Production of high density proton currents for colliding beam accelerators

A head-on collision of two 15 GeV protons produces an energy in the centre of mass equivalent to about 500 GeV with the target at rest. To achieve significant production of secondaries, intense beams of particles are required, and this is obtained by `stacking' successive pulses of particles at high energy. Limitations imposed by the accelerating system are examined and upper limits are given to the densities that can be obtained under different assumptions.