Internal transport barriers (ITBs) in tokamak plasmas are characterized by the reduction of transport in one or more of the particle, momentum, or energy channels in the core plasma region. On Alcator C-Mod, significant contributions to ITB studies were made possible with the core Thomson scattering (TS) diagnostic, which measures profiles of electron temperature (0.03

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.

The formation of internal transport barriers (ITB) has been observed in the core region of Alcator C-Mod under a variety of conditions. The improvement in core confinement following pellet injection (pellet enhanced performance or PEP mode) has been well documented on Alcator C-Mod in the past. Recently three new ITB phenomena have been observed which require no externally applied particle or momentum input. Short lived ITBs form spontaneously following the high confinement (H) to low confinement (L) mode transition and are characterized by a large increase in the global neutron production (enhanced neutron or EN modes.) Experiments with ICRF (ion cyclotron range of frequencies) power injection to the plasma off-axis on the high field side results in the central density rising abruptly and becoming peaked. The ITB formed at this time lasts for 10 energy confinement times. The central toroidal rotation velocity decreases and changes sign as the density rises. Similar spontaneous ITBs have been observed in ohmically heated H-mode plasmas. All of these ITB events have strongly peaked density profiles with a minimum in the density scale length occurring near r/a = 0.5 and have improved confinement parameters in the core region of the plasma. Keywords: Alcator C-Mod; confinement; tokamaks; transport phenomena; neutrons.

(cont.) Our study shows that although the short wavelength turbulence in the ETG range is unstable in the linear ohmic regime, the nonlinear simulation with k[theta][rho]s up to 4 does not raise the electron thermal diffusivity to the experimental level, where k[theta] is the poloidal wavenumber and [rho]s is the ion-sound Larmor radius. The H-Mode studies focus on plasmas before and during internal transport barrier formation in an enhanced D[alpha], H-Mode plasma. The simulated fluctuations from GYRO agree with experimental measurements in the ITG regime. GYRO also shows good agreement in transport predictions with experimental measurements after reducing the ion temperature gradient (~15%) and adding ExB shear suppression, all within the experimental uncertainty.

In this book, we introduced the concept of Internal transport barrier (ITB) which is generally indulged to grow up the regions of reduced transport which can be helpful for tokomak operation. Internal transport barriers are created near the low order rational surface. i.e. (q = 2, 5/2 and 3). These ITBs are reactive to some conditions with regards to MHD instabilities, which generate a considerable plasma flow, creating magnetic islands, which are wished as probable mechanisms for the development of ITB. Double tearing modes become active due to electron viscosity and the MHD flows in the development the modes is extended from quasi-linear solution in a slab geometry to nonlinear solution in cylindrical geometry for the torus of large aspect ratio. The DTM mediated by electron viscosity is considered to trigger an ITB with non-monotonic q- profiles in advance tokomak (AT) operation.