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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.

Progress in fusion research shows an increasing demand for diagnostics with high temporal and spatial resolution in order to study small and fast phenomena in fusion plasmas. Therefore, a new ten-channel pulsed radar reflectometer is under development for the TEXTOR Tokamak, to measure electron density profiles in the range of 0.4-4 X 10(19) m(-3) at a high repetition rate of 2 MHz. An additional feature of this diagnostic is the possibility to perform correlation measurements at 10 MHz repetition rate to investigate density fluctuations. The reflectometer will scan the plasma with 1-ns-long radar pulses in the 18-57 GHz range. In spite of the long flight time of the pulses of about 100 ns, the above mentioned high repetition rates could be achieved by time multiplexing the generation and reception of the radar pulses. Temporal selection of the received pulses is performed by fast switching of the local oscillators inputs of the heterodyne receiver, to generate time windows in which the reception of the reflected pulses is expected. An embedded VME controller will manage the system and store the data with a speed of 20 Msamples/s, up to a maximum of 64 Mbyte data per plasma shot. In order to facilitate the handling of such a huge amount of data, an advanced data reduction scheme is being developed. Remote operation with a fast data link from FZ Julich (Germany) to FOM Nieuwegein (The Netherlands) will be possible. (C) 1999 American Institute of Physics. [S0034-6748(99)68001-7].

Different operational limits of tokamaks are reviewed. The finally limiting processes are hard or soft plasma disruptions or a strong x-ray source. The underlying events are explained as far as the physical causes are known. Treated are the density limit, the low-q-limit, the runaway limit, the {beta}-limit, and the vertical displacement limit. In addition, the spontaneous MHD-instability and a hollow temperature profile situation are presented. The graphical displays are experimental results from the TEXTOR tokamak. 15 refs., 5 figs.

A 2 mm microwave scattering system was used on TEXTOR to observe density microfluctuations with good space and time resolution. In ohmic heating conditions, a turbulent sawtooth precursor was discovered and studied in detail near the q = 1 surface, the large fluctuation spike occurring at the sawtooth crash was precisely located in the mixing region, and it was shown that a turbulence pulse propagates together with the outgoing heat pulse. In sawteeth-free neutral beam injection heating discharges, evidence of a toroidal Doppler effect was collected suggesting short parallel wavelength fluctuations near the magnetic axis. In sawtoothing neutral beam injection discharges, a strong transition of the MHz fluctuation levels during the sawtooth development appears as new evidence of the link between confinement and fluctuations.

The theory of electromagnetic wave propagation in a cold plasma is only briefly treated. Optical approximations made for diagnostic applications are mentioned. Refractive index measurements via FIR-interferometry are discussed and an electron density profile evaluation is illustrated. The principle of microwave reflectometry is presented and examples from fixed frequency and pulsed radar reflectometry are shown. The theoretical background of electron cyclotron emission is given in a simplified version and examples of electron temperature measurements on TEXTOR are presented. Scattering of mm-waves from high temperature plasmas is shortly sketched. An example of a rapidly growing density fluctuation in a critical operational situation is illustrated by collective wave scattering on a turbulent plasma. Magnetic field fluctuation studies carried out by cross-polarization scattering of microwaves are reported. 14 refs., 12 figs.

A new fullerene co-crystal C60*SiH(C6H5)3 has been synthesized by crystallisation from a melt of SiH(C6H5)3 and C60 in sealed duran tubes at 80°C. X-ray investigations led to triclinic symmetry, space group P1̄, a = 10.086(1), b = 14.431(2), c = 14.911(2) Å, α = 79.13(1), β = 74.943(9), γ = 88.33(1)°, V = 2058.0(4) Å3, Z = 2. The crystal structure consists of isolated fullerene and silane molecules. At -80°C, C60 is still rotationally disordered. Temperature dependent Guinier powder diagrams do not show any phase transformation between 25 and -165°C. The compound is stable in air.

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.

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.

