Charge-exchange recombination spectroscopy (CXRS) has been used to compare results on ion temperatures from several diagnostics at TEXTOR-94. The question of whether the typically measured width of impurity spectral lines is representative for the main ion temperature , is addressed by applying CXRS to the Balmer-alpha spectrum of deuterium. The importance of the halo effect is found not to be severe for the measurements. is lower than the impurity temperatures for low-density discharges with neutral beam heating. The time evolution of and the toroidal rotation were also measured during sawtooth oscillations. From this a lower bound for the ion heat diffusivity of has been deduced.

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.

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)

A stationary high level of edge radiation (gamma = P-rad/P-tot up to similar to 90% with peak radiation up to similar to 1 MW/m(3)) 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 hows with a figure of merit f(H)/q(a) = 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 TRANSP 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.

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 experiments involving the interaction of radio-frequency heating with fast particles injected by neutral beams on TEXTOR are reviewed. The focus point of the present study is the comparison between experiments and theory. Theory involves the modelling of RF diffusion in velocity space, absorption of radio-frequency waves by non-maxwellian populations, computation of drift trajectories to take trapping and first-orbit losses into account. A more specific theoretical development used in this context is the reconstruction of charge exchange spectroscopy spectra. Three types of experiments are examined. The synergetic increase of beam current drive by radio-frequency power, the large neutron production in third harmonic heating of D beam ions and the control of central fast helium concentration by radio-frequency induced diffusion.

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.

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 3He 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.

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.

A set of experiments was carried out with TEXTOR neutral beams inside the torus to obtain detailed knowledge about the beam for later use in experiments such as charge exchange recombination spectroscopy and for energy deposition calculations. A segmented graphite plate and an infrared camera were used to gain information about the total beam power profile for different beams (H, D, He) and particle energies. Spatially resolved measurements of Doppler-shifted Hα line intensities were used to calculate the relative widths and fractional powers of the three beam species (for H and D) at different energies (this report mainly deals with this method). By these measurements the beams are fully characterized so that charge exchange spectroscopy could be employed to yield impurity concentration profiles in spite of an extended beam and lines of sight not tangential to the plasma surfaces. The results for a 50 keV hydrogen beam are also taking into account the calorimetric measurements inside the injector box. The beamlets are composed of two Gaussian profiles. The broader profile (divergence of 3° and a power fraction of 30%) is mainly scraped off in the injector itself. The composition of the smaller profile, which almost exclusively determines the power to TEXTOR, is calculated from the measured results. The power fractions for the three species (E, E/2, E/3) are 50%, 37.6% and 12.4% respectively. The divergences horizontally are 0.87, 0.92, 0.95 degrees and vertically 0.91, 0.98, 1.02 degrees respectively. The exact position of the beam inside TEXTOR and its direction are determined as well.

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.

The Rutherford scattering diagnostic at TEXTOR was used to determine the central toroidal rotation speed of the hydrogenic bulk ions in the plasma. During neutral beam injection speeds of the order of 10(5) ms-1 were found. With balanced injection no rotation was observed. A first comparison with charge exchange recombination spectroscopy showed, within the accuracy of the methods, no difference in the derived speeds, in agreement with neoclassical theory. The momentum confinement time of the central hydrogenic ions was found to decrease with the neutral beam power as approximately 1/square-root P(NBI).

Experiments have been performed to measure the conductance for backstreaming particles in the throat of the pump limiter ALT-II on TEXTOR under different discharge conditions. The how inside the throat is analysed by injecting a steady helium flux towards the neutralizer plate and by measuring the helium partial pressure. The amount of helium escaping into the plasma volume has been found to be strongly dependent on the local electron density and electron temperature. At low densities the backstreaming conductance corresponds to the classical value, at medium densities its value is reduced, whereas at the highest densities (> 5 x 10(18) m(-3)) an increasing how of escaping helium is found. It is shown that a reversal of the plasma flow in the throat of ALT-II can be established by exposing one blade (the other 7 retracted) to high density discharges with additional heating.

