Abstract: Edge Transport Barrier Studies On the Alcator C-Mod Tokamak

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 < T_e[eV] < 800) and density (0.3 < n_e[10²⁰m^-3] < 5) with 1.3-mm spatial resolution near the LCFS. Profiles of T_e, n_e, and p_e = n_eT_e are fitted with a parameterized function, revealing typical pedestal widths D of 2-6mm, with D_{T_e} > D_{n_e}, on average.

Pedestals are examined to determine existence criteria for the enhanced D_a (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 n^* giving the EDA regime. Further evidence suggests that increased |grad-p_e| may favor the QCM; thus EDA may have relevance to low-n^* reactor regimes, should sufficient edge pressure gradient exist.

Scaling studies of pedestal parameters and plasma confinement in EDA H-modes varied operational parameters such as current I_P and L-mode target density n_e,L. At fixed plasma shape, widths show little systematic variation with plasma parameters. Scalings are however determined for pedestal heights and gradients. The p_e pedestal height and gradient both scale as I_P², similar to scalings found on other tokamaks, though with differing pedestal-limiting physics. It is seen that the density pedestal value n_e,PED scales linearly with I_P, and more weakly with n_e,L, indicating that neutral fueling plays a relatively limited role in setting H-mode density. Plasma stored energy scales in a linear fashion with the p_e pedestal, such that empirical confinement scalings are affected by edge pedestal scalings.

Empirical determination of neutral density and ionization source was made across the pedestal region, enabling inferrence of neutral gradient scale length L₀ and effective diffusivity D_eff. The D_eff well is comparable in width to the pedestal, and L₀ tends to be less than D_{n_e}. Computation of L₀ in discharges with varying n_e,L yields a similar result, suggesting that D_{n_e} is generally set by the ETB extent and not neutral penetration length. Puffing gas into an existing H-mode edge yields no significant change in the values of n_e,PED, grad-n_e, which is qualitatively consistent with simulations using a coupled fluid-kinetic neutral model. Experiment and modeling indicate the importance of thermal equilibration of neutrals with ions, particularly in high density (collisional) plasmas.

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On 29 Jun 2005, 11:26.