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The current flowing between the new toroidal pump limiter of Tore Supra and the inner wall components was investigated. The response of the current to a change of the wall clearance as well as to a variation of the poloidal and toroidal magnetic field was studied. A simple estimate of the thermoelectric currents resulting from the radial temperature gradient is presented. We find, that the dependance of the measured current on the wall clearance can be reproduced by prescribing a deviation of the plasma potential from the sheath potential.

A simple analytic fluid model based on particle and momentum conservation describes the response of the scrape-off layer to an applied radial electric field. The model explains why the variation of plenum pressure during limiter biasing in TEXTOR shows the opposite behaviour as divertor biasing in Tokamak de Varennes: the mechanical baffles that define the throats of the pump limiter are aligned along the magnetic field, thus preventing collection of the induced perpendicular flux when the drift is directed towards the scoop. Rather, the optimal pumping is expected when the drift is directed away from the scoop due to the increase of the parallel flux needed to satisfy the Bohm-Chodura criterion. Qualitative agreement is obtained with measurements of the parallel flow near the limiter throat, and with measurements of the plenum pressure during negative biasing for both directions of toroidal magnetic field. The results, extrapolated to Tore Supra, predict that the actual design with throats should provide close to optimal pumping with minor modification of the natural radial electric field, but greater performance could be obtained with negative biasing if a toroidally symmetric neutralizer geometry were adopted.

Particle collection by the ALT-II belt limiter in the TEXTOR-94 tokamak is exclusively limited to the parallel outflux because the scoop walls that are oriented parallel to the field lines obstruct the poloidal E×B mass flow. With normal B tor direction (E×B towards the scoops), a threefold decrease of plenum pressure is measured during negative biasing, while with reversed B tor (E×B away from the scoops), a 60% pressure increase is observed. This behaviour is exactly opposite to that observed in X-point divertors. A simple fluid model explains this apparent contradiction, and gives good quantitative agreement with measurements of the parallel Mach number in the SOL. The essential physics is governed by the Bohm-Chodura criterion.

The superconducting tokamak Tore Supra will be equipped with an actively cooled toroidal pump limiter (TPL), in the framework of the CIEL (Composants Internes Et Limiteurs) project, dedicated to plasma facing component design for steady state operation. The TPL is equipped with throats, located only on the high field side, for particle collection allowing the control of plasma density which is essential for long plasma discharges. The present design work of the CIEL includes a biasing system in order to enhance the particle pumping. A fluid model, based on the classical fluid equation, is used to estimate the effects of the electric field on the particle flows in the Scrape-Off Layer (SOL). The modifications of the density, the particle flow (toroidal and poloidal) and the position of the stagnation point are discussed as a function of the bias voltage. The model clearly illustrates the different resulting effects on particle pumping for a divertor and a limiter configuration which are designed respectively for poloidal or parallel particle collection.

Particle and energy transport are modelled in TdeV using the multispecies finite element simulation code TOPO. For every charged species, equations are solved for the conservation of particles, parallel momentum, and energy. For neutral species, equations are solved for the conservation of particles and energy. For simplicity, all neutral fluxes are assumed to be diffusive. The transport equations are discretised with finite elements on a triangular unstructured mesh. This allows a realistic description of the TdeV divertor geometry, with segmented divertor plates arbitrarily oriented with respect to the local flux surfaces. The use of an unstructured mesh also allows local mesh refinement in the vicinity of the X point, and at selected locations near the divertor plates. Comparisons between simulated and experimentally measured density profiles are presented. Detailed comparisons between calculated and measured D α specific intensities are also presented. Finally, an assessment is made of the single ion temperature approximation commonly made in other edge transport models.

Steady-state H-modes have been obtained on TdeV both with ECRH and Lower Hybrid heating. Power thresholds are comparable with those predicted by the ITER scaling laws. A very pronounced threshold in edge density is observed, below which no L–H transitions occur. H–L transitions occur at significantly lower values of edge density. When the edge density exceeds the threshold, H-modes are produced both with an outboard limiter and with the divertor baffle within one density scrape-off layer of the separatrix. In the former case, confinement was degraded, in the latter, it was improved.