C70Br14 wurde durch Bromierung von C70 in Brom bzw. in den Losungsmitteln 1,1,2,2-Tetrachlorethan oder CS2 dargestellt. Die Verbindung wurde mittels Elementaranalyse, thermischer Analyse, 13C-Festkorperkernresonanzspektroskopie und Pulverdiffraktometrie charakterisiert. Die Gitterkonstanten betragen a = 32,09(2), b = 10,731(7), c = 18,135(6) A, β = 110,92(4)·, V = 5834(4) A3, Z = 4. Die Verbindung zerfallt langsam bei Raumtemperatur in bromarmere Addukte. Mittels thermischer Analyse (Aufheizgeschwindigkeit 5°/min) wurde der Zerfall in C70 und Br2 bei 134,9°C detektiert. Synthesis and Characterization of C70Br14 C70Br14 was obtained by bromination of fullerene C70 in liquid bromine or the solvents 1,1,2,2-tetrachlorethane and CS2, respectively. The compound was characterized by elemental analysis, thermal analysis, 13C-solid-state-NMR and powder diffraction. The lattice parameters are a = 32.09(2), b = 10.731(7), c = 18.135(6) A, β = 110.92(4)°, V = 5834(4) A3, Z = 4. At room temperature C70Br14 decomposes slowly to less bromine containing samples. Thermal analysis (heating rate 5°/min) shows decomposition to C70 and Br2 at 134,9°C.

Stationary high energy confinement is observed on TEXTOR-94 for times limited only by the flux swing of the transformer using strong edge radiation cooling. Necessary tools are the feedback control of the radiated power and of the plasma energy content. At the highest densities obtained (up to 1.2 times the Greenwald limit), energy confinement exceeds the edge-localized-mode-free H-mode scaling ITERH93-P by more than 20%. beta limits of TEXTOR-94 are reached with f(H89)/q(a) approximate to 0.6 No detrimental effect of the seeded impurity is seen. These high confinement discharges meet many conditions necessary for a fusion reactor regime.

Two methods of quantitative analyses of the fullerenes C60 and C70 are described. Quantitative Infrared Spectroscopy permits the determination of the concentrations of pure and mixed compositions of C60 and C70 in carbondisulfide solutions. Alternatively, the ratio of C60/C70 is analyzed by evaluating the lattice parameters of a solid solution of the fullerene species and cyclohexane according to Vegard's Law. Both methods show high accuracy and are suited for calibration of mass spectrometric analyses of fullerene samples.

Collective millimetre wave scattering and electron cyclotron emission techniques were used to study precursor instabilities to forthcoming tokamak disruptions. A 150 GHz O-mode scattering diagnostic installed on TEXTOR is briefly described. Tearing modes and MARFEs were observed prior to discharge disruption. Collective scattering spectra were evaluated from scattered radiation from different events and radial regimes on the tokamak plasma TEXTOR. The evolution of the scattering spectra demonstrates the development of magnetohydrodynamic turbulence. This turbulence produces highly enhanced energy and particle fluxes and terminates abruptly the confined plasma state. The disruptive evolution of a tokamak plasma is a big handicap for future fusion machines and either must be avoided or at least it must be reduced to a small number of events.

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.

The behavior of magnetohydrodynamic phenomena associated with disruptions of ohmically heated density limit discharges (qa = 3.9) in the TEXTOR tokamak has been investigated in detail. A minor disruption is demonstrated to be only due to an m = 2 mode instability, possibly due to the magnetic reconnection of an m = 2 island. When the m = 2 mode oscillations cease, the internal m = 1 kink mode instability is drastically enhanced at once, twisting the plasma core considerably. If the twisting is too strong, the twisted plasma core is inevitably displaced to the outside and brings cold plasma to the central region, leading to the energy quench at a major disruption.

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.

