Table of contents

Recent experimental results on the DIII-D tokamak have demonstrated that with off-axis electron-cyclotron heating (ECH) there is a large inward electron heat flux up the temperature gradient. We propose that owing to the finite drift parameter for the toroidal current, enhanced by the presence of trapped electrons, the isotropic component of the distribution function is driven non-Maxwellian. The enhanced number of energetic electrons leads to an extra and nonlinear inward heat flux in quantitative agreement with experiment. Furthermore, this model permits an understanding of profile consistency or resilience in tokamaks.

Reflectometry applied to the measurement of density profiles on fusion plasmas has been subject to many recent developments. After a brief reminder of the principles of reflectometry, the theoretical accuracy of reflectometry measurements is discussed. The main difficulties limiting the performance, namely the plasma fluctuations and the quality of the transmission lines, are analysed. The different techniques used for reflectometry are then presented grouped into three different categories, depending on the frequency spectrum of the probing wave: single frequency, few discrete frequencies, or broad spectrum. The present status and achievements of actual implementations of these techniques are demonstrated, with an analysis of their respective limitations and merits, as well as foreseen developments. Finally, a discussion of the various reflectometry techniques is made, in particular their ability to cope with plasma fluctuations and complex transmission lines, in view of the application to next step machines and very severe environments.

The Heliotron E device is characterized by a large aspect ratio, , large rotational transform, and , high shear, , an and M = 19 helical coil, two vertical field coils and 19 toroidal coils (19 toroidal coils were removed to increase neutral beam injection (NBI) heating power up to 4 MW in 1985), where is the rotational transform, is the average minor radius, R is the major radius, is the pole number and M is the helical period number of the magnetic field. Theoretical characteristics of Heliotron E are summarized in section 2. Initial experiments in 1980 - 3 were mainly for studying ohmically heated plasmas. Topics relating to current-carrying plasmas are reviewed in section 3. In Heliotron E the first currentless plasma was produced by ECRH using a gyrotron with a frequency of 28 GHz in 1982. Then the gyrotron frequency was increased to 53 GHz to produce higher density currentless plasmas. After NBI heating of the target plasma produced by ECRH was successful, currentless plasmas were studied intensively in Heliotron E. In section 4, the production and heating of currentless plasmas by ECRH, NBI and ion cyclotron range of frequency (ICRF) are summarized. MHD properties of currentless Heliotron E plasmas are summarized in section 5. Confinement and transport are discussed for ECRH, NBI and ICRF plasmas in section 6. Characteristics of edge plasma and plasma - wall interaction in Heliotron E are summarized in section 7. Configuration improvement and future directions based on Heliotron E results are discussed in section 8.

The radiation and transport characteristics of ASDEX Upgrade discharges with a neon-driven radiative mantle are modelled using a one-dimensional radial impurity transport code that has been coupled to a simple divertor model describing particle recycling and pumping. The code is well suited to describing the measured impurity line radiation, total, soft x-ray and bremsstrahlung radiation in regions of the plasma that are not dominated by two-dimensional effects. The recycling and pumping behaviour of neon as well as the bulk transport of neon for radiative boundary scenarios are discussed.

A new technique for the analysis and interpretation of reversed-field pinch discharges is presented, using the fast electron population in the plasma core as an indicator of the level of magnetic turbulence. The method combines x-ray measurements of the superthermal electron distribution in the plasma core with a self-consistent three-dimensional Fokker - Planck code. This technique is illustrated by an application to low-density discharges of the MST device, using measured soft x-ray spectra to constrain the simultaneous computation of the electron distribution function and of the magnetic fields. The consistency of the experimental results, analysed in this way with both kinetic and MHD dynamo theories is discussed. The best agreement between the code and the measured x-ray spectrum occurs when an MHD dynamo field of the appropriate level and shape is included with radial diffusion due to fluctuating fields %.

Characteristics of the internal transport barrier (ITB) were studied. The region of steep and , i.e. the ITB front, propagated from the core outwards. The thickness of the ITB front was about 3 cm. The ITB worked as a particle transport barrier as well as a thermal transport barrier for ions. The threshold heating power for ITB formation strongly increased with electron density and was independent of the toroidal magnetic field. ITB with was sustained for twice the global energy confinement time . A repetitive relaxation phenomenon at ITB was observed, which induced spikes like ELMs but had a different poloidal distribution.