In this paper, we report on the effects of density, divertor geometry and ECRH heating on divertor pressure and helium enrichment. Low, medium and high (semi-detached) densities were investigated with outboard pumping (OB) and private region pumping (PR). For OH discharges, density and geometry effects are compared with B2-EIRENE simulations. There is general agreement with experiments, except for the high density, private region pumping case where the experimental enrichment is lower than predicted. We studied the effect of H- and L-modes on He pumping. L-modes gave the highest divertor pressures and enrichment. H-modes did not affect appreciably the total pressure but the enrichment decreased. Finally, the effects of baffling, biasing and divertor fuelling are discussed; and a comparison is made between a two-reservoir and four-reservoir model for particle balance.

In the TdeV tokamak, the convection of the plasma towards the divertors can be studied by changing the radial electric field in the scrape-off-layer (SOL) through biasing the divertor plates relative to the vacuum vessel wall. Electrostatic probes were used to determine the plasma parameters on both the outboard midplane and the bottom of the discharge. From these measurements, poloidal differences in the electron kinetic pressure were seen to arise due to the application of the biasing voltage on the neutralizer plates and that these pressure differences were correlated to the changing radial electric field. By using a linear relation between the poloidal kinetic pressure drop and the radial electric field averaged between the two observation points, an experimental value of 8.8 × 10 −3 (Ω m) −1 is obtained for the perpendicular conductivity. A comparison with current models would tend to favor inertia and viscosity over friction with neutrals to counterbalance the j × B force during biasing although the scaling of the conductivity with the toroidal magnetic field, plasma density and plasma current should be verified to confirm this conclusion.

A visible spectroscopic imaging system has been developed on Tokamak de Varennes (TdeV) that provides profile measurements of the ion temperature and plasma rotation velocity. Multiple poloidal and tangential chords were coupled to the entrance slit of a spectrometer equipped with a back-illuminated charge coupled device camera at its exit slit. The camera records spatially and spectrally resolved emission profiles with real time digitization and up to 70 ms/frame as temporal resolution. Line-integrated radiation from different ionization stages in the visible range (B V, C III, He II, and N IV) were chosen as representative of the plasma in their regions of maximum emission. Three types of plasma discharges are compared: ohmic and electron cyclotron resonance heated L and H modes. Preliminary results show that with an L–H transition, there is a 20%–30% increase of the ion temperature at midradius, without a pedestal and an increase of the toroidal velocity shear at r/a∼0.8.

Ionization near the target plate is shown to play an important role in biasing experiments. Our previous SOL model, which calculates the induced radial electric field, is found to be inadequate to treat the new divertor geometry of TdeV. When recycling is included via the measured emission near the plate, the upgraded model correctly reproduces all the observed electric currents and fields during biasing in the new divertor configuration. A simple divertor model using this calculated field has been developed to simulate the evolution of the divertor ion and neutral parameters under the action of neutralization plate biasing. Using a 1D adiabatic fluid model for the divertor ions, a 1D convective representation for the SOL neutrals and a 0D calculation for the plenum pressure, this divertor model satisfactorily simulates most of the TdeV biasing experiments at all biasing voltages and all toroidal field directions at low line-averaged densities. The weaker agreement at high densities is largely a consequence of the crudeness of the general divertor physics rather than of the deficiency of the biasing physics implemented in the model. The model is finally used to explain the polarity asymmetries observed in divertor efficiencies during biasing, and to demonstrate that no mechanism other than plate current saturation is required to interpret the saturation of toroidal rotation observed in the SOL at large biasing voltages of either polarity.

