S. H. Batha

Enhancement of Tokamak Fusion Test Reactor performance by lithium conditioning

Wall conditioning in the Tokamak Fusion Test Reactor (TFTR) [K. M. McGuire et al., Phys. Plasmas 2, 2176 (1995)] by injection of lithium pellets into the plasma has resulted in large improvements in deuterium–tritium fusion power production (up to 10.7 MW), the Lawson triple product (up to 1021 m−3 s keV), and energy confinement time (up to 330 ms). The maximum plasma current for access to high‐performance supershots has been increased from 1.9 to 2.7 MA, leading to stable operation at plasma stored energy values greater than 5 MJ. The amount of lithium on the limiter and the effectiveness of its action are maximized through (1) distributing the Li over the limiter surface by injection of four Li pellets into Ohmic plasmas of increasing major and minor radius, and (2) injection of four Li pellets into the Ohmic phase of supershot discharges before neutral‐beam heating is begun.

Local transport barrier formation and relaxation in reverse-shear plasmas on the Tokamak Fusion Test Reactor

The roles of turbulence stabilization by sheared E×B flow and Shafranov shift gradients are examined for Tokamak Fusion Test Reactor [D. J. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)] enhanced reverse-shear (ERS) plasmas. Both effects in combination provide the basis of a positive-feedback model that predicts reinforced turbulence suppression with increasing pressure gradient. Local fluctuation behavior at the onset of ERS confinement is consistent with this framework. The power required for transitions into the ERS regime are lower when high power neutral beams are applied earlier in the current profile evolution, consistent with the suggestion that both effects play a role. Separation of the roles of E×B and Shafranov shift effects was performed by varying the E×B shear through changes in the toroidal velocity with nearly steady-state pressure profiles. Transport and fluctuation levels increase only when E×B shearing rates are driven below a critical value that is comparable to the fastest line...

q‐profile measurements in the Tokamak Fusion Test Reactor*

The q‐profile plays a key role in determining plasma equilibrium and stability in tokamaks. With the development of the motional Stark effect (MSE) diagnostic, accurate q(R,t) profiles have been measured and utilized for equilibrium and stability analysis. A multichannel MSE polarimeter system on the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Fusion 33, 1509 (1991)] has obtained data for several plasma conditions including L‐mode, supershot, current ramps, and high βp. For sawtoothing discharges on TFTR, it is found that q(0)∼0.7, well below one, and remains below one throughout the entire evolution of the sawtooth cycle with an increase in q(0) of ≤0.1 after a sawtooth crash. During high βp operation or coinjection of neutral beams on TFTR a significant broadening of the current profile and an increase of q(0) is observed.

The effect of Er on motional-Stark effect measurements of q, a new technique for measuring Er, and a test of the neoclassical Er

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) [Phys. Plasmas 2, 2176 (1975)] 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 are 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.

Equilibrium reconstruction of the safety factor profile in tokamaks from motional Stark effect data

The motional Stark effect (MSE) diagnostic has been used to obtain accurate measurements of the internal magnetic pitch angle tan−1(BZ/BT) in finite‐pressure tokamaks. The MSE data, together with external magnetic probe data, are used to reconstruct self‐consistently the equilibrium safety factor (q) profile and, hence, the plasma current density, in the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Nucl. Fusion Res. 1, 51 (1986)] and the Princeton Beta Experiment‐Modified (PBX‐M) tokamak [Phys. Fluids B 2, 1271 (1990)]. An efficient computational scheme, based on an inverse coordinate representation of the magnetic field, has been developed to solve the coupled nonlinear equations describing both the magnetohydrodynamic (MHD) equilibrium and the q profile, which best match all the experimental data.

Enhanced performance of deuterium--tritium-fueled supershots using extensive lithium conditioning in the Tokamak Fusion Test Reactor

In the Tokamak Fusion Test Reactor (TFTR) [K. M. McGuire et al., Phys. Plasmas 2, 2176 (1995)] a substantial improvement in fusion performance has been realized by combining the enhanced confinement due to tritium fueling with the enhanced confinement due to extensive conditioning of the limiter with lithium. This combination has resulted in not only significantly higher global energy confinement times than have previously been obtained in high current supershots, but also in the highest central ratio of thermonuclear fusion output power to input power observed to date.

High‐frequency core localized modes in neutral beam heated plasmas on TFTR

A band of high‐frequency modes in the range 50–150 kHz with intermediate toroidal mode numbers 4<n<10 are commonly observed in the core of supershot plasmas on TFTR [R. Hawryluk, Plasma Phys. Controlled Fusion 33, 1509 (1991)]. Two distinct varieties of magnetohydrodynamic (MHD) modes are identified, corresponding to a flute‐like mode predominantly appearing around the q=1 surface and an outward ballooning mode for q≳1. The flute‐like modes have nearly equal amplitude on the high‐field and low‐field side of the magnetic axis, and are mostly observed in moderate performance supershot plasmas with τE<2τL, while the ballooning‐like modes have enhanced amplitude on the low‐field side of the magnetic axis and tend to appear in higher performance supershot plasmas with τE≳2τL, where τL is the equivalent L‐mode confinement time. Both modes appear to propagate in the ion diamagnetic drift direction and are highly localized with radial widths Δr∼5–10 cm, fluctuation levels n/n, Te/Te<0.01, and radial displacemen...

CORE POLOIDAL ROTATION AND INTERNAL TRANSPORT 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.

Toroidal Alfvén eigenmodes in TFTR deuterium–tritium plasmas

Purely alpha-particle-driven Toroidal Alfven Eigenmodes (TAEs) with toroidal mode numbers n=1-6 have been observed in Deuterium-Tritium (D-T) plasmas on the Tokamak Fusion Test Reactor [D.J. Grove and D.M. Meade, Nucl. Fusion 25, 1167 (1985)]. The appearance of mode activity following termination of neutral beam injection in plasmas with q(0)>1 is generally consistent with theoretical predictions of TAE stability [G.Y. Fu et al., Phys. Plasmas 3, 4036 (1996]. Internal reflectometer measurements of TAE activity is compared with theoretical calculations of the radial mode structure. Core localization of the modes to the region of reduced central magnetic shear is confirmed, however the mode structure can deviate significantly from theoretical estimates. The peak measured TAE amplitude of delta n/n~10(superscript -4) at r/a~0.3-0.4 corresponds to delta B/B~10-5, while dB/B~10(superscript -8) is measured at the plasma edge. Enhanced alpha particle loss associated with TAE activity has not been observed.

Ion cyclotron range of frequency experiments in the Tokamak Fusion Test Reactor with fast waves and mode converted ion Bernstein waves

Recent experiments in the ion cyclotron range of frequencies (ICRF) in the Tokamak Fusion Test Reactor [Fusion Technol. 21, 13 (1992)] are discussed. These experiments include mode conversion heating and current drive, fast wave current drive, and heating of low (L)‐ mode deuterium–tritium (D–T) plasmas in both the hydrogen minority and second harmonic tritium regimes. In mode conversion heating, a central electron temperature of 10 keV was attained with 3.3 MW of radio‐frequency power. In mode conversion current drive experiments, up to 130 kA of current was noninductively driven, on and off axis, and the current profiles were modified. Fast wave current drive experiments have produced 70–80 kA of noninductively driven current. Heating of L‐mode deuterium and D–T plasmas by hydrogen minority ICRF has been compared. Finally, heating of L‐mode D–T plasmas at the second harmonic of the tritium cyclotron frequency has been demonstrated.