A diagnostic helium (He) beam is planned that applies collisionally induced fluorescence of the beam atoms in metastable states to measure the time evolution of density fluctuations and the profiles of plasma density and electron temperature in magnetic confinement experiments. The metastable helium atoms are formed when a beam of He ions is neutralized by resonant charge exchange with atoms in a gas cell. In the process of neutralization, about 1 to 10% of the beam atoms are produced in the metastable 2'S and 2'S levels. Because the excitation rate from the metastable level is typically a factor of 10' to 104 larger than transitions from the ground state, collisionally induced fluorescence from the beam propagating through a plasma is significantly increased by the enhanced population of metastable states. The fluorescence signal from the probe beam is proportional to the local density and is used to measure the density fluctuations locally in a plasma. Both temporal and spatial information about the fluctuations should be obtainable. In addition, the excitation rates of some specific spectral transitions of He are very temperature dependent. Thereby, the ratio of the line intensity of the He transition that is temperature independent to one that is highly temperature dependent could yield a sensitive measurement of the electron temperature. These measurements could provide temporal and spatial data leading to a better understanding of tokamak plasma equilibrium, stability, and transport. Phase I of this project evaluates this diagnostic technique and performs a conceptual design of a diagnostic system for use on a tokamak, such as the Poloidal Beam Experiment-Modified, which could be implemented in Phase II.Anticipated Results/Potential Commercial Applications as described by the awardee:The successful completion of this research would lead to the development of a diagnostic technique for measuring of the fluctuation level of the plasma density and the electron temperature profile. This diagnostic capability has been requested by the national fusion program and would be suitable for the present generation of magnetic fusion devices as well as future generation devices. The planned work could lead to commercialization of an on-line instrument for the control of fusion reactor plasmas.