The radial electric field inside the separatrix has been deduced from spectroscopic measurements of impurities on TdeV (Tokamak de Varennes), using the reduced radial momentum balance and two neoclassical models [R. D. Hazeltine, Phys. Fluids 17, 961 (1974) and Y. B. Kim, P. H. Diamond, and R. J. Groebner, Phys. Fluids B 3, 2050 (1991)]. The results from all three models are in fair agreement. Furthermore, the electric field has been deduced using the same models both with and without biasing the divertor plates relative to the machine wall, showing an inward propagation of the effect of the biasing created in the scrape-off layer (SOL). Undeniably, the electric field has been modified well inside the separatrix (0.6≲r/a≲0.9), revealing the possibility of modifying the internal electric field by external means.

The most promising concept for deep fuelling a reactor is by the injection of compact toroid (CT) plasmoids. The first results showing CT fuelling of a tokamak plasma, without any adverse perturbation to the tokamak discharge, are reported. The Compact Toroid Fueller (CTF) device was used to inject a CT-spheromak plasmoid into the TdeV tokamak. Following the CT penetration, the tokamak particle inventory increased by 16%, the loop voltage and the plasma current did not change, and there was no increase in magnetohydrodynamic (MHD) activity. The number of injected impurities was low and dominated by non-metallic elements. The plasma diamagnetic energy and the energy confinement time increased by more than 35%

At high densities, the plasma detaches from the outboard divertor plates in TdeV. The signatures are a reduction of the ion flux to the divertor plate, movement of the radiating zone from the plate toward the X-point, a pressure gradient between an ionization front and the target plate, and strong cross-field transport in the divertor. A toroidally-viewing TV imaging system allows us to observe local interactions between the divertor plasma and the different divertor plates. As the plasma detaches, the gas pressure in the divertor continues to rise, and there is evidence for molecular processes in the cold plasma near the divertor plates. Auxiliary heating increases the power and particle flow across the separatrix; our results suggest that detachment depends on the energy transported per particle. Simulations using the B2/EIRENE and DIVIMP codes give reasonable agreement with the measurements for the attached phase.

A single channel AM reflectometer, operating in the frequency range of 33.0⩽f⩽50.0 GHz, has been developed to provide edge electron density profiles on TdeV. The X-mode cut-off layer, covering densities ne⩽1019 m−3 for 1.5 T operation, is used to measure a portion of the plasma edge region 0.9⩽r/a⩽1.2. Briefly, our design consists in launching a microwave beam amplitude modulated at 201 MHz. The reflected wave is detected and downconverted to 1 MHz to facilitate filtering and phase detection. Finally, a linear phase detector compares the phase of the resulting signal (1 MHz) with that of the reference source. The design and construction of the reflectometer will be described and preliminary results presented.

An atom injector using laser blow off of lithium is installed on TdeV to obtain the radial edge profile of electron density near the throat of the top divertor. The profile is reconstructed from the emissivity of the LiI (2p 2P→2s 2S) transition at 670.8 nm. A short review of the diagnostic is given describing the experimental setup, the measurement procedures, and the density profile reconstruction method. Effects of plasma biasing on the edge electron density profile and the response of the edge profile to the line-average central electron density are shown to illustrate the diagnostic capabilities.

Experimental data, analysis and simulations are used to describe the physics of divertor detachment and He exhaust under detached conditions in TdeV. With increasing density, the plasma is found to detach progressively from the outboard divertor plates with a marked reduction of the ion flux to the plates, the generation of a pressure gradient between an ionization front and the target plate, and strong cross-field transport in the divertor. Local interactions between the divertor plasma and the plates are described, with evidence for carbon sputtering and molecular processes near the divertor plates. Divertor exhaust and retention continue to increase through detachment and He exhaust is not affected although the divertor He enrichment remains low but constant at about 0.2. A moderate density of seems to be sufficient both for efficient peak power load reduction at the divertor plate and good He exhaust through the divertor. Simulations of the edge and divertor plasmas using the B2/EIRENE and DIVIMP codes give reasonable agreement with the measurements and indicate possible divertor geometry improvements

