The beam lines rely on focusing the L2 and/or L3 laser into a gas jet or a gas cell. For an appropriate choice of experimental parameters (laser intensity, laser spot size and duration, and electron density in the gas), electrons are accelerated to relativistic energies by plasma wakefield acceleration and wiggled by the plasma itself (Betatron source) or by a second laser pulse (Compton source). This results in the emission of intense femtosecond X-ray or gamma-ray beams emitted from a micron-size source. The features of the radiation produced will depend on the needs of the end user. It will be possible to deliver either narrow spectrum (10% energy spread) or broadband radiation in a spectral range from keV to a few MeVs.
The following table summarizes performance requirements, which depend on the energies of incoming lasers L3 and L2 (for staging the output power, see more in the section on lasers).
Betatron Phase A
Betatron Phase B
100 keV–5 MeV
Pulse duration @ source
Chamber design and optical configurations
Two configurations are considered. The focal length of the focusing optics is chosen to reach a sufficient a0 parameter (a0 = 4–5) and to match the conditions for efficient laser wakefield acceleration in the bubble regime.
For the production of Betatron radiation, only one laser pulse (L3) will be used (the green laser in Figure 3). The laser is focused using a spherical mirror with a 3–5-meter focal length onto a gas jet or a gas cell. The choice of the focal length is defined to reach a laser strength parameter of a0 = 4 considering a 30 J/30 fs laser pulse. The electron density of the plasma will be in the order of 1018 cm–3 . Deciding to choose a gas jet or a gas cell will depend on the experiment. Gas cells allow for larger propagation distances to be reached and have tunable lengths. Gas jets are simple to implement and are supersonic with sharp density gradients. The gas length determines the length of acceleration and therefore the electron energy and the energy and flux of the radiation.
L3 and L2 laser pulses will be used for the Compton source. The first (the green laser in Figure 3) is used to accelerate electrons. The second is focused onto the electron beam. The radiation is produced at the collision of the laser and the electrons and is emitted in the direction of the electron beam. A third low energy laser pulse will be used in some cases to trigger the electron injection in the laser plasma accelerator. This will be used to inject electrons locally into the wakefield to produce nearly mono-energetic electrons with tunable energy. The energy will be adjusted by varying the collision position within the gas target. The Compton source requires the temporal overlap of L2 and L3 beams in the femtosecond range.
The following pictures illustrate two configurations for Betatron/Compton beam lines (Phase A and Phase B). In Phase A, the L3 (Stage 1) laser will be used for the Betatron beam line. To produce appropriate energy in gas, the "short" focusing off-axis parabola (f = 2 m) will be used.
The source of Betatron X-rays are electrons that are trapped in the ion cavity. They are positioned in the wake of the laser pulse—which is propagating in a gas—and perform oscillations caused by electrostatic forces generated by charge separation. These oscillations generate broadband radiation in the X-ray spectral domain.