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The paper reviews the results from tokamak experiments and measurements on electron beam ion traps for the line ratios x/w, y/w, and z/w in the spectra of helium-like ions with Z in the range from 14 to 28. With the exception of the DITE experiments, where these line ratios were found to be in agreement with theoretical predictions, all other tokamak experiments yielded values that were significantly greater than predicted. The reasons for these discrepancies are not yet understood. It is possible that radial profile effects were not properly taken into account in the majority of the tokamak experiments. The paper also gives a short historical review of the X-ray diagnostic developments and presents very recent data from a new type of X-ray imaging crystal spectrometer, which records spatially resolved spectra with a spatial resolution of about 1 cm in the plasma. These new data can be Abel inverted, so that it will be possible to determine line ratios at each radial position in the plasma. Effects of radial profiles, which may have affected the chord-integrated measurements of the past, will thus be eliminated in the future. PACS Nos.: 52.55.Fa, 32.30.Rj

Experiments in the Tokamak Fusion Test Reactor (TFTR) 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-l{sub i}). 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 (q{sub a} {approx} 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-l{sub i} 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 q{sub 0} > 1 and weak magnetic shear in the central region, a toroidal Alfven 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.

The HCN-interferometer/polarimeter on TEXTOR has been extended by a horizontal line of sight in the equatorial plane. The achieved signal to noise ratio and the time resolution of this polarimeter channel are sufficient to resolve the m = 1 magnetic precursor oscillation before the sawtooth collapse. The growth rate of the precursor oscillation in a sawtoothing ohmically heated discharge has been determined to be . The magnetic perturbation, determined from the Faraday rotation integral, assuming a kink-mode eigenfunction, is 13 mT just before the sawtooth crash and the `displacement' of the core is about 0.9 cm or . The amplitude of a stationary m = 1 mode observed in an additionally heated discharge with balanced neutral beam injection amounts to 26 mT.

Experiments with plasmas having nearly equal concentrations of deuterium and tritium have been carried out on TFTR. To date (September 1995), the maximum fusion power has been 10.7 MW, using 39.5 MW of neutral beam heating, in a supershot discharge and 6.7 MW in a high beta P discharge following a current ramp-down. The fusion power density in the core of the plasma has reached 2.8 MW/m3, exceeding that expected in the International Thermonuclear Experimental Reactor (ITER). The energy confinement time tau E is observed to increase in DT, relative to D plasmas, by 20% and the n1(0).T1(0). tau E 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 beta P discharges. First measurements of the confined alpha particles have been performed and found to be in good agreement with TRANSP simulations assuming classical confinement. Measurements of the alpha ash profile have been compared with simulations using particle transport coefficients from helium gas puffing experiments. The loss of energetic 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. ICRF heating of a DT plasma, using the second harmonic of tritium, has been demonstrated. DT experiments on TFTR will continue both to explore the physics underlying the ITER design and to examine some of the physics issues associated with an advanced tokamak reactor

Routine tritium operation in TFTR has permitted investigations of alpha particle physics in parameter ranges resembling those of a reactor core. ICRF wave physics in a DT plasma and the influence of isotopic mass on supershot confinement have also been studied. Continued progress has been made in optimizing fusion power production in TFTR, using extended machine capability and Li wall conditioning. Performance is currently limited by MHD stability. A new reversed magnetic shear regime is being investigated with reduced core transport and a higher predicted stability limit.

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.

The deuterium-tritium (D-T) experimental program on the Tokamak Fusion Test Reactor (TFTR) is underway and routine tritium operations have been established. The technology upgrades made to the TFTR facility have been demonstrated to be sufficient for supporting both operations and maintenance for an extended D-T campaign. To date fusion power has been increased to {approximately}9 MW and several physics results of importance to the D-T reactor regime have been obtained: electron temperature, ion temperature, and plasma stored energy all increase substantially in the D-T regime relative to the D-D regime at the same neutral beam power and comparable limiter conditioning; possible alpha electron heating is indicated and energy confinement improvement with average ion mass is observed; and alpha particle losses appear to be classical with no evidence of TAE mode activity up to the P{sub FUS} {approximately}6 MW level. Instability in the TAE mode frequency range has been observed at P{sub FUS} > 7 MW and its effect on performance is under investigation. Preparations are underway to enhance the alpha particle density further by increasing fusion power and by extending the neutral beam pulse length to permit alpha particle effects of relevance to the ITER regime to be more fullymore » explored.« less