The particle confinement time of ohmic double-null discharges in Tokamak de Varennes (TdeV) is determined by two different techniques, the conventional method and a new technique based on the temporal decay of the total core population following the injection of a gas puff. Both methods show a confinement time increasing with density up to a maximum of 13 ms at and decreasing at larger densities, with very little dependence on plasma current. Particle transport is analysed using fast gas puffing and Abel inversion of the seven-chord submillimetre (SMM) interferometer together with the source profiles determined by measurements. The incremental transport coefficients are obtained by testing the standard form of the particle flux function against the data during the transitory period towards equilibrium. Both perturbed diffusion and convection coefficients are found to vary approximately as the inverse of the density and almost proportionally to the plasma current. The equilibrium transport coefficients are then deduced from the experimental equilibrium density profiles and the measured incremental coefficients using a transport model developed from the data. The model is finally used to predict confinement times to be compared with experiment. The effect of divertor plate biasing on transport is also discussed.

Effective control of the particle and power flux distributions between the upper and lower divertors has been demonstrated on TdeV, using biasing of the divertor plates, in ohmic discharges with a double null magnetic configuration. Negative biasing of the divertor plates with respect to the walls significantly increases the pressure and the emitted power from the plasma in the active divertor into which E*B drift is directed, but it has little influence on the opposite divertor. This enhanced power dissipation is the result of improved particle retention in the active divertor. The incident power on the plates in the active divertor also increases with biasing, but at a slower rate. The Franck-Condon and charge exchange neutrals contribute significantly to the power loss in the divertors of TdeV in the present operating mode. Positive biasing of the divertor plates has little influence on the total losses from the divertor regions

Preliminary results concerning the influence of negative biasing of the divertor plates on particle recycling and on power loss distribution in single null discharges of TdeV during lower hybrid (LH) current drive and heating experiments are presented. The beneficial effects of negative biasing of the divertor plates, such as the ability to control power and particle fluxes in the SOL, remain effective in the presence of auxiliary heating and current drive. Up to 0.7 MW of auxiliary power were injected in these experiments. With a negative biasing of 150 V, and the E ⊗ b flow vector pointing towards the outer divertor chamber, a roughly 2 fold increase in the divertor pressure and the radiation from the divertor region is observed.

A simple model based on non-ambipolar radial transport and planar sheath physics is used to describe the generation of radial electric fields and currents in the scape-off layer of the Tokamak de Varennes (TdeV) during divertor plate biasing. In general, the calculated predictions compare favourably with TdeV results over a variety of plasma conditions and divertor magnetic configurations. Validated by the experiment, the model is used to study the scaling laws of perpendicular ion mobility and to test existing related theories. Finally, the model is proposed as a useful tool for the design and upgrade of biased divertors through optimization of the plate and throat geometry

An interferometer/polarimeter has been developed and tested for operation on the TdeV tokamak. The system operates at 393.6 μm on 7 channels equally spaced over the plasma diameter. The interferometer has operated routinely for six years and is used for density feedback control on TdeV, with very little supervision and maintenance. A sensitivity of 2.5×10−16 m−2 and a response time of 5 μs enables small density perturbations (Δn≥0.005ne) and fast transients to be resolved. With the appropriate choice of assumptions and boundary conditions, the system can supply credible electron density profiles together with their temporal evolution, using as isocontours the flux surfaces obtained from equilibrium calculations. When operated as a polarimeter, the system now routinely provides current‐density profile measurements as well. Optical cross‐talk between channels, identified as the main obstacle to the extension of the interferometer to polarimetry, was mastered using optical isolation techniques. The system c...

