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MHD phenomena in ASDEX Upgrade in standard shear discharge regimes as well as in weak and reversed shear discharges are investigated. The onset of neoclassical tearing modes leads to the most serious β limit at ASDEX Upgrade. The βp value for the onset of neoclassical tearing modes is found to be proportional to the poloidal ion gyroradius for collisionless plasmas, as proposed by the ion polarization current model. Larger collisionalities have a stabilizing effect. Sawtooth crashes or fishbones can trigger the mode, and in a few cases it appears spontaneously. The fractional energy loss due to a (3,2) mode saturates for large pressures at around 25%, in agreement with the result of theoretical calculations. Owing to its large and flexible heating power, ASDEX Upgrade is well equipped for investigations of fast particle driven or influenced MHD modes. The observed TAEs have been shown to agree well with theoretical predictions regarding the frequency as well as the drive by fast particles. Fishbones have been shown to not only affect the fast particle distribution but also directly to affect the background plasma. For low and reversed shear discharges, theoretical predictions for the growth of neoclassical tearing modes in regimes with a large fraction of bootstrap current at the rational surface, as well as for neoclassical double tearing modes, are given. In discharges with large impurity accumulation in the plasma centre, unusual MHD phenomena such as cascades of high n tearing modes and modes driven by positive pressure gradients have been found at ASDEX Upgrade. For similar discharges, double tearing modes have been investigated at TEXTOR.

Observations on ASDEX Upgrade show that fishbones can have a much stronger effect on the background plasma parameters than expected from an ideal, fast particle driven instability. They lead to a redistribution of electron temperature and impurity densities, similar in form and magnitude to sawteeth, albeit with the plasma parameter variation occurring on a significantly longer time-scale. Simulations of the current diffusion in discharges with favourable core confinement require fishbones to be furthermore associated also with magnetic reconnection to yield consistency between the conductivity and current profile information derived from ECE, bremsstrahlung, MSE and the localization of m = 1,n = 1 activity.

The early phase of the neoclassical (3,2) + (2,2) tearing mode at ASDEX Upgrade has been investigated in order to study the seed island. Sawtooth crashes or fishbones can trigger this mode, and in few cases it appeared spontaneously.

The βp value for the onset of the neoclassical (3, 2) tearing mode at ASDEX Upgrade is found to be proportional to the ion gyro-radius as proposed in the ion polarization model. This scaling law has been proven by varying the ion temperature, the magnetic field and the mass of the plasma ions.

If the ion Bernstein wave (IBW) heating power in an H mode discharge of the PBX-M experiment exceeds a threshold power of about 200 kW, a core transport barrier is created in the central region of the plasma. At lower neutral beam injection (NBI) powers, the core barrier is accompanied by an edge L mode. The high edge localized mode (ELM) repetition frequency (1 kHz) prevents the creation of a strong barrier, so the edge first has to make an H-to-L transition before a strong core transport barrier can be created. At higher NBI powers, the ELM repetition frequency is lowered to less than 200 Hz, which allows the immediate creation of a strong core barrier. Edge localized mode loss, which propagates radially first on a fast (non-diffusive) and then on a slow (diffusive) time-scale all the way to the plasma core, is strongly reduced in the core barrier region. Correlated with the reduced ELM loss, the fluctuations in the core barrier region are also strongly reduced, both during the ELM and during the quiet periods between the ELMs. There is strong evidence that the IBW induced poloidal flow shear is responsible for the stabilization of core turbulence and the creation of the core transport barrier. The large perpendicular E ? B flow shear component of the measured toroidal velocity in co-injection neutral beam heated discharges seems to be largely cancelled by the ion diamagnetic drift shear produced by large ion pressure gradients in the core barrier region. The value of IBW induced poloidal flow has not been experimentally determined, but its numerical value is found to be a factor of 4 larger than either the toroidal velocity or the ion diamagnetic drift shear components, leaving only IBW induced flow shear as the most probable cause for the turbulence stabilization. The core turbulence suppression and the creation of the core transport barrier is also consistent with expectations from a comparison between the E ? B flow shear rate and a rough estimate of the linear ion temperature gradient (ITG) growth rate. The presence of the core barrier region also strongly modifies the other MHD events: crashes on the q = 1.5, 2 surfaces and the disruption.

Two new types of high frequency (HF) MHD mode activity are observed when a core transport barrier is formed during ion Bernstein wave (IBW) heating in PBX-M. These modes, localized in the core transport barrier, seem to be mainly responsible for the soft saturation in the strength of the core transport barrier. The two types of HF mode activity are determined by the presence or absence of a strong 1/1 mode. The n = m modes are observed with the 1/1 mode, while the n = m - 1 modes are seen when the 1/1 mode is absent. The m numbers of both of these modes vary between 4 and 13. They all seem to be coupled and rotate as a rigid body. The n = m - 1 modes seemed to be narrowly localized on their corresponding rational surface, i.e. q = m/(m - 1). The modes are ballooning-like, and their amplitudes increase with increasing β. Calculations with the CASTOR code show that the n = m - 1 modes are most probably pressure driven tearing modes, but the possibility of kinetic ballooning modes (KBMs) cannot be fully excluded. The n = m modes appear on or around the q = 1 surface, and are localized at the X point of the 1/1 mode island. The frequency offset of the n = m modes, which is too low for a TAE, is shown to be consistent with β-induced toroidal Alfven eigenmodes (BAEs).

