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Fusion alpha particle diagnostics for ITER remain a challenging task. Standard escaping alpha particle detectors in present tokamaks are not applicable to ITER and techniques suitable for fusion reactor conditions need further research and development [1] , [2] . The activation technique is widely used for the characterization of high fluence rates inside neutron reactors. Tokamak applications of the neutron activation technique are already well developed [3] whereas measuring escaping ions using this technique is a novel fusion plasma diagnostic development. Despite low alpha particle fluence levels in present tokamaks, promising results using activation technique combined with ultra-low level gamma-ray spectrometry [4] were achieved before in JET [5] , [6] . In this research work, we use new advanced detector materials. The material properties beneficial for alpha induced activation are (i) moderate neutron cross-sections (ii) ultra-high purity which reduces neutron-induced background activation and (iii) isotopic tailoring which increases the activation yield of the measured activation product. Two samples were obtained from GERDA [7] , an experiment aimed at measuring the neutrinoless double beta decay in 76Ge. These samples, made of highly pure (9 N) germanium highly enriched to 87% in isotope Ge-76, were irradiated in real D–D fusion plasma conditions inside the TEXTOR tokamak. Comparison of the calculated and the experimentally measured activity shows good agreement. Compared to previously investigated high temperature ceramic material [8] , this candidate detector offers better prospects for signal to background S/B ratio, energy resolution and particle selectivity due to a unique alpha particle signature. Applicability to ITER is discussed. Finally, research needs for further development of this diagnostic technique are outlined.

This paper focuses on encouraging results obtained on the characterization of RF produced plasmas during pulsed-mode wall conditioning discharges in ion cyclotron resonance frequency (ICRF) regime in the limiter tokamak TEXTOR. Recent Ion Cyclotron Wall Conditioning (ICWC) experiment carried out in TEXTOR tokamak, lead to the identification of various dependences of the antenna-plasma coupling efficiency on the plasma parameters for possible ICWC-discharge cleaning in ITER at half field. Our ICWC experiments emphasize on (i) study of antenna coupling during the mode conversion scenario, (ii) reproducible generation of ICRF plasmas for wall conditioning, by coupling RF power from one or two ICRF antennas and (iii) effect of application of an additional (along with toroidal magnetic field) stationary vertical (BV ≪ BT) or oscillating poloidal magnetic field (Bp ≪ BT) on antenna coupling and relevant plasma parameters.

Ion cyclotron wall conditioning (ICWC) discharges, in pulsed-mode operation, were carried out in the limiter tokamak TEXTOR to explore safe operational regimes for the experimental parameters for possible ICWC-discharge cleaning in International Thermonuclear Experimental Reactor (ITER) at half field. Antenna coupling properties obtained during the ion cyclotron range of frequencies (ICRF) wall conditioning experiments performed in helium–hydrogen mixture in TEXTOR were analysed in relation to the obtained ICWC-plasma characterization results. Satisfactory antenna coupling in the mode conversion scenario along with reproducible generation of ICRF plasmas for wall conditioning, were achieved by coupling radio frequency (RF) power from one or two ICRF antennas. The plasma breakdown results obtained in the TEXTOR tokamak have been compared with the predictions of a zero-dimensional RF plasma production model. The present study of ICWC emphasizes the beneficial effect of application of an additional (along with toroidal magnetic field) stationary vertical (B V ≪ B T) or oscillating poloidal magnetic field (B P ≪ B T) on antenna coupling and relevant plasma parameters.

This paper focuses on a study of the principal operation aspects of standard ICRF heating antennas in the ion cyclotron wall conditioning (ICWC) mode: (i) ability of the antenna to ignite the cleaning discharge safely and reliably in different gases including those most likely to be used in ITER – He, H 2 , D 2 and their mixtures, (ii) the antenna capacity to couple a large fraction of the RF generator power (>50%) to low density (∼10 16 –10 18 m −3 ) plasmas and (iii) the RF power absorption schemes aimed at improved RF plasma homogeneity and enhanced conditioning effect. The ICWC discharge optimization in terms of RF plasma wave excitation/absorption resulted in successful simulation of the conditioning scenarios for ITER operation at full field (JET) and half-field (TEXTOR, TORE SUPRA, ASDEX Upgrade).