Bei der Kristallisation von Fullerenen aus unpolaren Losungsmitteln (C6H12, CCl4, CHBr3, CHCl3, H2CCl2) entstehen Kokristallisate der Zusammensetzung C60 · 12C6H12, C70 · 12C6H12, C60 · 12 CCl4, C60 · 2CHCl3, C60 · 2CHBr3 und C60 · 2H2CCl2. Die Gitterparameter wurden durch Rontgenbeugung an Pulvern bestimmt, nach Einkristalluntersuchungen an C60 · 12C6H12, C60 · 12CCl4 und C60 · 2CHBr3 liegt Orientierungsfehlordnung von C60 vor. C60 · 12C6H12, kubisch, a = 28,167(1) A; C70 · 12C6H12, kubisch, a = 28,608(2) A; C60 · 12CCl4, kubisch, a = 27,42(1) A; C60 · 2CHBr3, hexagonal, a = 10,212(1), c = 10,209(1) A; C60 · 2CHCl3, hexagonal, a = 10,08(1), c = 10,11(2) A; C60 · 2H2CCl2, tetragonal, a = 16,400(1) A, c = 11,645(7) A. Synthesis and Characterization of the Fullerene Co-Crystals C60 · 12 C6H12, C70 · 12 C6H12, C60 · 12 CCl4, C60 · 2CHBr3, C60 · 2CHCl3, C60 · 2H2CCl2 By crystallization of fullerenes from non-polar solvents (C6H12, CCl4, CHBr3, CHCl3, H2CCl2) compounds of the following compositions were obtained: C60 · 12C6H12, C70 · 12C6H12, C60 · 12CCl4, C60 · 2CHCl3, C60 · 2CHBr3 and C60 · 2H2CCl2. Lattice parameters have been determined by X-ray diffraction of powder samples; according to single-crystal examinations on C60 · 12C6H12, C60 · 12CCl4 and C60 · 2CHBr3 the fullerene is orientationally disordered. C60 · 12C6H12, cubic, a = 28.167(1) A; C70 · 12C6H12, cubic, a = 28.608(2) A; C60 · 12CCl4, cubic, a = 27.42(1) A; C60 · 2CHBr3, hexagonal, a = 10.212(1), c = 10.209(1) A; C60 · 2CHCl3, hexagonal, a = 10.08(1), c = 10.11(2) A; C60 · 2H2CCl2, tetragonal, a = 16.400(1) A, c = 11.645(7) A.

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

Impurity fuelling has been studied in TEXTOR by gas puffing under a wide range of experimental conditions. First, neon has been injected into discharges with a range of densities during both Ohmic and neutral injection heating. Then, helium, neon and argon have been injected into similar discharges and the increase in radiated power and Zeff studied as a function of the number of impurity atoms injected. Similar experiments using carbon monoxide and methane yielded markedly different results from those of the rare gases. The fraction of injected atoms entering the confined plasma has been estimated and found to vary widely, from less than 1% for carbon to about 100% for helium. Detailed Monte Carlo calculations performed using the LIM code to simulate the impurity behaviour have shown that good agreement with experiment can be obtained for all species except oxygen, where charge exchange processes are thought to be important

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

Impurity gases have been introduced into TEXTOR at well defined rates to study the fuelling process and the effect they have on the overall energy balance. It is found that carbon and oxygen, introduced as CO and CH 4 gases have radically different behaviour. Oxygen has a high recycling coefficient ∼-0.99 whereas carbon has a low recycling coefficient ∼-0.3. The radial position of the gas source was varied from the wall to the limiter radius but this made little difference to the effect on the global plasma parameters. For very high impurity influxes of both impurities drastic changes in the edge plasma were observed and disruptions could be induced.

In the TMI tokamak current spikes into elements of the poloidal limiter were observed during disruptions Currents flowed between limiter elements located at the outboard side and inboard side of the torus as well. Simultaneously, the variation of the plasma column vertical position is observed. The direction of this shift coincided with that of a force produced by currents between poloidal limiter elements and the toroidal magnetic field. The correlations between vertical shift, current flowing to limiters and the shift along the major radius were investigated in the small TF tokamak. During major disruptions in the TEXTOR tokamak the currents flowing between the ALT-II toroidal belt limiter and the liner were recorded.

Density limits and the evolution of disruptions were studied on TEXTOR for different wall materials facing the plasma surface. A systematic extension of the density limit was found over a large current range when the metallic wall was first exchanged by a carbon coated wall and later by a boron-carbon coated wall. The maximum attainable density nearly doubled when metallic impurities were replaced by low Z impurities such as boron and carbon. With a neutral beam additional heating power of PNI=3.2 MW, average densities exceeding ne=1020 m-3 were obtained. The maximum density observed scales like the square root of the auxiliary heating power. The sequence of processes leading to a density limit disruption was experimentally analysed in detail for ohmically heated plasmas

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.