Experiments in which the divertor plates are biased with respect to the grounded vacuum chamber were conducted in the Tokamak de Varennes, with ohmic discharges and boronized walls. A comprehensive set of diagnostics was used and those effects relating to plasma-surface interactions and their consequences, namely recycling and impurity production, are reported here. The main conclusion is that the bias voltage actively controls the balance between the particle fluxes to the walls and the diverters, with a weak influence on the confinement. The quantities associated with plasma surface interactions, i.e. the impurity influx, the plasma contamination and the global recycling coefficient, increase as a function of the voltage applied between the plates and the wall. Typically, the quantities of interest vary by a factor of two or more in the range from -240 to 290 V. The poloidal fluxes to the upper or lower divertor are enhanced by negative or positive biasing, respectively, in agreement with the sign of the E*B drift. For negative biasing the reduction in the wall flux and increase in the poloidal flux combine to improve greatly the divertor efficiency: the divertor pressure and the impurity retention time are increased by factors of up to 5 and 8, respectively. The above effects, together with the SOL density profiles and the biasing current-voltage characteristic, are consistent with a model in which the radial transport in the SOL is limited by the ion mobility; a mobility of 0.04 m2/V.s is found. Besides allowing active density and impurity control, it is suggested that biasing could facilitate the attainment of a radiative divertor by injection of well retained impurities in the divertor

Electrically insulated divertor plates are used on TdeV (Tokamak de Varennes) [18th EPS Conference on Controlled Fusion and Plasma Physics Berlin (European Physical Society, Petit‐Lancy, 1991), Vol. 15C, Part I, pp. 1–141] to produce various biasing configurations, which can be decomposed into two basic modes. Plasma biasing, with a radial electric field Er in the scrape‐off layer (SOL), is most promising for divertor applications. The Er field is produced with a particular divertor plate geometry, causing a nonambipolar radial current and a particle flow in the Er×BT direction, toward one of the divertors (the active divertor). The pressure and impurity retention in the active divertor are shown, in the Ohmic regime, to be strongly increased by biasing. He exhaust through this divertor is increased by a factor of almost 3 with modest biasing voltages and currents scalable to larger devices. Biasing also modifies the power repartition between the divertors, with the active divertor also receiving a larger...

The fact that the inner and outer divertor plates of the Tokamak de Varennes can be biased independently in a double-null or a single-null configuration makes it possible to characterize various biasing schemes. Current injection, in which the potential difference is applied on the same flux surface, and plasma biasing, in which the potential difference appears between the separatrix and the wall, are generic modes of particular interest. Current-voltage characteristic curves and induced electric fields are discussed for each mode. Finally, this study indicates that biasing only one electrode with respect to both the vessel and the other strike points of the separatrix yields a hybrid behaviour resembling current injection when the electrode is negative and resembling plasma biasing when the electrode is positive

A large cross-section Mach-Zehnder interferometer is illuminated by a 30-nsec CO(2) laser pulse. The line-integrated density sensitivity is 2 x 10(19) m(-2). Spatial features down to 150 microm can be identified within a usable field of view of 50 mm. An example of a typical interference pattern obtained on a plasma column is shown together with its corresponding Abel deconvoluted density profile.

A retroreflection system is described that produced a 64-fold increase in the brightness of a N/sub 2/ laser. (AIP)

The properties of diffraction focusing systems composed of a series of irises, coaxially distributed along a laser beam, are analyzed. Based on diffracted-ray methods, complete design equations and optimization procedures are given. Focusing experiments on a He–Ne laser beam are reported with two types of focusing structures, and the results are found to agree completely with the theoretical predictions.

A novel technique that uses a single master oscillator to serve both the injection and the stabilization function in a TEA-CO(2) laser transmitter is described. The method, based on a two-polarization scheme, provides sufficient optical isolation between the TEA laser pulse and the probing laser beam to prevent anydamage or saturation of the controlling system. Reproducible and stable SLM operation in the MW power levels is achieved on a long term basis.

Preparation of the internal walls of tokamaks by plasma enhanced chemical vapour deposition (PECVD) of boron containing films has now been implemented on several machines since its development on TEXTOR. More recently, such films were deposited on the internal walls of TdeV using not only this procedure but also two new approaches: solid target boronization (STB) which consisted in inserting a low-density boronized carbon-carbon (C-C) composite into the tokamak plasma and TMB fuelling where trimethylboron was used as fuelling gas during the plasma discharge. These approaches resulted in a rapid shot to shot improvement of important parameters such as the volume averaged resistivity and radiated power over the first dozen shots when the boron source is present. Typically, the resistivity is reduced from ∼4.0×10 −7 to ∼2.5×10 −7 Ωm, comparable to the resistivity obtained with PECVD. The radiated power relative to the ohmic power is reduced by a factor of 2 from 20 to 10%. When the boron source, present during STB or TMB fuelling, is removed however, these plasma parameters start increasing. Within a few tens of shots, they have reverted to their preconditioning values, a situation which requires hundreds of shots after PECVD.

