Table of contents

The transport of neutral helium atoms near divertor or limiter target plates in fusion devices has been studied using Monte Carlo simulation techniques. The atomic processes of ionization of helium atoms by electron impact and elastic scattering with plasma ions are included as well as the thermal motion and the streaming of the ions along the magnetic field. The results obtained show significant effects of elastic collisions below about 10 eV, causing a substantial fraction of the helium atoms to be reflected back to the target plate. This effect can be beneficial for the pumping of helium from the discharge chamber. A two-dimensional Monte Carlo code has been used to study helium recycling near a vented target plate. It is found that a substantial pumping efficiency can be obtained if the transparency of the vented plate is high (≥30%) and the ports are aligned nearly parallel to the magnetic field.

For ICRH scenarii where the majority cyclotron resonance intersects the plasma core, mode conversion of the fast magnetosonic wave to an Alfvén wave takes place at the plasma boundary on the high field side. Simple analytical estimates of the converted power for this mode conversion process are derived and compared with numerical calculations including finite electron inertia and kinetic effects. The converted power is found to depend on the local value of the wave field as well as on plasma parameters at the Alfvén wave resonance. The interference with the reflected wave will therefore modify the mode conversion. If the conversion layer is localized near the wall, the conversion will be strongly reduced. The conversion coefficient is found to be strongest for small density gradients and high density and it is sensitive to the value of the parallel wave number. Whether it increases or decreases with the latter depends on the ion composition. Analysis of this problem for ICRH in JET predicts that a large fraction of the power is mode converted at the plasma boundary for first harmonic heating of tritium in a deuterium-tritium plasma.

Several plasma instabilities driven by a heavy ion beam impinging on a target are considered. Potential instabilities include longitudinal electrostatic (two-stream) instabilities and transverse electromagnetic (filamentation or Weibel) instabilities. Since the ion beam may be accompanied by electrons, both electron beam and ion beam instabilities are considered. The electrostatic beam instabilities saturate by formation of an electron tail. In this paper the filamentation instability driven by this electron energy anisotropy is shown to be benign; this confirms the results of earlier studies in which the slower ion filamentation instability was shown to be unimportant. The beam ion, target electron instability is found to be suppressed by the gradient in the target density. Moreover, even if it were to grow, it would saturate at a benign level. In conclusion, no significant problem due to corona plasma instabilities in heavy ion fusion applications has been found.

The absorption of lower hybrid slow (LHS) waves in a thermonuclear plasma is investigated by accounting for both (self-consistent) quasi-linear electron Landau damping and (linear) absorption by fusion alpha particles. The full cold plasma wave dispersion properties are taken into account. A cylindrical geometry is adopted and it is assumed that the poloidal component of the wave vector can be disregarded. Analytical expressions for both absorption coefficients are presented, with emphasis on the case where the parallel refractive index of the LHS wave is near the accessibility value. In this case, electromagnetic effects are important. Numerical results for the profile of the radial power deposition on both electrons and alpha particles as well as the driven current density are given and discussed for parameters typical of a NET-like plasma. In particular, the fraction of the power absorbed by the alpha particles is evaluated. Conditions which ensure the absence of wave absorption by alpha particles are discussed.

A technique has been developed which makes it possible to measure in a non-destructive way the fill pressure of a transparent direct drive fusion shell. The accuracy of these measurements is within 2% of the best known values for the system D₂/DT. By raising the temperature of the shells and allowing them to leak for a certain time between the pressure measurements, their leak rate and permeability can be determined accurately. The technique is simple emough to be used as a tool for quality control to eliminate shells with a leak rate faster than that set by established standards; the surviving shells will then be automatically characterized regarding their fill pressure and their leak rate.

