Saeid Houshmandyar

Time-resolved measurements of double layer evolution in expanding plasma

Observations in steady-state plasmas confirm predictions that formation of a current-free double layer in a plasma expanding into a chamber of larger diameter is accompanied by an increase in ionization upstream of the double layer. The upstream plasma density increases sharply at the same driving frequency at which a double layer appears. For driving frequencies at which no double layer appears, large electrostatic instabilities are observed. Time-resolved measurements in pulsed discharges indicate that the double layer initially forms for all driving frequencies. However, for particularly strong double layers, instabilities appear early in the discharge and the double layer collapses.

Enhanced neutral depletion in a static helium helicon discharge

Laser-induced fluorescence (LIF) measurements of plasma opacity are used as a novel diagnostic to determine the absolute density of a metastable state of neutral helium atoms in a helicon plasma. The absorption scale length at a wavelength of 587.725?nm (vacuum) is determined from measurements of fluorescence intensity as a function of distance along the laser path. With a collisional?radiative model of the state populations, the absolute ground state neutral helium density is estimated from the metastable state density measurement. This paper expands upon previous work through measurements of neutral density, temperature and flow at different radial positions. The measured neutral density decreases by two orders of magnitude from the edge of the plasma to the axis of the plasma source. When the helicon source is operated in a static mode (i.e. no active gas pumping) the on-axis neutral density decreases by 69% from the pumping case and the on-axis plasma density increases by 42%; yielding an ionization fraction of approximately 90%.

Ducted kinetic Alfven waves in plasma with steep density gradients

Given their high plasma density (n ∼ 1013 cm−3), it is theoretically possible to excite Alfven waves in a conventional, moderate length (L ∼ 2 m) helicon plasma source. However, helicon plasmas are decidedly inhomogeneous, having a steep radial density gradient, and typically have a significant background neutral pressure. The inhomogeneity introduces regions of kinetic and inertial Alfven wave propagation. Ion-neutral and electron-neutral collisions alter the Alfven wave dispersion characteristics. Here, we present the measurements of propagating kinetic Alfven waves in helium helicon plasma. The measured wave dispersion is well fit with a kinetic model that includes the effects of ion-neutral damping and that assumes the high density plasma core defines the radial extent of the wave propagation region. The measured wave amplitude versus plasma radius is consistent with the pile up of wave magnetic energy at the boundary between the kinetic and inertial regime regions.

Measurements of neutral helium density in helicon plasmas.

Laser-induced-fluorescence (LIF) is used to measure the density of helium atoms in a helicon plasma source. For a pump wavelength of 587.725 nm (vacuum) and laser injection along the magnetic field, the LIF signal exhibits a signal decrease at the Doppler shifted central wavelength. The drop in signal results from the finite optical depth of the plasma and the magnitude of the decrease is proportional to the density of excited state neutral atoms. Using Langmuir probe measurements of plasma density and electron temperature and a collisional-radiative model, the absolute ground state neutral density is calculated from the optical depth measurements. Optimal plasma performance, i.e., the largest neutral depletion on the axis of the system, is observed for antenna frequencies of 13.0 and 13.5 MHz and magnetic field strengths of 550-600 G.

High sensitivity, low noise Mirnov coil array on Prairie View rotamaka)

An array of 32 Mirnov coils with novel features of high sensitivity and low noise has been installed on the outside chamber surface of Prairie View rotamak. This B(R)-oriented coil array has proven to be very reliable in the plasma driven by rotating magnetic field; it can resolve magnetic perturbation signals of 0.1 G. With this new diagnostic, the n = 1 tilt, radial shift, and kink modes are observed for the first time in rotamak plasmas.

High resolution ion Doppler spectroscopy at Prairie View Rotamak.

A fast ion Doppler spectroscopy (IDS) diagnostic system is installed on the Prairie View Rotamak to measure ion temperature and plasma flow. The diagnostic employs a single channel photomultiplier tube and a Jarrell-Ash 50 monochromator with a diffraction grating line density of 1180 lines/mm, which allows for first order spectra of 200-600 nm. The motorized gear of the monochromator allows spectral resolution of 0.01 nm. Equal IDS measurements are observed for various impurity emission lines of which carbon lines exhibit stronger intensities. Furthermore, the diagnostics is examined in an experiment where plasma experiences sudden disruption and quick recovery. In this case, the IDS measurements show ~130% increase in ion temperature. Flow measurements are shown to be consistent with plasma rotation.