Table of contents

Selective electron cyclotron emission and absorption by fast electron tails are discussed by using the concepts of momentum space spectra and collective resonance velocity. This is defined from the maximum of the emission and absorption profiles in the momentum space for a system of electrons and represents the counterpart of the one-particle resonance velocity. It is shown that the collective resonance is sensitive to the superthermal momentum distribution and is, therefore, a useful parameter in the diagnosis of the fast electron tail. A simple relation between the collective resonance and the emission and absorption coefficients for propagation normal to the magnetic field is found. This theoretical framework is used for a new diagnostic method based on the localized (in phase space) perturbation of the electron tail by an electron cyclotron wave and emission and transmission of radiation at higher harmonics. A numerical application is presented for the case of a lower hybrid sustained tail, using the appropriate Fokker-Planck and ray tracing codes.

A quiescent plasma is sustained for 80 energy confinement times by only gas fuelling and neutral beam heating in an axisymmetric region of the Tandem Mirror Experiment Upgrade (TMX-U). This plasma should be unstable because of the bad magnetic curvature and the absence of ion cyclotron heating which previously provided ponderomotive stabilization to sustain plasmas in bad-curvature regions of other axisymmetric mirror experiments. The TMX-U data are consistent with stabilization by a symbiosis between two mechanisms – line tying, which reduces the growth rate, and finite Larmor radius edge stabilization, which can result in quiescent operation.

The absorption characteristics of fast waves excited by a phased four-loop antenna array have been investigated experimentally in JFT-2M. The frequency of the fast waves is 200 MHz, which corresponds to approximately the tenth harmonic of the ion cyclotron frequency of hydrogen. The fast wave power is absorbed mainly by the bulk thermal electrons. It is shown that the absorption efficiency of the excited fast waves is improved with increasing density and temperature, and with decreasing phase velocity of the fast wave. The results are consistent with the theoretical predictions obtained from ray-tracing calculations. The power deposition profile is obtained through synchronous detection of the electron cyclotron emission modulated by a periodic heat source. In this modulation experiment with a limiter plasma on JFT-2M the electron thermal diffusivity is 2-3 m²·s⁻¹ and the convection velocity is 20-40 m·s⁻¹ at _e = 2 × 10¹⁹ m⁻³ and I_p = 230 kA. The resultant power deposition profile has a peak at the plasma centre and agrees well with that calculated with the ray-tracing code. The absorption efficiency calculated from the power deposition profile is 0.3-0.4 at _e = 2 × 10¹⁹ m⁻³ and T_e0 = 0.7 keV, which agrees roughly with that estimated from the initial rise of the plasma stored energy. The electron heating efficiency estimated from the absorption efficiency is (4–5) × 10¹⁹ eV·m⁻³·kW⁻¹ and the incremental confinement time is 8-10 ms, which is slightly longer than that in L-mode plasmas heated by neutral beam injection and/or ion cyclotron heating in JFT-2M.

When a plasma interacts with a material surface subject to an applied voltage, a sheath results. A one-dimensional model, specific to the magnetized ion sheath, is developed and applied to the radiofrequency (RF) sheath which forms near the Faraday screen of an ion cyclotron heating antenna. The RF sheath rectification of the applied voltage is shown to provide a large DC potential drop which can accelerate tons and cause sputtering. Numerical estimates of the high-Z impurity influx are compared with fast wave experiments, and it is concluded that the DC acceleration mechanism is a plausible explanation for the observed high-Z impurity release. Means of controlling the sputtering are examined, including operating with low densities at the Faraday screen.

During runaway discharges in TEXTOR, intense infrared (IR) radiation is-emitted in the electron flow direction. This can only be explained by synchrotron radiation of fast electrons. The observed spectral dependence is consistent with electrons of 25-30 MeV energy; the intensity corresponds to about 10¹⁶ electrons or to an electrical current of 40 kA. From the spatial structure of the observed IR pattern, new insight into the spatial distribution of the runaway electrons and their perpendicular momentum can be gained. The runaway electrons populate a torus with a diameter of 0.5-0.6 m, which is slightly larger than the plasma radius; the perpendicular momentum is determined from the vertical extent of the IR pattern and amounts to about 5 m₀c. The transformation rate of electrons to runaways can be estimated from the time delay of the IR signal as 2 × 10⁻⁴ s⁻¹; this agrees with theoretical expectations derived from the ratio of the electrical field strength to the critical field strength. In TEXTOR, runaways are confined up to energies of 50 MeV, which is just below the limit where a phase should exist in which runaways radiate as much energy as they gain per turn.

