Formation and Structure of Internal and Edge Transport Barriers

The phenomenology of transport barrier formation is reviewed with a focus on physics that may be common to the edge and the core. To this end, the framework of E x B velocity shear reduction of turbulence-induced fluxes, applied to the edge for some time, is studied in light of measurements of core bifurcation dynamics and recent tests of causality. Also, the possible role of the magnetic shear structure in facilitating core barrier formation is examined. Experimental and theoretical challenges for developing predictive capability for reactor-relevant conditions are highlighted by recent observations of spontaneous electric field shear generation far removed from edge effects, and efforts to characterize the plasma edge at and across the L-H transition

Improvements in the CHERS system for DT experiments on TFTR

Improvements in the charge exchange recombination spectroscopy (CHERS) system have resulted in accurate measurements of T{sub i} and V{sub {phi}} profiles during DT experiments. These include moving the spectrometer detector array and electronics farther away from the tokamak to a low neutron flux location. This relocation has also improved access to all components of the system. Also, a nonplasma-viewing calibration fiber system was added to monitor the change in fiber transmission due to the high flux DT neutrons. Narrowband filtered light transmitted through the calibration fiber is now used as a reference for the VO measurement. At the highest neutron flux of {approximately} 2.5 {times} 10{sup 18} neutrons/see (fusion power {approximately} 6.2 MW) a modest 5% decrease in fiber transmission was observed. Corrections for transmission loss are made and T{sub i} (r,t) and absolute V{sub phi} (r,t) profiles are automatically calculated within four minutes of every shot

Measurements of the production and transport of helium ash on the TFTR Tokamak

Helium ash production and transport have been measured in TFTR deuterium-tritium plasmas using charge-exchange recombination spectroscopy. The helium ash confinement time, including recycling effects, is 6--10 times the energy confinement time and is compatible with sustained ignition in a reactor. The ash confinement time is dominated by edge pumping rates rather than core transport. The measured evolution of the local thermal ash density agrees with modeling, indicating that alpha particle slowing-down calculations used in the modeling are reasonable

Core Poloidal Rotation and Internal Trnasport Barrier Formation in TFTR

Impurity poloidal rotation velocities have been measured in the core of TFTR plasmas using a new spectroscopic diagnostic. Two types of transitions to enhanced confinement in reversed shear plasmas are examined. A bifurcation in carbon poloidal rotation is observed to occur before the transition to enhanced confinement for one of these types, while other measured plasmas parameters remain constant. A narrow radial region with reversed poloidal rotation and rotational shear is established 60-100 ms before the transition, and is associated with a large negative radial electric field

New understanding of poloidal rotation measurements in a tokamak plasma

Significant atomic physics corrections to the measured poloidal velocity using charge exchange spectroscopy have been neglected when interpreting impurity poloidal velocity. In the presence of a magnetic field, the gyro motion of the impurity ion along with the finite lifetime of the observed state results in an additional line shift that scales with ion temperature and magnetic field strength. Effects of collisions on the lifetime of excited states, cascades from higher longer-lived n levels, and charge exchange from excited beam neutrals are calculated to determine necessary corrections to the measured poloidal velocity. The accuracy of computed corrections is tested utilizing symmetric upward and downward views from the TFTR poloidal rotation diagnostic

Transport simulations of ITER helium exhaust using recent data from TFTR, TEXTOR and JT-60

Insufficient helium exhaust can seriously degrade the performance of any fusion reactor and has recently been identified as a potentially significant issue for ITER. The effects of variations of the ratios D{sup He}/{chi}i, D{sup He}/D{sup D}, v{sup He}/D{sup He} and of the sawtooth period on helium exhaust in ITER are studied with BALDUR code transport simulations. Recent measurements of D{sup He}/{chi}i, at TFTR, of D{sup He}/D{sup D} at TEXTOR of v{sup He}/D{sup He} at TFTR, TEXTOR, and JT-60 are found compatible with sustained ignition and helium exhaust requirements for ITER. Ignition is found to be critically sensitive to the ratio v{sup He}/D{sup He}, particularly at the plasma edge, and to reduced sawtooth period. These critical dependencies are moderated for reduced helium recycling at the separatrix and for hollow particle diffusivity profiles. 17 refs., 9 figs., 1 tab

The effect of E{sub r} on MSE measurements of q, a new technique for measuring E{sub r}, and a test of the neoclassical electric field

Previous analysis of motional-Stark Effect (MSE) data to measure the q-profile ignored contributions from the plasma electric field. The MSE measurements are shown to be sensitive to the electric field and require significant corrections for plasmas with large rotation velocities or pressure gradients. MSE measurements from rotating plasmas on the Tokamak Fusion Test Reactor (TFTR) confirm the significance of these corrections and verify their magnitude. Several attractive configurations are considered for future MSE-based diagnostics for measuring the plasma radial electric field. MSE data from TFTR is analyzed to determine the change in the radial electric field between two plasmas. The measured electric field quantitatively agrees with the predictions of neoclassical theory. These results confirm the utility of a MSE electric field measurement

Theory, simulation, and experimental studies of zonal flows

The authors report on current theoretical understanding of the characteristics of self-generated zonal flows as observed in nonlinear gyrokinetic simulations of toroidal ITG turbulence [Science 281, 1835 (1998)], and discuss various possibilities for experimental measurements of signature of zonal flows