The final hardware modifications for tritium operation have been completed for the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324 (1992)]. These activities include preparation of the tritium gas handling system, installation of additional neutron shielding, conversion of the toroidal field coil cooling system from water to a FluorinertTM system, modification of the vacuum system to handle tritium, preparation, and testing of the neutral beam system for tritium operation and a final deuterium–deuterium (D–D) run to simulate expected deuterium–tritium (D–T) operation. Testing of the tritium system with low concentration tritium has successfully begun. Simulation of trace and high power D–T experiments using D–D have been performed. The physics objectives of D–T operation are production of ≊10 MW of fusion power, evaluation of confinement, and heating in deuterium–tritium plasmas, evaluation of α‐particle heating of electrons, and collective effects driven by alpha particles and testing of diag...

During lower hybrid current drive on tokamaks, suprathermal electrons are generated that emit hard x‐ray bremsstrahlung. A pinhole camera has been installed on the PBX‐M tokamak that records 128×128 pixel images of the hard x‐ray plasma bremsstrahlung emitted in the 30–200 keV photon energy range. The camera has a temporal resolution of 3 ms and a spatial resolution of 3–4 cm in the plasma. The detector is a 230‐mm‐diam hard x‐ray image intensifier. This instrument has identified hollow radiation profiles on PBX‐M, indicating off‐axis current drive. A detailed account of the construction of the hard x‐ray camera, its operation, and its performance is given.

Recent research on TFTR has emphasized optimization of performance in deuterium plasmas, transport studies and studies of energetic ion and fusion product physics in preparation for the D-T experiments that will commence in July of 1993. TFTR has achieved full hardware design parameters, and the best TFTR discharges in deuterium are projected to QDT of 0.3 to 0.5.The physics phenomena that will be studied during the D-T phase will include: tritium particle confinement and fueling, ICRF heating with tritium, species scaling with tritium, collective alpha-particle instabilities, alpha heating of the plasma and helium ash buildup. It is important for the fusion program that these physics issues be addressed to identify regimes of benign alpha behavior, and to develop techniques to actively stabilize or control instabilities driver by collective alpha effects.

Measurements of the toroidal rotation speed vφ(r) driven by neutral beam injection in tokamak plasmas and, in particular, simultaneous profile measurements of vφ, Ti, Te, and ne, have provided new insights into the nature of anomalous transport in tokamaks. Low‐recycling plasmas heated with unidirectional neutral beam injection exhibit a strong correlation among the local diffusivities, χφ≊χi>χe. Recent measurements have confirmed similar behavior in broad‐density L‐mode plasmas. These results are consistent with the conjecture that electrostatic turbulence is the dominant transport mechanism in the tokamak fusion test reactor tokamak (TFTR) [Phys. Rev. Lett. 58, 1004 (1987)], and are inconsistent with predictions both from test‐particle models of strong magnetic turbulence and from ripple transport. Toroidal rotation speed measurements in peaked‐density TFTR ‘‘supershots’’ with partially unbalanced beam injection indicate that momentum transport decreases as the density profile becomes more peaked. In hi...

Magnetohydrodynamic (MHD) activity within three zones (core, half‐radius, and edge) of TFTR [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. 1, p. 51] tokamak plasmas are discussed. Near the core of the plasma column, sawteeth are often observed. Two types of sawteeth are studied in detail; one with complete, and the other with incomplete, magnetic reconnection. Their characteristics are determined by the shape of the q profile. Near the half‐radius the m/n=3/2 and 2/1 resistive ballooning modes are found to correlate with a beta collapse. The pressure and the pressure gradient at the mode rational surface are found to play an important role in stability. MHD activity is also studied at the plasma edge during limiter H modes. The edge localized modes (ELM’s) are found to have a precursor mode with a frequency between 50–200 kHz and a mode number m/n=1/0. The mode does not show a ballooning structure. While these instabilities have been studied on many other machines, ...