In the present paper, inter-machine studies of Ion Cyclotron Wall Conditioning (ICWC) have been performed in limiter (TEXTOR) and divertor (ASDEX Upgrade, AUG) tokamaks in the presence of toroidal (0.2–2.35 T) and vertical (0–0.04 T) magnetic fields using the conventional ICRF antennas without modifications in hardware. The ICWC effect on both machines was studied by analyzing the removal rate of marker gases which have been loaded to the walls by glow discharge beforehand. Several factors were identified which could have a crucial impact on the conditioning efficiency: (i) RF power coupled to the plasmas; (ii) RF power absorption scheme; (iii) superimposing an additional vertical magnetic field on the toroidal field ( B V B T ). All the observed effects are analyzed in terms of RF plasma wave excitation/absorption and compared with the predictions from 1-D RF and 0-D transport codes. ICWC scenarios for ITER are proposed and analyzed.

Measurements of plasma rotation and electric field are crucial for the study of plasma confinement and transport. The present paper is devoted to experimental observations of poloidal asymmetry in perpendicular plasma rotation with correlation reflectometry on TEXTOR.

The paper describes a new attempt to measure fusion-reaction protons emitted from TEXTOR tokamak plasma. To perform test measurements, a small ion pinhole camera and solid-state nuclear track detectors (SSNTDs) of the PM-355 type were used. After the irradiation the detector was subjected to the interrupted etching procedure. The tracks of 3 MeV fusion-reaction protons were analyzed with an optical microscope. The main aim of the experiment was to determine the number and spatial distribution of the recorded fusion protons. Other PM-355 detector samples were irradiated with mono-energetic protons to determine calibration characteristics of the detectors. The measurements have been supported by the computation of proton trajectories in the TEXTOR facility and that of expected efficiency of the measuring setup for the 3 MeV proton detection. The computer simulations of different trajectories of charged particles (on the basis of motion equations), as well as those of the detection efficiency, have been performed with the Gourdon code.

Inter-machine studies of wall conditioning with the ICRF discharges have been performed in the limiter (TEXTOR) and divertor (ASDEX Upgrade (AUG)) tokamaks in the presence of a toroidal magnetic field (⩾2 T) using the conventional ICRF antennas without modifications in hardware. The vessel oxidation treatment by pulsed ICRF discharges in (He + O 2 )-mixture (TEXTOR) is analyzed in terms of ratios of the RF pulse length to the O 2 -puff duration. A successive set of deuterium and helium ICRF discharges was developed for post-oxidation wall cleaning and analyzed in the light of TEXTOR recovery to the normal plasma operation. A new scenario of ICRF wall conditioning in (He + H 2 )-mixture at two frequencies was applied in AUG and compared with the standard glow discharge in terms of outgassing efficiency. Modeling of the absorbed RF power was done to clear up a role of the H 2 concentration in the homogeneity of ICRF plasmas and the generation of high-energy ions.

A new attempt to measure fusion-protons produced within the TEXTOR tokamak has recently been undertaken by the IPJ team operating in a frame of the Association EURATOM-IPPLM, Poland, in the collaboration with the Belgian and German EURATOM Associations. To record fusion-produced protons in one of diagnostic ports of the TEXTOR, operated at IPP in Julich, a miniature ion-pinhole camera was constructed at IPJ in Swierk. The camera was equipped with a PM-355 type nuclear track detector and it was fixed upon a movable water-cooled manipulator, which was constructed by the German partner. The whole system was located inside the TEXTOR vacuum vessel. The detector was irradiated during several discharges performed under chosen experimental conditions. The irradiated detector was etched at the standard conditions, and tracks of fast (> 3:0 MeV) fusion-protons were analyzed with a microscope. The whole proton-pinhole image was analyzed quantitatively to determine the number and spatial distribution of the recorded fusion-protons. To understand the obtained experimental date, the results have been compared with those of computer simulations carried out by another team, as described in another paper.

Due to rearrangement of the diagnostic positions resulting from the dynamic ergodic divertor installation on TEXTOR, a new antenna pair has been recently installed. This antenna has been designed to test the load resilient “conjugate-T” (CT) mode of operation. The paper describes the calibration of the system and shows from impedance matrix measurements that the load resilience can be maintained in presence of the mutual coupling between the pair. The practical tuning procedure to obtain the load resilience of the system is also described.