Disruptions remain a major concern for tokamak devices, particularly for large machines. The critical issues are the induced (halo) currents and the resulting forces, the excessive heating of exposed surfaces by the instantaneous power release, and the possible occurrence of highly energetic runaway electrons. The key topics of the investigations on TEXTOR in the recent years concerned (a) the power deposition pattern recorded by a fast infrared scanner, (b) the runaway generation measured by synchrotron radiation in the infrared spectral region, (c) method development for "healing" discharges that are going to disrupt, and (d) massive gas puffing for mitigating the adverse effects of disruptions.

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.

An experimental investigation into electron temperature profile perturbations due to MHD tearing modes is performed on the TEXTOR tokamak. By means of an ECE diagnostic with a good temporal and radial resolution, m = 1 and m = 2 precursors to sawtooth crashes and major disruptions, respectively, can be studied. The specific temperature perturbations due to a rotating m = 2 magnetic island prior to a disruption have been studied. The phase of the rn = 2 MHD oscillation reverses at the rational q surface. The amplitude distribution of the perturbation has been determined. A frequency analysis of the MHD oscillations on different radial positions close to the rational surface shows an increase of higher harmonics in the frequency spectrum due to the difference in magnetic field topology. Temperature profiles over magnetic islands could be measured. A secondary temperature maximum in the centre of the m = 2 magnetic island has been found. Because of the good radial resolution of the ECE diagnostic an estimate of the island width and the position of the q = 2 surface could be obtained. In one case a movement of the q = 2 surface, due to peaking of the current profile, of more than 7 cm, i.e. 15% of the minor radius, was found. Island widths of w/a = 0.07-0.2 have been observed.

Density profile perturbations due to large rotating m/n = 2/1 magnetic islands have been studied by means of a four channel pulsed radar reflectometer in TEXTOR-94. The time of flight of the microwave pulses was strongly affected by the shallow density gradients over the magnetic islands. A detailed analysis showed the existence of secondary reflection lavers inside the magnetic island, which is an indication of a strong density peaking. This may be explained by assuming that the particle transport is reduced inside the magnetic island.

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)

An experimental investigation into rotating MHD modes has been performed in the TEXTOR tokamak. The effects on the stability of the MHD tearing modes of coupling between m/n = 2/1 and 1/1 modes and of the slowing down of the mode rotation by wall friction have been studied. Tangential neutral beam injection (NBI) has been used to change the toroidal rotation to observe the influence of plasma rotation on the development of MHD modes. Two phases in the rotation frequency behaviour of the modes can be distinguished: a slow reversible decrease and a fast exponential decay, which starts with a short transient increase in frequency. The last phase finally results in mode locking, which mostly leads to a disruption. By injecting low power NBI in the last phase of the slowing-down process, mode locking could be prevented. The mode amplitude and rotation frequency have been stabilized, without major changes to the pressure profile. By decreasing the central plasma velocity, using NBI, a coupling of the two modes has been forced. The coupling of the modes had a destabilizing effect on the MHD activity.

Disruptions are studied in TEXTOR using two infrared cameras. In the thermal quench phase, fast changing heat fluxes are observed, each delivering energies larger than 1 kJ/m(2) to the limiter. These bursts are correlated with an electron temperature pulse near the limiter and an increased release of impurities. Further bursts are observed in the current decay phase. For the first time a disruption generated beam of runaway electrons, accelerated in about 10 ms to about 20 MeV, is observed inside the plasma by the infrared synchrotron radiation. This beam carries a current of about 20 kA and is lost within approximately 100 mu s. The heat deposition on the limiter due to the runaways is comparable to the power flux by the heat bursts.

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.

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.

Electron heat pulse propagation following sawtooth collapse has been simultaneously observed along two opposite radial directions. For a given flux surface, the arrival of the heat pulse at the high field side (HFS) is delayed by up to 1 ms as compared with the arrival at the low field side (LFS). The observed delay agrees with an estimate of the time required for parallel electron heat transport along the magnetic field lines connecting the corresponding LFS and HFS isobars. This suggests that the primary radial heat transport occurs on the LFS, whereas the HFS is fed subsequently from the LFS by temperature equilibration along the connecting magnetic field lines

The properties of the N41-1D oscillations (Mirnov Oscillations) in ohmic TEXTOR tokamak plasmas are systematically described in this paper based on experimental data, including mainly the principal modes of the MHD oscillations and the correlaion among the oscillations, the propagating direction of the MHD oscillations, as well as the correlative properties among the oscillative temperature, density and density fluctuation of the m=2/n=1 mode.

