B. Giesen

Overview of radiative improved mode results on TEXTOR-94

The radiative improved (RI) mode is a tokamak regime offering many attractive reactor features. In the article, the RI mode of TEXTOR-94 is shown to follow the same scaling as the linear ohmic confinement regime and is thus identified as one of the most fundamental tokamak operational regimes. The current understanding derived from experiments and modelling of the conditions necessary for sustaining the mode is reviewed, as are the mechanisms leading to L-RI mode transition. The article discusses the compatibility of high impurity seeding with the low central power density of a burning reactor, as well as RI mode properties at and beyond the Greenwald density.

Influence of boronization on the plasma performance in TEXTOR

The plasma-facing surfaces of TEXTOR have recently been coated (plasma chemical deposition) by a thin layer of hydrogen-rich carbon and boron (ratio 1:1). The paper describes the improved plasma performance which resulted when this coating replaced the amorphous carbon (a-C:H) layer used before. The main differences were a pronounced reduction of the radiation losses, a decrease of Zeff from 1.4 to 1.2 and a significant change in the hydrogen recycling: the recycling coefficient was smaller than unity during current flat top at a wall temperature Tw=150 degrees C whereas this could only be achieved at Tw>or=300 degrees C with a-C:H. Density excursions during ICRH pulses were avoided even at a power level of more than 2 MW. Possible explanations for the reduction of the oxygen concentration and for the change in hydrogen recycling are suggested and discussed.

Overview of experiments with radiation cooling at high confinement and high density in limited and diverted discharges

An overview is presented of recent experiments with radiating mantles on limiter and divertor machines, realizing simultaneously high confinement and high density at high-radiation levels. A variety of operational regimes has been observed and the characteristics of each are documented. High-performance plasmas (i.e. edge localized mode (ELM)-free H-mode confinement quality and normalized beta values simultaneously) with radiating mantles have been demonstrated under quasistationary conditions during the maximum flattop time of the machine (equal to tens of confinement times) on DIII-D and TEXTOR-94. Maximum values for up to 4 and for the advanced tokamak confinement-stability product up to 13, have been obtained in very high confinement mode (VH-mode) like discharges with radiating mantles in DIII-D. There is a striking similarity between improved ohmic confinement discharges (with or without Ne seeding) and radiating mantle discharges, indicating a possible common origin for the confinement improvement observed. Possible scenarios for the application of radiating mantles on larger machines such as JET and JT-60U are indicated.

The high-power (3-MW) long-pulse (3-s) radio-frequency system for ion cyclotron resonance heating experiments on TEXTOR

A multimegajoule ion cyclotron resonance heating (ICRH) experiment was installed on the Torus Experiment for Technology-Oriented Research (TEXTOR) tokamak. The system consists of two independent power lines each designed to generate and launch 1.5 MW of radio-frequency (rf) power into the machine during a 3-s period in the 25- to 29-MHz frequency range. Each power line consists of the following items: a 1.5-MW transmitter, a transmission line system, and an interface linking the transmission line to the antenna of the shielded strip-line type placed along the tokamaks hot liner. Details of the line and antenna diagnostics and data acquisition system together with the subsequent impedance characteristic calculations are given. The rf radiation shielding for the ICRH experiment is explained. The control of the rf setup as a TEXTOR sub-system and the generator pulse control and operation modes are outlined. The antenna loading and power limitation in the presence of plasma and the conditioning procedure are discussed. Finally, the new rf system compatible with the toroidal pump limiter Advanced Limiter Test-II is presented.

A scheme for a dynamic ergodic divertor in TEXTOR

A helical multipolar coil system for dynamic edge ergodization of the TEXTOR plasma is proposed being energized by a three phase ac-current. This would permit a sweeping of the perturbation pattern with the aim (I) to establish a uniform distribution of the heat load at the wall components, (II) to increase the thickness of the scrape-off layer, (III) to improve the helium exhaust and to enhance impurity shielding for controlled edge radiation cooling, and (IV) to possibly influence plasma rotation. Resonant interaction with internal modes may be foreseen in a later phase.

