Table of contents

A direct measurement of the impurity radial flux in the FTU plasma core has been obtained by observation of soft x-ray spectra from highly ionized charge states of intrinsic molybdenum. This measurement has been performed using the equation of continuity for Mo31+ and the radial density profiles of Mo32+, Mo31+ and Mo30+. The profiles of these ions have been obtained by inversion of the measured signal along six different lines of sight in a series of reproducible discharges. Other ions, such as Fe and Ni, which emit in the VUV range, are also monitored at different chords and confirm the picture given by the soft x-ray measurements. A collisional - radiative model developed at LLNL and JHU allowed the reconstruction, from the spectra, of the radial profiles for the three molybdenum ions. The resulting flux shows that the transport is negligible (within the experimental errors) in the central region r < 10 cm, but is very rapid in the intermediate region 10 < r < 20 cm, and is more than one order of magnitude greater than the neoclassical prediction. Defining an effective `radial drift velocity' , it is found that has its maximum at half radius and there its value is about 0.4% of the thermal velocity.

A neutral particle analyser of MeV energy range has been used to measure atomic hydrogen fluxes in ICRF and combined ICRF+NBI-heated JT-60U plasmas. Successful application of a pulse height analysis system to separate the particle signal from a noise produced by DD neutrons and -rays is demonstrated. The energy dependence of the neutron sensitivity and energy resolution of the NPA detectors are comparable with calibration data. Charge exchange on and ions is considered as the dominant process for neutralization of fast ICRF-driven protons. A simplified model based on analysis of the steady-state ion balance equation system is applied to estimate and target densities. Model calculations are used to interpret the observed difference for the relatively low (E < 0.4 MeV) and high (E > 0.6 MeV) energy parts of the atomic hydrogen spectrum. It is confirmed that the model calculations can well explain the dependence of the atomic fluxes on the relative toroidal angle between the injected beam and the analyser, and the energy dependence of the flux ratio between `active' (ICRF + NBI) and `passive' (ICRF alone) phases. The role of the charge-exchange target effect in the plasma density scan is also discussed.

Plasmas produced by heavy-ion beams as well as laser-driven indirect ICF target physics in a hohlraum-capsule system have attracted considerable attention recently. The study of the transport phenomena of these media, in their extreme conditions, still requires some important refinements. For that, a quantum collective approach is used to analyse the electron conductive opacity of strongly coupled plasma taking into account several non-ideality effects in addition to electron - electron (e - e) collisions, which make an important contribution.

The basic formalism rests upon suitable extension of the Boltzmann - Bloch quantum transport equation with a unified scheme treating both short- and long-range collisions. The e - e interactions are shown to contribute to the conductive opacity in the low coupling regime. The extensively used Lorentz gas approximation cannot be justified for inertially confined plasmas or for plasmas of astrophysical interests.

Results are presented under analytical forms with comparisons with existing theories.

Detection of non-Maxwellian electron energy distribution functions (EEDFs) by Langmuir probes in the presence of radiofrequency (RF) fluctuations in the plasma potential is possible if the probe is properly RF compensated. If the non-thermal electrons are created by wave acceleration, however, they usually are bunched at one phase of the RF. In this case, the usual method of forcing the probe to follow fluctuations in the floating potential will be inappropriate, since itself will be time-dependent in such a way as to ensure that the fast electron tail is not detected. The probe must be made to follow fluctuations in the space potential. Computations are given to show the effect of a RF on distributions with high-energy tails with and without the proper compensation. Singular EEDFs arising in the case of constant energy gain are also treated for the first time.

The effects of light impurities, such as helium, boron or carbon, on Alfvén wave dispersion characteristics are studied. It is shown that a small population of light impurities in a hydrogen plasma can strongly modify the dispersion of the global Alfvén waves and the Alfvén continuum. It is also shown that the global Alfvén wave enters into the Alfvén continuum. Under these conditions, it is possible to heat the plasma core efficiently by using an antenna with a narrow toroidal wavelength spectrum. The relationship between impurity concentration and the efficiency of Alfvén wave heating is demonstrated. This effect is explored via a series of numerical calculations of the heating conditions specifically for the Tokamak Chauffage Alfvén wave heating experiment in Brazil (TCA/BR) (1996 Nucl. Fusion 30 503).

