Dust particles with submicron radii have been observed in the scrape-off layer (SOL) and divertor region of DIII-D during normal plasma operation. The particles are observed via their relatively large cross section for Rayleigh scattering of the Nd : YAG laser used for Thomson scattering measurements of electron density and temperature. In the outer region of the SOL particles are rare, with an average density of about 6 × 10 3 particles m −3 . The particle density falls sharply going toward the last closed flux surface, being ∼0 at and inside the separatrix. Modelling of the observed pulse height distribution indicates an average particle radius of ∼80 nm. These measurements indicate that dust created by normal plasma operation is not a likely source of core carbon contamination on DIII-D, nor is it a serious contributor to redeposition of carbon films.

We introduce optical feedback frequency stabilized cavity ring-down spectroscopy (OFFS-CRDS), a near-shot-noise-limited technique that combines kilohertz resolution with an absorption detection sensitivity of 5×10(-13) cm(-1) Hz(-1/2). Its distributed feedback laser source is stabilized to a highly stable V-shaped reference cavity by optical feedback and fine-tuned by means of single-sideband modulation. The stability of this narrow laser is transferred to a ring-down (RD) cavity using a new fibered Pound-Drever-Hall (PDH) locking scheme without a dedicated electro-optic phase modulator, yielding several hundred RD events per second. We demonstrate continuous coverage of more than 7 nm with a baseline noise of 5×10(-12) cm(-1) and a dynamic range spanning six decades. With its resonant intracavity light intensity on the order of 1 kW/cm2, the spectrometer was used for observing a Lamb dip in a transition of carbon dioxide involving four vibrational quanta. Saturating such a weak transition at 160 μW input power, OFFS-CRDS paves the way to Doppler-free molecular overtone spectroscopy for precision measurements of hyperfine structures and pressure shifts.

We report a subkilohertz-linewidth distributed-feedback diode laser that is optical-feedback locked to a highly stable V-shaped cavity with drift rates below 20 Hz/s. This source is continuously tunable over 1 THz around 1590 nm by selecting a cavity mode and using an innovative single-sideband modulation scheme, which allows for frequency shifting over up to 40 GHz with millihertz accuracy. This robust setup achieves high performance without advanced vibration isolation and will be a powerful tool for metrological applications, in particular a redetermination of the Boltzmann constant by molecular spectroscopy.