Suppression of Fermi acceleration in composite particles

• We study the motion of a composite particle in a one dimensional billiard.
• If the billiard walls are fixed the motion is chaotic for low energies.
• If the walls move periodically Fermi Acceleration may occur.
• The internal degrees of freedom slow down the particle.
• The composite particle gains energy at a smaller rate than a point particle.

We study the motion of a composite particle in a one-dimensional billiard with a moving wall. The particle is modeled by two point masses coupled by a harmonic spring. We show that the energy gained by the composite particle is greatly reduced with respect to a single point particle. We show that the amount of energy transferred to the system at each collision with the walls is independent of the spring constant. However, the presence of the spring is responsible for the energy suppression because it diminishes the number of collisions by storing part of the system’s energy and reducing the velocity of the particle’s center of mass.