The paper presents the results of an experimental study of the wave fields excited in a tokamak plasma under Alfvén wave heating conditions, with a large number of Alfvén resonance surfaces present in the plasma. The wave fields excited by Faraday shielded antennas with current elements oriented in the poloidal direction were measured by an array of magnetic probes positioned along a vertical diameter of the plasma. Pronounced enhancement of the wave fields at the radii of Alfvén resonance surfaces was observed. The dependence of the wave field profiles on the plasma parameters and on the toroidal and poloidal positions is analysed with the help of a simple theoretical model. Evidence of the excitation of the outward propagating surface electrostatic wave is presented. The profiles excited by a number of phasings of an array of three toroidally spaced antennas are compared. The results provide unambiguous evidence that the peaks in the profiles are due to mode conversion of the compressional Alfvén wave to the torsional Alfvén wave at Alfvén resonance surfaces. Measurements of the resistive loading of the antenna indicate that up to 50% of the total wave power was deposited at Alfvén resonance surfaces well inside the plasma and 40% at a single Alfvén resonance surface.

The paper describes a novel, highly elongated, ellipsoidal shell tokamak (EST) configuration which has favourable high pressure magnetohydrodynamic Mercier and bellooning stability properties for a range of shell geometries. Calculations of the magnetic field (|B|) contours also suggest that an EST can be broadly omnigenous. The considerations leading to the choice of an ellipsoidal shell configuration, the equilibrium and ballooning stability properties and the axisymmetric stability are discussed and the parameters of a sample design for a modest proof-of-principle EST are presented.

Second harmonic electron cyclotron heating for the extraordinary mode in lower hybrid current driven plasma has been investigated experimentally in the JFT-2M tokamak. Effective electron heating is observed at largely downshifted cyclotron frequencies as well as at the usual cyclotron resonance frequency. An absorption efficiency of 15% at f₀/2f_c0 = 0.76 is estimated from measurements of the Shafranov parameter and the diamagnetic flux (f₀ is the frequency of the RF wave and f_c0 is the cyclotron frequency at the plasma centre). From a measurement of the transmission of radiated microwaves the absorption efficiency is deduced. The dependences of the absorption efficiency on the toroidal magnetic field obtained from the two measurements are consistent with each other within experimental error. From measurements of the soft X-rays and the electron cyclotron emission it is deduced that there is selective perpendicular heating of the fast electrons which satisfies the relativistic cyclotron resonance condition. The absorption efficiency obtained is in reasonable agreement with a theoretical prediction taking mildly relativistic electrons into account. Selective cyclotron heating of 70 kW produces a 65 kA/s ramp-up of the lower hybrid driven plasma current; a resulting ramp-up efficiency of 30% is deduced.

The consequences of beta driven equilibrium changes on toroidal ion temperature gradient driven instabilities are considered. A semi-analytic method for evaluating the stability of finite aspect ratio, shaped tokamaks at finite beta is presented. The appearance of higher order eigenmodes with relatively high growth rates is observed. Stabilization is achieved at high poloidal beta.

The asymmetric electron distribution generated during lower hybrid current drive has been computed using a 3-D Fokker-Planck code. The superthermal tail and the resulting current are generally a combination of two components streaming in opposite toroidal directions. An appropriate diagnostic method for experimental investigation of the two superthermal populations is wave transmission of two equivalent rays with equal and opposite values of the refractive index. These equivalent rays can be realized by launching the waves from symmetric positions with respect to the equatorial plane at equal and opposite angles in the toroidal direction. Using an appropriate ray tracing code, the damping of the two rays is computed and it is shown that it results from electrons with opposite parallel velocities. The differential transmission is then a measure of the overall asymmetry of the electron momentum distribution.

An attached poloidal current, which flows in a circuit lying partly in the vacuum vessel wall and partly in the scrape-off layer of the plasma, is observed during vertical instabilities in the DIII-D tokamak. A direct measurement of the current, using Rogowski loops on several protective tiles at locations where the plasma contacts the wall, is in good agreement with the value determined from MHD equilibrium reconstructions using measured values of magnetic field and flux. This attached current, which can reach transient peaks of several hundred kiloamperes, interacts with the toroidal magnetic field to create a large vertical force on the vacuum vessel. The predicted motion of the vessel resulting from the measured currents agrees well with the observed displacement of the vacuum vessel.