Core electron temperature measurements have been performed in the large, high-current RFX reversed field pinch experiment (a = 0.46 m, R = 2 m, target plasma current = 2 MA) with the double filter technique. With this technique the soft x-ray emission is measured by two Si detectors, covered by different Be-filters, and viewing the same region of the plasma. By choosing the filters in such a way that only the emission from the continuum part of the spectrum is measured, the ratio of the two signals is a function of the highest electron temperature along the line of sight of the detectors.

With this diagnostic, the electron temperature has been measured in RFX with a bandwidth of approximately 2 kHz. This allows the direct measurement, for the first time in a RFP on a single shot basis, of coherent oscillations, associated with relaxation events and showing under some conditions a sawtooth-like character.

The measurements of electron temperature obtained with this method are in good agreement with those performed with other diagnostics.

The electron density profile has been analysed using the maximum entropy method (MEM) in signal processing of the X-mode microwave reflectometry carried out in the GAMMA 10 tandem mirror. Although the detected reflectometry signals included the noise components due to density fluctuations in addition to signal components, the signal components were spectrally separated from the noise components using MEM. Reconstructed density profiles are consistent with that obtained from the millimetre wave interferometry measurements assuming a parabolic profile. The present MEM analysis will also be applicable to microwave reflectometry experiments on tokamaks or stellarators where signal levels were significantly affected by plasma movement or density perturbations.

Colorimetry was introduced into the DIII-D tokamak to measure in situ the growth and erosion of transparent wall coatings (a-C:H) on the divertor. The colorimetric measurement system consisting of a halogen light source, a set of three filters and a black/white camera is described together with a first erosion measurement. An insertable graphite sample with a diameter of 4.7 cm was precoated with a 300 nm thick amorphous carbon film and was exposed in the divertor for several discharges with its surface coplanar to the surrounding graphite tiles. For each of the discharges the plasma strike point was moved onto the sample for 1 s to erode the coating. Between the discharges a camera signal with each filter was recorded and the film thickness was evaluated along a radial line across the DIMES sample. Thus it has been possible for the first time to measure erosion and deposition of divertor material in situ and shot-by-shot. The average peak heat flux with the strike point on DIMES was about 110 W . The measurement shows a strong decrease in the film thickness almost over the entire sample with an average erosion rate of .

This paper describes the poloidal and toroidal spin-up of an isothermal plasma in toroidal equilibria. The mechanism is the Stringer spin-up mechanism which will be generalized to an arbitrary toroidal equilibrium using the Hamada coordinate system. Viscous damping or magnetic pumping balances the accelerating forces and determines the threshold of spin-up. The accelerating forces arise from the Coriolis forces which couple the radial flow to the poloidal flow velocity. The theory presented here is a continuation of a previous paper and applies to a multi-species plasma with impurities. In the two-fluid model - or including impurities - in a multi-fluid model, the flux-friction relations derived on every magnetic surface are the basic equations for computing the poloidal and toroidal flow velocities of the particle species. Furthermore, the effect of turbulent forces on the mean flow is analysed. The Reynolds stresses resulting from anisotropic turbulence provide a force in the tangential direction and thus contribute to the poloidal and toroidal spin-up. Furthermore, turbulence increases the Stringer spin-up mechanism and introduces enhanced plasma losses. The final result is a set of differential equations describing the poloidal and toroidal flow on every magnetic surface. The formalism is valid in every regime of collisionality; however, to obtain specific results, appropriate approximations must be found for every regime. This will be outlined for the collisional regime where the viscous forces are given by Braginskii's equations. The relation between plasma rotation and the zonal circulation in planetary atmospheres will be discussed.

A mathematical error invalidates the main conclusion of a paper by DiVita and Brusati.

Lazzaro and Minardi have claimed that a mathematical error in an equation invalidates the main result of an earlier paper. It is now shown that the equation is physically justified and mathematically correct.

An error in the journal production process led to figures 6 and 7 being incorrectly printed. The correct versions are given below.

