R. Uhlemann

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.

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 influence of plasma-edge properties on high confinement discharges with a radiating plasma mantle at the tokamak TEXTOR-94

The radiative improved mode obtained on the limiter tokamak TEXTOR-94 combines the possibility of power exhaust by a radiating plasma boundary (with a fraction of the radiated power with respect to the total input power up to 90% with neon or argon cooling) with improved energy confinement (as good as in the ELM-free H-mode in divertor tokamaks) at high plasma densities (line-averaged central-electron density equal to or even above the Greenwald density limit nGW) in quasi-stationary discharges. An overview is given of the substantial changes in plasma-edge properties occurring at high radiated power levels . These changes are characterized by a reduction of the plasma-edge density and temperature, a reduction of particle transport out of the confined plasma volume and an increase of the penetration depth of deuterium and impurity atoms. As a consequence, the particle confinement time increases and the electron-density profiles steepen. The transition to improved confinement takes place as soon as the density peaking reaches a critical threshold. An internal transport barrier is observed in the bulk of RI-mode plasmas (at r=a6 0:6) characterized by an increase of the pressure gradient and of the shear of the toroidal velocity compared to discharges without additional impurity seeding. The dilution at the plasma boundary is strongly increased by the seeded impurities whereas the central dilution is only weakly affected.

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

Confinement transitions with radiation cooling in TEXTOR-94

New experiments performed at high power with radiation cooling on TEXTOR-94, upgraded for long pulses, have shown the following. (i) Improved confinement conditions at high density (for which the plasma is mainly thermal) lasting for several seconds up to the present maximum flat top time of the upgraded machine with a low and stationary central impurity content. Note that the radiation occurs near the plasma edge, mainly inside the last closed magnetic flux surface. (ii) The existence of confinement transitions during the radiatively cooled phase of the discharge: the already observed transitions from higher to lower confinement regimes have been confirmed but, remarkably, transitions from lower to higher confinement are now also present, leading to an improvement of the confinement time of up to a factor of three.

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.

Review of combined ICRH‐NBI results in TEXTOR

The synergism observed between NBI and ICRH is theoretically interpreted for the interaction at the second and third ion cyclotron harmonic. It is also shown that the performances of supershot‐like discharges obtained with balanced injection can be substantially improved by beam‐RF interaction both at 2 ωCD and 3 ωCD.

Actively cooled high-intensity heat shield (form-locked) design analysis

Abstract A feasibility study is performed with 2D-mathematical models for heat shield designs using carbon fibre composites and tungsten monoblocks, which are form-locked to the coolant tube instead of metallurgically bonded. The geometrical boundary conditions and the overall shape correspond to the design for the ITER divertor vertical target. Steady state thermo-mechanical analyses have been carried out for a limited number of cases to investigate the effects of the particular features of the proposed connection. The results demonstrate the principal feasibility of this approach and open prospects for further improvements.

High-intensity non-brazed heat shield for safe steady-state operation

Many plasma-facing components in todays experiments are inertially cooled. Graphite tiles are mounted on a water-cooled steel plate using a spring and bolt mechanism. When exposed to high heat flux, such a shield can take loads only for a few seconds without becoming overheated. To operate in high intensity long pulsed/steady-state conditions, new actively cooled heat shields of brazed and non-brazed types are under development. With clamped (form-locked) monoblock tiles a prototype of non-brazed heat shield structure underwent its first high heat flux tests for divertor applications in the Julich MARION ion source [1]. The test results are encouraging, and validated a finite element model as discussed in this paper.

Thermomechanical behaviour of actively cooled divertor elements with internal coolant flow return

Abstract For future fusion devices, target element specimens with a reversal of the coolant flow have been designed, manufactured and tested. Two parallel OFHC-Cu tubes connected with another at one end by a Cu flow reversal element brazed onto blocks of a 2-D Carbon Fibber reinforced Carbon (CFC) have been selected to realize such divertor elements. To estimate the thermal behaviour, heat load experiments were performed in the high heat flux ion beam facility MARION. Peak power densities up to 13 MW/m2 for 5, 10 and 20 s of the elliptic Gaussian shaped hydrogen ion beam have been applied to the target element specimens. After extreme heat loads, cracks in the 2-D CFC block occurred leading to a strong deformation of the CFC and a partial detachment of the Cu tubes. By using a castellated CFC materials and a large number of braze depots for the target elements no cracking of the CFC material took place and a good thermal performance up to power loads of more than 10 MW/m2 has been achieved.