Cavity ring-down spectroscopy to measure negative ion density in a helicon plasma source for fusion neutral beams.

Cavity Ring Down Spectroscopy (CRDS) is used to measure the D- absolute density produced in the helicon plasma reactor RAID (Resonant Antenna Ion Device) at the Swiss Plasma Center. The birdcage geometry of the helicon antenna produces a homogeneous, high-density plasma column (n e ≅ 1.5 × 1018 m-3 in H2 and D2 at 0.3 Pa and 3 kW of input power) 1.4 m long. We present the CRDS experimental setup, its positioning on the RAID reactor, and how the mechanical and thermal effects of the plasma affect the measurement. First results in deuterium plasma confirm the production of negative ions (D-) with a significant density: an average value of 3.0 × 1016 m-3 of D- is obtained at 0.3 Pa and 5 kW of power input in Cs-free plasma. This result is in good agreement with calculations performed with the collisional radiative code YACORA.

Millimeter-Wave Beam Scattering by Field-Aligned Blobs in Simple Magnetized Toroidal Plasmas.

The first direct experimental measurements of the scattering of a millimeter-wave beam by plasma blobs in a simple magnetized torus are reported. The wavelength of the beam is comparable to the characteristic size of the blob. In situ Langmuir probe measurements show that fluctuations of the electron density induce correlated fluctuations of the transmitted power. A first-principles full-wave model, using conditionally sampled 2D electron density profiles, predicts fluctuations of the millimeter-wave power that are in agreement with experiments.

Optically isolated millimeter-wave detector for the Toroidal Plasma Experiment.

We have designed and built an optically isolated millimeter-wave detection system to prevent interference from a nearby, powerful, 2.45 GHz microwave source in millimeter-wave propagation experiments in the TORoroidal Plasma EXperiment (TORPEX). A series of tests demonstrates excellent system noise immunity and the ability to observe effects that cannot be resolved in a setup using a bare Schottky diode detector.

Cross-field motion of plasma blobs in an open magnetic field line configuration.

The radial propagation of blobs generated from plasma instabilities is investigated in an open magnetic field line configuration. Blob cross-field velocities and sizes are obtained from internal probe measurements using pattern recognition. By varying the ion mass, the normalized vertical blob scale a[over] is scanned from a[over] < 1 to a[over] > 1. An analytical expression for the blob velocity including cross-field ion polarization currents, parallel currents to the sheath, and ion-neutral collisions is derived and shows good quantitative agreement with the experimental data. In agreement with previous theoretical studies, this scaling shows that, for a[over] < 1, the blob velocity is limited by cross-field ion polarization currents, while for a[over] > 1 it is limited by parallel currents to the sheath.

Universal statistical properties of drift-interchange turbulence in TORPEX plasmas.

A unique parabolic relation is observed to link skewness and kurtosis of around ten thousand density fluctuation signals, measured over the whole cross section of a toroidal magnetized plasma for a broad range of experimental conditions. All the probability density functions of the measured signals, including those characterized by a negative skewness, are universally described by a special case of the Beta distribution. Fluctuations in the drift-interchange frequency range are necessary and sufficient to assure that probability density functions can be described by this specific Beta distribution.