Table of contents

Owing to the complexity of the exact calculation, synchrotron losses are usually estimated in system studies, with expressions derived from a plasma description using simplifying assumptions on the geometry, radiation absorption, and density and temperature profiles. In the present article, a complete formulation of the transport of synchrotron radiation is performed for realistic conditions of toroidal plasma geometry with elongated cross-section, using a quasi-exact method for the calculation of the absorption coefficients, and for arbitrary shapes of density and temperature profiles. The effects of toroidicity and temperature profile on synchrotron radiation losses are analysed in detail. In particular, when the electron temperature profile is almost flat in the plasma centre as, for example, in internal transport barrier confinement regimes, synchrotron losses are found to be much stronger than in the case where the profile is represented by its best generalized parabolic approximation, though both cases give approximately the same thermal energy content. Such an effect is not included in presently used approximate expressions. As an illustration, it is shown that in the case of an advanced high temperature plasma envisaged for a steady state commercial reactor, synchrotron losses represent approximately 20% of the total losses, so that this term becomes significant in the power balance of such a plasma. Finally, the authors propose a seven variable fit for the fast calculation of synchrotron radiation losses. This fit is derived from a large database which has been generated using a code implementing the complete formulation, and is optimized for massively parallel computing.

The Helicity Injected Torus programme has made progress in understanding relaxation and helicity injection current drive. Helicity conserving MHD activity during the inductive (ohmic) current ramp demonstrates the profile flattening needed for coaxial helicity injection (CHI). Results from cathode and anode central column CHI pulses are consistent with the electron locking model of current drive from a pure n = 1 mode. Finally, low density CHI, compatible with ohmic operation, has been achieved. Some enhancement of CHI discharges with the application of ohmic heating is shown.

The centrally localized alpha heating that allows the attainment of ignition in a toroidal plasma is shown to lead to a consistent plasma pressure profile. Such a profile turns out to depend much more on the achievement of ignition than on the transport assumptions. This feature is observed in a series of dynamic simulations relevant to the Ignitor experiment.

SXR cross-sectional distributions measured in the European reversed field pinch (RFP) experiment RFX by means of a tomographic diagnostic are described. Various reconstruction techniques have been compared. A correction technique to minimize the systematic effect due to the non-identical thicknesses of the material filters has been developed. A number of experimental situations have been explored, and a study of the correlation between the MHD and SXR properties of the plasma has been made. The analysis concentrates both on standard discharges and on the recently discovered quasi-single-helicity states, whose signature is the growth of a sizeable m = 1 structure in the emissivity distributions. Detailed imaging of the plasma in these conditions is presented.

Physics issues are discussed for compact stellarator configurations which achieve good confinement by the fact that the magnetic field modulus |B| in magnetic co-ordinates is dominated by poloidally symmetric components. Two distinct configuration types are considered: (1) those which achieve their drift optimization and rotational transform at low β and low bootstrap current by appropriate plasma shaping; and (2) those which have a greater reliance on plasma β and bootstrap currents for supplying the transform and obtaining quasi-poloidal symmetry. Stability analysis of the latter group of devices against ballooning, kink and vertical displacement modes has indicated that stable β values on the order of 15% are possible. The first class of devices is being considered for a low β near term experiment that could explore some of the confinement features of the high β configurations.

Direct measurements of localized electric fields have been made by the laser induced fluorescence (LIF) method using the Stark effect in the central cathode core region of an inertial electrostatic confinement fusion (IECF) neutron (proton) source. These are expected to have various applications, such as luggage security inspection, non-destructive testing, land mine detection and positron emitter production for cancer detection, currently producing continuously about 10⁷ n/s D-D neutrons. Since 1967, when the first fusion reaction was successfully proved to have taken place in a very compact IECF device, potential well formation due to the space charge associated with spherically converging ion beams has been a central key issue remaining to be clarified in beam-beam collision fusion, which is the major mechanism of the IECF neutron source. Many experiments, although indirect, have been done so far to clarify the nature of the potential well, but none of them has produced definitive evidence. The results found by the present LIF method show a double well potential profile with a slight dip for ion beams with relatively larger angular momenta, whereas for ions with smaller angular momenta, a much steeper potential peak develops.

Restrictions on magnetic diagnostics are discussed. Being related to the integral nature of the measurable quantities, these follow from the fundamental laws of electromagnetism. A series of examples demonstrating the strength of these restrictions is analysed. The general rule is emphasized that information obtained from external magnetic measurements is insufficient for reliable evaluation of plasma current and pressure profiles in tokamaks and in stellarators. The underlying reason is that outside the plasma the self-field of the equilibrium plasma currents is determined by the boundary conditions on the plasma surface alone.

The Large Helical Device (LHD) is a plasma physics experimental device with a magnetic stored energy of 960 MJ, consisting of two superconducting (SC) helical coils and six SC poloidal coils. The trial operation and the first plasma discharge of the Phase I project for LHD were completed on 31 March 1998 as initially planned. The second experimental campaign was conducted with additional heating using two NBI devices. The third campaign started in June 1999 and was completed in January 2000. Many plasma heating tests with a plasma field of up to 2.90 T were carried out. Major test results on the SC magnet system for LHD are as follows: (1) The LHD cryogenic system was successfully operated for 13 400 h and proved its high reliability. (2) A central field of 2.91 T at a radius of 3.60 m was achieved at an inner helical coil current of 11.08 kA, a middle helical coil current of 11.83 kA and an outer helical coil current of 12.02 kA. (3) All six poloidal coils were excited stably. (4) Nine flexible SC bus lines with a total length of 497 m were operated stably and safely.

