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Non-neutral electron plasmas confined in a uniform magnetic field have been studied as 2 dimensional (2D) fluids [1–6]. Assuming the correspondence between the 2D drift-Poisson equation and the 2D Euler equation [1], non-neutral electron plasmas are treated as ideal 2D (inviscid) Euler fluids. It means for non-neutral electron plasmas that the cyclotron motion of electrons in the magnetic field is neglected by the guiding center approximation, that the collisions between electrons are neglected, and that the effects of confining potentials are negligible. In fact, when the system is approximated as two point vortices [3, 4], it was confirmed experimentally that the system followed the Hamiltonian formulation by Onsager [7]. Thus, a trajectory of vortex motion is restricted to a constant Hamiltonian. Also, various point vortex crystals were studied with and without background vorticity [6, 8–11]. Merging processes of vortices were also studied with special emphasis on the geometrical structure [2, 10, 11] and viscosity [12]. In Ref. [12], a global viscosity is estimated with enstrophy and palinstrophy. The presence of viscosity during the vortex merging process suggests that 2D Euler picture is not applicable. Here, a similar vortex merging process is investigated experimentally from different view points, i.e., an intensity (vorticity) histogram and a phase space volume. The experimental apparatus is composed of an electron gun, solenoidal coils, 45 ring electrodes and phosphor screen with CCD camera. Electrons are confined inside a set of ring electrodes with the inner diameter of 70 mm in a uniform axial magnetic field Bz ∼ 250 G. Potential well formed by negative voltages applied to both ends of electrodes provides axial confinement and the magnetic field provides radial confinement. An initial state of hollow den-

Phase Space Volumes of Non-Neutral Electron Plasmas during Vortex Merging Processes