[en] Direct observation of ICRF (∼80 MHz) waves (k_p≅0.5 cm^-1-10 cm^-1) is now possible in Alcator C-Mod using an optical heterodyne technique on the Phase Contrast Imaging (PCI) system, which uses a CO₂ laser to observe electron density fluctuations. The PCI observations are vertical chord averages, so the full-wave ICRF code TORIC [1] has been used to simulate the wave fields in these plasmas to aid in interpretation. Mode-converted ion Bernstein waves (IBW) in plasmas composed of H, D and ³He at 6 T have been observed at both high (1 MA) and low (400 kA) current. The fast magnetosonic wave launched from the low-field side has been observed in high density (∼5x10²⁰ m^-3) D(H) plasmas with off-axis ICRH at 4.5 T. Comparison between PCI measurements and code results are presented. The measured wave numbers are in good agreement with the local dispersion relations for both types of waves

[en] A new array of loop probes has been installed behind the inner wall tiles, opposite one of the RF antennas. These probes measure the power transmitted through the plasma core. During fundamental minority fast-wave heating in D(H), the transmission factor decreases with increasing minority concentration (scanned shot-to-shot), in agreement with the predictions of a simple analytic theory and of the FELICE full wave code. During a shot the transmission is observed to vary synchronously with the sawtooth instability in the plasma center. The sense of this variation is observed to reverse as the minority concentration is raised. Both observations are consistent with the ejection of minority ions from the core during a sawtooth crash

[en] Phase contrast imaging (PCI) diagnostic has been used to study mode conversion physics of ion cyclotron range of frequency waves [E. Nelson-Melby et al., Phys. Rev. Lett. 90, 155004 (2003)], plasma turbulence [A. Mazurenko et al., Phys. Rev. Lett. 89, 225004 (2002); N. Basse et al., Phys. Plasmas 12, 052512 (2005)], and Alfven Cascades [M. Porkolab et al., IEEE Trans. Plasma Sci. 34, 229 (2006)] in Alcator C-Mod. The C-Mod PCI system measures line integrated electron density fluctuations along 32 vertical chords with a sampling frequency of 10 MHz and wavenumber resolution up to 30 cm^-1. Although PCI normally lacks localization along the chords, the vertical variation of the magnetic field pitch angle allows for localized measurements for large k_{perpendicular} fluctuations. A system consisting of a partially masked phase plate on a rotatable stage has been installed and quasicoherent modes with wave number ∼5 cm^-1 associated with the enhanced DαH mode at the top and bottom of the plasma have been differentiated. In future experiments, for k∼30 cm^-1, a spatial resolution of r/a∼0.3 can be achieved under ideal conditions

[en] Mode converted ion cyclotron wave (ICW) has been observed with phase contrast imaging (PCI) in D-³He plasmas in Alcator C-Mod. The measurements were carried out with the optical heterodyne technique using acousto-optic modulators which modulate the CO2 laser beam intensity near the ion cyclotron frequency. With recently improved calibration of the PCI system using a calibrated sound wave source, the measurements have been compared with the full-wave code TORIC, as interpreted by a synthetic diagnostic. Because of the line-integrated nature of the PCI signal, the predictions are sensitive to the exact wave field pattern. The simulations are found to be in qualitative agreement with the measurements.

[en] Rapid changes in the loading resistance of fast wave antennas can limit high power operations of heating systems in the ion cyclotron range of frequencies (ICRF). Although novel matching techniques are being developed to reduce their effects, understanding the physics involved in these variations is of interest to guide and facilitate the design effort. We have studied the dependence of the loading resistance upon plasma parameters for the three ICRF antennas in the Alcator C-Mod tokamak. In contrast with similar studies in JET and Tore Supra, the evanescent decay term was not found to play an important role. The dominant variations could be related to changes in the shape of the electron density profiles in the propagating region. In H-mode, the loading resistance decreases as the density at the top of the pedestal is increased, and increases for higher scrape-off-layer (SOL) densities. This dependence on global plasma parameters is generally identical for the three antennas, up to a proportionality constant, while local changes in front of an individual antennas could explain the residual discrepancy. To link the observations with theory, the surface impedance at the Faraday shield was calculated by solving the wave equation in a slab geometry using experimental radial density profiles. This approach leads to a good agreement with measurements over a wide range of operating conditions in L-mode, ELM-free and EDA H-mode plasmas, and it can be interpreted qualitatively in terms of impedance transformation in the SOL region. Implications for more complex modelling approaches are also discussed

