X-ray circular dichroism (XMCD), one of the main tools to study magnetism, benefits enormously from the capability of a fast alterable helicity of circularly polarized X-ray photons. Here we present a method for boosting the alternating frequency between right- and left-handed photons to the MHz regime, more than three orders of magnitude faster than state-of-the-art technologies. The method is based on a twin elliptical undulator installed in an electron storage ring being operated in a novel mode where the electron optics is tuned close to a resonance with electrons captured in transverse resonance island buckets. Propagating through the twin undulator, electrons from different islands emit photons of the same wavelength but of opposite helicity. These two helicity components can be alternated as fast as 2 ns. In a proof-of-principle experiment at BESSY II, we demonstrate XMCD at the L₂,₃ absorption edges of Ni with an 800 ns helicity flip.

X射线圆二色性（XMCD）是研究磁性的主要工具之一，它从圆偏振X射线光子的快速可变螺旋度的功能中受益匪浅。在这里，我们提出了一种将左右手光子之间的交流频率提高到MHz的方法，比最新技术快三个数量级。该方法基于安装在电子存储环中的双椭圆波荡器，该双椭圆波荡器以一种新颖的模式工作，其中电子光学器件被调谐到与横向共振岛桶中捕获的电子接近共振的状态。通过双波荡器传播，来自不同岛的电子发射相同波长但螺旋度相反的光子。这两个螺旋度分量可以交替高达2 ns的速度。₂，₃的Ni的吸收边缘与一个800纳秒螺旋翻转。