The confinement and scaling features of the S-1 spheromak have been investigated using magnetic, spectroscopic and Thomson scattering data in conjunction with numerical modelling. Results from the multi-point Thomson scattering diagnostic show that the central beta remains constant (β_t0 ∼ 5%) as the plasma current density increases from 0.68 to 2.1 MA·m⁻². The density is observed to increase slowly over this range, while the central electron temperature increases much more rapidly. Analysis of the global plasma parameters shows a decrease in the volume average beta and the energy confinement as the total current is increased. The power balance has been modelled numerically with a zero-dimensional non-equilibrium time dependent coronal model and is consistent with the experimental observations. In addition to energy losses from radiation, the model includes convection and conduction. The conduction includes magnetic field fluctuation induced energy loss.

An experimental investigation of metallic impurities has been made in the ASDEX divertor in order to better understand their likely sources and production mechanisms. The impurities were collected on rotatable carbon collector strips which were exposed to the scrape-off layer plasma near the upper outer divertor plate under various discharge conditions. Quantitative ion beam analysis of exposed samples was subsequently carried out by Rutherford backscattering spectroscopy, proton induced X-ray emission and nuclear reaction spectroscopy. Time resolved measurements and spatial profiles of collected impurity fluxes reveal an increase in collected metallic impurities during auxiliary neutral beam heating of plasma discharges causing strong erosion of the copper divertor plate. To a rough approximation, 0.2% of this auxiliary heating power is invested in the production of stainless steel impurities. Measurements made for single-null divertor discharges show a copper flux of 6 × 10¹⁷ atoms·s⁻¹ in the scrape-off layer which is not directly sputtered from the target plates. Collected impurity fluxes in the divertor chamber are found to increase with both the main plasma current and the density. Measurements made after boronization reveal an initial reduction in collected stainless steel impurities although boronization does not have any long term effect on the suppression of such impurities. Some limitations of the collector probe technique are also pointed out, and planned improvements of the existing setup are outlined.

A fusion ignition experiment (IGNITEX) is described, the original concept for which was proposed by Rosenbluth, Weldon and Woodson. In this concept, a single-turn-coil tokamak device produces a self-sustained fusion reaction of the type T (d, n) α. The basic idea is to employ a very high magnetic field and a very high plasma current to heat the plasma ohmically to thermonuclear temperatures and then to produce a stable ignited plasma. The experiment will permit the scientific study of a new regime of physics: alpha heated ignited plasmas.

In the Texas Experimental Tokamak, a discharge regime characterized by persistent peaked profiles was observed to be induced by pellet fuelling, and its transport properties were studied. The hallmark of the regime is the suppression of sawteeth, and the regime was attained by injecting hydrogen pellets to promptly double the plasma density. In each of several pairs of experiments, a pellet fuelled discharge was compared with an edge fuelled discharge with similar averaged electron density, plasma current and toroidal magnetic field in order to characterize the transport change and to look for causal changes in the plasma turbulence. The impurity, radiation and working gas particle profiles were more peaked for the pellet fuelled case. The values of rjt derived from measured ion temperature and density profiles for high density edge fuelled and pellet fuelled discharges indicate that ion pressure gradient driven turbulence should be reduced in the pellet fuelled case. The macroscopic effects were accompanied by microscopic changes. Measurements of turbulent density fluctuations in high density edge fuelled discharges give strong evidence that a component of the turbulence propagates in the ion diamagnetic direction and that this particular mode is reduced in pellet fuelled discharges. The effects of the reduction of an ion mode turbulence were sought in the energy confinement of the discharges, but it was found that for these experiments (tailored for the turbulence diagnostics) the energy flowing in the ion channel was not large enough to affect the energy confinement. Simulations were used to interpret some of the results. Discharge simulations which include the pellet injection can reproduce the sawtooth suppression. This result and known properties of discharges in which sawteeth are suppressed suggest that some of the profile effects (including peaking of the working gas particles) induced by pellet injection are due to sawtooth suppression. The particle peaking may cause the observed reduction in the turbulence which follows pellet injection.