Deuterium fuelling through a carbon test limiter has been applied to maximize the plasma density in the Radiative Improved Mode. The impact of the fuelling on the local plasma edge properties has been investigated, by analyzing the spectral emission of both deuterium atoms and molecules, which indicates the creation of a cold and dense plasma cloud with a local electron density at the last closed flux surface up to 8 × 10 19 m −3 , about 4 times higher than the electron density far away from the puffing location at the same plasma radius. The local electron temperature decreases to less than 10 eV. The experimental data can be reproduced by a model for the development of the cold plasma cloud and the critical fuelling rate to initiate the process based on the heat balance in the cloud. The correlation of the resulting local perturbation with the global confinement properties is discussed.

The original work done on TEXTOR on ion cyclotron resonance heating of the plasma is reviewed. After a brief introduction outlining the principles, the radio-frequency (rf) system is described, with its substantial evolution during time. Then, the different physics results are re- viewed. Ion cyclotron heating has been performed in a large number of scenarios and under a wide range of conditions. Aside from the various minority or mode- conversion scenarios, the interaction with beam ions and the possibility of controlling fast-ion diffusion with rf have been deeply investigated. Both the interaction with the wall or edge plasma and the impact on improved core confinement were studied in detail. Pioneering work was done to demonstrate efficient heating with unshielded antennas and plasma production in a tokamak by rf alone for plasma startup assistance or wall conditioning.

This paper reports on experiments in TEXTOR with electron cyclotron resonance heating (ECRH) of radiatively improved (RI) mode discharges. With ECRH the energy content of RI-mode discharges can be increased without the normally observed power degradation in confinement time. The experiments are described and the effects of ECRH on global confinement and local plasma parameters of RI-mode discharges are discussed; the favourable scaling of energy content is due to a zone of low electron thermal transport just outside the sawtooth inversion radius. Moreover, the heating effect of ECRH in the RI-mode is compared with the effect in L-mode; this comparison sheds some light on the physics of electron thermal transport in RI-mode discharges.

The paper reports on some applications of Solid-State Nuclear Track Detectors, particularly of the PM-355 type, for diagnostics of fusion reaction protons (3 MeV) and other fast ions emitted from high-temperature plasmas. The measurements with the use of these detectors were performed at the PF-1000 Plasma-Focus facility in Warsaw, TEXTOR tokamak in Juelich, and PALS lasersystem in Pargue.

The 110 GHz and the new 140 GHz gyrotron systems for electron cyclotron resonance heating (ECRH) and ECCD on TEXTOR are described and results of ECRH experiments with the 110 GHz system are reported. Central ECRH on Ohmic plasmas shows the presence of an internal electron transport barrier near q = 1. This is confirmed by modulated ECRH experiments. A central barrier is also indicated by ECRH in radiatively improved (RI) mode discharges and up to two barriers are seen with ECRH during the current ramp phase. ECRH control of sawteeth is reported for both Ohmic and RI mode target plasmas.

The radiative improved (RI) mode at TEXTOR is a high confinement regime, which is obtained by the seeding of neon into deuterium plasmas. Recent experiments were aimed to study the influence of external gas fuelling on the confinement properties of the RI mode. In particular, it was found that a hydrogen puff into such plasmas leads to lower confinement compared with the discharges fuelled with the same amount of deuterium gas. This paper attempts to explain the reduction of confinement in RI plasmas with an external hydrogen puff and its relation to the level of impurity concentration, which is a critical parameter for RI mode confinement.

The knowledge of the flux of energetic ions to the first wall and their energy and pitch angle distributions are important for ITER. We study the possibility of using charged particle-induced activation which provides an attractive, simple, and robust technique to measure escaping million electron volt ions. For time resolved measurements, prompt gamma rays emitted by the target are directly viewed with a spectrometer. In this preliminary study we assess the possibility to fulfill the requirements necessary for the ITER alpha loss detectors. Test experiments are under preparation and simulations with the Monte Carlo MCNP4C code are used to assess the performance (signal-to-noise ratio) of the detectors.

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.

