Table of contents

A novel method of electron cyclotron wave current startup has been proposed, in which a discharge between a pair of electrodes is introduced and forms a magnified toroidal plasma current in a strong toroidal field and a weak vertical field. The method has been demonstrated on the CT-6B tokamak. The experimental results are consistent with a model that is suggested.

Runaways generated during disruptions in tokamaks may have damaging consequences in large machines because of the high power generated by their localized deposition on the vessel walls. In an investigation of the runaways generated in disruptions in JET, detailed time and space resolved X ray images of the runaway beam in flight have been obtained for the first time and these allow a detailed diagnosis and analysis of the production and movement of the runaways. These measurements are now possible because of the development, for the JET DT campaign, of radiation protected soft X ray cameras. The measurements show that the runaways are generated at the vessel centre in a region with small minor radius and they then move to interact with a small area of the wall. The observed radiation power both in detectors installed in the torus and in the radiation protected cameras shows reasonable agreement with values calculated from the runaway current and energy. The current density profile and q profile of the runaway beam are also determined. After the start of the disruption there is a delay before the onset of runaway generation and this may offer control possibilities.

Theories predict that neutrals play a role in the low to high (L-H) confinement mode transition in tokamak plasmas via charge exchange damping and other effects. Previous estimates of neutral damping have been based on calculations of the edge neutral density. This work introduces a new method of measuring the neutral density near the X point, where simulations predict it to be a maximum. The technique employed uses D_α light from a TV camera reconstructed onto a poloidal plane, along with Thomson scattering measurements of the electron temperature and density. Measured neutral densities span the range 10⁹-10¹³cm^-3. Good agreement, considering the neutral density error bars, is found between the measurements and the 2-D simulations. This work represents the first step in verifying previous 2-D simulations and in corroborating previous conclusions that the neutral damping is large enough to play a role in the L-H transition process.

The penetration process of a rotating magnetic perturbation into a rotating tokamak plasma has been investigated, taking account of the mode structure of the perturbations, and using a small magnetic probe in the small research tokamak CSTN-IV. It was observed that the radial component of the perturbation was amplified in the plasma when magnetic islands are formed, while the poloidal component was attenuated but, deep inside the plasma, amplified. Using a resistive MHD code, such modifications of external perturbation were found to be caused by a redistribution of plasma current due to the formation of magnetic islands. Considering the phase velocity of both the perturbation and the intrinsic plasma rotation, it was also found that the sideband component substantially affects the penetration process with magnetic islands forming. Therefore, in dynamic ergodic divertor operation, it is necessary to put the emphasis not only on the frequency of the perturbation fields but also on their direction, mode structure and the radial profile of the intrinsic plasma rotation at the same time.

A catalytic regime for tritium in deuterium-tritium (DT_x) fusion is investigated, including ion-electron collisions, mechanical expansion, bremsstrahlung radiation and inverse Compton scattering losses. It is shown that the consideration of the three main nuclear reactions of the hydrogen isotopes (D(D,n)³He, D(D,p)T and T(D,n)⁴He) only leads to erroneous results for internal tritium breeding. Among the secondary reactions in DT fusion, the reaction ³He(D,p)⁴He is the most important in the tritium catalytic regime, due to its large contribution to the plasma heating. When all the nuclear reactions are taken into account, internal tritium breeding is found to be possible in a short range of parameters defining the target performance (tritium content parameter x, areal density and ignition temperature).

First measurements of turbulence levels and turbulence induced transport in the outer edge of the plasma of TEXTOR-94 during radiative improved mode discharges show a reduction by a factor of 4-7 of the radial particle turbulent transport. The quenching is most evident on the normalized potential fluctuations and is strongest above 100 kHz. Non-linear gyrokinetic particle-in-cell simulations of these discharges show impurity induced suppression of the electrostatic fluctuations associated with the ion temperature gradient driven mode over most of the cross-section, including the edge. Such a mechanism is proposed as the explanation for the improved confinement and turbulence reduction. The reduction in the edge turbulent transport levels is consistent with increased particle confinement time and the reduction of the SOL thickness. Particle and energy fluxes to the limiter are reduced by an order of magnitude. A concomitant increase of the measured energy and particle confinement times τ_E and τ_p versus radiated fraction suggests a common underlying suppression mechanism.

