Simultaneous measurements of radial and poloidal wavenumbers using a heavy ion beam probe

Experimental studies of density, temperature, or potential fluctuations have been limited by the inability of the available diagnostics to determine radial and poloidal wavenumbers at the same plasma location. Laser scattering, for example, is only sensitive to the wavenumber perpendicular to the beam. Heavy ion beam probes obtain wave number information by observing two nearby locations along the beam (with two detectors) and thus are only sensitive to the wavenumber parallel to the beam. The authors report a technique under investigation that can permit the beam probe to determine both components of the wavenumber by adding a rapid, small amplitude, square wave sweep voltage modulation that moves the measurement point between two points displaced slightly from the beam axis. With four measurement points displaced along and perpendicular to the beam, k can be determined for fluctuation frequencies limited only by the beam sweep modulation and detector electronics