Figure 6. Power deposition profiles from high-frequency power modulation experiments (points) (Hartfuss H J 1994 Plasma Phys. Control. Fusion 36 B17) and a maximum entropy analysis of the ordinary heat pulses (dotted) compared with the profile required in order to describe the power-stepping experiments with a continuous model for × (dashed).

Figure 7. Illustration of a heat pulse as produced by a non-locally reacting ×.

8 - 18 August 1996 International Summer School on Plasma Physics and Technology La Jolla, CA, USA Contact: Mr V Stefan, Institute for Advanced Physics Studies, PO Box 2964, La Jolla, CA 92038, USA. Tel +1-619-456-5737.

26 - 30 August 1996 Joint Varenna - Lausanne International Workshop on Theory of Fusion Plasmas Villa Monastero, Varenna, Italy Contact: Centro di Cultura Villa Monastero, 1 Piazza Venini, 22050 Varenna (Lecco), Italy. Tel +39-341-831261, Fax +39-341-831281. Application and abstract deadline: 15 June 1996.

2 - 5 September 1996 EU - US Workshop on Transport in Fusion Plasmas Villa Monastero, Varenna, Italy Further information: G Gorini, ISPP, 16 Via Celoria, I-20133 Milano, Italy. Tel +39-2-2392637, Fax +39-2-2392205, E-mail ggorini@mi.infn.it.

Administrative contact: Centro di Cultura Villa Monastero, 1 Piazza Venini, 22050 Varenna (Lecco), Italy. Tel +39-341-831261, Fax +39-341-831281. Application and abstract deadline: 15 June 1996.

9 - 13 September 1996 International Conference on Plasma Physics Nagoya, Japan Contact: Conference Secretariat, c/o Prof. Hiromu Momota, National Institute for Fusion Science, Nagoya 464-01, Japan. Tel +81-52-789-4260, Fax +81-52-789-1037, E-mail icpp96@nifs.ac.jp. Abstract deadline: 31 March 1996.

16 - 20 September 1996 19th Symposium on Fusion Technology Lisbon, Portugal Contact: Professor Carlos Varandas, Centro de Fusão Nuclear, 1096 Lisboa Codex, Portugal. Fax +351-1-8417819, E-mail cvarandas@cfn.ist.utl.pt. General information will be available via WWW with URL http://www.cfn.ist.utl.pt.

25 - 29 September 1996 Summer University of Plasma Physics Garching, Germany Contact: Ms Ch Stahlberg, Max-Planck-Institut für PlasmaPhysik, Boltzmannstr 2, D-85748 Garching, Germany. Tel +49-89-3299-2232, Fax +49-89-3299-1001.

2 - 4 October 1996 6th European Conference on Fusion Theory Utrecht, The Netherlands Contact: T J Schep, FOM - Institute for Plasma Physics, Postbus 1207, NL-3430 BE Nieuwegein, Netherlands. Tel +31-3402-312-24.

11 - 15 November 1996 38th Annual Meeting of the Division of Plasma Physics, APS Denver, CO, USA Contact: Dr Richard Hazeltine, University of Texas, Institute for Fusion Studies, RLM 11.314, Austin, TX 78712. Tel +1-512-471-1322, E-mail stewart@hagar.ph.utexas.edu.

17 - 18 February 1997 Plasma '97: 21st Australian Institute of Nuclear Science and Engineering Plasma Science and Technology Conference Sydney, Australia Contact: Margaret Lanigan, Conference Manager, PMB 1, MENAI NSW 2234, Australia. Fax +61-(0)2-439-6561, E-mail ainse@ansto.gov.au.

6 - 11 April 1997 10th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating Ameland, The Netherlands Contact: J Hamers-Smit, FOM - Instituut voor Plasmafysica 'Rijnhuizen', Postbus 1207, 3430 BE Nieuwegein. Tel +31-30-6096999, Fax +31-30-6031204, E-mail ec10@rijnh.nl. Application and abstract deadline: 17 January 1997.

8 - 12 September 1997 12th International Conference on Gas Discharges and their Applications Greifswald, Germany Contact: Dr G Babucke, Inst. f. Niedertemperatur-Plasmaphysik, Robert-Blum-Str. 8 - 10, 17489 Greifswald, Germany. Tel +49-3834-554411, Fax +49-3834-554301, E-mail gd97@public.inp.uni-greifswald.de.