Extended Data Fig. 2: Constant betatron-mismatch bands for variable plasma density.

Energy spectra of the unscraped probe bunch of a working point similar to that of Fig. 2 as a function of delay after the start of the plasma-generating high-voltage discharge. The timing is scanned over a ~3 μs range, corresponding to a density decay of ~10¹⁷ to ~10¹⁵ cm⁻³. The far-left spectra of high intensity at t < 0 represent discharge times after the beam has already traversed the capillary—effectively a plasma off state. The betatron-mismatch bands of increased intensity at constant energy—highlighted by orange dashed lines—indicate that the betatron oscillations of the driving-beam head are independent of plasma density and may therefore be used as a signal for the curvature of the channel.