In toroidal magnetically confined plasmas, eigenmodes of Alfvén waves can be destablized by energetic ions with velocities comparable to the Alfvén velocity. With the advent of tokamak experiments in which populations of energetic ions can be introduced by neutral beam injection, radio frequency wave heating or by fusion reactions, major advances have been made in Alfvén eigenmode research in the past 10 years. After introducing the basic concepts on the Alfvén eigenmode instability, data on this subject from various toroidal devices are described, emphasizing the interplay between experiment and theory. Experimental results on mode identification, instability drive, mode damping and saturation, and energetic ion redistribution are compared with theory.

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A universal integral equation has been derived and solved for the nonlinear evolution of collective modes driven by kinetic wave particle resonances just above the threshold for instability. The dominant nonlinearity stems from the dynamics of resonant particles that can be treated perturbatively near the marginal state of the system. With a resonant particle source and classical relaxation processes included, the new equation allows the determination of conditions for a soft nonlinear regime, where the saturation level is proportional to the increment above threshold, or a hard nonlinear regime, characterized by explosive behavior, where the saturation level is independent of the closeness to threshold. In the hard regime, rapid oscillations typically arise that lead to large frequency shifts in a fully developed nonlinear stage. The universality of the approach suggests that the theory applies to many types of resonant particle driven instabilities, and several specific cases, viz. energetic particle driven Alfvén wave excitation, the fishbone oscillation, and a collective mode in particle accelerators, are discussed.

After many years of fusion research, the conditions needed for a D–T fusion reactor have been approached on the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324 (1992)]. For the first time the unique phenomena present in a D–T plasma are now being studied in a laboratory plasma. The first magnetic fusion experiments to study plasmas using nearly equal concentrations of deuterium and tritium have been carried out on TFTR. At present the maximum fusion power of 10.7 MW, using 39.5 MW of neutral-beam heating, in a supershot discharge and 6.7 MW in a high-βp discharge following a current rampdown. The fusion power density in a core of the plasma is ≊2.8 MW m−3, exceeding that expected in the International Thermonuclear Experimental Reactor (ITER) [Plasma Physics and Controlled Nuclear Fusion Research (International Atomic Energy Agency, Vienna, 1991), Vol. 3, p. 239] at 1500 MW total fusion power. The energy confinement time, τE, is observed to increase in D–T, relative to D plasmas, by 20% and the ni(0) Ti(0) τE product by 55%. The improvement in thermal confinement is caused primarily by a decrease in ion heat conductivity in both supershot and limiter-H-mode discharges. Extensive lithium pellet injection increased the confinement time to 0.27 s and enabled higher current operation in both supershot and high-βp discharges. Ion cyclotron range of frequencies (ICRF) heating of a D–T plasma, using the second harmonic of tritium, has been demonstrated. First measurements of the confined alpha particles have been performed and found to be in good agreement with TRANSP [Nucl. Fusion 34, 1247 (1994)] simulations. Initial measurements of the alpha ash profile have been compared with simulations using particle transport coefficients from He gas puffing experiments. The loss of alpha particles to a detector at the bottom of the vessel is well described by the first-orbit loss mechanism. No loss due to alpha-particle-driven instabilities has yet been observed. D–T experiments on TFTR will continue to explore the assumptions of the ITER design and to examine some of the physics issues associated with an advanced tokamak reactor.

Experimental evidence is reported of an internal kink instability driven by a new mechanism: barely trapped suprathermal electrons produced by off-axis electron cyclotron heating on the DIII-D tokamak. It occurs in plasmas with an evolving safety factor profile q(r) when q(min) approaches 1. This instability is most active when ECCD is applied on the high field side of the flux surface. It has a bursting behavior with poloidal/toroidal mode number = m/n = 1/1. In positive magnetic shear plasmas, this mode becomes the fishbone instability. This observation can be qualitatively explained by the drift reversal of the barely trapped suprathermal electrons.