An overview of the results obtained so far for the radiative I-mode regime on the upgraded Torus Experiment for Technology Oriented Research (TEXTOR-94) [Proceedings of die 16th IEEE Symposium on Fusion Engineering (Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1995), Vol. 1, p. 470] is given. This regime is obtained under quasistationary conditions with edge neon seeding in a pumped limiter tokamak with circular cross section. It combines high confinement and high beta (up to a normalized beta, beta(n) = 2) with low edge q values (down to q(a) = 2.8) and high density even above the Greenwald limit together with dominant edge radiative heat exhaust, and therefore shows promise for the future of fusion research. Bulk and edge properties of these discharges an described, and a detailed account is given of the energy and particle confinement and their scaling. Energy confinement scales linearly with density as for the nonsaturated Ohmic Neo-Alcator scaling, but the usual degradation with total power remains. No deleterious effects of the neon seeding on fusion reactivity and plasma stability have been observed. (C) 1997 American Institute of Physics.

A fast computer code for the simulation and analysis of neutron signals is used to map out parameter ranges where relevant plasma parameters can be determined from neutron rate measurements of plasmas heated by neutral beam injection (NBI). The comparison with experimental values from the TEXTOR database identifies a small number of NBI-heated high-density discharges where the measurement of the central electron temperature by using second harmonic ECE emission becomes in principle impossible although its determination by using third harmonic emission is possible. The calculations show that the electron temperatures can be determined with sufficient accuracy from neutron emission rate measurements. As an example of this novel application, core electron temperatures are evaluated for TEXTOR discharges.

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.

A stationary high level of edge radiation ( gamma =Prad/Ptot up to ~90% with peak radiation up to ~1 MW/m3) has been obtained in TEXTOR by using silicon and/or neon as radiating impurities. The confinement and neutron reactivity are not degraded but can even be improved at high plasma densities. Stationary reactor relevant heating and radiated power flows with a figure of merit fH/qa=0.6 have been achieved. The interpretation of these results shows a reduction of the bulk transport in the presence of edge radiation cooling. The properties of the radiatively cooled discharges are interpreted or modelled mainly by the self-consistent radiative transport code RITM, and also by the codes TRANS and PRETOR. From these modelling studies an enhancement of the bulk confinement is found in terms of the reduction of the convective losses and the decrease of the edge electron temperature, which results in a peaking of the current profile. The code RITM also predicts self-consistently the detailed properties of the radiating layer for different injected impurities as a function of their incoming flux, and shows that the optimal conditions to obtain confinement improvement as well as minimum fuel dilution by the radiating impurity are obtained at high density

It has been shown recently on TEXTOR that it is possible to obtain stable discharges with good thermal confinement under quasi-stationary conditions with a cold radiative boundary, by injecting the impurities Ne and/or Si. In the first part the main experimental results are summarized. In the second part a first attempt is presented for the modelling of the basic transport processes in these discharges. This is done by using the codes TRANSP for interpretative simulations and RITM for predictive simulations. We find that the bulk confinement is favorably influenced by the application of edge cooling and a satisfactory agreement is obtained between the predictions of RITM and the experimental results.

Controlled application of radiating impurities in the boundary layer can help to solve the problem of power exhaust in a fusion reactor. Experiments in the Torus Experiment for Technological Oriented Research (TEXTOR) [J. Nucl. Mater. 145–147, 3 (1987)] are presented, which show that impurities with sufficiently high atomic number (≥10) are well suited for this purpose. Injection of neon, a gas recycled at the wall, enabled the establishment of a quasistationary radiating boundary layer, from which more than 90% of the input power was emitted. The required neon density was established by means of a feedback control for the neon influx, which was made possible by the toroidal pump limiter Advanced Limiter Technology (ALT‐II) [J. Nucl. Mater. 162–164, 115 (1989)]. Alternatively, or in addition silicon was introduced as a condensing element—either by surface reactions from siliconized walls or by silane [SiH(D)4] injection—which revealed self‐controlling mechanisms effective with changing plasma parameters. In neither case was a significant increase in central impurity concentration observed and good energy confinement time was maintained up to the highest plasma densities. Based on the information from various refined edge diagnostics, the underlying mechanisms for the buildup of a radiating plasma mantle and the interdependences of neon and silicon on other impurities are discussed.