Improved confinement in TEXTOR

A new regime of enhanced confinement (I-mode) is found in plasmas with circular cross-sections in the pump limiter tokamak TEXTOR with boronized walls. This regime is obtained with three types of auxiliary heating, namely NBI co-injection, NBI counter-injection+ICRH and NBI co-injection+NBI counter-injection, and has many similarities with the H-mode regime obtained in divertor tokamaks. The energy confinement times obtained in these discharges scale as favourably as those in stationary H-mode discharges with edge localized modes. A detailed analysis of the scaling of the confinement time with plasma current, heating power and plasma density is presented. Characteristic electron density and temperature profiles are observed, with large central values and well developed edge pedestals. They are compared with those found in H-mode discharges and supershots. A poloidal beta limit of 1.6 is found in the I-mode discharges of TEXTOR. The maximum toroidal beta values obtained reach nearly 1%, i.e. 0.7 times the Troyon limit in TEXTOR. I-mode confinement is always linked with low recycling and absence of MHD activity. If these conditions are not met. L-mode scaling is retrieved. MHD activity, which is more likely to occur at low plasma densities and currents, can cause a sudden drop to L-mode scaling. So far, no transition from the L-mode scaling to the I-mode scaling has been observed

Technical lay-out of the dynamic ergodic divertor

Abstract A description of the layout for the dynamic ergodic divertor (DED) is presented. It is proposed to install a perturbation coil system consisting of four quartets of coils and two compensation coils helically wound on the high field side of TEXTOR-94. The windings follow the magnetic field lines at q =3 surface for one toroidal turn. To obtain a static solution and a rotating field solution, the system can be operated at DC, 50 Hz and in the band between 1 and 10 kHz. As power supplies, convertor units using 4-quadrant bridges with IGBTs (insulated gate bipolar transistor) are foreseen. The required frequency synchronization of the units will be enforced by a central control unit.

Recent results on ion cyclotron and combined heating of TEXTOR

Abstract The recent experimental activity in the field of auxiliary heating and related topics on TEXTOR is reviewed. TEXTOR is equipped with up to 4 MW of ion cyclotron heating power and 3.4 MW of neutral beam injection. The combination of the radiating boundary concept with high auxiliary power has extended the improved confinement domain to the large density regime and demonstrated the viability of the radiating boundary concept for long pulse high power operation. Improved confinement was also achieved in third harmonic heating, characterised by predominant coupling of the RF to the beam ions. Operation of an unshielded antenna with insulated limiters proved that RF sheaths are taking place on the side limiters and are suppressed by insulation. Control of the helium flux by the RF was successfully demonstrated using the interaction of the RF with fast 3 He ions injected by neutral beam. Preliminary tests with a high Z limiter indicate compatibility, and even a positive effect, of the RF. Experience gained in operating unshielded antennas is also commented on.

Sensitivity Study of Mechanical Behavior of Busbar System Designed for Wendelstein 7-X Stellarator

The Wendelstein 7-X stellarator is now under construction in Greifswald, Germany. The superconducting busbar system for the stellarator coils is designed and manufactured at the Forschungszentrum Jiilich, Germany. The electromagnetic forces on the long busbars, considerable relative tilting of the coils carrying the busbar supports, lack of available space required for the robust design of these supports, collisions with other systems during operation as well as the cooling down process had to be taken into account during design phase. Having a complex layout and complying with different, often contradictory requirements, the busbar system turned out to be sensitive to the manufacture and assembly tolerances. The modular structure of the magnet system implied the modular sequence of the busbar system production. The mechanical behavior of the busbar system has been numerically simulated for the module 5. The developed global model reflects main important features of the complex system in a relatively simple way. It proved to be useful tool for frequent iterations during design period and for numerous tolerance studies. The paper describes the main features of the numerical model and some results of the sensitivity study of the busbar system.

Eddy currents calculation in the TEXTOR 94 modified liner/vacuum vessel system during plasma disruption

Abstract The Dynamic Ergodic Divertor—to be installed inside the vacuum vessel of the TEXTOR 94 tokamak—requires a new liner design. The new liner represents a thin toroidal shell with numerous holes. One-third of the liner shell, facing the ergodic coils, is cut out for purposes of the ergodic divertor structures. To sustain the electromagnetic loads the cut-off edges of the inconel liner shell are reinforced with massive steel structures. Edges of the liner holes are reinforced with different types of steel structures. Calculation of the induced currents in the liner and vacuum vessel and electromagnetic forces on the structures due to interaction of the induced currents with the electromagnetic field during a plasma disruption is presented. The eddy currents in the segments of the vacuum vessel separated by bellows form closed loops. Because of numerous ports in the vacuum vessel and numerous holes in the liner, there are currents flowing perpendicular to the toroidal magnetic field. This leads to bending and twisting of the structures. The calculated electromagnetic forces were used for the subsequent structural analysis.