The evaporation of frozen deuterium pellets injected into Alcator C plasmas is examined using a specialized time dependent plasma hydrodynamic code which includes flux limited thermal conduction and neglects magnetic field effects. Ablatant characteristics (density, temperature, ionization) are temporally and spatially resolved and are in good agreement with experimental measurements and with values predicted by other models. The hydrogen Balmer-alpha (H_α line radiation measured experimentally from the ablating pellet is quantitatively reproduced. A thin neutral region of millimetre radius and a partially ionized region of centimetre radius are calculated to evolve. The neutral region does not play a dominant role in determining the pellet lifetime. Flux limited thermal conduction in the cold, dense ablatant naturally 'shields' the pellet from the hot plasma, relatively independently of the degree of ionization. For the flux limit parameter an estimate of f ≈ 0.2 is obtained, which is close to the 'expected' value of 0.32. Finally, it is found that the time dependence of the H_α signal is not directly proportional to the mass ablation rate, but is more nearly proportional to the thermal flux from the tokamak plasma to the ablatant.

The loading resistance ΔR of a two-ion species tokamak plasma was measured for fast wave heating with poloidal phasing. The dependence of ΔR on the relative antenna phasing is the same as that of the power absorption calculated by a two-dimensional ICRF code. It is for the first time that the effectiveness of poloidal phasing as a control of polarization could be demonstrated on a high density toroidal plasma with a low aspect ratio.

Allowing for the three-dimensional inhomogeneity of a real vacuum magnetic field, the author uses numerical methods to investigate Pfirsch-Schlüter currents in the magnetic configurations of an ℓ = 2 torsatron as a function of the winding law applied to the helical winding. The results allow him to assess the possibility of minimizing these currents, in the configurations considered, by selecting an appropriate winding law. The case of small beta is considered.

Microtearing modes are shown to be more unstable for H-mode profiles than for L-mode profiles of plasma near the edge of DIII-D. This indicates that microtearing is not responsible for the L-H transition. This result is mainly attributed to the large H-mode density gradient which outweighs the stabilizing effect of magnetic shear. A similar result is obtained for drift instabilities, for which two modes are found near the edge. The characteristics of the microtearing modes and the experimentally observed quasicoherent fluctuations are very similar, suggesting that they are closely related.

For interaction studies, impurity pellets of stainless steel and plastic carbon with a diameter of 0.5 mm and a velocity of 400 ± 100 m·s⁻¹ have been injected into plasmas driven by fast wave current, with a sustained plasma current of 35-50 kA and an electron density of (2-5) × 10¹² cm⁻³. The density rise is (6-8) × 10¹² cm⁻³ for stainless steel pellets and 4 × 10¹² cm⁻³ for plastic carbon pellets. At pellet injection, the current driven plasmas show no disruption, whereas all of the Ohmic discharges are disruptive. These phenomena are interpreted by a difference in the collision time with ablated pellets between thermal and non-thermal electrons. From measurements of the temporal evolution of the soft X-ray emission, the decay time of the injected impurity is estimated to be 25 ms. The effective charge states of the material of the injected pellets are calculated from the density rise and it is found that they are in the range of 0.8-1.5.

By setting up radial electric fields in the edge of a tokamak plasma by means of electrodes it is possible to trigger edge rearrangements which are similar to L-H transitions. The radial currents due to the finite orthogonal conductivity provide a poloidal torque which spins up the plasma against the viscous force opposing the poloidal rotation. It is found, in agreement with theoretical predictions, that this force first increases with the rotation speed and then, above a critical rotation speed, decreases. Under conditions in which the applied torque exceeds the maximum opposing force, a bifurcation from low field to high field is inevitable and results in many of the typical L-H transition signatures. The measured radial conductivity is compared with neoclassical estimates.

A compound saawtooth consists of a subordinate relaxation followed by a main relaxation; it has approximately twice the period and the amplitude of a normal sawtooth. High resolution 2-D images, produced by the soft X-ray tomography system of the Tokamak de Varennes, are used to compare the different types of sawtooth relaxations. It appears that a reconnection process is involved for all collapses, although the reconnection is incomplete during the partial relaxation.

This biennial meeting is sponsored by the Institute. of Electrical and Electronics Engineers (IEEE). The 13th Symposium on Fusion Engineering was attended by about 450 persons from nine different countries. About 370 technical papers were presented, including 5 plenary talks, 120 oral presentations and 245 poster presentations. The emphasis was on practical results for magnetic and inertial fusion engineering. Also covered were technical results from operating experiments in two invited oral sessions. Other invited oral sessions were devoted to the design status and development issues in next-generation devices (CIT, ITER) and fusion reactors (ARIES).