MeV particle loss measurements from fusion plasmas, in particular alpha particles, remain difficult in large fusion devices and further R&D is needed for ITER. This paper describes the first attempt to measure 3 MeV escaping fusion protons emitted from TEXTOR tokamak plasmas using activation technique. This technique was successfully demonstrated, initially, in 2006 on the JET tokamak. An ion camera equipped with a collimator and several types of activation detectors was installed inside the TEXTOR vacuum vessel to perform these measurements. After irradiation, the detectors were analyzed using ultra low level gamma-ray spectrometry at the HADES underground laboratory. 3 MeV escaping fusion protons were detected in larger number −∼6 times more - compared to earlier measurements using this technique on JET. Another major progress was the reduction of the cooling time by a factor of 50, which made possible to detect radionuclides with half-life of less than 90 min.

Improved confinement is achieved on TEXTOR under high power conditions (up to 4 MW of additional heating with NBI-co+ICRH, NBI-co+counter or NBI-co+counter+ICRH) with edge radiative cooling employing silicon or neon as the radiating impurities. It is shown that in quasi-stationary conditions up to 85% of the input power can be radiated. Such high power fractions offer the possibility of utilizing these techniques to facilitate the power exhaust problem for a Tokamak reactor. Discharges with edge radiative cooling exhibit enhanced confinement properties at high densities, e.g. at a central line averaged electron density of 7.5*1013 cm-3, an enhancement factor of 1.7 over ITER L89-P confinement scaling is obtained with an edge q value as low as 2.7. Stable discharges have been obtained even with the q=2 surface located inside the radiating zone. Furthermore, for radiatively cooled discharges heated with balanced NBI-co+counter with or without ICRH, supershot-like peaked electron density profiles, with central density values above 1.0*1014 cm-3 are observed. The present results show that there is no impurity accumulation in the centre and the Ne and/or Si concentration is so low that the reactivity of the plasma remains unaffected

The possibility of increasing the coupling of ICRH power to plasmas on TEXTOR by gas injection was investigated, for a given voltage applied at the input of an antenna pair. The antenna pair was operated in the conjugated T mode, D(H) heating was used. Modeling of the antenna by transmission line theory shows that load resilience is maintained in a broad range of independent resistance variation for each strap. It is found that the absolute value of the reflection coefficient can be maintained below 0.2 for typical values of resistance measured in plasma shots (2–10 Ω/m) in good matching conditions. During gas injection, the loading resistance showed a clear increase with increasing line average electron density measured close to the plasma edge. Evidence of fast wave eigenmodes was also found, as characteristic resonant behavior of loading resistance and antenna self‐inductance, due to poor absorption in the plasma caused by high H minority concentration.

This contribution reports on isotope exchange studies with both Ion Cyclotron Wall Conditioning (ICWC) and Glow Discharge Conditioning (GDC) in TEXTOR and TORE SUPRA. The discharges have been carried out in H2, D2 (ICWC and GDC) and He/H2 mixtures (ICWC). The higher reionization probability in ICWC compared to GDC, following from the 3 to 4 orders of magnitude higher electron density, leads to a lower pumping efficiency of wall desorbed species. GDC has in this analysis (5–10) times higher removal rates of wall desorbed species than ICWC, although the wall release rate is 10 times higher in ICWC. Also the measured high retention during ICWC can be understood as an effect of the high reionization probability. The use of short RF pulses (∼1 s) followed by a larger pumping time significantly improves the ratio of implanted over recovered particles, without severely lowering the total amount of removed particles.

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

ITER and future superconducting fusion machines need efficient wall conditioning techniques for routine operation in between shots in the presence of permanent high magnetic field for wall cleaning, surface isotope exchange and to control the in-vessel long term tritium retention. Ion Cyclotron Wall Conditioning (ICWC) based on the ICRF discharge is fully compatible and needs the presence of the magnetic field. The present paper focuses on the principal aspects of the ICWC discharge performance in large-size fusion machines: (i) neutral gas RF breakdown with conventional ICRF heating antennas, (ii) antenna coupling with low density (similar to 10(17) m(-3)) RF plasmas and (iii) ICWC scenarios with improved RF plasma homogeneity in the radial and poloidal directions. All these factors were identified as crucial to achieve an enhanced conditioning effect (e.g. removal rates of selected "marker" masses). All the observed effects are analyzed in terms of RF plasma wave excitation/absorption and compared with the predictions from I-D RF full wave and 0-D RF plasma codes. Numerical modeling and empirical extrapolation from the existing machines give good evidence for the feasibility of using ICWC in ITER with the main ICRF antenna.