The efficiency of plasma current drive with lower hybrid waves is reflected in the distribution of suprathermal electrons formed during this process. Past measurements of the bremsstrahlung radiation from these electrons depended on arrays of hard x‐ray detectors at different toroidal and poloidal locations around tokamaks [Nucl. Fusion 25, 1515 (1985)]. To avoid the space requirements and the complex correlation and inversion procedures associated with such systems, a single 23‐cm‐diam hard x‐ray image intensifier tube is being installed for two‐dimensional imaging on PBX‐M. It will be optically coupled to an image processor already in place for a soft x‐ray pinhole camera [Rev. Sci. Instrum. 59, 1831 (1988)], and analysis algorithms similar to those already developed for this diagnostic will be used.

A peak fusion power production of 9.3±0.7 MW has been achieved on the Tokamak Fusion Test Reactor (TFTR) in deuterium plasmas heated by co and counter injected deuterium and tritium neutral beams with a total power of 33.7 MW. The ratio of fusion power output to heating power input is 0.27. At the time of the highest neutron flux the plasma conditions are: Te(0)=11.5 keV, Ti(0)=44 keV, ne(0)=8.5×1019 m−3, and 〈Zeff〉=2.2 giving τE=0.24 s. These conditions are similar to those found in the highest confinement deuterium plasmas. The measured D‐T neutron yield is within 7% of computer code estimates based on profile measurements and within experimental uncertainties. These plasmas have an inferred central fusion alpha fraction of 0.2% and central fusion power density of 2 MW/m3 similar to that expected in a fusion reactor. Even though the alpha velocity exceeds the Alfven velocity throughout the time of high neutron output in most high power plasmas, MHD activity is not substantially different from that in co...

A review of TFTR plasma transport studies is presented. Parallel transport and the confinement of suprathermal ions are found to be relatively well described by theory. Cross-field transport of the thermal plasma, however, is anomalous with the momentum diffusivity being comparable to the ion thermal diffusivity and larger than the electron thermal diffusivity in neutral beam heated discharges. Perturbative experiments have studied nonlinear dependencies in the transport coefficients and examined the role of possible nonlocal phenomena. The underlying turbulence has been studied using microwave scattering, beam emission spectroscopy and microwave reflectometry over a much broader range in k perpendicular to than previously possible. Results indicate the existence of large-wavelength fluctuations correlated with enhanced transport.

Disruptive instabilities in PLT have been analysed by using an array of surface barrier X-ray detectors viewing the plasma cross-section along 20 chords. A wide variety of phenomena has been observed. A system of classification has been attempted, based upon: (1) the severity of the disruption, (2) the dominant precursor oscillations, and (3) the location of the onset of the disruption. Minor disruptions, in which the disruption does not appear at the location of the island of the precursor oscillation, have been observed, sometimes accompanied by seemingly independent higher-frequency oscillations of different helicity localized near the point of the disruption. Major disruptions exhibit flatter central q-profiles, slowing of the oscillations, asymmetry with respect to the centre of the discharge, and a correlation with high-Z impurity radiation.

During the 1987 run, TFTR reached record values of QDD, neutron source strength, and Ti(0). Good confinement together with intense auxiliary heating has resulted in a plasma pressure greater than 3*105 Pascals on axis, which is at the ballooning stability boundary. At the same time improved diagnostics, especially ion temperature profile measurements, have led to increased understanding of tokamak confinement physics. Ion temperature profiles are much more peaked than previously thought, implying that ion thermal diffusivity, even in high ion temperature supershot plasmas, is greater than electron thermal diffusivity. Based on studies of the effect of beam orientation on plasma performance, one of the four neutral beamlines has been re-oriented from injecting co-parallel to counter parallel, which will increase the available balanced neutral injection power from 14 MW to 27 MW. With this increase in balanced beam power, and the addition of 7 MW of ICRF power it is planned to increase the present equivalant QDT of 0.25 to close to break-even conditions in the coming run.

Ion-temperature results are deduced from Doppler-broadening measurements of the $K\ensuremath{\alpha}$ ($1s\ensuremath{-}2p$) resonance line emitted from heliumlike iron impurity ions in the hot central core of PLT (Princeton Large Torus) tokamak discharges. The measurements were performed using a high-resolution Bragg-crystal spectrometer with a multiwire proportional counter.

This paper gives a brief description of the geometry and parameters of the PLT tokamak, reviews some of the last four years' results that are particularly relevant to plasma-boundary interactions, and then concentrates on two specific problems.

