M. Okabayashi

Attainment of high confinement in neutral beam heated divertor discharges in the PDX tokamak

Abstract The PDX divertor configuration has recently been converted from an open to a closed geometry to inhibit the return of neutral gas from the divertor region to the main chamber. Since then, operation in a regime with high energy confinement in neutral beam heated discharges (ASDEX H-mode) has been routine over a wide range of operating conditions. These H-mode discharges are characterized by a sudden drop in divertor density and H α emission and a spontaneous rise in main chamber plasma density during neutral beam injection. The confinement time is found to scale nearly linearly with plasma current, but can be degraded due either to the presence of edge instabilities or heavy gas puffing. Detailed Thomson scattering temperature profiles show high values of T c near the plasma edge (∼ 450 eV) with sharp radial gradients (∼ 400 eV/cm) near the separatrix. Density profiles are broad and also exhibit steep gradients close to the separatrix.

Impurity levels and power loading in the pdx tokamak with high power neutral beam injection

Abstract The PDX tokamak provides an experimental facility for the direct comparison of various impurity control techniques under reactor-like conditions. Four neutral beam lines inject > 6 MW for 300 ms. Carbon rail limiter discharges have been used to test the effectiveness of perpendicular injection, but non-disruptive full power operation for > 100 ms is difficult without extensive conditioning. Initial tests of a toroidal bumper limiter indicate reduced power loading and roughly similar impurity levels compared to the carbon rail limiter discharges. Poloidal divertor discharges with up to 5 MW of injected power are cleaner than similar circular discharges, and the power is deposited in a remote divertor chamber. High density divertor operation indicates a reduction of impurity flow velocity in the divertor and enhanced recycling in the divertor region during neutral injection.

Initial results from the scoop limiter experiment in PDX

Abstract A particle scoop limiter with a graphite face backed by a 50 liter volume for collecting particles was used in PDX. Experiments were performed to test its particle control and power handling capabilities with up to 5 MW of D° power injected into D+ plasmas. Line average plasma densities of up to 8 × 1013 cm−3 and currents up to 450 kA were obtained. Plasma densities in the scoop channels greater than 2 × 1013 cm−3 and neutral densities in the scoop volume greater than 5 × 1014 cm−3 were observed. There is evidence that recycling may have occurred in the scoop channels for several discharges with large line-averaged plasma density. At beam powers up to 2.5 MW, energy confinement times above 40 ms were deduced from magnetics measurements and from transport analysis. Pressures in the vacuum vessel were in the 10 −5 Torr range, and recycling source neutral densities in the central plasma were low.

Electron thermal conductivity in the trapped electron regime in the FM‐1 spherator

The parametric dependence of electron thermal conductivity was studied in the trapped electron regime by utilizing a localized heating method in a shear stabilizing configuration in the FM‐1 spherator. The electron thermal conductivity K⊥ and the particle diffusion coefficient D⊥ show similar dependence on the shear length, Ls, and the electron temperature Te[K⊥ ∼ (1/400 ∼ 1/800)(Ls/a) (kTe/16eB)]. The conductivity was larger when the electron temperature gradient was parallel to the density gradient compared with the antiparallel case. The fluctuations associated with the heat flow are compared with the prediction of the trapped electron particle instability. Relevance to tokamak experiments is discussed.

Hot‐electron plasma in the levitated spherator

In the hot‐electron plasma experiment of the levitated spherator, it is found that hot electrons initially trapped in the mirror section are diffused out by electron‐neutral collisions resulting in the establishment of the equilibrium between the trapped hot electrons and circulating hot electrons. The confinement time reaches 6 sec at the neutral pressure of 2 × 10−7 Torr, which is about one‐third of the classical decay time across the magnetic field. The time dependence of the density of the circulating hot electrons is in good agreement with the numerical calculations based on the Fokker‐Planck equation. However, the existence of circulating hot electrons is sensitive to the magnetic field error of 1%. The establishment of the inductive current of the hot electron plasma is also investigated.

Trapped‐electron drift instability caused by bad curvature

The effect of curvature on the trapped electron drift instability is considered. If the particles are trapped in bad curvature regions, the instability is seen to have a growth rate comparable to that of the temperature gradient instability treated by Kadomtsev and Pogutse. On the other hand, particles trapped in good curvature regions have a stabilizing influence, although a lower growth rate mode (which has a node at the magnetic field minimum) can still exist.

Confinement properties in levitated spherator

In the levitated spherator, a plasma confinement time of 150u2009∼u2009200 msec was obtained at a plasma density of 2 × 1011cm−3 and an electron temperature of 1 eV. This confinement time is 1/3−1/6 of the classical decay time. It was observed that the confinement time increased with an increase in the electron temperature in the range of 0.1 to 5.0 eV. At higher electron temperature the plasma decay time was reduced with an increase in the electron temperature.

Heating of the relativistic electron plasma in a levitated spherator

Noncyclotron resonant microwave heating preferentially heats relativistic electrons in a plasma. In the FM‐1 device, a levitated spherator, plasma may be produced by cyclotron resonant microwave heating at a base pressure of 3 × 10−8 Torr in helium. Under these conditions, plasma lifetimes are of the order of 100 sec. Nonresonant microwave heating at power levels of up to 1.5 kW produced electron temperatures of up to 900 keV with 4 MeV electrons not at all uncommon, as measured by wall bremsstrahlung produced at the limiter. Anomlaous loss mechanisms drop the energy to about 140 keV within 200 msec after the turnoff of the nonresonant heating, and the temperature remains roughly this value for 100 sec. Nonresonant heating at the cyclotron harmonics is as efficient as heating below the fundamental, which is explained by observing that synchrotron emission may be used to compute the nonresonant absorption.