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Stationary discharges with $H$-mode edge and internal transport barrier for energy, momentum, and particle transport have been obtained on ASDEX Upgrade. At a line averaged density of $4\ifmmode\times\else\texttimes\fi{}{10}^{19}{\mathrm{m}}^{\ensuremath{-}3}$ an $H$ factor of ${H}_{\mathrm{ITER}89\ensuremath{-}P}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.4$ and ${\ensuremath{\beta}}_{N}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2$ could be maintained for 6 s corresponding to 40 confinement times, limited only by the possible discharge duration. The $q$ profile is flat in the center and close to 1. Current diffusion calculations suggest that reconnection is required to clamp the safety factor at 1, which in the absence of sawteeth is explained by the strong fishbone activity.

High confinement [with $H\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$, ITERH- $98(y,2)$] at ${\ensuremath{\beta}}_{N}\ensuremath{\approx}2.3$ has been found in ASDEX Upgrade discharges with existing $(m,n)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(3,2)$ neoclassical tearing modes (NTMs). The reason for the high confinement at high ${\ensuremath{\beta}}_{N}$ values is a transition of the NTMs into the so-called frequently interrupted regime. In this regime, the NTM growth is frequently interrupted by sudden drops in amplitude. Because of the long NTM growth time, the resulting averaged NTM amplitude remains much smaller than the corresponding saturated value. As this behavior with the beneficial effect on energy confinement has been confirmed at JET, such a high confinement regime at ${\ensuremath{\beta}}_{N}g2$ might also be expected for ITER.

High confinement [with H = 1, ITERH- 98( y,2)] at beta(N) approximately 2.3 has been found in ASDEX Upgrade discharges with existing (m,n) = (3,2) neoclassical tearing modes (NTMs). The reason for the high confinement at high beta(N) values is a transition of the NTMs into the so-called frequently interrupted regime. In this regime, the NTM growth is frequently interrupted by sudden drops in amplitude. Because of the long NTM growth time, the resulting averaged NTM amplitude remains much smaller than the corresponding saturated value. As this behavior with the beneficial effect on energy confinement has been confirmed at JET, such a high confinement regime at beta(N)2 might also be expected for ITER.

For the tokamak magnetic confinement concept, theory has predicted a distinct relationship between the plasma shape, the internal magnetic structure, and the presence or absence of fast ion losses in the presence of plasma instabilities. We have, for the first time, carried out measurements of the magnetic safety factor profile, [ital q]([ital r]), in plasmas unstable to a specific instability, the so-called fishbone'' mode. The experimental equilibria reconstructed from these data have been used to demonstrate that when the plasma is unstable to fishbones, the fast ion confinement properties depend strongly on the radius of the magnetic surface where [ital q]([ital r])=1.

Neutral-beam heating of plasmas in the Tokamak Fusion Test Reactor at low preinjection densities (n/sub e/(0)approx. =10/sup 19/ m/sup -3/) were characterized by T/sub e/(0) = 6.5 keV, T/sub i/(0) = 20 keV, n/sub e/(0) = 7 x 10/sup 19/ m/sup -3/, tau/sub E/ = 170 msec, ..beta../sub t//sub h//sub e//sub t//sub a/ = 2, and a d(d,n)/sup 3/He neutron emission rate of 10/sup 16/ sec/sup -1/. The ion temperature and the deuterium-fusion neutron yields were significantly higher than for previous tokamak experiments. The low initial densities were achieved by operation of the Tokamak Fusion Test Reactor with low plasma currents (less than or equal to1 MA) and by extensive limiter conditioning.

Strong magnetohydrodynamic activity has been observed in PDX neutral-beam--heated discharges. It occurs for ..beta../sub T/q> or =0.045 and is associated with a significant loss of fast ions and a drop in neutron emission. As much as 20%--40% of the beam heating power may be lost. The instability occurs in repetitive bursts of oscillations of < or =1 msec duration at 1--6 msec intervals. The magnetohydrodynamic activity has been dubbed the ''fishbone instability'' from its characteristic signature on the Mirnov coils.

Use of the ASDEX divertor permits the production of stable low-density discharges (n/sub e/> or approx. =10/sup 12/ cm/sup -3/) with extremely low resistivity lasting for more than 10 s. While the distribution functions of electrons and ions show suprathermal tails, runaway electrons in the megaelectronvolt range are found to disappear with decreasing density. There are indications that in these discharges the energy confinement is improved compared with ALCATOR scaling.

Plasma heating by near-perpendicular injection of up to 7.2 MW of neutral-beam power has been studied in the PDX tokamak. Collisionless plasmas with central ion temperatures up to 6 keV have been obtained. The total plasma energy, which is dominated by contributions from beam and thermal ions, rises linearly with increasing beam power. The ion heating efficiency in PDX is comparable to that measured in PLT with tangential injection.

Bean-shaped configurations favorable for high ..beta.. discharges have been investigated in the Princeton Beta Experiment (PBX) tokamak. Strongly indented bean-shaped plasmas have been successfully formed, and beta values of over 5% have been obtained with 5 MW of injected neutral beam power. These high beta discharges still lie in the first stability regime for ballooning modes, and MHD stability analysis implicates the external kink as responsible for the present ..beta.. limit.

Initial operation of the tokamak fusion test reactor has concentrated upon confinement studies of Ohmically heated hydrogen and deuterium plasmas. Total energy confinement times (${\ensuremath{\tau}}_{E}$) are 0.1-0.2 s for a line-average density range (${\overline{n}}_{e}$) of (1-2.5)\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{m}}^{\ensuremath{-}3}$ with electron temperatures of ${T}_{e}(0)\ensuremath{\sim}1.2\ensuremath{-}2.2$ keV, ion temperatures of ${T}_{i}(0)\ensuremath{\sim}0.9\ensuremath{-}1.5$ keV, and ${Z}_{\mathrm{eff}}\ensuremath{\sim}3$. A comparison of Princeton large torus, poloidal divertor experiment, and tokamak fusion test reactor plasma confinement supports a dimension-cubed scaling law.

Tangentially coinjected deuterium beam ions were accelerated from 82 up to 150 keV during a major-radius compression experiment in the tokamak fusion test reactor. The ion energy spectra and the variation in fusion yield were in good agreement with Fokker-Planck code simulations. In addition, the plasma rotation velocity was observed to rise during compression.

During tangential neutral-beam injection into the PBX tokamak, bursts of two types of instabilities are observed. One instability occurs in the frequency range 120-210 kHz and the other oscillates predominantly near the frequency of bulk plasma rotation (20-30 kHz). Both instabilities correlate with drops in neutron emission and bursts in charge-exchange neutral flux, indicating that beam ions are removed from the center of the plasma by the instabilities. The central losses are comparable to the losses induced by the fishbone instability during perpendicular injection. © 1986 The American Physical Society.