Ion cyclotron heating on TEXTOR has now reached the Megajoule level. The heating scenario is normally mode conversion but occasionally minority heating in a D-(H) plasma. With appropriate wall conditioning by carbonization more than 1 MW of RF power has been injected for long pulse durations ( approximately 1 s). The ICRF heated plasma is characterized by a quasi-stationarity of all plasma parameters, little if no impurity increase and a loop voltage reduction resulting in the total power coupled to the plasma reaching six times the remaining ohmic power input. Evidence of the coupling of the RF power to the plasma is obtained from the increase of the thermal load on the limiters and central energy deposition is supported from analysis of the sawtooth heating rate.

The paper summarizes the main results obtained during the first six months of experimental work on TEXTOR with ohmically heated plasmas. The machine(R = 175 cm; a = 45-50 cm) was operated at wall temperatures up to 303 oC with hydrogen gas, a toroidal field of 2 T and a maximum plasma current of 500 kA, yielding central electron temperatures of 1.2 keV, maximum line-averaged densities of 4 x 1013 cm-3 and discharge durations of up to 2.7 sec (for Ip≤340 kA). Besides providing long-lasting reproducible plasmas for forthcoming plasma wall interaction research, the following tasks were accomplished: (1) control of low Z impurities by effective wall cleaning and conditioning; (2) accurate plasma positioning for extended pulse durations; (3) determination of the current density distribution by way of Faraday rotation measurements; (4) investigation of plasma limiter interaction including spectroscopic determination of released neutral particle fluxes and measurement of the power density distribution in the scrape-off layer by infrared-thermography.

Four new low field side antennae grouped in pairs have been installed on TEXTOR. It is found that the interaction with the wall (density rise, impurity generation) is significantly reduced when operating each pair out of phase ( pi ) as opposed to in phase (0). The beneficial effect in the pi configuration is obtained without drop in plasma loading. This experimental property is shown, from theory, to be explained by the judicious choice of the geometrical configuration. A further improvement in the wall interaction is made possible by an appropriate choice of wall conditioning (wall carbonization with liner at 400 degrees C or, above all, boronization). As a result record low values of Prad/Ptotal were achieved during ICRH. The large reduction in wall interaction during ICRH allows routine long pulse (>1 s) ICRH operation at the maximum power level available ( equivalent to 2.5 MW).

TEXTOR, a tokamak primarily devoted to plasma-wall interaction studies, has been operated during the last 2–3 years under a wide variety of discharge conditions. Discharges with Ohmic heating only and with additional ICR heating in the MW-range were produced over an extended range of plasma parameters. Different limiter materials were used — Inconel, stainless steel, graphite, TiC coated graphite. The surface of the first wall (liner) consisted either of Inconel or of a carbon layer deposited upon the Inconel. Characteristic features of the discharge have been modified by making use of the modular pump limiter ALT-I and by applying a localized perturbation coil for “ergodization” of the magnetic field structure in the boundary. The characteristics of plasma-wall interaction under these conditions and their influence on the global performance of the TEXTOR plasma are described and discussed. Use has been made of novel diagnostics specifically developed for edge plasma investigation.

The response of a stationary weakly ionized plasma to a density perturbation in the neutral gas component was studied in a neon plasma with the following typical properties: electron density ¯N e≈8×1012 cm−3, electron temperature on the axis of the vesselT e0≈3.0 eV; neutral gas densityN n≈1×1017cm−3 and neutral gas temperatureT n0≈600 °K. A neutral density perturbation, generated 50 cm apart from the plasma, produces a fluctuation in the ion density and a sharp spike in the differential voltage of a floating double probe. The experimental observations demonstrate the propagation of an ion sheath and of an electric field perturbation together with the neutral density perturbation. An interpretation of the plasma response to acoustic wave pulses has been proposed by Ingard and Schulz in a theory on acoustic wave modes in a weakly ionized gas. The experimental results are in good agreement with the theoretical expectations.