The experimental results of three different reversed field pinch machines at the Electrotechnical Laboratory in Tsukuba are compared. In particular, the results are summarized in terms of the difference in the configuration of the conductive shell system. The three configurations correspond to TPE-1RM15 (b/a = 1.18), TPE-1RM20 (b/a = 1.08) and TPE-1RM20mod (b/a = 1.12), where b/a is a measure of the shell proximity, and a and b are the minor radii of the plasma and the inner surface of the shell closest to the plasma, respectively. The magnetic fluctuations, global confinement properties and mode locking events induced by the external field error at the thick shell gap are studied. It is shown that the magnetic fluctuation amplitude increases with b/a, as expected from the linear growth rate of the dominant MHD modes, and that a multilayered, close fitting shell system improves the performance under extreme operating conditions in the high pinch parameter region and when external field errors are considered, while the performance under normal operating conditions is comparable within the shot to shot deviations of the data.

Operating a tokamak at plasma densities near the empirical Greenwald limit _e^Gw in H mode could yield significant advantages for a fusion reactor. Trying to avoid the strong confinement degradation observed with gas puff refuelling, pellet injection from the magnetic high field side was applied. Sufficient pellet particle flux was supplied to achieve persistent density rampup and to enable density control in H mode at a level beyond _e^Gw for the first time. The pellet induced density increase decays in a fast phase with τ = 10 ms until about half of the latest pellet inventory remains, and decays thereafter to the base density on the particle confinement timescale with τ = 120 ms. The fast decay is the result of strong ELM events following each injected pellet, accompanied by a loss of energy, causing a transient reduction of the plasma energy content by convective heat flux. Recovery of the plasma energy after the ELM sequence takes place with τ = 25 ms, enabling transient operation at appropriately high densities without significant confinement degradation. To reach this scenario, however, confinement degradation caused by other factors must be inhibited. Other factors causing confinement degradation were found to be the increase of neutral gas pressure by pellet born gas puff at insufficient pumping speed or the occurrence of neoclassical tearing modes triggered by pellets when the temperatures close to rational surfaces were reduced too strongly.

The roles of `bumpy' field on collisionless particle confinement in helical axis heliotrons are investigated with model magnetic field and particle orbit calculations in Boozer co-ordinates. The mod B_min contours can be shifted in the major radius direction with control of the bumpy field, where B_min is the minimum value of |B| in the toroidal direction within one field period. The area of closed mod B_min contours is a useful measure with which to evaluate global collisionless particle confinement as long as the mod B_min contours connect toroidally. The negative value of the ratio between the bumpy component and the helicity component contributes to obtaining the largest area of closed mod B_min contours for a finite ratio between the toroidicity component and the helicity component. The radially increasing bumpy field contributes to the realization of a toroidally localized mod B_min structure. It also produces a region with a large derivative of |B| with respect to the toroidal magnetic flux, which is well aligned to the toroidally localized mod B_min structure. This enhances the large poloidal drift of the guiding centre orbits, while the radial drift is unaffected, effectively improving the particle confinement.

The three dimensional dynamics of a spheromak-like compact toroid (SCT) plasmoid, which is injected into a magnetized target plasma region, is investigated by using MHD numerical simulations. It is found that the process of SCT penetration into this region is much more complicated than that which has been analysed so far by using a conducting sphere (CS) model. The injected SCT suffers from a tilting instability, which grows with a similar timescale to that of the SCT penetration. The instability is accompanied by magnetic reconnection between the SCT magnetic field and the target magnetic field, which disrupts the magnetic configuration of the SCT. Magnetic reconnection plays a role in supplying the high density plasma, initially confined in the SCT magnetic field, to the target region. The penetration depth of the SCT high density plasma is also examined. It is shown to be shorter than that estimated from the CS model. The SCT high density plasma is decelerated mainly by the Lorentz force of the target magnetic field, which includes not only the magnetic pressure force but also the magnetic tension force. Furthermore, by comparing the SCT plasmoid injection with the bare plasmoid injection, magnetic reconnection is considered to relax the magnetic tension force, i.e. the deceleration of the SCT plasmoid.