Table of contents

Models for studying radio frequency heating in tokamaks are either based on the trajectory integral along the unperturbed particle orbit or on the Hamiltonian approach. Although they describe the same physics and should thus be equivalent, this equivalence is not immediate. In this letter the link between them is established and generalization to other magnetic geometries is discussed.

Ion cyclotron resonance heating (ICRH) of JET high-performance (optimized shear and hot-ion H-mode) deuterium discharges, characterized by strong neutral beam injection (NBI), high ion and electron temperatures and high deuterium-deuterium fusion reactivity, is analysed using numerical simulations. The main aim is to investigate under which conditions good bulk ion heating can be obtained with ICRH tuned to the fundamental hydrogen resonance, coinciding with the second-harmonic deuterium resonance, near the plasma centre. The analysis has been facilitated by adding NBI source terms in the ion cyclotron range of frequencies (ICRF) code PION, which calculates the time evolution of the ICRF power deposition and the resonating ions distribution functions in a self-consistent way. The results show that damping ICRF power on deuterons results in a more central bulk ion heating profile than minority hydrogen ions, which is an advantage. The highest bulk ion heating fractions of up to 50% of the total ICRF power were obtained by using multiple frequencies, the effects of which have been simulated in detail for the first time. Furthermore, operating with high plasma densities has been found to be beneficial for maximizing the bulk ion heating.

We present evidence for high harmonic fast wave (HHFW) heating in the core of the CDX-U spherical torus (aspect ratio R/a1.5) at the Princeton Plasma Physics Laboratory. We use a spectroscopic technique combining fast, multichordal measurements of the ultrasoft x-ray line emission from intrinsic C and O. The results show that in a few cases substantial (nearly 50%) increases in the core electron temperature can be obtained with only about 100 kW of radio-frequency power, compared to 150 kW ohmic input. Modifications of the core magnetohydrodynamic activity consistent with the estimated T_e increase are also observed. The majority of the discharges exhibit only modest core T_e increases however, with most of the heating occurring outside r/a0.5. A proposed explanation for this effect consistent with the experimental observations is based on a neoclassical transport instability for low-Z impurities, similar to an effect recently described for high-Z impurities in large tokamaks.

The phenomenon of striation observed during pellet ablation in hot magnetized plasma is investigated experimentally injecting aluminum micro-pellets into MT-1M and TEXTOR-94 tokamak and W7-AS stellarator plasmas. Changes in pellet cloud shape and temporal fluctuations of total emitted line radiations are studied. Special attention is paid to the investigation of the dependence of fluctuation frequency on pellet velocity. The results show that the light fluctuation frequency is characterized by a characteristic time rather than by a characteristic length, and that the striation is caused by a phenomenon that occurs during the evolution of the ionized-pellet cloud. The compatibility of these experimental observations with different theories of striation is discussed.

The interferometer version of the enhanced scattering diagnostic (IES) is theoretically analysed and applied to diagnose lower hybrid test waves, revealing its ability in principle to provide the wavenumber spectrum of coherent small-scale waves or fluctuations in a magnetized plasma. Both the spectrum as well as the spatial structure of density perturbations obtained from these measurements are in reasonable agreement with the results of radio frequency (RF) probe measurements if one takes into account the different sensitivities of the two methods with respect to the wavenumber.

ELMy H-modes are currently a promising scenario envisaged for the operation of a future fusion reactor. The reliable extrapolation of the present experimental data to a reactor requires a model which is capable of explaining the variety of experimental phenomena observed in high-density H-modes. This paper attempts to construct a model based on the assumption that the behaviour of high-density ELMy H-modes can be explained through the similarity of edge transport mechanisms.

We have identified three dimensionless parameters as the most representative for the high-density H-mode operation: (a) F = q²R /f(s) representing the ideal ballooning limit, (b) collisionality ^*_e = Z_effnqR/T²_e which is postulated to be responsible for the transition from type I to type III edge localized modes (ELMs) and (c) the L-H transition boundary represented by F_L-H = T³_e/(B²LZ_eff/(m_i)^1/2). Fixing any two out of these three parameters allows one to find scalings for the main operational points in the edge n_e-T_e diagram and reproduce the Greenwald/Hugill dependences: _e~B/qR for density limits.

More detailed scalings for the type I to type III ELM transition point, which may be of particular interest for a reactor, show that the critical separatrix density should scale as n_e~B/qR, where 1, >1, <1 (but being close to unity) and q is assumed to be the safety factor at 95% of the flux, q₉₅. Good agreement is found between experimental results on JET for the density at the top of the pedestal and the scaling _e~B/R^3/4q^5/4 for the critical separatrix density at the transition, in the conditions where the two densities are expected to be proportional to each other.

2-6 August 1999 14th International Symposium on Plasma ChemistryPrague, Czech Republic Contact: V Sember. E-mail: sember@ipp.cas.cz, Website: http://www.ipp.cas.cz/NTP/ispc-14/index.html

26-28 August 1999 International Workshop on Plasma Diagnostics http://ovpserv4.rug.ac.be/AppliedPhysics/Workshop/Belgium

6-10 September 1999 International Topical Conference on Plasma Physics: New Frontiers of Nonlinear SciencesFaro, Portugal Contact: Mr R Guerra, Department of Physics, University of Algarve, Faro, Portugal. E-mail: rguerra@mozart.si.ualg.pt. Registration deadline: 30 November 1998, abstract deadline: 15 February 1999.

12-17 September 1999 First International Conference On Inertial Fusion Sciences And ApplicationsUniversity Bordeaux 1, France Contact: William J Hogan, e-mail: IFSA99@llnl.gov

27-29 October 1999 8th European Fusion Theory ConferenceVilla Olmo, Como Contact: M Lontano (Chairman), Tel : ++39 02 66173 260; D Farina, Tel : ++39 02 66173 232, C Allocchio (Administrative Secretary), e-mail: fusion99@ifp.mi.cnr.it, fax: ++39 02 66173 239

15-19 November 1999 41th APS Division of Plasma Physics Annual MeetingSeattle, Washington e-mail: meetings@aps.org, URL: http://www.aps.org/meet/index.html