Derek S. Thompson

Confocal Laser Induced Fluorescence with Comparable Spatial Localization to the Conventional Method

We present measurements of ion velocity distributions obtained by laser induced fluorescence (LIF) using a single viewport in an argon plasma. A patent pending design, which we refer to as the confocal fluorescence telescope, combines large objective lenses with a large central obscuration and a spatial filter to achieve high spatial localization along the laser injection direction. Models of the injection and collection optics of the two assemblies are used to provide a theoretical estimate of the spatial localization of the confocal arrangement, which is taken to be the full width at half maximum of the spatial optical response. The new design achieves approximately 1.4 mm localization at a focal length of 148.7 mm, improving on previously published designs by an order of magnitude and approaching the localization achieved by the conventional method. The confocal method, however, does so without requiring a pair of separated, perpendicular optical paths. The confocal technique therefore eases the two window access requirement of the conventional method, extending the application of LIF to experiments where conventional LIF measurements have been impossible or difficult, or where multiple viewports are scarce.

Models, assumptions, and experimental tests of flows near boundaries in magnetized plasmas

We present the first measurements of ion flows in three dimensions (3Ds) using laser-induced fluorescence in the plasma boundary region. Measurements are performed upstream from a grounded stainless steel limiter plate at various angles ( ψ=16° to 80°) to the background magnetic field in two argon helicon experiments (MARIA at the University of Wisconsin-Madison and HELIX at West Virginia University). The Chodura magnetic presheath model for collisionless plasmas [R. Chodura, Phys. Fluids 25, 1628 (1982)] is shown to be inaccurate for systems with sufficient ion-neutral collisions and ionization such as tokamak scrape off layers. A 3D ion fluid model that accounts for ionization and charge-exchange collisions is found to accurately describe the measured ion flows in regions where the ion flux tubes do not intersect the boundary. Ion acceleration in the E→×B→ direction is observed within a few ion Larmor radii of the grounded plate for ψ=80°. We argue that fully 3D ion and neutral acceleration in the plasm...

Spatial structure of ion beams in an expanding plasma

We report spatially resolved perpendicular and parallel, to the magnetic field, ion velocity distribution function (IVDF) measurements in an expanding argon helicon plasma. The parallel IVDFs, obtained through laser induced fluorescence (LIF), show an ion beam with v ≈ 8000 m/s flowing downstream and confined to the center of the discharge. The ion beam is measurable for tens of centimeters along the expansion axis before the LIF signal fades, likely a result of metastable quenching of the beam ions. The parallel ion beam velocity slows in agreement with expectations for the measured parallel electric field. The perpendicular IVDFs show an ion population with a radially outward flow that increases with distance from the plasma axis. Structures aligned to the expanding magnetic field appear in the DC electric field, the electron temperature, and the plasma density in the plasma plume. These measurements demonstrate that at least two-dimensional and perhaps fully three-dimensional models are needed to accur...

Pressure dependence of an ion beam accelerating structure in an expanding helicon plasma

We present measurements of the parallel ion velocity distribution function and electric field in an expanding helicon source plasma plume as a function of downstream gas pressure and radial and axial positions. The ion beam that appears spontaneously in the plume persists for all downstream pressures investigated, with the largest parallel ion beam velocities obtained for the lowest downstream pressures. However, the change in ion beam velocity exceeds what would be expected simply for a change in the collisionality of the system. Electric field measurements confirm that it is the magnitude of the potential structure responsible for accelerating the ion beam that changes with downstream pressure. Interestingly, the ion density radial profile is hollow close to the end of the plasma source for all pressures, but it is hollow at downstream distances far from the source only at the highest downstream neutral pressures.

Ion heating and flows in a high power helicon source

We report experimental measurements of ion temperatures and flows in a high power, linear, magnetized, helicon plasma device, the Resonant Antenna Ion Device (RAID). Parallel and perpendicular ion temperatures on the order of 0.6 eV are observed for an rf power of 4 kW, suggesting that higher power helicon sources should attain ion temperatures in excess of 1 eV. The unique RAID antenna design produces broad, uniform plasma density and perpendicular ion temperature radial profiles. Measurements of the azimuthal flow indicate rigid body rotation of the plasma column of a few kHz. When configured with an expanding magnetic field, modest parallel ion flows are observed in the expansion region. The ion flows and temperatures are derived from laser induced fluorescence measurements of the Doppler resolved velocity distribution functions of argon ions.

Direct measurements of classical and enhanced gradient-aligned cross-field ion flows in a helicon plasma source using laser-induced fluorescence

Direct laser induced fluorescence measurements are shown of cross-field ion flows normal to an absorbing boundary that is aligned parallel to the axial magnetic field in a helicon plasma. We show Langmuir and emissive probe measurements of local density and plasma potential in the same region, as well as floating probe spectra near the boundary. With these measurements, we investigate the influence of ion-neutral collisionality on radial ion transport by varying the ratio of the ion gyro-radius, ρi, to the ion-neutral collision length, λ, over the range 0.34 ≤ ρiλ−1 ≤ 1.60. Classical drift-diffusion transport along density and potential gradients is sufficient to describe flow profiles for most cases. For two parameter regimes (ρiλ−1 = 0.65 and 0.44), low-frequency electrostatic fluctuations (f < 10 kHz) and enhanced cross-field bulk ion flow to the boundary are observed.

Demonstration of confocal laser induced fluorescence at long focal lengths

We add a collection path obscuration to a confocal telescope and confirm theoretical predictions of significant improvement in the longitudinal spatial localization. The improvements of spatial localization permitted an extension of the confocal telescopes focal length from 150 mm to 500 mm. At this longer focal length, millimeter-scale spatial localization is confirmed by comparing radial profiles of metastable state density obtained via confocal and conventional optical arrangements in a helicon source. The long focal length arrangement enables the measurement of argon neutral velocity distribution functions in the conventionally inaccessible region under a helicon source antenna.

Zeeman splitting measurements of magnetic fields in iodine plasma

Iodine is an attractive propellant for next generation ion thrusters. Laser induced fluorescence (LIF) is widely used with other propellant species as a non-perturbative technique for measuring flow for thruster prediction models. We apply LIF methods recently demonstrated for singly-ionized iodine to a magnetized plasma environment similar to those found in ion thrusters and in magnetically confined laboratory plasmas. We demonstrate the feasibility of remotely determining the local magnetic field from the Zeeman effect-split spectrum of I+.

Micro-spectrometer for fusion plasma boundary measurements

In situ probes are being developed to make direct, spatially resolved measurements of the ion energy spectra in the edge of tokamak plasmas while being easily replaced and requiring minimal resources. The ion spectrometers will consist of a combined collimator and energy analyzer fabricated from silicon and mated to a detector to yield a form factor of approximately 2.0 cm × 1.5 cm × 0.2 cm. Results of fabrication and testing of the combined collimator and energy analyzer element are presented.