In this contribution, we report on experimental results on edge transport in limiter H-mode plasmas in TEXTOR under the influence of the Dynamic Ergodic Divertor (DED). These plasmas are characterized by a pedestal structure mainly visible in the electron density, resulting in increased electron pressure gradients of up to 30 kPa/m over a pedestal width of 25 mm at high pedestal collisionalities (nu(e.) = 1 - 10), and with high frequency ELMs in the range of 300-1500 Hz. Under the influence of DED the pedestal pressure is gradually reduced and completely collapses to L-mode when the laminar zone extends all the way across the pedestal width. Toroidal plasma rotation is maintained at H-mode levels by the torque introduced by DED in the stochastic region. The perturbed magnetic topology has been optimized to access conditions with a density pump-out which are strongly governed by wall pumping capabilities in TEXTOR. (C) 2009 Elsevier B.V. All rights reserved.

Ion Cyclotron Wall Conditioning (ICWC) discharges, in pulsed‐mode operation, were carried out in TEXTOR to simulate a scenario of ITER wall conditioning at half‐field. The ICWC discharges were performed in a continuous Helium flow with adjunction of molecular Hydrogen, puffed after the ICRF ignition thus achieving better antenna coupling in the Mode Conversion scenario with improved ICWC performance. Reproducible generation of ICRF plasmas and reliable wall conditioning, after proper wall preloading with Deuterium glow, were achieved by coupling the RF power at 29 MHz from one or two ICRF antennas at different toroidal magnetic fields (different cyclotron harmonics). The antenna coupling properties were analyzed in relation to wall conditioning output (removal rates) for selected marker masses. Present study of ICWC at low and high power density per particles (at different gas pressures) emphasizes on the efficiency of the application of stationary, oscillating or rotating poloidal magnetic field. These e...

An experiment was performed on the TEXTOR tokamak which proves that the plasma can be heated by ion cyclotron resonance using antennas without a Faraday shield. The heating performance of the unshielded antenna was found to be similar to that of a shielded antenna. Although the radiated power was somewhat higher with the unshielded antenna, no impurity problems were encountered. These results are explained on the basis of radiofrequency sheath theory and by the almost perfect shielding of parallel electric fields by the plasma. Implementation criteria for unshielded antennas were obtained.

The excitation of fast-wave global eigenmodes is investigated both experimentally for TEXTOR and theoretically using a slab coupling mode. A brief comparison with a cylindrical model shows that in the TEXTOR situation no essential geometrical effect is left out by the slab code. The resonant behaviour associated with eigenmodes is studied for several signals: the resistance and inductance of the antennae the r.f. magnetic and electric field components picked up by probes at the plasma edge, and the power coupled to a fast-wave antenna used as a receiver. It is shown that all the major features of these signals linked to the existence of eigenmodes can be explained using the theoretical models.

A diagnostic for measuring the energy spectrum of fast charged particles has been developed at TEXTOR-94. It consists of a pair of solid state silicon passivated implanted planar silicon detectors which are actively water cooled. The detectors are mounted on a manipulator and are inserted into the vacuum vessel during plasma operation. The preamplifiers are located just behind the detectors. The noise level is strongly minimized by careful grounding and rf shielding. Collimators placed in front of the detectors allow to obtain radial and pitch angle-resolved measurements. Fast particles coming from the center of the plasma are detected by one detector. The second detector can be used as a background monitor or can provide an additional measurement at another position. Orbit calculations in a realistic magnetic field predict the detection efficiency. The calculated detection efficiency shows good agreement with our experiments. The stable temperature and excellent noise immunity of our system allow to measure the energy spectrum of the 3 MeV protons and 1 MeV tritons produced by the DD fusion reactions in discharges with different heating scenarios. High resolution energy spectra are even obtained in discharges heated with injection of deuterium in a deuterium plasma.

The paper reports on measurements of the space distribution of fusion protons of energy equal to about 3-MeV, originating from the D(d, p)T reactions. The measurements were carried out on the TEXTOR facility by means of a small ion pinhole camera, which was equipped with a solid-state nuclear track detector of the PM-355 type. The results obtained in two series of successive discharges are compared. The first series was performed with an additional heating of TEXTOR plasmas with NBI of fast deuterons, whereas in the second series plasma was heated by ICRF and NBI of hydrogen neutrals. Computer simulations of different trajectories of charged particles have been performed with the Gourdon code and the detection efficiency has been calculated for various orientations of the measuring assembly.

