Table of contents

The beam ion thermalization process during tangential neutral beam injection in the ISX-B tokamak is investigated. The classical model is tested in co- and counter-injected discharges at low plasma current, a regime where large orbit width excursions enhance the importance of the loss regions. To test the model, experimental charge exchange spectra are compared with the predictions of an orbit following Monte Carlo code. Measurements of beam-plasma neutron emission and measured decay rates of the emission following beam turnoff provide additional information. Good agreement is found between theory and experiment. Furthermore, beam additivity experiments show that, globally, the confinement of beam ions remains classical, independently of the injected beam power. However, some experimental evidence suggests that the fast ion density in the plasma core did not increase with beam power in a way consistent with classical processes.

Numerical calculations of the heavy ion beam interaction with the low density plasma (corona) surrounding a small fusion pellet are discussed. As heavy ions enter the coronal plasma, they lose some of their electrons, and these stripped electrons, called 'drop-offs', constitute an electron beam with approximately the velocity of the ions. The interaction among the heavy ions, the drop-off electrons and the background electrons is investigated. The coronal density and temperature profiles are determined from one-dimensional hydrodynamic calculations, assuming classical ion deposition. The electron-electron and ion-electron cold two-stream instabilities are examined with one-dimensional particle-in-cell calculations. The results suggest that a strong coupling among the heavy ions, the drop-off electrons and the background electrons can exist; this leads to the creation of a non-Maxwellian electron plasma and significant electron heating. The ion beam energy loss is sensitive to the emittance of the beam, to the saturation level of the electric fields generated and to the background density scale length; these effects are discussed. Estimates of ion energy coupling to the corona are made, including the possible impact of target charge buildup; also, the coronal coupling effect on target physics is addressed.

The evolution of current and temperature profiles in JET has been analysed using an advanced equilibrium analysis code and absolutely calibrated ECE measurements. It is found that during the discharge 'flat-top' the evolution of the current is well described by neoclassical resistivity. However, during the discharge rise phase, the current penetration can be more rapid. The MHD processes influencing the rapid penetration are discussed qualitatively by analysing the trajectories of several discharges in the (q₀, q_a) plane.

A simulation model is described for lower hybrid (LH) current drive, rampup, heating, and sawtooth stabilization. The model incorporates a one-dimensional radial transport code, parallel velocity Fokker-Planck calculation, and a toroidal ray tracing code. For steady LH current drive it is found that the RF current generation is accurately predicted by a fast electron confinement time of the form ms in the density range of 3 × 10¹⁹ m⁻³ ≲ _e ≲ 7 × 10¹⁹ m⁻³ (where ± distinguishes electrons moving parallel (antiparallel) to the current drive direction). Also in this range, the theoretically predicted wave absorption and experimentally measured electron temperatures and stored energy were found to be consistent with an electron thermal diffusivity whose magnitude is independent of n_e. To reproduce the experimentally measured values of LH rampup efficiency at _e = 3 × 10¹⁹ m⁻³, it was necessary to take . For LH heating at densities of _e ≌ 1.4 × 10²⁰ m⁻³, the power lost due to collisional damping of the LH ray trajectories at the plasma periphery was found to be significant, because of higher edge densities. Studies of LHRF sawtooth stabilization experiments with RF current drive indicated the possibility of creating stable profiles of the safety factor, q, via the generation of positive RF current near the q = 1 surface, thus producing a current 'pedestal'.

Non-inductive current drive using second harmonic ECRH at both 28 GHz and 60 GHz has been studied in the CLEO tokamak. At 60 GHz, RF driven currents of up to 5 kA have been observed at e = 4 × 10¹⁸m⁻³ for 185 kW of injected power, indicating an efficiency of η ≡ _eI_RFR₀/P_RF = 0.001 (10²⁰ m⁻³, A, m, W⁻¹). The RF driven current scaled linearly with total plasma current in the range of 5-15 kA and was maximized when the cyclotron resonance was located near to the centre of the plasma. Sawtooth activity was normally strongly affected and transient sawtooth stabilization was often observed. With detailed theoretical studies it is possible to reproduce both the high absorption efficiencies and the scaling of RF driven current with resonance position seen in the 60 GHz experiments. However, the magnitude of the observed current is a factor of about three below that theoretically predicted. At 28 GHz, no evidence of RF driven current could be detected. Possible reasons for this are discussed.

Effects of nuclear elastic scattering (NES) on the slowing down of charged fusion products in a typical deuterium plasma and the burn dynamics of ignited Cat-D and D-D-³He plasmas are investigated on the basis of a linear Boltzmann-Fokker-Planck equation. A multigroup method is used to obtain the distribution functions for the fusion products and their recoils. It is shown that an adequate treatment of the slowing-down process, especially the consideration of NES and of the slowing-down time delay, is essential for an accurate prediction of the dynamic behaviour and the thermal instability of plasmas. NES enhances the thermal instability and accelerates the temperature excursion from the equilibrium. The inclusion of NES results in a 20-30 keV increase in the critical temperature for thermal stability.