The screening effect of the divertor plasma against the backflow of gas into the plasma core is studied on TdeV, a double-null tokamak with biasable neutralizing plates. Accumulation of intrinsic impurities in the divertor chamber is observed, and negative biasing of the plates causes a strong enhancement of the effect. The divertor gas composition is correlated with the line radiation emitted in the vicinity of the plates and with the plasma impurity content. Using a simple particle balance model, it is concluded that H 2 flows unimpeded through, the divertor slit, but that the backflow of other gases is significantly weaker than predicted by the vacuum conductance, e.g. by a factor of ≥4 in the case of CO.

The application of a bias voltage to a neutralization plate of the upper divertor with respect to the vacuum chamber in the Tokamak de Varennes (TdeV) influences the plasma well inside the separatrix. In particular, the unbiased Ohmic poloidal rotation edge velocity measured by visible spectroscopy is found to be in the electron diamagnetic drift direction (2-3 km/s) and increases by a factor of two for Vbias = 100 V. This coincides with a major reduction of the microturbulence signal at low frequencies (50 kHz < f < 1 MHz) and for all wave numbers probed (20 cm-1 < k < 60 cm-1), as determined from coherent laser scattering measurements. One possible explanation is that the turbulence signal is simply Doppler shifted to frequencies outside the accessible range. This scenario is, however, difficult to reconcile with some observations. Another explanation invokes a reduction of the turbulence level. The variation of the turbulence signal as a function of the applied bias voltage can indeed be reproduced with a theoretical model based on radial and poloidal decorrelation mechanisms, the latter corresponding to poloidal velocity shear stabilization. This model also explains the steepening of the k-spectrum decay during biasing. Biasing also modifies the electron density profile inside the separatrix. These changes of ∇ne cannot explain the behaviour of microturbulence in terms of their effect on the free energy term driving the instability. On the contrary, the observed turbulence behavior, when explained in terms of stabilization, would agree with the plasma maintaining a steeper electron density gradient

Bias voltages applied to the divertor plates in TdeV (Tokamak de Varennes) permit fine control of several main plasma parameters, including the poloidal rotation velocity near the separatrix, microturbulence, the sawtooth period and the heat pulse propagation speed. Biasing also improves the divertor efficiency in either the top or bottom closed divertor chamber depending on polarity, in agreement with E × B drifts. Negative biasing reduces carbon and CO fluxes from the wall, the loop voltage and the X-ray emissivity, all indicative of decreasing impurity contamination.

Laser emission on the sequence bands of CO2 has been achieved in a TEA amplifier using either a hot-CO 2 cell or a Michelson interferometer to prevent oscillation of the regular bands. It has been found that the insertion losses of the hot-CO2 filter severely limit the energy extraction efficiency on the sequence bands. With the Michelson arrangement, output energies of 140 mJ on the sequence P(17) have been produced in a 33-cm-long TEA amplifier. The relatively low efficiencies obtained suggest that generation of high-intensity laser beams at the sequence band frequencies would require the use of oscillator-amplifier combinations.

VUV spectroscopy and mass spectrometry measurements imply that the concentration of neon — an artificially introduced impurity in the Tokamak de Varennes — decreases in the central plasma early in the discharge. The missing atoms are found in the volume between the plasma and the vessel walls. The measured transport coefficients of neon are similar to those of oxygen, and reduced penetration of recycled neon into the hot plasma by the screening effect of the scrape-off layer plasma is postulated to explain its depletion.