[en] To utilize ion cyclotron range of frequency (ICRF) heating for ITER and future fusion reactors where high Z metallic plasma facing components (PFCs) will be employed, impurity production needs to be minimized and controlled. With high Z PFCs, the acceptable fractional concentration of high Z material in the plasma, tungsten ∼10^-5 and molybdenum ∼10^-4, is significantly more restrictive compared to low Z material, carbon ∼0.02. Furthermore, high power ICRF heating needs to be compatible with the use of low Z coatings, e.g. boronization, which in current tokamaks is used to control plasma radiation that has been shown to be very important for high performance H-modes, particularly in devices with high Z PFCs. In Alcator C-Mod, we have investigated the compatibility of high power ICRF heating with high performance plasmas and high-Z PFCs with, and without, boronization. Without boronization, excess radiation particularly from Mo, a strong edge radiator, resulted in lower H-factors. Upon boronization, record C-Mod stored energy and world record plasma pressures were achieved but the beneficial effect of boronization degrades after ∼50 MJ of injected power. The erosion rate is estimated to be quite significant at ∼10-15 nm/s. Areas outside the divertor were identified as the important Mo source and B erosion locations and found to be isolated to the active antenna. Furthermore, we observed that erosion rate associated with ICRF heating was unaffected by the heating scenario's single pass absorption

[en] Experimental studies of Mode Conversion Current Drive (MCCD) on Alcator C-Mod are reported. By sweeping the mode conversion layer through the inversion radius in D(³He) plasmas, the sawtooth period was changed from 3 to 12 ms and its evolution was found to be consistent with localized current drive. This demonstrates that MCCD can be used for sawtooth control. Loop voltage measurements of net MCCD on Alcator C-Mod did not yield conclusive results. This was attributed to low MCCD efficiencies in these plasmas and to the presence of large sawtooth oscillations, which complicate the loop voltage analysis

[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] Access to advanced operating regimes in the EAST tokamak will require a combination of electron-cyclotron resonance heating (ECRH), neutral beam injection (NBI) and ion cyclotron range frequency heating (ICRF), with the addition of lower-hybrid current drive (LHCD) for current profile control. Prior experiments at the EAST tokamak facility have shown relatively weak response of the plasma temperature to application of ICRF heating, with typical coupled power about 2 MW out of 12 MW source. The launched spectrum, at n_φ = 34 for 0-π -0-π phasing and 27 MHz, is largely inaccessible at line-averaged densities of approximately 2 × 10"1"9 m"−"3. However, with variable antenna phasing and frequency, this system has considerable latitude to explore different heating schemes. To develop an ICRF actuator control model, we have used the full-wave code TORIC to explore the physics of ICRF wave propagation in EAST. The results presented from this study use a spectrum analysis using a superposition of n_φ spanning −50 to +50. The low density regime typical of EAST plasmas results in a perpendicular wavelength comparable to the minor radius which results in global cavity resonance effects and eigenmode formation when the single-pass absorption is low. This behavior indicates that improved performance can be attained by lowering the peak of the k_|_| spectrum by using π/3 phasing of the 4-strap antenna. Based on prior studies conducted at Alcator C-Mod, this phasing is also expected to have the advantage of nearly divergence-free box currents, which should result in reduced levels of impurity production. Significant enhancements of the loading resistance may be achieved by using low k_|_| phasing and a combination of magnetic field and frequency to vary the location of the resonance and mode conversion regions. TORIC calculations indicate that the significant power may be channeled to the electrons and deuterium majority. We expect that implementation of these recommendations in EAST will yield substantial improvements in the net absorbed power that will greatly assist in the attempt to access advanced tokamak operating regimes

[en] Significant changes in the sawtooth period have been observed on the Alcator C-Mod tokamak during phased ion cyclotron range of frequencies (ICRF) operation in the mode conversion regime. As the mode conversion layer was swept outwards through the q = 1 surface in D(³He) plasmas, the sawtooth period was found to increase and then decrease for counter-current drive phasing. For co-current drive and heating phasings, it was observed to decrease and then increase. With 2 MW ICRF power, the period varied from 3 to 12 ms. The observed evolution is consistent with localized current drive by mode converted waves in the vicinity of the q = 1 surface. Simulations with the full wave code TORIC indicate that the electron heating and current drive are due to mode converted ion cyclotron waves. The observed evolution for symmetric (heating) phasing is difficult to attribute to localized heating, since temperature profile stiffness prohibits large changes in the resistivity gradient at the q = 1 surface. An alternative explanation is found in TORIC simulations, which predict co-current drive for symmetric phasing due to a strong up-down asymmetry in the ICW wave field