The paper presents a model of the plasma boundary region that has been developed to study the flow of impurities generated at the target plates in the pumped divertor configuration planned for JET. For the flow equations the model is one-dimensional, parallel to the magnetic field, but the neutral species is treated in two dimensions. All ionization stages of an impurity can be treated separately or, alternatively, an average charge impurity species can be considered. The validity of this approximation is assessed. The presented results show that impurities can be retained near the target plates by a forced flow of plasma generated by recirculating, across the separatrix from the main plasma to the scrape-off layer, a fraction of the flow reaching the target plates.

Predictions with the 1½-D BALDUR transport code, using recent theory based models for thermal and particle transport, are compared with measured profiles of electron plasma density and electron and ion temperatures for Ohmic, L-mode and supershot discharges on TFTR. The profile consistent drift wave model is found to overestimate the ion temperatures at high heating powers so that a third mode or loss process is needed in addition to drift wave transport (trapped electron mode and η_i mode) and an edge loss model. None of several versions of the local multiple mode models, using the 1989 Carreras-Diamond resistive ballooning mode, gives T_e, T_i within 20% for all three TFTR regimes studied.

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.

Plasma models are described and used to calculate numerically the transport confinement (nτ_E) requirements and steady state operation points for both the reversed field pinch (RFP) and the tokamak. The models are used to examine the CIT tokamak ignition conditions and the RFP experimental and ignition conditions. Physics differences between RFPs and tokamaks and their consequences for a D-T ignition machine are discussed. Compared with a tokamak, the ignition RFP has many physics advantages, including Ohmic heating to ignition (no need for auxiliary heating systems), higher beta, lower ignition current, less sensitivity of ignition requirements to impurity effects, no hard disruptions (associated with beta or density limits) and successful operation with high radiation fractions (f_RAD ∼ 0.95). These physics advantages, coupled with important engineering advantages associated with lower external magnetic fields, larger aspect ratios and smaller plasma cross-sections, translate to significant cost reductions for both ignition and reactor applications. The primary drawback of the RFP is the uncertainty that the present scaling will extrapolate to reactor regimes. Devices that are under construction should go a long way towards resolving this scaling uncertainty. The 4 MA ZTH is expected to extend the nτ_E transport scaling data by three orders of magnitude above the results of ZT-40M, and, if the present scaling holds, ZTH is expected to achieve a D-T equivalent scientific energy breakeven, Q = 1. A base case RFP ignition point is identified with a plasma current of 8.1 MA and no auxiliary heating.

The influence of radial transport of electrons on radiofrequency and Ohmic current drive in tokamaks is discussed. It is found that such processes can reduce the efficiency of current drive in tokamaks in which the energy confinement time is comparable with or less than the collision time of the heated electrons and that, even if this is not the case, they significantly broaden the driven current profile.

The emission of light from hydrogenic pellet material has been studied in a special experimental set-up. The measurements show that the intensity of light from particle bombarded solid hydrogens is very small and that none of the well known lines for the gas phase are emitted from the solid. The solid hydrogens do not contribute to the strong H_α(D_α) signal observed from the pellet cloud during penetration into the plasma.

According to the magnetic diffusion equation, the ratio of the normalized current density and the temperature sawteeth should lie between 2/3 and 3/4 on the tokamak magnetic axis if, as is the case in TEXTOR, the q = 1 magnetic surface remains still during the sawtooth ramp phase, and if the sawtooth amplitude is moderately small and rises linearly with time. We present experimental confirmation of this theoretical result in TEXTOR. The implication is that the electrical conductivity is neoclassical over most of the ramp phase.