High-resolution spectra of Fe xxv have been observed from Princeton Large Torus tokamak discharges and are used for a detailed comparison with theoretical predictions of Bely-Dubau, Gabriel, and Volont\'e for the ($1{s}^{2}nl\ensuremath{-}1s2pnl$, $ng~3$) dielectronic satellites at 1.85 \AA{}. The theory predicts a significant apparent broadening of the resonance lines of high-$Z$ heliumlike ions in high-temperature plasmas due to the dielectronic capture of electrons into outer atomic shells. The experimental results and theoretical predictions are in good general agreement.

A curved-crystal Bragg x-ray spectrometer has been used to measure $K\ensuremath{\alpha}$ or $1s\ensuremath{-}2p$ radiation from highly stripped Fexviii-Fexxv impurity ions in the Princeton large torus tokamak. The spectrometer has sufficient energy resolution (\ensuremath{\lesssim}4 eV at 6400 eV) to distinguish between the different ionization states of iron by measuring the energy shift of the $K\ensuremath{\alpha}$ x rays. The measured wavelengths agree well with theory and with spectra from solar flares and from laser-produced plasmas. The distribution of Fe charge states in the center of the discharge has been inferred from a comparison of the measured x-ray spectrum with theory. The shape of the spectrum depends strongly on electron temperature (${T}_{e}$) in the range ${T}_{e}=800\ensuremath{-}1500$ eV. Within the factor of 2 uncertainty in $L$-shell ionization cross sections, measured intensities agree with theory, which is based on coronal equilibrium, indicating that the ion lifetime in the center of the plasma is approximately equal to or greater than the equilibration time.

Experimental results from high-power neutral-beam--injection experiments on the Princeton Large Torus tokamak are reported. At the highest beam powers (2.4 MW) and lowest plasma densities (n/sub e/(0) = 5 x 10/sup 13/cm/sup -3/), ion temperatures of 6.5 keV are achieved. The ion collisionality v/sub i/* drops below 0.1 over much of the radial profile. Electron heating of ..delta..T/sub e//T/sub e/ approx. = 50% has also been observed, consistent with the gross energy-confinement time of the Ohmically heated plasma, but indicative of enhanced electron-energy confinement in the core of the plasma.

Dielectronic satellite spectra of Fe XXV near 1.8500 A have been observed from PLT (Princeton Large Torus) tokamak plasma discharges for electron temperatures in the range from 1.5 to 3 keV and an electron density of 2 x 10/sup 13/ cm/sup -3/. The electron temperature was independently determined from the electron cyclotron radiation emitted by the plasma. The quality of the spectra allows a detailed comparison with theoretical prediction, which is of importance in view of diagnostic applications.

TEXTOR, a tokamak primarily devoted to plasma-wall interaction studies, has been operated during the last 2–3 years under a wide variety of discharge conditions. Discharges with Ohmic heating only and with additional ICR heating in the MW-range were produced over an extended range of plasma parameters. Different limiter materials were used — Inconel, stainless steel, graphite, TiC coated graphite. The surface of the first wall (liner) consisted either of Inconel or of a carbon layer deposited upon the Inconel. Characteristic features of the discharge have been modified by making use of the modular pump limiter ALT-I and by applying a localized perturbation coil for “ergodization” of the magnetic field structure in the boundary. The characteristics of plasma-wall interaction under these conditions and their influence on the global performance of the TEXTOR plasma are described and discussed. Use has been made of novel diagnostics specifically developed for edge plasma investigation.

An arrangement is described whereby, employing a simple microscope, the magnitude of the stage-noise is reduced, independent of the cell-length, such that it has no influence any longer on the determination of the scattering angle. The unsensitivity to shock or vibration permits the use of an ocular-eye-piece screw-micrometer. An auxiliary device for convenient alignment of the nuclear plates is also described.

MEASUREMENTS of the neutron spectra from the PLT tokamak plasma are reported here which provide information on the centre-of-mass velocity of the reacting deuteron pairs. The observation in PLT of no significant directed velocity yields additional evidence that the neuterons are produced by a thermonuclear process. Previous spectral measurements on other fusion devices have often determined that the neutron emission was non-thermonuclear and resulted from a small ‘tail’ component of energetic ions1–3. The neutron spectra indicated a directed velocity of the reacting deuterons and thus non-thermonuclear neutron emission if the injected neutral beam ions were deuterium. In this condition, the neutron spectra were consistent with the expected beam-induced d(d,n) 3He reactions. However, if the heating beam used hydrogen, then the neutron spectra are consistent with a thermonuclear neutron emission resulting from the beam heating of the bulk deuterons in the plasma.

