John Heard

Evolution of the parallel and perpendicular ion velocity distribution functions in pulsed helicon plasma sources obtained by time resolved laser induced fluorescence

The temporal evolution of parallel and perpendicular ion velocity distribution functions (ivdf ) in a pulsed, helicon-generated, expanding, argon plasma is presented. The ivdf’s temporal evolution during the pulse was determined with time resolved (1 ms resolution), laser induced fluorescence. The parallel ivdf measurements indicate that, in the expansion region of the plasma and for certain operational parameters, two ion populations exist: a population moving at supersonic speeds (1.1 Mach) resulting from acceleration in an electric double layer (EDL) and a slow moving population (0.7 Mach) generated by local ionization. After 100 ms, although present, the EDL is not fully developed and has not reached a steady-state. Measurements of the perpendicular ivdf indicate constant radial expansion, with ion speeds of ≈ 400 ms −1 , in the expansion region.

Laser induced fluorescence in a pulsed argon plasma

A time-resolved laser induced fluorescence (LIF) technique for pulsed argon plasmas is described. A low power, tunable diode laser pumps a three level Ar II transition sequence at a wavelength of 668.6138 nm. With a standard LIF system designed for steady-state plasmas (e.g., 4 kHz optical chopper, 20 kHz band-width detector, and a lock-in amplifier), we demonstrate that the evolution of the ion velocity distribution can be resolved with a time resolution of 1 ms through a combination of time-series averaging and post-acquisition digital signal processing.

Upper limit on turbulent electron temperature fluctuations in the core of Alcator C-Mod

Electron cyclotron emission correlation radiometry is used to measure the local turbulent electron temperature fluctuations in the plasma core of the Alcator C-Mod tokamak. Using standard analysis techniques, we see no evidence of the broadband fluctuations seen in other devices. From an analysis of the diagnostic resolution, coupled with these data, we can place an upper limit on the measurable fluctuation amplitude and set bounds for the wave number spectrum required for the electrostatic turbulence model of anomalous heat flux to be valid.

High resolution ECE radiometer for electron temperature profile and fluctuation measurements on Alcator C-Mod

Abstract A broadband heterodyne radiometer has been installed on Alcator C-Mod to measure second harmonic electron cyclotron emission at 234–306 GHz. The high-resolution diagnostic is now operational with 32 channels separated by

A 300 GHz collective scattering diagnostic for low temperature plasmas

A compact and portable 300 GHz collective scattering diagnostic employing a homodyne detection scheme has been constructed and installed on the hot helicon experiment (HELIX). Verification of the homodyne detection scheme was accomplished with a rotating grooved aluminum wheel to Doppler shift the interaction beam. The HELIX chamber geometry and collection optics allow measurement of scattering angles ranging from 60 degrees to 90 degrees. Artificially driven ion-acoustic waves are also being investigated as a proof-of-principle test for the diagnostic system.