Turbulent transport of a heliotron/torsatron is investigated near the radius where the pitch of the magnetic field line is minimum or maximum. The current diffusive interchange mode turbulence is analysed. The curvature of the pitch of the magnetic field line dictates the level of the turbulence and the turbulent transport. The enhanced turbulence and the suppression of the pressure gradient are shown to occur in the vicinity of the radius where the magnetic shear vanishes. The dependence of the nonlinear decorrelation rate of the turbulence on the pressure gradient is found to be independent of the magnitude of the shear, ( being the normalized pressure gradient multiplied by the gradient of the magnetic field).

A simple zero-dimensional computer code has been prepared for predicting the neutron emission in the initial phase of beam-heated hot-ion H-mode discharges in the Joint European Torus (JET). It performs very well for discharges with low particle recycling up to the onset of MHD-related activity, which usually occurs within one second of commencing full power heating. It can also be used to model the entire discharge, including the termination, when fuelling of the plasma volume uniformly with cold neutrals is adopted as a first-order approximation for edge recycling effects. Specifically, given the time traces for the beam heating power, the global neutron emission, average electron density, and signals, the code can satisfactorily reconstruct the core volume rotation rate, ion temperature and electron temperature for the entire duration of the discharge, and it also provides an explanation for the anomalously low burnup of d - d fusion product tritons that is sometimes observed. The code provides insight into the relative intensities of the three contributing reaction mechanisms to the total neutron emission (thermal, beam - thermal and beam - beam reactions) as an aid to interpretation of data from the various neutron diagnostics on JET, including the neutron profile monitor and the neutron spectrometers. Tritium fuelling and beam injection are also implemented, so the code can be used to extrapolate the neutron emission from existing d - d discharges to that expected from similar d - t discharges.

The properties of protons under acceleration by an ion cyclotron range of frequency (ICRF) waves with the second to fourth hydrogen harmonics have been investigated in the JT-60U tokamak at the Japan Atomic Energy Research Institute (JAERI). Protons have been accelerated up to 1 MeV in the presence of an ICRF wave of fixed frequency, neutral beams (NB), and a fixed toroidal magnetic field which is scanned through several plasma discharges. The tail temperature of the protons, which is evaluated in the range 0.32 - 0.86 MeV, has been observed to increase in the second to third harmonics, however increase of the tail temperature in the third to fourth harmonics has not been observed clearly. Furthermore, the dependence of tail temperature on the harmonic number has been found to be in qualitative agreement with results from a simulation code analysis based upon the one-dimensional Fokker - Planck equation coupled with the kinetic wave equation. Experimental values for the stored energy of the accelerated ions have shown, however, that the response of stored energy to changes in absorbed ICRF power is much stronger than the response to changes in harmonic number. Also, the incremental energy confinement times for heating discharges matching the third and fourth harmonics ( and ) of hydrogen have been observed to be less than half that for those matching the second harmonic. It has been found that suppression of the absorbed ICRF power accompanied with the occurrence of cavity resonance in the and heating discharges reduces the stored energy of the accelerated ions and the incremental energy confinement time.

High-Z materials as tungsten are intended to be used in future fusion reactors due to their low sputtering rates and high melting points. In this context the important question is whether the use of high-Z materials is compatible with the concept of a cold radiative boundary. To investigate the local release and transport behaviour of the high-Z impurities, Mo and W test limiters were used in auxiliary heated discharges under different radiation scenarios with neon seeding. In addition, laser blow-off of tungsten as well as xenon gas puffing were performed. In some particular discharge conditions impurity accumulation was observed in the plasma core, which in the case of ohmic discharges led to minor disruptions. The connection between the accumulation and the evolution of the current density profiles is discussed. A transport analysis is made, in order to compare the observations with the neoclassical theory. It is shown that with neon the impurity - impurity driven fluxes enhance the high-Z concentration in the plasma core. However, if the source of the high-Z elements is significantly reduced, by increasing the plasma density, a development of an accumulation instability can be avoided.