Fast ions created in the fusion processes will provide up to 70% of the heating in ITER. To optimize heating and current drive in magnetically confined plasmas insight into fast-ion dynamics is important. First measurements of such dynamics by collective Thomson scattering (CTS) were recently reported [Bindslev , Phys. Rev. Lett. 97, 205005 2006]. Here we extend the discussion of these results which were obtained at the TEXTOR tokamak. The fast ions are generated by neutral-beam injection and ion-cyclotron resonance heating. The CTS system uses 100-150 kW of 110-GHz gyrotron probing radiation which scatters off the collective plasma fluctuations driven by the fast-ion motion. The technique measures the projected one-dimensional velocity distribution of confined fast ions in the scattering volume where the probe and receiver beams cross. By shifting the scattering volume a number of scattering locations and different resolved velocity components can be measured. The temporal resolution is 4 ms while the spatial resolution is similar to 10 cm depending on the scattering geometry. Fast-ion velocity distributions in a variety of scenarios are measured, including the evolution of the velocity distribution after turnoff of the ion heating. These results are in close agreement with numerical simulations.

Runaway electrons represent a serious problem for the reliable operation of the future experimental tokamak ITER. Due to the multiplication factor of exp(50) in the avalanche even a few seed runaway electrons will result in a beam of high energetic electrons that is able to damage the machine. Thus suppression of runaway electrons is a task of great importance, for which we present here a systematic study of runaway electrons following massive gas injection in TEXTOR. Argon injection can cause the generation of runaways carrying up to 30% of the initial plasma current, while disruptions triggered by injection of helium or of mixtures of argon (5%, 10%, 20%) with deuterium are runaway free. Disruptions caused by argon injection finally become runaway free for very large numbers of injected atoms. The appearance/absence of runaway electrons is related to the fraction of atoms delivered to the plasma centre. This so-called mixing efficiency is deduced from a 0D model of the current quench. The estimated mixing efficiency is 3% for argon, 15% for an argon/deuterium mixture and about 40% for helium. A low mixing efficiency of high-Z impurities can have a strong implication for the design of the disruption mitigation system for ITER. However, a quantitative prediction requires a better understanding of the mixing mechanism.

Neutral beam injection (NBI-co and NBI-counter), ICRH and their combinations have been compared on TEXTOR for what concerns the heating and their effect on the energy anisotropy and the confinement. In most of the cases, stationary heated plasma conditions have been obtained with boronized wall. The main results are: (i) Production of a hot ion mode with NBI-co. It is characterized by a large energy anisotropy, by a large noninductively driven toroidal current and by the stabilization of the sawteeth (of the monster type) due to the hot ion tail. (ii) Significant enhancement of these effects when ICRH is added to NBI-co. The neutron yield is also increased by the addition of ICRH. Peak electron and ion temperature around 3 keV have been obtained and 70% of the total current has been non inductively driven. (iii) Large excess of the total energy content (up to 2.3) with respect to the L-mode scaling predictions has been obtained with the combinations NBI-co+ICRH or NBI-co+NBI-counter. (iv) Up to 6 MW of additional power has been coupled to the plasma leading to beta p=1.5 and beta t equal to 70% of the Troyon limit.

The first results of the Dynamic Ergodic Divertor in TEXTOR, when operating in the m/n=3/1 mode configuration, are presented. The deeply penetrating external magnetic field perturbation of this configuration increases the toroidal plasma rotation. Staying below the excitation threshold for the m/n=2/1 tearing mode, this toroidal rotation is always in the direction of the plasma current, even if the toroidal projection of the rotating magnetic field perturbation is in the opposite direction. The observed toroidal rotation direction is consistent with a radial electric field, generated by an enhanced electron transport in the ergodic layers near the resonances of the perturbation. This is an effect different from theoretical predictions, which assume a direct coupling between rotating perturbation and plasma to be the dominant effect of momentum transfer.

