Abstract
Experiments in the Tokamak Fusion Test Reactor (TFTR) [Phys. Plasmas 2, 2176 (1995)] have explored several novel regimes of improved tokamak confinement in deuterium–tritium (D–T) plasmas, including plasmas with reduced or reversed magnetic shear in the core and high-current plasmas with increased shear in the outer region (high li). New techniques have also been developed to enhance the confinement in these regimes by modifying the plasma-limiter interaction through in situ deposition of lithium. In reversed-shear plasmas, transitions to enhanced confinement have been observed at plasma currents up to 2.2 MA (qa≈4.3), accompanied by the formation of internal transport barriers, where large radial gradients develop in the temperature and density profiles. Experiments have been performed to elucidate the mechanism of the barrier formation and its relationship with the magnetic configuration and with the heating characteristics. The increased stability of high-current, high-li plasmas produced by rapid expansion of the minor cross section, coupled with improvement in the confinement by lithium deposition has enabled the achievement of high fusion power, up to 8.7 MW, with D–T neutral beam heating. The physics of fusion alpha-particle confinement has been investigated in these regimes, including the interactions of the alphas with endogenous plasma instabilities and externally applied waves in the ion cyclotron range of frequencies. In D–T plasmas with q0>1 and weak magnetic shear in the central region, a toroidal Alfvén eigenmode instability driven purely by the alpha particles has been observed for the first time. The interactions of energetic ions with ion Bernstein waves produced by mode conversion from fast waves in mixed-species plasmas have been studied as a possible mechanism for transferring the energy of the alphas to fuel ions.
Highlights
Since the Tokamak Fusion Test ReactorTFTR1 began its deuterium–tritiumD–Tphase of operation in December 1993, more than 1.2 GJ of D–T fusion energy has been produced
For the last 18 months, considerable effort has been devoted to developing new operational regimes which offer the possibility of increased plasma stability while preserving the good confinement and extremely high fusion reactivity of existing TFTR regimes
This figure makes use of the fact that in TFTR supershots, in which the plasma energy is dominated by the ion component, a very constrained relationship is observed between the plasma energy and the fusion power output in both D and D–T plasmas
Summary
Since the Tokamak Fusion Test ReactorTFTR1 began its deuterium–tritiumD–Tphase of operation in December 1993, more than 1.2 GJ of D–T fusion energy has been produced. A similar regime was discovered in the DIII-D tokamak at about the same time and has since been studied in several tokamaks, including JT-60U4 and the Joint European TorusJET.5 It is produced by a different heating method, namely neutral beam injection, the ERS regime has strong similarities to two other regimes of improved confinement involving modification of the q profile, namely the pellet enhanced performance mode in JET6 and that occurring in Tore-Supra with lower-hybrid current drive.. A second line of investigation grew out of previous experiments to improve plasma stability by creating more highly peaked current profiles through current rampdown.8 This technique, which increases the internal inductance parameter, li , of the plasma, and produces what is called the high-li regime, had already achieved high normalized- and significant fusion power, but was limited operationally in its extrapolability to higher performance. VI we describe the experiments with heating by waves in the ion-cyclotron range of frequenciesICRFin various plasma and wave coupling regimes
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