The Multi-level 3D (M3D) project simulates plasmas using multiple levels of physics, geometry and grid models in one code package. The M3D code has been extended to fundamentally non-axisymmetric and small aspect ratio, R/a >~1, configurations. Applications include the non-linear stability of the NSTX spherical torus and of the spherical pinch, as well as the relaxation of stellarator equilibria. The fluid level physics model has been extended to evolve the anisotropic pressures p_j|| and p_j⊥ for the ion and electron species and has been applied to magnetic island evolution.

The present work addresses the issue of identifying the major non-linear physics processes which may regulate drift and drift-Alfvén turbulence using a weak turbulence approach. Within this framework, on the basis of the non-linear gyrokinetic equation for both electrons and ions, an analytic theory is presented for non-linear zonal dynamics described in terms of two axisymmetric potentials, δϕ_z and δA_{|| z}, which spatially depend only on a (magnetic) flux co-ordinate. Spontaneous excitation of zonal flows by electrostatic drift microinstabilities is demonstrated both analytically and by direct 3-D gyrokinetic simulations. Direct comparisons indicate good agreement between analytic expressions of the zonal flow growth rate and numerical simulation results for ion temperature gradient driven modes. Analogously, it is shown that zonal flows may be spontaneously excited by drift-Alfvén turbulence, in the form of modulational instability of the radial envelope of the mode as well, whereas in general, excitations of zonal currents are possible but have little feedback on the turbulence itself.

The article concerns studies of energetic ion beams and their correlation with pulsed relativistic electron beams. Time integrated measurements were performed with an ion pinhole camera equipped with solid state nuclear track detectors, and time resolved studies were carried out with a scintillation detector, allowing determination of the ion energy spectrum on the basis of the time of flight technique.

Quasi-steady-state high β_p H mode discharges performed by suppressing neoclassical tearing modes (NTMs) are described. Two operational scenarios have been developed for long sustainment of the high β_p H mode discharge: NTM suppression by profile optimization, and NTM stabilization by local electron cyclotron current drive (ECCD)/electron cyclotron heating (ECH) at the magnetic island. Through optimization of pressure and safety factor profiles, a high β_p H mode plasma with H_89PL = 2.8, HH_y,2 = 1.4, β_p ≈ 2.0 and β_N ≈ 2.5 has been sustained for 1.3 s at small values of collisionality ν_e* and ion Larmor radius ρ_i* without destabilizing the NTMs. Characteristics of the NTMs destabilized in the region with central safety factor above unity are investigated. The relation between the beta value at the mode onset β_N^on and that at the mode disappearance β_N^off can be described as β_N^off/β_N^on = 0.05-0.4, which shows the existence of hysteresis. The value of β_N/ρ_i* at the onset of an m/n = 3/2 NTM has a collisionality dependence, which is empirically given by β_N/ρ_i* ∝ ν_e*^0.36. However, the profile effects such as the relative shapes of pressure and safety factor profiles are equally important. The onset condition seems to be affected by the strength of the pressure gradient at the mode rational surface. Stabilization of the NTM by local ECCD/ECH at the magnetic island has been attempted. A 3/2 NTM has been completely stabilized by EC wave injection of 1.6 MW.

In order to supply plasma fuel confined in spheromak-like compact toroids (SCTs) to a fusion device, the SCTs must be successfully guided through a drift tube region, in which they might be influenced by the magnetic field leaking from the fusion device. To reveal the SCT dynamics in a drift tube, MHD numerical simulations, where the SCTs are accelerated in a co-axial perfectly conducting cylinder with an external magnetic field, are carried out. In addition, the effect of an extended central electrode is examined by changing the length of the inner conducting cylinder. It is revealed that the SCT penetration depth is shorter than that estimated from the conventional conducting sphere model and that the SCTs are further decelerated by extending the inner conducting cylinder. These results are consistent with the results of the compact toroid injection experiment performed on the TEXT Upgrade tokamak. Finally, the deceleration mechanism of the SCTs is discussed by comparing the simulation result with the proposed theoretical model.

In the Large Helical Device, stable discharges lasting longer than one minute have been obtained using the complete heating scheme, including ECH. The plasma is sustained with NBI or ICRF of 0.5-1 MW. The central plasma temperature is higher than 1.5 keV with a density of (1-2) × 10¹⁹m^-3 maintained until the end of the pulse. Full installation of the carbon divertor has contributed to this achievement. This provides a sufficient basis for physics and technology studies for the next campaign. Long pulse operation opens new possibilities in diagnostics and in physics studies. Higher accuracy and reliability are obtained with a diagnostics parameter scan, longer integration of signals or two dimensional measurement. The mechanism of a slow oscillation called `breathing' is discussed. Hydrogen recycling analysis has been carried out and preliminary results have been obtained. On the basis of these results, the future programme is divided into two categories: (i) physics and technology experiments utilizing long pulse discharges lasting up to 5 minutes, and (ii) extension of the pulse length up to one hour.