  A key challenge in particle accelerators is to achieve high peak intensity.
Space charge is particularly strong at lower energy such as during injection
and typically limits achievable peak intensity. The beam stacking technique can
overcome this limitation by accumulating a beam at high energy where space
charge is weaker. In beam stacking, a bunch of particles is injected and
accelerated to high energy. This bunch continues to circulate, while a second
and subsequent bunches are accelerated to merge into the first. It also allows
the user cycle and acceleration cycles to be separated which is often valuable.
Beam stacking is not possible in a time varying magnetic field, but a fixed
field machine such as an Fixed Field Alternating Gradient Accelerator (FFA)
does not sweep the magnetic field. In this paper, we describe experimental
demonstration of beam stacking of two beams at KURNS FFA in Kyoto University.
The momentum spread and intensity of the beam was analysed by study of the
Schottky signal, demonstrating stacking with only a slight increase of momentum
spread of the combined beams. The intensity of the first beam was, however,
significantly reduced. RF knock-out is the suspected source of the beam loss.