Fueling and gas evolution have been investigated in the Tokamak de Varennes using time-resolved mass spectrometric gas analysis during and after the shot, gas flow measurements, and standard plasma diagnostics. The evolution of the H and impurity content, both in the plasma and the neutral gas, is used to compare the recycling behaviour of H2 and CO. An accounting of the exhaust of H2 and other gases after the shot is given, together with a discussion of the outgassing mechanisms. The scaling of the fueling efficiency with density and current has been studied and shows a remarkable increase at high current due to improved particle confinement and possibly an increased recycling coefficient.

Experimental results obtained on the Tokamak de Varennes during a series of reproducible ohmic discharges are presented. Profiles of basic plasma parameters are constructed and compared with theoretical predictions. In particular, the measured plasma resistivity agrees with the neoclassical scaling rather than with Spitzer resistivity. The study of electron-density fluctuations indicates a linear dispersion relation with a propagation velocity of 3.0 × 104 cm s−1. Particle transport investigations are initiated, giving experimental diffusion and convection coefficients across the plasma radius for electrons and impurity ions.

Simulation results obtained with the axisymmetric Princeton Tokamak Simulation Code TSC are compared with experimental results for the time evolution of the Tokamak de Varennes plasma. The code correctly predicts experimental conditions; of special interest are the favourable comparisons of the simulation with experiments during fast current ramp-down (from 250 kA in 40 ms, three times faster than the L/R time).

A movable Langmuir/Mach probe apparatus has been successfully operated in the Tokamak de Varennes. Comprised of three probe elements, a tip probe acting as a standard Langmuir probe and two side probes acting as a Mach probe, this apparatus has been used to acquire simultaneous probe characteristics in the SOL of the tokamak. Densities and Mach numbers computed from four different probe models have been compared. The densities calculated from the various models are similar in magnitude and compare favorably with the extrapolated density from the interior of the plasma. However, when the same models are used to compute the Mach numbers, they vary from subsonic to supersonic for the same set of measurements. Preliminary results in a separate experiment which measures the plasma flow indicate that the Hutchinson model is in better agreement than the other models. Finally, the preliminary results of flow measurements in the TdeV indicate the presence of an appreciable poloidal velocity near the last closed flux surface.

Electron density fluctuations propagating poloidally in the Tokamak de Varennes are investigated using CO 2 laser scattering. During a series of reproducible ohmic discharges, a linear dispersion is observed for the large wave numbers probed ( kϱ i >3), indicating a propagation velocity of 3.0 × 10 4 cm/s.

A system consisting of magnetically shielded ion gauges and a differentially pumped quadrupole mass spectrometer is described. It provides time‐resolved measurements of the neutral gas fluxes during discharges in the Tokamak de Varennes. A response time of 15 ms for hydrogen and ≊50 ms for impurity gases was obtained by a judicious choice of distance (to avoid magnetic disturbances), conductance, and pumping speed. An interface between the spectrometer and the data acquisition system of the tokamak allows fast (ms) selection of the mass and real‐time (≊10 ms) adjustment of the gain to optimize the signal‐to‐background ratio. The usefulness of this diagnostic is illustrated by its use in three experiments: propagation of a plasma density perturbation following pulsed (5 ms) gas puffing, exchange of the discharge gas from H2 to He, and chemical impurity (CO, CO2 ) production during discharges at the density limit and qa≊2. The correlation of low impurity fluxes with absence of disruptions in these last shot...

Large current profile modifications (−0.4

Current drive experiments have been performed on TdeV over a wide range of parameters. It is found that the current drive efficiency {eta} does not scale with {ital N}{sub {parallel}} as expected from simple theory. Simulations with the ACCOME code suggest that the reduced efficiency at low {ital N}{sub {parallel}} values is mostly due to a large toroidal upshift in the LH wave spectrum. The loss of suprathermal electrons to the divertor plates was monitored during CD experiments. The power loss increases with decreasing {ital N}{sub {parallel}} values and can be reach up to 15{percent} of the injected rf power in low density plasmas. {copyright} {ital 1996 American Institute of Physics.}

In this review paper the effects of an electric field are addressed. The focus is on open field lines in a tokamak. Electrically biasing the divertor plates of TdeV with respect to the vacuum vessel sets up a radial electric field and a radial current between the separatrix and the wall. It has been demonstrated that the resulting E×B flow and the j×B force produce fundamental phenomena, affecting confinement on closed field lines, parameters of the SOL and the performance of a divertor.

