Comment by londons_explore

> There's a big mismatch in coefficients of thermal expansion and silica is brittle.

The way these are manufactured together means the silica with the lower CTE solidifies first - giving a tube filled with molten aluminium. Next the aluminium solidifies. Then the whole thing cools down and the aluminium probably delaminated from the walls of the tube, leaving a gap of a few hundred micrometers. The aluminium also ends up stretching slightly (one time).

During use, the inner core will heat up and cool down, fairly substantially (perhaps by 100C), using that gap that formed as the cable was manufactured.