Beam-heated Tokamaks Research Articles

Multichannel electron cyclotron emission radiometry for magnetohydrodynamic mode studies in TEXTOR

Localized magnetohydrodynamic modes occur frequently as precursors to dangerous plasma disruptions in tokamaks. Fast electron cyclotron emission (ECE) radiometry permits a space-resolved study of the dynamic behavior of these instabilities. Two types of multichannel, heterodyne ECE systems working on TEXTOR are briefly described. Temperature profile perturbations caused by tearing modes are shown. Typical Te fluctuations generated by rotating magnetic islands are presented. An early disruption warning system was built and tested on TEXTOR discharges. A real-time cross correlation calculation of two ECE signals is used to identify an m=2 mode. An analog alarm signal is generated then early during the instability evolution. An experimental test result is presented from a beam-heated tokamak plasma.

Asymmetries in the density at the edge of a tokamak plasma

Several Langmuir probes were used simultaneously to observe density asymmetries at the edge of the beam-heated tokamak ISX-B. The large number of identical discharges required for the joint JET/ISX-B beryllium limiter experiment offered the opportunity to perform to perform a careful edge study of the scrape-off layer using probes at various poloidal, toroidal, and radial locations. Floating, double, Langmuir probes were used at 3 separate locations. Edge density asymmetries were observed and correlated with the local connection length along a field line between material surfaces in the scrape-off layer. The effects of limiter movement were also explored with limiters other than the main limiter. It was observed that the asymmetries were altered somewhat when either the magnetic field or the plasma current was reversed. These spatial variations were accentuated locally when a certain critical density was exceeded and may be due to enhanced recycling at the main limiter just below the disruption-limited maximum density.

Measurements of plasma ion temperature and rotation velocity using the He II 4686-Å line produced by charge transfer

Spatially and temporally resolved values of the ion temperature and toroidal rotation velocity near the plasma axis have been obtained in neutral beam-heated plasmas in the Doublet III tokamak from measurements of the Doppler broadening and Doppler shift of the He II 4686-Å line, excited by charge transfer reactions between fast hydrogen atoms in the beam and thermal helium ions in the plasma. The measurement technique, which uses visible optics and a multichannel detector, promises to be a straightforward method to obtain ion temperature profiles in beam-heated tokamaks.

Ion-temperature-gradient instability in toroidal plasmas

The stability of the ion-temperature gradient mode in a toroidal plasma is investigated. Using the ballooning-mode formalism, an ordinary difference-differential equation is derived which includes full kinetic effects. The equation is then examined in various limits where it reduces to an ordinary differential equation. Analytic and computational studies show that, for ηi ≡d ln Ti /d ln n≫1, toroidal effects further destabilize the mode. However, it is also found that toroidal effects give rise to higher ηi threshold compared to the slab case. Extensive numerical calculations over a wide range of parameters are carried out to delineate the regions of instability. It is concluded that the ηi instability is of relevance to present-day beam-heated tokamaks.

Development of low-Z surface coatings for JT-60 first wall

Impurity control is one of the essential elements in realizing the most favorable operating parameters of a beam-heated tokamak [I]. In JAERI, development of surface coatings of low-Z materials is intensively underway to take advantage of these materials in reducing the undesirable effect of wall-released impurities in JT-60. Work in this area includes development of coating techniques as well as evaluation of candidate coatings. The coating development plan consists of two stages [2,3]. The aim in the first stage is to develop low-Z coatings which are suitable for the attainment of operating parameters for the break-even condition. The goal of the second stage is to develop advanced materials which are applicable to fusion experimental reactors. The coating development includes precoating carried out before wall installation and in-situ coating carried out after the completion of the vacuum vessel.

Impurity Evolution in a Beam-Heated Tokamak

Impurity evolution during neutral beam injection has been investigated in a large tokamak with the first wall made of low-Z (carbon), medium-Z (titanium) and high-Z (molybdenum), based on a simple model of wall-released impurity behavior. The use of low-Z material seems helpful in reducing impurity contamination in a high-temperature plasma with material limiters. A divertor is effective for controlling of wall-sputtered impurities for any of the wall materials.

Beta limits in beam-heated tokamaks

Consideration is given to some practical features associated with beta optimization in neutral-beam-heated tokamaks. Previous ideal MHD stability calculations of beta limits have employed ad hoc pressure and current density profiles. A coupled 1 – 1/2-D transport-ballooning eigenmode model was used to match observable experimental parameters and to simulate those not directly measured (such as the beam deposition and heating profiles), including neutral gas and impurity effects. With this model it was found that: (1) beta values exceeding the ideal MHD limits have been obtained, and (2) strong effects of direct beam contributions to the pressure, or indirect effects of strong local beam heating, can outweigh the relatively remote vacuum field shaping in well-penetrated discharges. Since beam-induced currents can produce significant changes in transport by changing the location and width of islands produced by saturated resistive tearing modes, these have been calculated self-consistently.