[en] A new two-dimensional full-wave code has been developed to simulate ordinary (O) mode reflectometry signals caused by plasma density fluctuations. The code uses the finite-difference time-domain method with a perfectly matched layer absorption boundary to solve Maxwell's equations. Huygens wave sources are incorporated to generate Gaussian beams. The code has been used to simulate the reflectometer measurement of the quasicoherent mode (60--250 kHz) associated with enhanced D_α (EDA) H modes in the Alcator C-Mod tokamak. It is found that an analysis of the realistic experimental layout is essential for the quantitative interpretation of the mode amplitude

[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 high spatial resolution CCD based one-dimensional imaging system to measure visible continuum emissivity profiles from Alcator C-Mod tokamak plasmas is described. The instrument has chordal resolution that is better than 1 mm for the edge region of the plasma, where very sharp (1 to 10 mm) gradient lengths in plasma parameters are observed after the formation of the H-mode transport barrier. Each image has up to 2048 pixels, and total spatial coverage goes from 2 cm inside of the magnetic axis to ∼4 cm outside of the last closed flux surface in the ∼22 cm horizontal minor radius plasmas. Time resolution can be varied from 0.21 ms to 4 ms; good signal to noise is achieved with 1 ms integration under typical plasma conditions. The emission over most of the plasma volume is dominated by free--free bremsstrahlung, and can be used to infer local values of the average ion charge (Z_eff). Toroidally localized puffing of deuterium, nitrogen, and helium reveals that a significant contribution to the signal in the scrape-off layer at the extreme edge of the plasma can come from diatomic molecular band pseudocontinuum emission

[en] An eight-channel beam-emission-spectroscopy (BES)1 system has been installed on the Alcator C-Mod tokamak, intended for use with a diagnostic neutral hydrogen beam (DNB). Capable of localized measurements from the plasma edge to the plasma core, the BES diagnostic collects light from the first Balmer transition (H_α) resultant from beam/plasma collisions. The H_α line splits into several components whose central wavelengths depend on the viewing geometry, the magnetic field, and the beam energy. This is due to the Doppler shifts from viewing the beam off perpendicular, the different velocities of the three mass components of the beam (H, H₂, H₃), and the large motional Stark effect. Optimal signal-to-noise requires collecting these components while attenuating all other emission: primarily bremsstrahlung and D_α radiation (from plasma D⁰/e^- collisions). Tunable bandpass filters are thus required. A BES simulation code has been developed that calculates the brightnesses (bremsstrahlung, D_α, H_α) versus wavelength using plasma profile data from the C-Mod MDSplus database,2 a computation of the beam penetration, the viewing and DNB geometries, and bandpass filter characteristics. The model was first used to estimate signal levels and choose the optimal BES bandpass filters; its ultimate purpose is to determine the shot-to-shot tuning requirements of the filters for different discharge conditions. Comparisons of measured and predicted background bremsstrahlung and D_α brightnesses are presented, as are first measurements and calculations of the beam emission. The code is written in the IDL programming language3 utilizing the ''widget'' graphical user interface. Designed for geometrical and spectral flexibility, it can be modified to simulate other beam diagnostics such as motional-Stark-effect plasma current measurements and charge-exchange recombination spectroscopy, as well as passive diagnostics measuring chord-averaged spectral emission

[en] Spontaneous toroidal rotation, self-generated in the absence of an external momentum input, exhibits a rich phenomenology. In L-mode plasmas, the rotation varies in a complicated fashion with electron density, magnetic configuration and plasma current and is predominantly in the counter-current direction. The rotation depends sensitively on the balance between the upper and lower null and plays a crucial role in the H-mode power threshold. Rotation inversion between the counter- and co-current directions has been observed following small changes in the electron density and plasma current, with very distinct thresholds. In contrast, the intrinsic rotation in H-mode plasmas has a relatively simple parameter dependence, with the rotation velocity proportional to the plasma stored energy normalized to the plasma current, and is nearly always directed co-current. In plasmas with internal transport barriers, formed either with off-axis ICRF heating or LHCD, the core rotation velocity increments in the counter-current direction as the barrier evolves.

[en] Plasma flow at near-sonic speed is observed in the high-field region of the scrape-off layer (SOL) in Alcator C-Mod [1]. The principal drive is identified as a ballooning-like cross-field transport mechanism: plasma streams along magnetic field lines from low- to high-field regions in response to poloidal pressure variations; these are maintained by poloidally asymmetric cross-field transport. Thus, the largest component of the parallel flow is a transport-driven flow. As a result of the drive mechanism, the poloidal location of an X-point or limiter contact point determines the magnitude and direction of the transport-driven flow. In single-null discharges with BxΔB pointing toward (away from) the X-point, the flow circulates the confined plasma toroidally in the co-current (counter-current) direction. Consequently, the SOL possesses co-current (or counter-current) volume-averaged toroidal momentum. Depending on discharge conditions, the momentum couples across the separatrix and affects the toroidal rotation of the confined plasma. Thus the SOL imposes a 'flow boundary condition,' accounting for a positive (negative) increment in central plasma co-rotation in L-mode discharges when BxΔB is toward (away from) the x-point. Experiments in ICRF-heated discharges suggest that this boundary condition and its Xpoint dependence may explain the sensitivity of L-H power threshold to X-point location: in a set of otherwise similar discharges, the L-H transition is seen to be coincident with central rotation achieving roughly the same value, independent of Xpoint location. For discharges with BxΔB pointing away from the X-point (i.e., with the flow boundary condition impeding co-current rotation), the same characteristic rotation can be achieved only with higher auxiliary input power. Remarkably, L-H power thresholds in lower-limited discharges are identical to those in lower X-point discharges; SOL flows are also similar, suggesting a connection. (Author)