The physics of current profile modification with lower hybrid waves has been investigated in the Princeton Beta Experiment-Modification tokamak by comparing experimental internal magnetic field profiles with ray tracing code predictions. When the n| spectrum of the launched waves was varied, local changes in the current profile were observed according to equilibria, reconstructed from motional Stark effect polarimetry measurements. Changes in the central safety factor (q) were also determined to be a function of the applied radiofrequency (RF) power. Calculations of the non-inductive current with the Tokamak Simulation Code/Lower Hybrid Simulation Code were unable to duplicate the measured current profiles. Instead, a current broadening mechanism like a finite fast electron diffusion coefficient had to be hypothesized to fit the data. This study was possible only with recent improvements in diagnostics and numerical analysis tools, and it constitutes an important contribution to our understanding of non-inductive current drive for future fusion reactors

The characteristics of high- beta , low-q disruptions have been studied in PBX-M, a device with a nearby conducting shell. The coupling between the wall and the plasma was varied by choosing different plasma shapes, including nearly circular plasmas, D-shaped plasmas and bean-shaped plasmas (indented on the midplane), and by increasing the effective coverage of the plasma by the shell. Disruption precursors were observed to have a strong dependence on the coupling between the plasma and the shell. Measured mode growth times vary from between several times the Alfven time-scale (~100 mu s) to the L/R time-scale of the wall (~20 ms). The behaviour of observed disruption precursors is interpreted in terms of the resistive wall mode theory of ideal plasmas, and a detailed calculation of the stability of a strongly coupled bean configuration using the NOVA-W linear stability code is presented. The experimental observations are in good agreement with the theoretical predictions

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

In many of the medium and high beta discharges in PBX-M, low n modes (n=1-4) with different n numbers are observed. The prevalence of low n modes decreases with increasing n number. Furthermore, if two modes of different frequency and n number (?1, and ?2; k1 and k2) are simultaneously present in the plasma, these modes interact non-linearly and create sidebands in frequency (?2 ? ?1) and wavenumber (k2 ? k1 or n2 ? n1 and m2 ? m1). It has also been observed that if these fundamental modes, ?1/k1 and ?2/k2, contain strong harmonics, the harmonics also interact non-linearly, creating more non-linear products: k?2 ? l?1 and kk2 ? lk1, where k and l are integers describing the harmonics. These multiple three wave couplings on a given rational surface can create several modes of the same m and n numbers but of different phase velocities. The multiple perturbations of the rational surfaces can increase the plasma losses. In fact, a decrease in neutron rate and an enhanced loss of medium energy ions are observed during this three wave coupling interaction. Although the three wave coupling has been observed primarily in medium or high beta discharges, it is a more general plasma phenomenon because it also occurs occasionally at low ?

High frequency pressure driven modes have been observed in high poloidal beta discharges in the Princeton Beta Experiment Modification (PBX-M). These modes are excited in a non-axisymmetric equilibrium characterized by a large, low frequency mt = 1/nt = 1 island, and they are capable of expelling fast ions. The modes reside on or very close to the q = 1 surface and have mode numbers with either mh = nh or (less probably) mh/nh = mh/(mh-1), with mh varying between 3 and 10. Occasionally these modes are simultaneously localized in the vicinity of the ml = 2/nl = 1 island. The high frequency modes near the q = 1 surface also exhibit a ballooning character, being significantly stronger on the large major radius side of the plasma. When a large mt = 1/nt = 1 island is present, the mode is poloidally localized in the immediate vicinity of the X point of the island. The modes occur exclusively in high beta beam heated discharges and are likely to be driven by the beam ions. They can thus be a manifestation of either a toroidicity induced shear Alfven eigenmode (TAE) at q = (2mh+1)/2nh, a kinetic ballooning mode, or some other type of pressure driven (high β) mode. Most of the data are consistent with the theoretical predictions for the TAE gap mode. Since the high frequency modes in PBX-M, however, are found exclusively on or in the immediate neighbourhood of magnetic surfaces with low rational numbers (q = 1, 2,...), other possibilities are not excluded

High poloidal beta discharges in PBX-M routinely enter the H mode regime: typically, a quiescent phase followed by an MHD-active phase characterize the H mode period. An analysis of the energy transport during these phases is conducted using the experimental data and the code TRANSP; effective thermal diffusivities are computed. The quiescent H phase is characterized by a decrease of the thermal ion energy transport and a flattening of the associated effective diffusivity profile. Enhanced fast ion losses are observed during the MHD-active phase; particles in the lower end of the fast-ion energy spectrum with a large perpendicular velocity component are predominantly affected. Folding these losses into the analysis permits to reproduce the measured stored energy and time evolution of the neutron production rate during the MHD-active phase. During the MHD-active phase, the ion thermal diffusivity maintained a profile (magnitude and shape) similar to that obtained at the end of the quiescent H phase, suggesting, within the uncertainty of this analysis, that the MHD activity does not affect the thermal plasma energy transport. The observed stored energy (beta) saturation results from the enhanced losses in the fast-ion population. An analysis without inclusion of enhanced fast ion losses fails to reproduce the experimental measurements. Throughout the H mode period the electron effective diffusivity remains lower than the ion effective diffusivity. An error analysis is presented

