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High resolution measurements on the Alcator C-Mod tokamak [I.H. Hutchinson et al, Phys. Plasmas 1, 1551 (1994)] of the transport barrier in the "Enhanced Da" (EDA) regime, which has increased particle transport without large edge localized modes, show steep density and temperature gradients over a region of 2-5 mm, with peak pressure gradients up to 12 MPa/m. Evolution of the pedestal at the LH transition is consistent with a large, rapid drop in thermal conductivity across the barrier. A quasi-coherent fluctuation in density, potential and Bpol, with fo%7E50-150 kHz and kq%7E 4 cm-1, always appears in the barrier during EDA, and drives a large particle flux. Conditions to access the steady-state EDA regime in deuterium include d> 0.35, q95> 3.5 and L-mode target line average density> 1.2 x 1020 m-3. A reduced q95 limit is found for hydrogen discharges.
The tokamak is the principal tool in controlled fusion research. This book acts as an introduction to the subject and a basic reference for theory, definitions, equations, and experimental results. The fourth edition has been completely revised, describing their development of tokamaks to the point of producing significant fusion power.
Experiments were carried out to study the similarity of H-mode (High confinement mode) physics on the Alcator C-Mod and DIII-D tokamaks comparing plasmas with matched local edge dimensionless parameters ... and shape ... It was observed that matching local values of the dimensionless parameters on top of the H-mode pedestals produces similar Te and ne profiles across the entire pedestal region, with pedestal width scaling linearly with the machine size. Furthermore, in H-modes with matched pedestals, similar edge fluctuations were observed. Discharges in DIII-D with scaled pedestal parameters similar to those of C-Mod EDA (Enhanced D-alpha) H-mode showed a quasicoherent mode localized to the outer half of the pedestal, similar to the C-Mod quasicoherent mode (QC mode). The wavenumber of the mode observed on DIII-D matches the wavenumber of the C-Mod QC mode if scaled with the machine size. For higher pedestal temperatures and pressures, small high frequency ELMs (Edge Localized Modes) appear in both machines. Keywords: edge plasmas; H-mode; tokamaks; fluctuations; transport phenonmena.
The wide range of plasma parameters available on Alcator C-Mod has led to the accessibility of many regimes of operation. Since its commissioning, C-Mod has accessed the Linear ohmic confinement, Saturated ohmic confinement, L-Mode and ELM-free, ELMy and Enhanced D[alpha] H-Mode regimes. Recently, another novel regime, the IMode, has been identified[1][2][3][4]. I-modes feature the presence of steep H-Mode-like electron and ion temperature gradients at the edge of the plasma with L-Mode-like density profiles. The I-Mode, in contrast to the Hl-mode, shows very weak degradation of energy confinement with increased input power, and routinely reaches H98 > 1 while operating at low edge collisionalities ... making it a good candidate for reactor relevant tokamaks. Also relevant for reactors, this regime can be sustained in steady state for more than -15 energy confinement times without the need for ELMs to regulate particle and impurity confinement. Changes in edge density, temperature and magnetic field fluctuations accompany the L-mode to I-mode transition, with reduction of fluctuations in the 50-150kHz range as well as the appearance of a Weakly Coherent Mode (WCM) in the 200-300kHz range, analogous to the Quasi-Coherent Mode (QCM) characteristic of the Enhanced D[alpha] H-mode. Previous work[4] has established a connection between the midrange fluctuation suppression and reduction in the effective thermal diffusivity, Xye, in the pedestal region. The mechanism in I-mode for maintaining sufficient particle transport to avoid impurity accumulation and instabilities has been unclear. The O-mode reflectometry system has been extensively used for the characterization and detection of the I-mode and the WCM, in part, enhanced by upgrades to the system which enabled the baseband detection of density fluctuations at an array of cutoff locations at the edge of the plasma[5] [6] [7]. Using a novel model, the autopower signals of reflectometry channels detecting the density fluctuations have been decomposed into a broadband component and a WCM component. The latter is then used to estimate the intensity of the WCM. In parallel, the particle transport across the LCFS in I-mode plasmas has been estimated using a volume integrated particle transport model, where ionization source measurements are acquired using D[alpha] profiles measured near the outboard midplane. This model takes into account the anisotropic ionization source density around the periphery of the plasma by introducing an asymmetry factor, [sigma], which is then estimated using a study of I-Mode to H-Mode transitions. The results imply that measurements at the outboard midplane overestimate the surface-averaged influx. Finally, a comparison has been made between the particle flux across the LCFS of the I-mode and the intensity of the WCM, which shows a generally positive correlation between the two. This is supporting evidence that the WCM is, in fact, responsible for maintaining particle and impurity transport across the edge of the I-mode energy transport barrier.
Edge transport barriers (ETBs) in tokamak plasmas accompany transitions from low confinement (L-mode) to high confinement (H-mode) and exhibit large density and temperature gradients in a narrow pedestal region near the last closed flux surface (LCFS). Because tokamak energy confinement depends strongly on the boundary condition imposed by the edge plasma pressure, one desires a predictive capability for the pedestal on a future tokamak. On Alcator C-Mod, significant contributions to ETB studies were made possible with edge Thomson scattering (ETS), which measures profiles of electron temperature (20 [leq] Te[eV] [leq] 800) and density (0.3 [leq] ne[10^20m^-3] [leq] 5) with 1.3-mm spatial resolution near the LCFS. Profiles of Te, ne, and pe = neTe are fitted with a parameterized function, revealing typical pedestal widths [delta] of 2-6mm, with [delta]Te [geq] [delta]ne , on average. Pedestals are examined to determine existence criteria for the enhanced D[alpha] (EDA) H-mode. A feature that distinguishes this regime is a quasi-coherent mode (QCM) near the LCFS. The presence or absence of the QCM is related to edge conditions, in particular density, temperature and safety factor q. Results are consistent with higher values of both q and collisionality [nu]* giving the EDA regime. Further evidence suggests that increased abs([nabla]pe) may favor the QCM; thus EDA may have relevance to low-[nu]* reactor regimes, should sufficient edge pressure gradient exist.
Internal transport barrier (ITB) plasmas with peak pressures of 0.25 MPa and pressure gradients as large as 2.5 MPa/m have been produced in Alcator C-Mod using off-axis ICRF heating. The onset of the ITBs is apparent when the pressure gradient exceeds 1.0 MPa/m, which is similar to the JET criterion of [rho]* s/L [equal to or greater than] 0.014. Concomitant with the peaking of the core pressure as the ITB develops is a drop of the toroidal rotation velocity profile inside of the barrier foot; the maximum of the velocity gradient coincides with the peak in the pressure gradient. The quasi-coherent (QC) mode, associated with the enhanced D[alpha] (EDA) H-mode plasmas which evolve the ITBs, breaks up and disappears as the barriers develop, even though the measured edge pedestal parameters remain fixed. The position of the ITB foot has been moved over a range of 1/3 of the plasma minor radius by varying the toroidal magnetic field. The peak in the calculated bootstrap current density profile has correspondingly been regulated over a similar range in plasma minor radius. The location of the density profile foot is found to expand as the toroidal magnetic field is reduced and the ICRF frequency is lowered. The density foot radius is relatively independent of q95, however, in a scan of the plasma current at fixed BT and wave frequency.