[en] Backscattering experiments to detect lower-hybrid (LH) waves have been performed in Alcator C-Mod, using the two modified channels (60 GHz and 75 GHz) of an ordinary-mode reflectometry system with newly developed spectral recorders that can continuously monitor spectral power at a target frequency. The change in the baseline of the spectral recorder during the LH wave injection is highly correlated to the strength of the X-mode non-thermal electron cyclotron emission. In high density plasmas where an anomalous drop in the lower hybrid current drive efficiency is observed, the observed backscattered signals are expected to be generated near the last closed flux surface, demonstrating the presence of LH waves within the plasma. This experimental technique can be useful in identifying spatially localized LH electric fields in the periphery of high-density plasmas.

[en] We have carried out a detailed study of the dependence of ICRF mode conversion flow drive (MCFD) on plasma and RF parameters. The flow drive efficiency is found to depend strongly on the ³He concentration in D(³He) plasmas, a key parameter separating the ICRF minority heating regime and mode conversion regime. At +90 deg. antenna phasing (waves in the co-I_p direction) and dipole phasing, the driven flow is in the co-I_p direction, and the change of the rotation velocity increases with both P_RF and I_p, and scales unfavorably vs. plasma density and antenna frequency. When MCFD is applied to I-mode plasmas, the plasma rotation increases until the onset of MHD modes triggered by large sawtooth crashes. Very high performance I-mode plasmas with H_ITER98,y2∼1.4 and T_e0∼8 keV have been obtained in these experiments.

[en] A three-chord polarimeter on Alcator C-Mod will make measurements of the poloidal magnetic field and plasma fluctuations. The beams from two frequency-offset, 200 mW, FIR lasers operating at 117.73 μm are combined to produce collinear, counter-rotating, circularly polarized beams. The beams are divided into three chords which are directed into the plasma at one toroidal location. Corner cube retro-reflectors mounted on the inside wall return the beam for a double pass. The mixing product of the two beams is detected both before (reference) and after (signal) the plasma using polarization sensitive detectors that produce a beat signal at ∼ 4 MHz. During the plasma discharge, the phase delay of the signal mixer, which depends on the Faraday effect, is evaluated with respect to the reference and produces line-integrated information on the poloidal magnetic field. Measurements on C-Mod require the phase error to be at the 0.1 degree level, and great care in the design of optical mounts, polarizers, beam-splitters, focusing optics, and acoustic and magnetic shielding was required. Development of new planar diode Schottky detectors was necessary to provide high sensitivity for a diagnostic that will eventually have at least six chords. Absorption of the FIR laser light by water vapor requires that the entire beam path be purged with dry air. Six retro-reflectors on the inner wall arranged in an ITER-like configuration provide poloidally viewing chords from near the mid-plane to well into the plasma scrape off layer. A pneumatically controlled shutter protects the in-vessel optics during boronizations and during limited discharges that might accelerate damage to the retro-reflector surfaces. Tests indicate there is no measurable signal contamination from the toroidal magnetic field due to the Cotton-Mouton effect. Polarization sensitivity of the wire mesh beamsplitters necessitated system calibration. Good agreement to EFIT reconstructions has been observed along with plasma fluctuations up to 400 kHz.

[en] Plasma flow drive via ICRF mode conversion (MC) has been demonstrated on Alcator C-Mod. The toroidal rotation in these D(³He) MC plasmas is typically more than twice above the empirically determined intrinsic rotation scaling in ICRF minority heated plasmas. In L-mode plasmas at 3 MW ICRF power input, up to 90 km/s toroidal rotation and 2 km/s localized (r/a∼0.4) poloidal rotation has been observed. The MC ion cyclotron wave (ICW) was detected by a phase contrast imaging system in heterodyne setup. Through TORIC 2-D full wave simulation, and comparison with other experimental evidence, we hypothesize that the interaction between the MC ICW and the ³He ions may be the mechanism for the observed MC flow drive. TORIC simulation suggests that similar flow drive scenario may be realized on JET D(³He) plasmas. The promising scenarios on ITER are the inverted minority scenario (T)D and high field launch for T-D-(³He) plasma. In non-radioactive phase, these correspond to (³He)-H and ⁴He(³He) plasmas respectively.