Measurements of the flux surface shapes in a tokamak with a non-circular cross-section have been made using a tangentially viewing soft X ray pinhole camera. These measurements allowed the derivation of the value of the axial safety factor, q(0), and the shape of the plasma current profile, during equilibrium modification experiments. Two different equilibrium modification techniques were used: neutral beam current drive and plasma current ramp. In the neutral beam current drive experiment, tangential counter-injection was found to modify the equilibrium significantly, broadening the current profile and raising the value of q(0) above unity. In tangential co-injection discharges, q(0) values as low as 0.4 were observed. Reproduction of the measured q(0,t) using a radial current diffusion model strongly suggests there was a non-classical component to the cross-field current diffusion. A plasma current ramp of 2.5 MA/s was also shown to broaden the current profile; however, the value of q(0) was not altered

H-mode plasmas with high poloidal beta values in the PBX-M tokamak often exhibit periods of edge localized mode (ELM) activity, with each ELM preceded by a short duration (≤ 350 μs) burst of high frequency (200-250 kHz) magnetic activity. The burst grows on a time-scale of 10 μs and disappears just before the rapid increase in the Dα emission that is characteristic of the ELM. The burst of activity is observed at all poloidal locations, with the largest amplitudes at coils on the inner major radius side, indicating that the mode is not outward ballooning in character. Stability calculations indicate that this high frequency ELM precursor is probably the pressure driven ideal kink mode.

The bremsstrahlung emission from the PLT tokamak during lower-hybrid current drive has been measured as a function of angle between the magnetic field and the emission direction. The emission is peaked strongly in the forward direction, indicating a strong anisotropy of the electron velocity distribution. The data demonstrate the existence of a nearly flat tail of the velocity distribution, which extends out to approximately 500 keV and which is interpreted as the plateau created by Landau damping of the lower-hybrid waves.

Intrinsic impurity behaviour and transport properties in neutral beam heated L- and H-mode PBX tokamak plasmas were studied with a variety of impurity diagnostics. Central impurity accumulation was most often observed in H-mode discharges; sometimes it resulted in a thermal collapse due to high central metallic radiation (~1.5 W • cm−3). The impurity accumulation was evident from peaked Zeff and radiated power profiles and was further substantiated by specific vacuum ultraviolet and X-ray spectroscopy measurements. It is shown that impurity accumulation was neither unique nor inevitable in H-mode discharges and it could be suppressed by sufficient gas puffing. Central impurity accumulation was also seen in L-mode plasmas even with co-injected neutral beams. This usually occurred at high beam power and relatively low density. While there was no significant difference in the degree of accumulation between L-mode and H-mode discharges, the Zeff profile itself was more peaked in the H-mode because the electron density profiles are flatter in H-mode plasmas than in L-mode plasmas. The degree of accumulation increased as Zeff(0) itself increased, which suggests that neoclassical convection and diffusion driven by both impurity-impurity and impurity-plasma ion collisions contribute to the central plasma particle transport.

Princeton Beta Experiment (PBX) discharges run at the lowest q and highest βt always terminated in a hard disruption. The discharges, with βt values of up to 5.5% and q-values down to 2.2, were obtained by employing large current ramps and large gas feed rates during neutral beam injection. Previous work has indicated that the achieved βt values were consistent with the limit imposed by the n = l ideal external kink with a conducting wall at b/a = 2. The authors of the paper investigate further the validity of ideal MHD theory in explaining the low qψ disruptions. In particular, the characteristics of the pre-disruption MHD activity in these low-q discharges, specifically the time-scale of growth and internal and external mode structures, are compared with those determined from theoretical calculations. The results of these comparisons indicate that non-ideal effects must be considered in order to obtain detailed agreement between theory and experiment.

Absorber foils have been installed in the TFTR X-Ray Imaging System to permit measurement of the electron temperature along 10 to 30 chords spaced at 5-12.5 cm with a time resolution of less than 100 ..mu..s. The technique uses the ratio of x-ray fluxes transmitted through two different foils. The ratio depends mainly on electron temperature. Simulations show that strong impurity line radiation can distort this ratio. To correct for these effects, special beryllium-scandium filters are employed to select the line-free region between 2 and 4.5 keV. Other filter pairs allow corrections for Fe L and Ni L line radiation as well as Ti K and Ni K emission. Good accuracy is also obtained with simple beryllium filters, provided that impurity corrections are incorporated in the analysis, taking line intensities from the x-ray pulse-height analysis diagnostic. A description of modeling calculations and a comparison of temperature values from this diagnostic with data from the x-ray pulse height analysis, the electron cyclotron emission, and the Thomson scattering diagnostics are presented. Several applications of the absorber foil electron temperature diagnostic on TFTR are discussed.