Fluctuations in x-ray intensity from the ST tokamak show a characteristic sawtooth behavior. This behavior is identified as an internal disruption. The internal disruptions are preceded by growing sinusoidal $m=1$, $n=1$ oscillations. The properties of these oscillations are compared with predictions for the $m=1$ internal kink mode.

This paper describes measurements of the central ion and electron temperature of tokamak plasmas from the observation of the 1s - 2p resonance lines, and the associated dielectronic (1s/sup 2/nl - 1s2pnl, with n greater than or equal to 2) satellites, of helium-like iron (Fe XXV) and titanium (Ti XXI). The satellite to resonance line ratios are very sensitive to the electron temperature and are used as an electron temperature diagnostic. The ion temperature is deduced from the Doppler width of the 1s - 2p resonance lines. The measurements have been performed with high resolution Bragg crystal spectrometers on the PLT (Princeton Large Torus) and PDX (Poloidal Divertor Experiment) tokamaks. The details of the experimental arrangement and line evaluation are described, and the ion and electron temperature results are compared with those obtained from independent diagnostic techniques, such as the analysis of charge-exchange neutrals and measurements of the electron cyclotron radiation. The obtained experimental results permit a detailed comparison with theoretical predictions.

The evolution of ultra-soft x-ray radiation (USX, h..nu.. approx. > 100 eV) is compared to that of the soft x-ray radiation (SX, h..nu.. approx. > 1000 eV) during several disruptions in PLT. Spatial resolution is obtained in both cases by arrays of silicon surface barrier detectors viewing along different chords. During some disruptions the USX behaves quite differently from the SX, and a classification is made based on the USX behavior. Different interpretations of the data are discussed, along with the possibility that these measurements may distinguish between the roles of temperature and impurity density changes during disruptions.

Tokamak discharges, which are studied for the magnetic confinement of fusion plasmas, emit soft x-ray radiation in the energy range from 1–20 keV. This radiation consists of a bremsatrahlung and recombination continuum, and characteristic line spectra of impurity ions from the plasma limiter and the walls of the vacuum vessel. The radiation represents a unique source of information and is increasingly used as a diagnostic of plasma parameters. This includes measurements of the electron and ion temperature, investigation of the ionization equilibrium and particle transport, and the study of MHD fluctuations and disruptions.

Quantitative vacuum ultraviolet (UV) measurements (lambda greater than or equal to 100 A) of various ST-Tokamak discharges show that the atom density of metal impurities is about 0.1 to 1.5 percent of the electron density and that the atom density of oxygen is about 1 to 10 percent. These concentrations yield an effective ionic charge, anti Z, consistent with the measured current, voltage, and profiles of electron temperature. The radial distribution of high-Z material is obtained from x-ray measurements in the range 1 A less than or equal to lambda less than or equal to 4 A; from these one concludes that there is no extreme concentration of high-Z material in the center, but cannot exclude variations within a factor of 2. The x-ray spectrum is dominated by recombination; using bremsstrahlung and recombination calculations the value of anti Z derived from x- ray intensity is comparable with the value from conductivity. An instrument is described for making measurements in the extreme UV region (5 less than lambda less than 20 A). In this range strong emission has been observed (radiated power about 10 percent of power input in the center), which is attributed to line emission from heavy-metal (Fe ormore » Mo) ions which are produced only at an electron temperature above about 1200 eV; the very narrow radial profile of observed intensity is consistent with measured temperature and density profiles. Rough estimates of the concentration of heavy metals required to account for the radiated power are consistent with vacuum UV and x-ray data. (auth)« less

PLT discharges are categorized in terms of energy balance into three types: radiation dominated (RD); transport dominated (TD), and an intermediate type (TR). Control of plasma edge conditions is essential for the formation of these types. Measurements and calculations show the relative importance of radiation, transport, and electron-ion coupling in different regions of the discharge. Ion heating for neutral beam injection is found to be effective in all types. Electron heating by neutral beam injection is only consistently obtained for TD discharges.

