  Ultrafast high-brightness X-ray pulses have proven invaluable for a broad
range of research. Such pulses are typically generated via synchrotron emission
from relativistic electron bunches using large-scale facilities. Recently,
significantly more compact X-ray sources based on laser-wakefield accelerated
(LWFA) electron beams have been demonstrated. In particular, laser-driven
sources, where the radiation is generated by transverse oscillations of
electrons within the plasma accelerator structure (so-called betatron
oscillations) can generate highly-brilliant ultrashort X-ray pulses using a
comparably simple setup. Here, we experimentally demonstrate a method to
markedly enhance and control the parameters of LWFA-driven betatron X-ray
emission. With our novel Transverse Oscillating Bubble Enhanced Betatron
Radiation (TOBER) scheme, we show a significant increase in the number of
generated photons by specifically manipulating the amplitude of the betatron
oscillations. We realize this through an orchestrated evolution of the temporal
laser pulse shape and the accelerating plasma structure. This leads to
controlled off-axis injection of electrons that perform large-amplitude
collective transverse betatron oscillations, resulting in increased radiation
emission. Our concept holds the promise for a method to optimize the X-ray
parameters for specific applications, such as time-resolved investigations with
spatial and temporal atomic resolution or advanced high-resolution imaging
modalities, and the generation of X-ray beams with even higher peak and average
brightness.
