the best real-world conductor of heat you will find. If you heat a miles-long pipe of molten salt on one end, that heat will make its way to a temperature rise on the other end in surprisingly rapid fashion. But this is only true when it is molten. Naturally, if molten salt encounters its solid salt, the solid melts pretty easily to expand the volume of molten salt.
Molten salt stores quite a bit of energy in its phase change, and of course as a salt solidifies, it becomes more dense. This means the top salt (at the collector) will always be the slowest to solidify.
Not sure where all this worry about "freezing to death" comes from. The salt's effectiveness comes from its conduction, not its flow.
I also think having solid salt inside your pipes isn't that bad; the stuff is water soluble at room temperature. To get rid of clogged pipes, I would consider pumping in hot water, replacing it when it gets too salty. That might take lots of time, but is fairly safe (I think you would need to dry the inside of your tubing very well afterwards; molten salt and water probably isn't a nice combination)