An electron‐free plasma consisting of negative ions (SF6−) and positive ions (Ar+), and negligible neutral‐ion collision frequencies has been created in the laboratory. This plasma has a mass ratio of approximately 3.5‐similar to many computer particle‐in‐cell simulated systems. A fluid description of this positive and negative ion confinement (PANIC) plasma is given and compared to experimental measurements of a beam–plasma instability for both beam species and a wide range of beam energies. The fluid dispersion relation and most growing modes are predicted to be insensitive to many parameters of the PANIC beam–plasma system, and found to the consistent with the data.