The experimentally observed persistence of the m = 1 mode following an internal disruption is described. The hot central region appears to spiral outward, being pared by interception with the radius of q = 1 (as deduced from the radial extend of the m = 1 mode). The outward motion of the axis can be arrested and the surviving helical filament can subsequently spiral inward, rather than being annihilated as expected from nonlinear resistive internal kink mode simulations.

The Tokamak Fusion Test Reactor (TFTR) (R. J. Hawryluk, to be published in Rev. Mod. Phys.) experiments on high-temperature plasmas, that culminated in the study of deuterium–tritium D–T plasmas containing significant populations of energetic alpha particles, spanned over two decades from conception to completion. During the design of TFTR, the key physics issues were magnetohydrodynamic (MHD) equilibrium and stability, plasma energy transport, impurity effects, and plasma reactivity. Energetic particle physics was given less attention during this phase because, in part, of the necessity to address the issues that would create the conditions for the study of energetic particles and also the lack of diagnostics to study the energetic particles in detail. The worldwide tokamak program including the contributions from TFTR made substantial progress during the past two decades in addressing the fundamental issues affecting the performance of high-temperature plasmas and the behavior of energetic particles. The progress has been the result of the construction of new facilities, which enabled the production of high-temperature well-confined plasmas, development of sophisticated diagnostic techniques to study both the background plasma and the resulting energetic fusion products, and computational techniques to both interpret the experimental results and to predict the outcome of experiments.

The Tokamak Fusion Test Reactor (TFTR) is a large tokamak which has performed experiments with 50:50 deuterium - tritium fuelled plasmas. Since 1993, TFTR has produced about 1090 D - T plasmas using about 100 grams of tritium and producing about 1.6 GJ of D - T fusion energy. These plasmas have significant populations of 3.5 MeV alphas (the charged D - T fusion product). TFTR research has focused on alpha particle confinement, alpha driven modes, and alpha heating studies. Maximum D - T fusion power production has aided these studies, requiring simultaneously operation at high input heating power and large energy confinement time (to produce the highest temperature and density), while maintaining low impurity content. The principal limitation to the TFTR fusion power production was the disruptive stability limit. Secondary limitations were the confinement time, and limiter power handling capability.

It has recently been proposed that stochasticity resulting from toroidal coupling could lead to a saturation of the m=1 internal mode in tokamaks. Results are presented from the nonlinear evolution of the m=1 mode with toroidal coupling that show that stochasticity is not enough to cause saturation of the m=1 mode.

It was previously shown that the confinement of plasma in Stellarators during the Ohmic heating process, after substantially full ionization is reached, is not in accord with classical collisional diffusion. Evidence is presented which shows that the enhanced diifusion rate also governs confinement during the ionization phase, where the neutral particle density is still high. The discharges considered were obtained with hydrogen in the B-3 Stellarator and were of the same character as those used to demonstrate the rapid loss of plasma after nearly full ionization. The peak electron density in each discharge is plotted as a function of the confining magnetic field for hydrogen pressures of 11.3 x 10/ sup -5/, 30 x lO/sup -5/, and 58.1 x lO/sup -5/ mm Hg at applied electric fields of 0.031, 0.105, and 0.31 v/cm, respectively. lt is concluded that there is some loss of plasma during the ionization phase of the discharges because the peak density becomes smaller as the confiniug field is reduced. The results are roughly consistent with a time constant of the order of 100 mu sec for diffusion across a magnetic field of 20 to 40 kilogauss, in general agreement with the times observed after peak density.more » (B.O.G.)« less

Steady‐state temperature and magnetic field profiles are derived in cylindrical geometry, including classical electron‐ion equilibration, Ohmic heating, and the classical ion thermal transport appropriate to tokamak geometry. To fit the experimental data, a direct electron heat loss must accompany any appreciable resistivity anomaly. The classical equilibrium can be thermally stable only if Ti/Te ≳ 2/3. Direct electron heat loss by radiation is further destabilizing; but for typical parameters, stability can be achieved by a moderate amount of anomalous electron thermal conduction. The growth time of thermal instabilities is generally limited by the resistive skin time. The presence of a runaway component of the current allows shorter growth times, and this may be related to the present high‐density limit in tokamaks.