  Attosecond bursts of coherent synchrotron-like radiation are found when
driving ultrathin relativistic electron disks in a quasi-one-dimensional regime
of wakefield acceleration, in which the laser waist is larger than the wake
wavelength. The disks of overcritical density shrink radially due to the
focusing wake fields, thus providing the transverse currents for the emission
of an intense, radially polarized, half-cycle pulse of about 100 attoseconds in
duration. The electromagnetic pulse first focuses to a peak intensity 10 times
larger ($7\times10^{20}\rm W/cm^2$) than the driving pulse and then emerges as
a conical beam. Saturation of the emission amplitudes is derived analytically
and in agreement with particle-in-cell simulation. By making use of gas targets
instead of solids to form the ultrathin disks, the new scheme allows for high
repetition rate required for applications.
