Highly peaked density and pressure profiles in a new operating regime have been observed on the Tokamak Fusion Test Reactor (TFTR). The qprofile has a region of reversed magnetic shear extending from the magnetic axis to r / u-0.3-0.4. The central electron density rises from 0.45 x lo2' m-3 to nearly 1.2 x lo2' m-' during neutral beam injection. The electron particle diffusivity drops precipitously in the plasma core with the onset of the improved confinement mode and can be reduced by a factor of N 50 to near the neoclassical particle diffusivity level.

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.

Long-wavelength (k ±pi < 1) density turbulence has been measured with good spatial localization in the core region of a high temperature tokamak plasma with auxiliary heating. Density fluctuations of h/n > 0.5% exist for /cj. < 2 cm "' with radial and poloidal correlation lengths typically 1-2.5 cm in the confinement region, corresponding to /:j.p/=^ 0.1-0.3. An anisotropic wave-number spectrum is observed, and estimates of the turbulence-driven transport are comparable to the anomalous transport observed in tokamaks.

A transport code (TRANSP) is used to simulate future deuterium-tritium (DT) experiments in TFTR. The simulations are derived from 14 TFTR DD discharges, and the modelling of one supershot is discussed in detail to indicate the degree of accuracy of the TRANSP modelling. Fusion energy yields and 01 particle parameters are calculated, including profiles of the 01 slowing down time, the 01 average energy, and the AlfvBn speed and frequency. Two types of simulation are discussed. The main emphasis is on the DT equivalent, where an equal mix of D and T is substituted for the D in the initial target plasma, and for the Do in the neutral beam injection, but the other measured beam and plasma parameters are unchanged. This simulation does not assume that 01 heating will enhance the plasma parameters or that confinement will increase with the addition of tritium. The maximum relative fusion yield calculated for these simulations is QDT-0.3, and the maximum a contribution to the central toroidal 0 is PJO)-0.5%. The stability of toroidicity induced Alfvkn eigenmodes (TAE) and kinetic ballooning modes (KBM) is discussed. The TAE mode is predicted to become unstable for some of the simulations, particularly after the termination of neutral beam injection. In the second type of simulation, empirical supershot scaling relations are used to project the performance at the maximum expected beam power. The MHD stability of the simulations is discussed.

A TFTR supershot with a plasma current of 2.5 MA, neutral beam heating power of 33.7 MW, and a peak DT fusion power of 7.5 MW is studied using the TRANSP plasma analysis code. Simulations of alpha parameters such as the alpha heating, pressure, and distributions in energy and Vparallel/ v are given. The effects of toroidal ripple and mixing of the fast alpha particles during the sawteeth observed after the neutral beam injection phase are modeled. The distributions of alpha particles on the outer midplane are peaked near fotward and backward Vparalel /

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.

Supershots in TFTR often suffer a performance deterioration characterized by a gradual decrease of the D-D fusion neutron yield and plasma stored energy after several hundred milliseconds of auxiliary heating. The correlation between this performance deterioration and the development of low m (the poloidal mode number), n (the toroidal mode number) MHD modes is studied through shot-to-shot comparisons and statistical data analyses. A good correlation is observed between performance deterioration and the appearance of strong 3/2 and 4/3 macroscopic modes (magnetic islands)in small major radius plasmas (R = 2.45 m). The magnetic island structures are observed using Mirnov and ECE diagnostics. The measured T_, Ti and n_ profiles show that development of the islands corresponds to a nearly constant decrement of these quantities over the core region r < rs, where rs is the mode rational surface, on a transport time scale (t > TE). The observed energy deterioration scaling, 5W/W ,,_ w/a, where w is the magnetic island width, agrees with both a local transport model and predictive numerical simulations. For larger major radius plasmas (R = 2.52, 2.60 m), a continuous increase of edge recycling rate during the neutral beam injection phase seems to have a larger effect on the performance deterioration than does the MHD.

A new method of actively modifying the plasma-wall interaction was tested on the Tokamak Fusion Test Reactor. A laser was used to introduce a directed lithium aerosol into the discharge scrape-off layer. The lithium introduced in this fashion ablated a n d migrated preferentially to the limiter contact points. This allowed the plasma-wall interaction to be influenced in situ and in real t i m e by external means. Significant improvement in energy confinement and fusion neutron production rate as well as a reduction in t h e plasma Z eff have been documented in a neutral-beam-heated plasma. The introduction of a metallic aerosol into the plasma e d g e increased the internal inductance of the plasma column and also resulted in prompt heating of core electrons in Ohmic plasmas. Preliminary evidence also suggests that the introduction of a n aerosol leads to both edge poloidal velocity shear and edge electric field shear. . I n t r o d u c t i o nIt has been well documented that the fusion performance of discharges in the Tokamak Fusion Test Reactor (TFTR) was strongly dependent on the physical and chemical condition of the graphite surface forming its limiter [1,2]. In particular, the highperformance supershot mode of operation was attainable at high currents (>2.0 MA) only when the graphite inner wall had previously been "scoured" by repeated discharges heated by high-power neutral beam injection (NBI) in order to render the limiter surface free of loosely-bound carbon and loosely-adsorbed hydrogenic material. At the end of a series of such pre-conditioning discharges, plasma fueling from the limiter typically reached a minimum.Moreover, only when this low-recycling condition had been attained did high levels of fusion performance become accessible.It has also been well documented that dramatic improvements in supershot fusion performance were attainable by the deposition of elemental lithium (Li) onto the limiter surface once it had been brought into the low-recycling condition. This deposition was carried out in earlier experiments by the injection and ablation of a few small (3 mg each) Li pellets [3,4,5,6]. In order to improve plasma performance during NBI, Li pellets were typically injected into the Ohmic phase of discharges and the ablated Li was allowed to 3 condense out of the plasma column and onto the limiter surface before the application of auxiliary heating. While the use of Li pellets did improve plasma performance, the technique was, nonetheless, highly perturbing. In order to reduce the perturbation to the plasma, brief but successful experiments with a Li effusion oven were also carried out on TFTR and are described elsewhere [7,8].While the use of an oven did result in an increase in the amount of Li deposited onto the inner wall as compared to pellet injection, deposition could only take place between discharges and not during the discharge of interest.In this work, an alternate wall-conditioning technique is described in which Li was injected into the scrape-off layer (SOL), during pl...