A transcollisional, electromagnetic fluid model, incorporating the parallel heat flux as a dependent variable, is constructed to treat electron drift turbulence in the regime of tokamak edge plasmas at the L - H transition. The resulting turbulence is very sensitive to the plasma beta throughout this regime, with the scaling with rising beta produced by the effect of magnetic induction to slow the Alfvénic parallel electron dynamics and thereby leave the turbulence in a more robust, non-adiabatic state. Magnetic flutter and curvature have a minor qualitative effect on the turbulence mode structure and on the beta scaling, even when their quantitative effect is strong. Transport by magnetic flutter is small compared to that by the flow eddies. Fluctuation statistics show that while the turbulence shows no coherent structure, it is coupled strongly enough so that neither density nor temperature fluctuations behave as passive scalars. Both profile gradients drive the turbulence, with the total thermal energy transport varying only weakly with the gradient ratio, . Scaling with magnetic shear is pronounced, with stronger shear leading to lower drive levels. Scaling with either collision frequency or magnetic curvature is weak, consistent with their weak qualitative effect. The result is that electron drift turbulence at L - H transition edge parameters is drift Alfvén turbulence, with both ballooning and resistivity in a clear secondary role. The contents of the drift Alfvén model will form a significant part of any useful first-principles computation of tokamak edge turbulence.

We report production of hydrogen plasma using a 2.45 GHz microwave source in a toroidal device BETA (Bora D 1989 Phys. Lett. 139A 308). The equilibrium density, floating potential and plasma temperature in the poloidal cross section have been measured. Contours of equal density and floating potential exhibit a slab nature in the vertical direction of the poloidal cross section. Plasma is formed with a peak density of . The electron temperature is about 4 - 7 eV. Our measurements reveal that the X-mode wave is mode converted to electrostatic modes near upper hybrid resonance (UHR), which propagates close to the electron cyclotron resonance (ECR) region where the entire energy of the waves is deposited. Thus we observe plasma profiles broadened towards the weaker magnetic field region up to the UHR region, with an inner boundary near ECR region. This is further supported by numerical calculation. The fluctuating component of the density and floating potential is also measured. Low-frequency (LF) electrostatic instabilities are observed to be radially localized. The presence of both flute and drift modes is reported. Our measurements reveal that the excitation of LF fluctuations depends strongly on the sign of the product where and are the radial density and magnetic field gradients, respectively. LF coherent fluctuating modes are observed in the region where and a turbulent spectrum is observed where the condition is satisfied. Our measurements reveal that collisional drift modes could be excited when (where ), whereas Rayleigh - Taylor modes could be excited when and the density gradient is antiparallel to the effective gravity `g'.

The transport processes in edge (collisional) plasmas of tokamaks with smooth profiles of macroscopic plasma parameters and induced poloidal and toroidal plasma flows are considered. The toroidal and poloidal velocities of particles, the radial electric field and the ion heat flux are derived. It is shown that forces, induced by radio frequency waves, plasma turbulence or neutral beam injection, can be used to control the poloidal and toroidal plasma velocities, as well as ion heat conductivity, in a wide range of these values.

10 - 14 November 1997 Topical Workshop on Plasma Physics: New Perspectives of Collective Effects Miramare-Trieste, Italy Contact: P K Shukla, International Centre for Theoretical Physics, PO Box 586, I-34100 Trieste, Italy. Tel +39-40-2240357, Fax +39-40-224163, E-mail smr1014@ictp.trieste.it. Application deadline: 31 March 1997.

22 May - 1 June 1998 Advanced Technologies based on Wave and Beam Generated Plasmas Sozopol, Bulgaria Contact: Professor Dr H Schlüter, Ruhr-Universität Bochum, Inst. f. Experimentalphysik II, NB 5/67, 44780 Bochum, Germany. Tel +49-234-7003670, Fax +49-234-7094643, E-mail hs@plasma.ep2.ruhr-uni-bochum.de. Online information and application at http://hera.ep2.ruhr-uni-bochum.de/ASI. Application deadline: 15 November 1997.