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

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

Co-injection (DO --> D+) applied to TEXTOR leads to a hot ion node regime with enhanced confinement. A synergistic increase of the beam effects is observed with the addition of ICRH at omega = 2omega(cD) = omega(cH) (H minority heating scenario) resulting, beside other reviewed effects, in a significant increase of the ion temperature and of the beam driven current (respectively larger than 30 % and 50 % for the addition of an RF power comparable to the NBI one). The large ion heating efficiency of ICRH also remains when ICRH is added to balanced injection and the hot ion mode regime remains up to the maximum achieved beta (=2/3 of the Troyan limit with more than 6MW of auxiliary heating). ICRH also leads to the formation of a more 'isotropic' tail. These results are interpreted with the help of a Fokker-Planck code which computes the beam distribution function in presence of RF and of TRANSP simulations. The amount of RF absorption by the H minority by the ion beam and die bulk plasma is theoretically evaluated.It is shown that a large part of the synergistic effects can be explained by the rise of the electron temperature due to the minority heating which increases the beam slowing down time and its critical energy. A smaller contribution to the effects is due to direct coupling of the RF power to the beam (less than 10 % of the total RF absorbed power) and to the decrease of the plasma toroidal rotation induced by the RF.ICRH has also been added to co-injection at omega = 3omega(CD). In this case no minority heating is present and the RF energy coupling to the beam is one of the dominant effects. It leads to the formation of a very energetic tail of the ion beam with a strong increase of the beam-target neutron reactivity.

The ion species fractions of the multimegawatt neutral beam ion sources for Torus Experiment for Technological Oriented Research (TEXTOR) were determined by H(alpha)/D(alpha)-light Doppler shift spectroscopy. The ion sources are modified Joint European Torus (JET) plug in neutral injectors (PINIs) with a three-grid acceleration system and a magnetic field in the plasma box produced by permanent magnets in checkerboard arrangement. The ion species mix of the extracted beam was measured for energies of 20-60 keV and beam currents of 13-100 A of hydrogen and deuterium produced by the two ion sources (PINI) mounted on the neutral beam injectors of TEXTOR. The maximum H+/D+ fraction at full power operation is 66%-69%. No significant difference between the two ion sources was found. The results of the hydrogen ion species mix are in good agreement with prior optical and momentum analysis made on the NI-Testbed. The additionally measured dependence of the extracted species mix from the source gas pressure is small in the pressure range above 3 mubar where the ion sources are usually operated. Measurements over the beam pulse length from 0.1 to 3 s show no change in the species mix. For calculating the deuterium ion species mix at low energies, several cross sections for H(alpha)-light excitation, charge exchange, and dissociation available from the literature were extrapolated to about 3 keV per nucleon. The correction factors used for calculating the ion species fraction from the ratio of the relevant integrated Doppler shift light peaks are given as a function of the gas target density in the neutralizer and of the energy per nucleon. The optical spectrometers specially designed and constructed for neutral beam H(alpha)-light Doppler shift spectroscopy give a sufficient resolution of 1 angstrom and enough light intensity also at smaller beam energies and currents. The subsequent injected neutral beam power fractions H-0(E), H-0(E/2), and H-0(E/3) and the molecular fractions H-2(0)(E) and H-2(0)(2/3E) are given as a function of acceleration voltage considering the beam line transmission and 90% of equilibrium gas target density. The injected full energy neutral beam component at 55 keV is for hydrogen 48% or 0.7 MW and 66% or 1.1 MW for deuterium.

The compatibility of radiative edge cooling by neon injection and He exhaust with the pump limiter ALT-II is studied on TEXTOR. It is demonstrated that in plasmas with strong auxiliary heating (2 MW NBI) and with the highest average electron densities (n(e)BAR = 5.5 X 10(19) m-3) the effective confinement time tau(P)* for He has a minimum. This good pumping performance is maintained even for cases in which up to 90% of the heating power is radiated from the plasma boundary (cold radiative edge). The processes inside the scoops of the pump limiter (neutral particle transport, re-ionization) and the variation of particle confinement in the main plasma dominate this behavior.