The toroidal Alfvén eigenmodes (TAE) are calculated to be stable in the presently obtained deuterium–tritium plasmas in the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Nucl. Fusion Res. 26, 11 (1984)]. However, the core localized TAE mode can exist and is less stable than the global TAE modes. The beam ion Landau damping and the radiative damping are the two main stabilizing mechanisms in the present calculation. In future deuterium–tritium experiments, the alpha-driven TAE modes are predicted to occur with a weakly reversed shear profile.

Wall conditioning in the Tokamak Fusion Test Reactor ͑TFTR͒ ͓K. M. McGuire et al., Phys. Plasmas 2, 2176 ͑1995͔͒ by injection of lithium pellets into the plasma has resulted in large improvements in deuterium-tritium fusion power production ͑up to 10.7 MW͒, the Lawson triple product ͑up to 10 21 m Ϫ3 s keV͒, and energy confinement time ͑up to 330 ms͒. 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 ͑1͒ distributing the Li over the limiter surface by injection of four Li pellets into Ohmic plasmas of increasing major and minor radius, and ͑2͒ injection of four Li pellets into the Ohmic phase of supershot discharges before neutral-beam heating is begun.

Alpha-particle-driven toroidal Alfvén eigenmodes (TAEs) have been observed for the first time in deuterium-tritium (D-T) plasmas on the tokamak fusion test reactor (TFTR). These modes are observed 100-200 ms following the end of neutral beam injection in plasmas with reduced central magnetic shear and elevated central safety factor ͓q͑0͒ . 1͔. Mode activity is localized to the central region of the discharge ͑r͞a , 0.5͒ with magnetic fluctuation levelB Ќ ͞B k ϳ 10 25 and toroidal mode numbers in the range n 2 4, consistent with theoretical calculations of a-TAE stability in TFTR.[S0031-9007 (97)02857-3] PACS numbers: 52.55.Fa, 52.35.Bj, 52.35.Py, 52.55.PiDeuterium-tritium (D-T) plasma operation on the tokamak fusion test reactor (TFTR) provides the first opportunity to investigate the interaction of fusion alpha particles with plasma waves under reactor relevant conditions. Such investigations are crucial for assessing the impact of plasma instabilities on the confinement of energetic alpha particles, which are required to sustain ignition in a D-T reactor. One candidate instability with the potential for affecting alpha particle confinement in tokamaks is the toroidal Alfvén eigenmode (TAE) [1]. This Letter describes the first observation of purely alpha-particledriven TAEs in TFTR with central b a as low as 0.02% (b a ϵ alpha particle pressure͞magnetic pressure), well below that expected in the International Thermonuclear Experimental Reactor (ITER) [central b a ϳ ͑0.5 1͒%].TAEs are discrete frequency modes occurring inside toroidicity induced gaps in the shear Alfvén spectrum which can be destabilized by the pressure gradient of energetic ions. These modes can potentially cause internal redistribution and enhanced loss of energetic alpha particles in a D-T reactor due to their extended radial structure, relatively low instability threshold, and resonant interaction with 3.5 MeV alpha particles near the Alfvén velocity [2,3]. The characteristics of TAEs and associated fast ion losses have been studied in experiments utilizing circulating neutral beam ions ͑E b # 100 keV͒ [4-6], deeply trapped minority ions in the MeV range of energy [7-9], nonlinear beat wave excitation using fast magnetosonic waves [10], and external excitation using saddle coils [11]. Until recently alpha-driven TAEs had not been observed in TFTR, even in the highest fusion power D-T "supershot" plasmas ͑P fus ഠ 10.7 MW͒ with b a ഠ 0.3% in the core of the discharge [12]. These results were consistent with theoretical calculations of alpha-driven TAE stability in TFTR, after taking into account beam ion Landau damping and radiative damping due to coupling to the kinetic Alfvén wave (KAW) [13]. However, a better comparison with theory requires the actual observation of purely alpha particle driven TAEs in D-T plasmas, as described in this Letter. The present experiment was motivated by recent theoretical calculations for low-n modes ͑n , 6͒ in TFTR indicating a significant reduction in the central b a required for destabilizing TAEs under condi...