The present paper reviews the recent results achieved in the ICRF‐DC experiments performed in helium/hydrogen mixtures in the non‐circular tokamaks ASDEX Upgrade and JET and first tests of the ICRF discharges in helium/oxygen mixtures in the circular tokamak TEXTOR. Special emphasis was given to study the physics of ICRF discharges. A new recipe for safe and reliable RF plasma production [〈ne〉∼(3–5)×1017 m−3, Te∼(3–5) eV] with improved antenna coupling efficiency (by 1.5–3 times) and improved radial/poloidal homogeneity was proposed and successfully tested: coupling the RF power in the FW‐IBW mode conversion scenario in plasmas with two ion species. The first results on ICRF wall conditioning in helium/hydrogen and in helium/oxygen mixtures are analyzed.

Recently, the dynamic ergodic divertor (DED) of TEXTOR has been studied in an m/n = 3/1 set-up which is characterized by a relatively deep penetration of the perturbation field. The perturbation field creates (a) a helical divertor, (b) an ergodic pattern and/or (c) excitation of tearing modes, depending on whether the DED current is static, rotating in the co-current direction or in the counter-current direction. Characteristic divertor properties such as the high recycling regime or enhanced shielding have been studied. A strong effect of the ergodization is spin up of the plasma rotation, possibly due to the electric field at the plasma edge. Tearing modes are excited in a rather reproducible way and their excitation threshold value, their motion and their reduction due to the ECRH/ECCD have been studied. The different scenarios are characterized by strong modifications of the toroidal velocity profile and by a reduced or enhanced radial transport.

A 110 GHz, 400 kW, 200 ms gyrotron has been employed on TEXTOR for ECRH. The highly localised electron heating allowed the identification of (multiple) electron transport barriers. The RTP q-comb model for the electron heat conductivity gives a good description of TEXTOR results with ECRH in Ohmic discharges. Central ECRH in the current ramp phase speeds up central current penetration due to the highly peaked Te profile. A stable evolution of these discharges with early heating required the combination of counter-NBI with central ECRH. In RI-mode, central ECRH resulted in an increase of the confined energy in proportion to the ECRH power, without signs of power degradation as is usual in RI-mode with NBI and ICRH. Efficient ECRH is observed up to the X-mode cut-off density. The gyrotron has also been successfully used for diagnosis of energetic ions by means of Collective Thomson Scattering.

Confinement deterioration and rollover from the radiative improved (RI) mode to the low (L) confinement mode in plasmas with a strong gas puff in the tokamak TEXTOR-94 are modelled by the one-dimensional transport code RITM (radiation from impurities in the transport model). The anomalous transport coefficients in the plasma core include contributions from the ion temperature gradient and dissipative trapped electron instabilities. This model allows us to reproduce the L-RI bifurcation with impurity seeding in good agreement with experimental observations. Whereas the transport at the plasma edge under the RI-mode conditions might be described by the electrostatic turbulence caused by electric currents in the scrape-off layer of the limiter, the present computations show that the level of the edge transport must be increased by at least a factor of three in order to reproduce the evolution of the plasma density profile and the effective ion charge during RI-to-L back transitions. An increase of this order is in agreement with reflectometer measurements and could tentatively be explained by the effect of neutrals on resistive drift modes.

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 f(H93) greater than or equal to 1 and normalized beta values beta(n) greater than or equal to 2 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 beta(n) up to 4 and for the advanced tokamak confinement-stability product beta(n) x f(L89) 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 experiments on plasma heating and confinement in a medium size tokamak are reported. A high neutron production is obtained due to reaching the I‐mode of the confinement operation better energy storage and the efficient use of RF reactor systems. (AIP)

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.

Radial profiles of electron temperature and density are measured at high spatial (∼1 mm) and temporal (≥10 μs) resolution using a thermal supersonic helium jet. A highly accurate detection system is applied to well-developed collisional-radiative model codes to produce the profiles. Agreement between this measurement and an edge Thomson scattering measurement is found to be within the error bars (≲20%). The diagnostic is being used to give profiles near the ion cyclotron resonant heating antenna on TEXTOR to better understand RF coupling to the core.

First measurements were performed to test a model prediction which states that the off axis coupling of ion-cyclotron waves to energetic helium ions can generate either an inward or an outward drift of these particles. Ion-cyclotron waves with a power of up to 1 MW are coupled to energetic particles at the high field side, the low field side, or in the center by varying the toroidal magnetic field. When changing the heating power or the resonance location, the variation of the concentration of the energetic helium in the plasma agrees qualitatively with the model predictions.