A portion of the light from the terawatt laser is used to generate electrons in the photocathode gun. These electrons are accelerated in the linac and focused to a very small focal spot at the interaction zone (IZ). The remainder of the light from the laser is amplified and compressed in time and then sent back along the path of the electron beam coming out of the linac. It too is focused to a very small spot at the IZ, where the electron beam and the laser beam collide head-on. X-rays are generated by this collision. They exit the machine in the direction that the electron beam was headed. Since the electron beam is tunable, the X-rays are tunable. Since the laser photons are all at the same frequency, the X-rays are all at the same wavelength.