Table of contents

The flux of impurity atoms into plasmas from limiting surfaces is considered. It is shown how the flux of an impurity released from a surface can be derived from spectroscopic measurements along a line-of-sight directed at the surface. A theoretical atomic level population model is developed to obtain the 'ionization per emitted photon' quantities which link the spectroscopic measurement to the flux. Metastable states and finite density plasma effects are taken into account and observations at visible wavelengths are emphasized. Detailed studies and calculations are performed for C⁺¹, C⁺², O⁺¹, O⁺², Cr, Cr⁺¹, Fe, Fe⁺¹, Ni and Ni⁺¹. Tabulations and graphs of relevant quantities are provided. The application of the theory to impurity influxes in the JET Tokamak is described.

Conditions under which ballooning modes are expected to be excited have recently been obtained in two different types of discharges in JET. In the first type, extremely large pressure gradients have been produced in the plasma core through pellet injection in the current rise phase followed by strong additional heating. In the second type, the total pressure of the discharge is approaching the Troyon limit (1988). The stability of these discharges with respect to the ideal MHD ballooning modes has been studied with the stability code HBT. The equilibria are reconstructed with the IDENTC code using the external magnetic measurements and the experimental pressure profile. The results show that the evaluated high beta discharge is unstable in the central region of the plasma. This instability is related to the low shear and not to a large pressure gradient, as expected at the Troyon limit. In the pellet discharges the regions with the large pressure gradients are unstable to ballooning modes at the time of the beta decay, which ends the period of enhanced performance. The maximum pressure gradient in these discharges is limited by the boundary of the first region of stability. The observed phenomena at the beta decay are similar to those observed at the beta limit in DIII-D and TFTR.

The ion cyclotron resonance heating of monster sawtooth (period greater than the energy confinement time) and pellet-fueled peaked-density profiles in limited discharges of JET Tokamak are studied. The monster sawtooth is a characteristic JET regime which is related to fast ions generated during the minority ion heating. In the ICRF heating of peaked-density profile discharges, the authors find typically that T_i0 is higher roughly by a factor of 2 and T_e0 roughly by 35% at a fixed P_TOT/n_e0 when compared to nonpeaked profile cases. T_e0 and T_i0 are central electron and ion temperatures, respectively, n_e0 is the central electron density and P_TOT is the total input power. The ion heating is improved in the pellet case, in part, due to a higher collisionality between the background ions and the energetic minority, but more significantly by a reduction of local ion energy transport in the central region. The transport-code simulation of these discharges reveals that there is a reduction of both chi _e and chi _i in the central region of the plasma in the ICRF heated peaked-profile discharges where chi _e and chi _i are the electron and ion heat conductivities, respectively.

The energy confinement characteristics of ICRF-heated Tokamak plasmas are studied at high input power density approximately 2 MW m^-3 volume averaged, on the JIPP T-IIU device (R=0.91 m, a=0.23 m). High electron and ion temperatures (T_e approximately 2.5 keV, T_i approximately 2.0 keV, at each maximum) have been achieved by the operation at a plasma current I_P of 280 kA, plasma line-averaged electron density n_e of 7*10¹³ cm^-3 and input power of 2 MW, with a suppression of total radiation loss (30-40% of the total input power) by a carbon coating on the vacuum vessel. The fraction of ICRF power absorbed by the plasma, alpha , is determined experimentally from the decay of the stored plasma energy just after the ICRF pulse is terminated. The value of alpha increases slightly with increasing electron density and decreases from 90 to 70% as the ICRF power is increased from 1 to 2 MW m^-3 volume averaged. The global energy confinement time tau _epsilon , defined by W_P/(P_OH+ alpha P_r.f), decreases by a factor of 2 approximately 3 from that in Ohmic plasmas as the heating power increases up to 2 MW. It is found that the energy confinement time has a strong line-averaged electron density dependence as tau _E varies as n_e^0.6, which is obtained by the use of the measured absorbed power, while the Kaye-Goldston scaling predicts tau _E varies as n_e^0.26.

The large scale hydrodynamics of laser accelerated thin disks (foils) are studied numerically using a 2-D, evolutionary Lagrangian fluid code. The laser and target parameters considered are relevant to direct drive laser fusion, in the classical interaction regime. Peculiar 2-D features observed in the experiments are recovered in the simulations; agreement with experimental data is found for a series of experiments concerning foils of different thickness. Scaling laws for the design of hydrodynamically equivalent experiments are tested, with the effects of non-scaled processes turning out to be fairly small, although observable, and qualitatively in agreement with the relevant theory. Simulations also indicate the adequacy of foil acceleration experiments driven by non-uniform (but controlled) beams to the quantitative study of the smoothing of the irradiation non-uniformities.

The behavior of M=1 kink modes of reversed field pinch plasmas is analyzed in the presence of asymmetric external perturbations, especially field errors at toroidal shell gaps due to vertical field penetration. Perturbations in the vacuum and in the plasma are respectively expressed by using a class of modified Bessel functions and Bessel functions with M=1 modes, and amplitudes are determined by matching those on the perturbed boundary. It has been shown that the M=1 perturbation is amplified and its stability is deteriorated by the response of the plasma to the shell-gap field errors. Comparison with experimental observations of magnetic field fluctuations on the TPE-1RM15 Reversed Field Pinch is also made.

Drift waves have been successfully launched from flag probes in a steady-state magnetized plasma, and the launching mechanism has been identified. Nonlinear interactions are observed between launched and intrinsic waves. A wide range of further experimental studies is thus made possible, of fundamental relevance to plasma confinement.

A simple model is developed to investigate if and under what circumstances the thin shell instabilities of a Reversed Field Pinch can be stabilized by a rotating secondary wall. The principles may be applicable to reactor designs that utilize a flowing liquid blanket.