[en] A major problem in the understanding of the airborne transport of radioactivity in mines is the lack of detailed knowledge of mass transfer to and fluid flow over rough walls under fully developed turbulent flow conditions. This paper will report the progress on a project that is designed to obtain that information

[en] 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 <1 cm and a frequency response of 1 MHz, measuring T_e on the low field side, for typical machine operation at B_T=5.4 T. We describe the operation of the radiometer based on two novel second harmonic mixers appropriate for these high frequencies. We discuss the design and implementation of the optical system, heterodyne mixer and intermediate frequency sections. First results are presented along with the calibration of the diagnostic and comparison to other ECE diagnostics. Future plans for the diagnostic as a correlation radiometer for temperature fluctuation measurements are also discussed

No abstract available

[en] A high-resolution Thomson scattering diagnostic is in operation on the Alcator C-Mod tokamak, measuring radial profiles of electron temperature and density at the plasma edge. Photons are scattered from a Nd--yttrium--aluminum--garnet laser beam pulsed at 30 Hz (1.3 J, 8 ns pulse), and are measured by a filter polychromator with four spectral channels. The polychromator measures T_e in the range of 15--800 eV and n_e of 0.3--3x10²⁰m^-3. Twenty scattering volumes are located about the last closed flux surface, spaced for a nominal resolution of 1.3 mm in midplane radial coordinates. High resolution is essential for measuring edge T_e and n_e profiles on C-Mod, since these quantities exhibit gradient scale lengths as small as 2 mm in H mode. The steep profiles at the H mode edge are fit to a parameterized pedestal function for ease of analysis. Measured profiles are compared with edge profiles from electron cyclotron emission and visible continuum diagnostics

[en] A Lower Hybrid Current Drive (LHCD) system has been installed on the Alcator C-MOD tokamak at MIT. Twelve klystrons at 4.6 GHz feed a 4x22 waveguide array. This system was designed for maximum flexibility in the launched parallel wave-number spectrum. This flexibility allows tailoring of the lower hybrid deposition under a variety of plasma conditions. Power levels up to 900 kW have been injected into the tokomak. The parallel wave number has been varied over a wide range, n_parallel ∼ 1.6-4. Driven currents have been inferred from magnetic measurements by extrapolating to zero loop voltage and by direct comparison to Fisch-Karney theory, yielding an efficiency of n₂₀IR/P ∼ 0.3. Modeling using the CQL3D code supports these efficiencies. Sawtooth oscillations vanish, accompanied with peaking of the electron temperature (T_e0 rises from 2.8 to 3.8 keV). Central q is inferred to rise above unity from the collapse of the sawtooth inversion radius, indicating off-axis cd as expected. Measurements of non-thermal x-ray and electron cyclotron emission confirm the presence of a significant fast electron population that varies with phase and plasma density. The x-ray emission is observed to be radially broader than that predicted by simple ray tracing codes. Possible explanations for this broader emission include fast electron diffusion or broader deposition than simple ray tracing predictions (perhaps due to diffractive effects).

[en] A high resolution edge Thomson scattering diagnostic is used in the Alcator C-Mod tokamak to study spatial structure of the edge plasma, with emphasis on the investigation of the edge transport barrier that forms in high confinement mode (H mode) of tokamak operation. The diagnostic employs a 30 Hz Nd:YAG laser (1.5 J, 8 nsec pulses) and a four spectral channel filter polychromator. Twenty scattering volumes, defined by collection optics geometry, span the region of the transport barrier, providing measurements of the electron density and temperature profiles with ∼1.5 mm spatial resolution. The system is capable of measuring values of T_e between 15 eV and 800 eV, and covers the electron density range from 3x10¹⁹ m^-3 to 5x10²⁰ m^-3. (author)

[en] An rf current drive system with an output power of 1 MW at 3.7 GHz is under development for the Tokamak de Varennes. The control system is based on an Ethernet local-area network of programmable logic controllers as front end, personal computers as consoles, and CAMAC-based DSP processors. The DSP processors ensure the PID control of the phase and rf power of each klystron, and the fast protection of high-power rf hardware, all within a 40 μs loop. Slower control and protection, event sequencing and the run-time database are provided by the programmable logic controllers, which communicate, via the LAN, with the consoles. The latter run a commercial process-control console software. The LAN protocol respects the first four layers of the ISO/OSI 802.3 standard. Synchronization with the tokamak control system is provided by commercially available CAMAC timing modules which trigger shot-related events and reference waveform generators. A detailed description of each subsystem and a performance evaluation of the system will be presented. (orig.)

[en] Design of the Alcator C-Mod Thomson scattering (TS) diagnostic is discussed and the results of the measurements are presented. The TS system has six spatial channels with observation volumes evenly distributed between the midplane and the edge of the plasma. Each channel is capable of measuring the electron density in the range N_e=5x10¹⁹ - 5x10²¹ m^-3 and temperature from T_e=200 eV to 10 keV. A 30 Hz, 1.5 J per pulse Nd-YAG laser is employed allowing the measurements of evolution of T_e and N_e profiles during plasma shot. A laser beam position control and feedback system provides for the beam alignment stability and reliable electron density measurements. Examples of the core density and temperature profiles measured at different stages of the plasma evolution are discussed. copyright 1999 American Institute of Physics

[en] ICRF provides Alcator C-Mod with up to 3.5 MW of power at 80 MHz using two double-strap antennas. By varying the magnetic field and the mix of ion species, a variety of heating regimes have been studied. To investigate electron heating regimes, mixtures of ³He and H were used in plasmas at 6.5T. Efficient (<80%) direct electron heating has been observed in this scenario and is attributed to mode conversion to the ion Bernstein wave (IBW). Measurements of the mode conversion power deposition using a nine channel grating polychromator (GPC) indicate that the IBW damping can be very strong, with central power densities >25MW/m³ and FWHM widths of ∼0.2a. The presence of >8% deuterium open-quotes impurityclose quotes in these plasmas is shown to significantly broaden the power deposition profiles. The GPC has also been used to study the heating mechanisms in the two standard C-Mod heating scenarios: D(H) and D(³He) minority heating at 5.3T and 7.9T respectively. Mode conversion can provide a significant fraction of the heating in D(³He) plasmas, with 60% efficiency and profiles which are peaked well off axis (r/a∼0.6) at the highest ³He concentrations (n_3He/n_e∼0.2). Data from D(H) experiments illustrate techniques to measure minority ion tails using electron temperature dynamics. In addition, evidence is presented for D(H) mode conversion heating at high hydrogen concentration. copyright 1997 American Institute of Physics

[en] A lower hybrid system is under development for current drive and heating on the Tokamak de Varennes. Near steady-state RF current drive and/or transformer recharge should allow long pulse operation of a diverted plasma for up to 30 sec at nearly full machine parameters (I_p∼240 kA, bar n_e∼3x10¹⁹ m^-3). The system design is for 1 MW of RF power at a relatively high N_double-prime (variable around 3.1) in a narrow spectrum (0.4 FWHM) at 3.7 GHz. Design considerations and technical solutions are presented