The total erosion yields of several types of graphites have been measured during high ion flux exposure in the PISCES plasma facility. Hydrogen and deuterium plasmas have been used to bombard Poco, ATJ, pyrolytic, and C-SiC-coated graphites, and a '4-D' C-C composite weave. Erosion experiments with ion fluxes of up to 2 × 10¹⁸ cm⁻² · s⁻¹, at ion energies of 50 to 200 eV and sample temperatures of 50 to 950°C, are reported. In the absence of redeposition, the erosion rates of all the graphites tested is essentially the same. The C-C composite weave material showed equal or lower erosion yields compared to the other graphites over the range of temperatures tested. The maximum total carbon erosion yield at 600°C increased by a factor of two for 50 eV ion bombardment and by a factor of five for 200 eV ion bombardment, compared to the reference values found for room temperature samples. At temperatures above 800°C, the chemical erosion is suppressed and the erosion yield reaches values expected for physical sputtering. A direct comparison with two ion beam experiments with an ion flux three orders of magnitude lower than used in PISCES showed total erosion rates within a factor of two, indicating that chemical sputtering is not significantly suppressed by high ion fluxes. The results show that the chemical sputtering of graphite limiters or divertor plates in tokamaks will vary by a factor of only two to three over a limiter temperature range of 0 to 800°C if the incident ion energy is less than 100 eV as is observed in tokamak experiments. The net erosion is also strongly reduced by reionization in the plasma and redeposition of both hydrocarbons and physically sputtered carbon.

A self-consistent transport code is used to evaluate how plasma confinement in tokamaks is influenced by the microturbulent fields excited by the dissipative trapped electron (DTE) instability. As shown previously, the saturation theory on which the code is based has been developed from first principles. The numerical results reproduce well the Neo-Alcator scaling law observed experimentally (for example in TEXTOR) in non-detached Ohmic discharges, the confinement degradation resulting when auxiliary heating is applied and a large number of other experimental observations. The potential impact of the toroidal ion temperature gradient (η_i) mode on energy confinement is assessed by estimating the ion thermal flux with the help of the mixing length approximation. The temperature and density profiles measured in TEXTOR (q_a ≃ 2.45) are compared at either variable mean density or variable additional power, and their stability against DTE and η_i modes is checked; in the latter case, a new criterion is used, which is valid for arbitrary curvature. All profiles examined are marginally unstable for the two modes, essentially between the q = 1 and q = 2 magnetic surfaces. The code results and the stability analysis lead to the following conclusions and suggestions: (1) The DTE instability is sufficient to explain the anomalous heat transport in low density discharges (attached plasmas) with or without additional heating; the marginal instability for the DTE mode thus follows from heat flux constraints. (2) The observed marginal instability against the η_i mode must then follow from particle flux constraints. (3) The condition that both γ_ηi and γ_DTE (γ = growth rate) must be small is the restriction which determines the profiles that correspond to the experimental conditions and which determines, to a large extent, profile consistency. (4) Finally, it is suggested that the deviation from Neo-Alcator scaling, the density limit and the phenomenon of plasma detachment are interrelated effects which arise at high densities, when the constraint on the electron heat flux becomes harder to satisfy.

The linear mirror geometry for a high fluence, neutron irradiation facility is evaluated by current studies. In this steady state, low-Q design, a two-component plasma is driven by neutral beams with mirror confinement of the hot ions and with no electrostatic axial reduction in the warm ion end losses. Warm ion fuelling and end-wall power denstiy could require substantial plasma density exterior to the mirror cell and neutral gas near the end wall. The paper evaluates to what extent the electron power loss parallel to the magnetic field along open field lines is a function of the escaping warm plasma collisionality and of the end-wall conditions. Within the limits of a one-dimensional model, equations are developed to calculate the electron temperature.

Oscillating MHD modes in JET are often observed to slow down as they grow and generally stop rotating (lock) when the amplitude exceeds a critical value, then continue to grow to large amplitudes (_r/B_θ ∼ 1%). The mode can grow early in the current rise or after perturbations, such as a pellet injection or a large sawtooth collapse, and maintain a large amplitude throughout the remainder of the discharge. Such large amplitude quasistationary MHD modes can apparently have profound effects on the plasma, including stopping central ion plasma rotation, reducing the amplitude and changing the shape of sawteeth, flattening the temperature profile around resonant q surfaces and reducing the stored energy. Perhaps most important, large amplitude locked modes are precursors to most disruptions. Some large amplitude modes can be avoided by proper programming of the q evolution. The apparent reasons for the mode locking in a particular location are discussed and a comparison with theory is made.

