Structure of luminous clouds near a sublimated hydrogen pellet

The spatial structure of a luminous cloud near a hydrogen pellet sublimated in a tokamak plasma is investigated experimentally. The luminous cloud has the form of a constant-intensity plateau extended along the magnetic field and is surrounded by an exponentially fading halo. The cross size of the plateau is determined by the boundary of the ionization region of atoms leaving the pellet surface. The plateau length is approximately determined by the balance between the pressures in the luminous cloud and the bulk plasma. Taking into account the structure described is important for the correct interpretation of the signals from alpha-particle diagnostics by charge-exchange neutral fluxes, because these signals depend on the characteristic dimensions of the cloud.