The time dependence of the electron density in the afterglow period of an electrodeless discharge in hydrogen was measured by means of 4- and 8-mm microwave interferometry. An exponential decay was observed in the late afterglow permitting the evaluation of a time constant in the density range from 1012–1010 electrons per cm3. The decay time of the plasma was influenced by the discharge conditions. Electron losses could be explained by ambipolar diffusion and attachment to impurities. Two diffusion coefficients were found correlated to the discharge duration. The mobility value H0=10.1±1.0 cm2/V · sec calculated from the diffusion coefficient found for short discharge pulses agrees with Saporoschenkos mobility value μ0=10.2 cm2/V · sec for the H 3 + -ion. A second mobility value μ0=14.8±1.2cm2/V · sec found for longer discharge pulses might refer to the H+-ion.

The expansion of plasma in the early stage of an inductive high frequency discharge in helium has been investigated experimentally. In a collision dominated case a non-stationary ambipolar diffusion current transfers a lightly ionized plasma from a source region into an initially neutral ambient gas. Time resolved measurements of electron density and electron temperature have been performed at the source and at a test point outside of the plasma source region by 4 mm-microwave interferometer and pulsed double probe techniques. A comparison of experimental data with calculations based on a simplified diffusion model for collision dominated plasma expansion is given.

The production of a highly ionized deuterium plasma at initial gas pressures of 0.5 to 2 mTorr has been investigated in a compact toroidal geometry (major radius of the torus vessel 20 cm, minor radius 10 cm). The gas, pre-ionized by a capacitively coupled RF-discharge, is further ionized (pre-heated) by toroidal and poloidal plasma currents which are induced by the discharge of a condenser bank (7.5 kJ maximum energy) into a helical coil system wound around the torus. At the end of the ionization phase, the current in the coil is crowbarred. Radial plasma density profiles are measured by using a movable 4-mm microwave probe. The plasma current, its energy, and the toroidal and poloidal magnetic field distributions are deduced from magnetic measurements, and the electron temperature from the Dβ-line-to-continuum-light-intensity ratio.At the low initial gas pressures studied here, the plasma formation occurs when the toroidal electric field amplitude on the torus axis lies below a critical value (< 6 V/cmmTorr). Using a programmed auxiliary perpendicular magnetic field, a plasma which is both highly ionized and well detached from the walls of the torus is obtained. Plasma equilibrium can be maintained during the time of observation (up to ≈ 40 μs after crowbar). Electron densities greater than 6 × 1013 cm−3 (cutoff density for 4-mm waves), a degree of ionization higher than 60%, and electron temperatures of 5 eV are reached starting from a pressure of 1 mTorr D2. The toroidal plasma current and the toroidal magnetic field are, respectively, 7 kA and 0.45 kG at the time of crowbar. The measured density and magnetic field profiles indicate that the magnetic axis lies significantly off the axis of the torus vessel.

A control system for TEXTOR tokamak has been designed using a detailed model including important eddy currents in machine components. Plasma displacements and current are controlled by 4 different coil currents. Realistic power supply models were provided. Actual values of controlled variables were evaluated by analogue computation from the tangential and normal component of the poloidal magnetic field. Magnetic sensors for these field components were developped from heat resistant steel cable. The feedback loops were optimized to stabilize the system against recurrent step function disturbances. Classical system design and decoupling allow the loops to be operated one by one.

A frequency modulated 155 GHz millimeter wave interferometer was built to perform real time electron density measurements on a Tokamak plasma machine with a plasma diameter of 1 meter. An intermediate frequency IF = 312 kHz was selected at a diode detector by a clocked sawtooth modulation of a reflex klystron oscillator. The zero crossing instants of this frequency were recorded. Since phase shifts of 30×2? and more are expected in the measurement branch of the interferometer a digital fringe counting system with analog conversion for real time applications was set up. A Camac interface allows a local data storage in a memory, data acquisition by a diagnostic computer and off-line display on a terminal. The accuracy and stability of the electronics were tested in a periodic experiment. The time resolution was tested on a pulsed toroidal plasma experiment. Density changes corresponding to 1 fringe in a 40 ?sec were registered. The accuracy limit of the registration system is a phase shift of ??=?/8.

ICRH and NB, Co‐injection, are compared at the same power of 1.4 MW and then combined. The diamagnetic energy increases linearly with the total power. At low plasma current, the combination of ICRH and NB, Co‐injection, is possibly synergetic and leads to nearly zero loop voltage.