Magnetic surface optimization studies have been conducted on the Auburn Torsatron. Winding errors in the original helical field coil resulted in a displacement and an up-down asymmetry in the magnetic surfaces. The displacement was initially corrected by unbalancing the outer Helmholtz coils. Computer modelling showed that the observed displacement and asymmetry are expected for a helical field coil winding error having a cos (θ) modulation. A helical correction coil with the appropriate modulation was designed and installed. Magnetic surface mappings with and without the helical correction coil were conducted. With the correction coil, there was a significant improvement in surface symmetry. The experimental results are in good agreement with the predictions of the computer modelling.

Excitation of the lower hybrid wave (slow wave) using a launcher designed for fast wave excitation has been observed under conditions where neither direct excitation nor mode conversion is expected to take place. The slow wave was revealed through the presence of its resonance cone. The experiment was performed with Ω_i ≪ ω ≪ Ω_e. The coupling of the fast wave into the slow wave is attributed to parasitic excitation, which is a linear mechanism involving electron × motion and charge separation in a density gradient. This process could be a significant loss mechanism for fast wave current drive in toroidal plasmas.

In a sawtooth oscillation, the resistive reconnection and the internal disruption are two distinct and sequential events. At the beginning of the reconnection of magnetic field lines around an m = 1 magnetic island, the mixing of the electron temperatures associated with the reconnection results in an increasing island width and a steep pressure gradient in the boundary layer adjacent to the reconnecting surface, leading to the destabilization of ideal MHD modes. The thickness of the boundary layer is determined by the ratio of electron cross-field thermal diffusivity and the speed of reconnection. This narrow boundary layer and the short onset time of a sawtooth crash can be supported by 'finegrained' transport processes which drive the electron energy loss in a tokamak plasma.

Lower hybrid current drive data obtained on the JT-60 tokamak at a density of _e ≌ 1 × 10¹⁹ m⁻³ are analysed. The parallel and perpendicular pressures of the superthermal electron tail are deduced from magnetic measurements. The energy stored in the superthermal electrons is found to be comparable to the bulk stored energy, and a significant pressure anisotropy of β_p||/β_p⊥ ≌ 3 is inferred. The electron tail distribution function can be modelled by a three-temperature Maxwellian with a flat tail extending to the accessibility energy in the forward direction, a perpendicular temperature of 150 keV, a backward parallel temperature of 250 keV, and a backward to forward ratio of 2. These results are consistent with expectations based on theory, the results of a detailed numerical modelling, and the model distribution functions inferred previously on smaller tokamaks.

Self-consistent equilibrium effects on tokamak magnetic field ripple have been calculated using a three-dimensional equilibrium code. The poloidal component of the plasma current is found to amplify the magnitude of the ripple. This effect is large enough that it should be included in tokamak ignition experiment design studies. An analysis of the results separates the contribution of the Shafranov shift to the ripple modification from the contributions of other finite pressure effects.

Experiments were carried out on the TRX field reversed theta pinch in which s, the number of ion gyroradii between the magnetic null and the separatrix of a field reversed configuration, was increased significantly compared with previous experiments where s ≤ 2. Sustained values of s > 4 were obtained in hydrogen discharges with configuration lifetimes as long as twenty tilt growth times, where the calculated growth rate reduction due to ion kinetic effects has been accounted for.

A poloidal ring limiter was installed at one toroidal location in the OHTE device operated as a reversed field pinch. The ring acts as the solitary limiter beyond the smooth carbon tile vacuum vessel wall. The mere presence of the poloidal ring limiter broadens the current profile, rather than narrowing it by the reduced aperture, and raises the necessary one-turn voltage needed to sustain the current. The magnetic fluctuation frequency spectrum for both m = 0 and m = 1 poloidal modes is shifted to higher frequency in discharges with the ring limiter. The observed voltage increase is compared with that predicted by two recent edge helicity loss models.

Negative plasma potentials as large as −400 V were measured in the magnetic fusion experiment Tandem Mirror Experiment-Upgrade (TMX-U). Normally, a plasma confined by an open ended magnetic field produces a positive potential. To measure the negative potentials, neutral beams were injected into the plasma region at angles within the loss cone. The greatest negative potentials were observed after the electron cyclotron heating power was turned off but while energetic ions were still being introduced into the plasma with large-angle neutral beams. These negative potentials are explained in terms of thermal ions electrostatically trapped by long lived, hot, mirror confined electrons.

The International Workshop on Laser Interactions and Related Plasma Phenomena is the oldest continuing series of meetings in the United States with published proceedings documenting much of the growth of this dynamic field. The meeting format is designed to provide an interactive atmosphere that encourages first disclosures along with in-depth critical reviews. The Eighth International Workshop was held at the Naval Postgraduate School at Monterey, California, 26-30 October 1986, under the continuing directorship of H. Hora (University of New South Wales) and G.H. Miley (University of Illinois). The local organizer was F. Schwirzke (Naval Postgraduate School).