Donald Umstadter and co-workers used a neodymium-glass laser to illuminate helium gas in a vacuum chamber. The team were able to focus 4 trillion watts of power on the gas. The electric field of the laser pulses ionizes the gas and causes the free electrons to oscillate back-and-forth in a straight line. The magnetic field of the laser interacts with the electrons through the Lorentz force. This force is normally very weak, but at high enough intensities the effects of the electric and magnetic fields on electron motion become comparable. The effect of the Lorentz force is to change the electron motion from oscillation in a straight line to a figure-of-eight pattern. This change causes the scattered light to be emitted as harmonics of the incident light. The researchers hope to use the technique to develop "table-top" X-ray sources.