We report the first results of nondisruptive, central fueling of a tokamak by the injection of an accelerated spheromak compact toroid (CT). Interferometry measurements indicate central fueling of the tokamak plasma on a fast time scale (0.5 ms), with more than 50% of the CT mass used for plasma fueling. The tokamak particle inventory increased by more than 30% without disruption.

The envelope and frequency characteristics of the output pulse of a high repetition rate hybrid TEA-CO 2 laser are presented. Both the intrapulse and interpulse laser frequency stabilities are experimentally determined at repetition rates up to 300 Hz. The recovery of the CW laser signal following the generation of the TEA laser pulse is analyzed theoretically and experimentally. Short term reproducibilities of ±2 MHz are observed at a pulse repetition rate of 300 Hz with initial chirp rates of about 1.5 MHz/μs. Improvements and limits on power and repetition rate are discussed.

Small‐signal gains as high as 1.5% cm−1 have been measured in a TEA‐CO2 amplifier on the 00°2‐ (1001, 0201)I,II sequence bands. The ratio of the gain coefficient of the sequence bands over the gain coefficient of the regular bands has been observed to be independent of either the CO2 or He proportion in the gas mixture. It has also been found to increase with the electrical energy dumped into the active medium and to reach a value of 1/3 at an excitation density of 140 J/l. Gain on the sequence bands has been found to be responsible for the anomalously high gain coefficient that is measured at low J values of the R branch of the regular 9.4‐μm transition in TEA amplifiers.

A CO2 oscillator–amplifier laser system generating short pulses of radiation in the gigawatt range is described. Emphasis is laid upon self‐oscillation problems associated with a high‐gain amplifier chain. The insertion of SF6, Freon, and C2H5OH cells between the amplifier modules is found efficient in suppressing any oscillation in the presence of a polyethylene target. The characteristics of the output radiation are given together with the reflection measurements of the target during the interaction with the laser pulse.

Experimental results on the operation of a Laflamme double‐discharge laser structure up to pressures of several atmospheres are presented. With an electrical energy input of 30 J/liter atm, a multimode pulse output of 1 J is obtained at 6 atm from a 1/3‐liter active volume, containing 7.5% CO2, 7.5% N2, and 85% He. It is found that a high level of energy deposition into the CO2 gas can be achieved, provided that the plasma E/p is optimized in both the main discharge and the preionizing discharge. On the basis of the understanding gained during this investigation, a ruggedized Laflamme structure capable of withstanding prolonged arcing conditions has been developed.

This paper reports on an extensive parametric investigation of a TEA CO2 laser operated in an unstable confocal resonator. The coupling mirror diameter and the cavity magnification, M, which give the better use of the laser medium for different applications, are experimentally determined. These results are compared to the diffractional and geometrical theories of unstable resonators. It is found that, in our experimental conditions, the energy density in the focal plane of a lens depends only weakly on the value of M while the cavities having a magnification smaller than 2 are the most efficient in extracting the energy out of the lasing medium.

We describe a pulsed CO2 laser scattering experiment set up to measure the ion temperature of a magnetically stabilized arc plasma (ne≂1021 m−3, Te≂5 eV). Sensitivity and spectral resolution considerations led us to use heterodyne detection with a separate local oscillator and a multichannel rf frequency analyzer. The forward scattering angle is chosen to be 5° in order to match the expected ion feature spectrum to the receiver bandwidth and also to eliminate spontaneous fluctuations from the small‐k larger‐amplitude signal. Experimental scattering spectra illustrate the apparatus sensitivity to discharge current and gas mixture. Ion‐acoustic resonances are resolved as well as the spectral signatures from various mixed ion species plasmas. The ion temperature appears to be determined to ±10% once Te and ne are known from other measurements. Applications to high temperature fusion plasmas are discussed.