Search

Radial electric field is known to be an important factor affecting transport and confinement in toroidal fusion plasmas. Langmuire probe measurements of peripheral radial electric field evolution in the presence of a rotating MHD island were performed on the TUMAN-3M tokamak in order to clear up the possible connection between the radial electric field and the island rotation, both in L and H-modes. The measurements showed that Er became positive, if the island was large enough, in spite of the constant direction of the island's rotation. Comparing similar ohmic H-mode discharges with or without a rotating MHD island, it was found that in the presence of the large island Er was always more positive. Possible explanations of this observation are discussed.

The main parameters and design of the Globus-M spherical tokamak are described, including the modifications introduced after construction was completed. The experimental program and possibilities for optimizing the operating regimes of the facility are discussed. The research program envisages beginning with Ohmic heating experiments and continuing with work on rf heating and generation of drag currents. Recent results of modeling the propagation of rf waves in the 10–50 MHz and 2.45 GHz ranges are presented and the possibility of using these to control the profiles of the plasma parameters is discussed. A brief description is given of the primary diagnostics and the further development of the diagnostic complex is discussed.

The focus of the TUMAN-3 and TUMAN-3M tokamaks programme is on issues of improved confinement. The transition from an ordinary ohmic regime into improved confinement mode has been found in circular limiter configuration in a vessel with all-metallic walls and limiters. The signatures of the H-mode in auxiliary heated tokamaks have been observed in this regime. The crucial role of the radial electric field was found in experiments with internal probe biasing. Other techniques were demonstrated to trigger H-mode: short increase of the working gas puffing rate, minor radius magnetic compression and pellet injection. The scaling of the energy confinement time in ohmic H-mode was obtained, which differs dramatically from the scaling for the ordinary ohmic regime. A strong dependence of on plasma current was found. The scaling for the ohmic H-mode is consistent with the scaling proposed for devices with powerful auxiliary heating (JET/DIII-D H-mode scaling). The result shows that H-mode physics is universal in tokamaks with different geometries and heating methods. In 1994 a new vacuum vessel was installed in the TUMAN-3 tokamak. The modified device, TUMAN-3M, is able to produce higher and , up to 2 T and 0.2 MA, respectively. During the first operational period a plasma current of 0.15 MA was achieved at T, which corresponded to . The impact of the quality of wall coating on confinement was asserted. The longest energy confinement time (30 ms) was observed under the conditions of best boronization.

The disregard of fast processes (e.g., the L-H transition) in a tokamak plasma can lead to large errors in the determination of the energy confinement time τE. A major upgrade in the electromagnetic diagnostic system for the analysis of plasma parameters and in the collection system has made it possible to take into account the influence of the transient character of the radial profile and the value of the plasma current IP as well as the stored energy W on the determination of τE from diamagnetic measurements and to investigate fast processes involved in the L-H transition. The energy confinement time τE is calculated from the equation τ E=W/[UPIP-d/dt(LIP2/2)-dW/dt] where UP is the plasma loop voltage (V), IP is the plasma current (A), and L is the total internal inductance (H). The total inductance L of the plasma column has been determined from measurements of the quantity βJ+Ii/2, where βJ is the ratio between the gas-kinetic pressure and the pressure of the poloidal magnetic field, and Ii is the internal inductance. The inclusion of transient behavior in the determination of τE from diamagnetic measurements gives a correction of up to 50%.

The difficulty of Heavy Ion Beam Probe (HIBP) application on the TUMAN‐3M (R=0.53m, a=0.22m, BT=0.8T, Ip=140kA, Te=0.5keV, n

The ‘‘TUMAN‐3’’ Tokamak programme concentrates on issues of improved confinement. In 1989 the transition from an ordinary Ohmic regime into an improved confinement mode was achieved. The signatures of the H‐mode in auxiliary heated tokamaks have been observed in this regime. The crucial role of the boundary radial electric field was found in the experiments with internal bias probe. Other techniques were demonstrated to disturb the boundary plasma which led to H‐mode triggering: short increase of working gas puffing, minor radius magnetic compression and pellet injection. The role scaling of the energy confinement time in the Ohmic H‐mode was obtained, which differs dramatically from the scaling for the ordinary Ohmic regime. There were found a strong dependence of τE on plasma current and a weak dependence on density. The maximum value of τE was 10 times longer than in the ordinary Ohmic region. The τE scaling for the Ohmic H‐mode is consistent with the scaling proposed for devices with powerful auxiliar...

MHD oscillations with m/n = 4/1 and 3/1 that arise at the periphery of the TUMAN-3M tokamak in the initial stage of a discharge are investigated. It is found that these oscillations lead to a significant modulation of the electron density n{sub e}, which is attributable to the accumulation of plasma within a magnetic island. Numerical simulations of the modulation structure made it possible to determine the radius of the resonant surface and the radial width of the island and to evaluate the characteristic density gradient in the island. The gradient was found to be ten times larger than that of the unperturbed profile of n{sub e}(r) near the resonant surface. This points to reduced plasma transport within the magnetic island.

The results of magnetic compression investigation in tokamaks are discussed. The latest experiments on minor and major radius compression on Tuman-3 tokamak are described in more detail.

The theory of stable plasma diffusion is discussed, and basic experimental data on diffusion in magnetic fields are surveyed. The analysis of experimental data showed that in many cases the diffusion of charged particles across a strong magnetic field is much faster than predicted by theory. In the presence of currents in the plasma or with strongly directed particle motion (in positive discharge cylinder, ~n discharge with oscillating elections, and in strongly ionized plasma) the diffusion agrees with the theoretical prediction only for small magnetic fields. With larger fields the diffusion rate is faster. The observed abnormally fast particle transport across magnetic fields was accounted for in the development of instabilities, oscillations, and noises. 122 references. (R.V.J.)

A review is given of studies of gas discharge.in the microwave regions. The review includes the publications of 1946 to 1957.