Parametric scalings of the intrinsic (spontaneous, with no external momentum input) toroidal rotation observed on a large number of tokamaks have been combined with an eye towards revealing the underlying mechanism(s) and extrapolation to future devices. The intrinsic rotation velocity has been found to increase with plasma stored energy or pressure in JET, Alcator C-Mod, Tore Supra, DIII-D, JT-60U and TCV, and to decrease with increasing plasma current in some of these cases. Use of dimensionless parameters has led to a roughly unified scaling with MA ∝ βN, although a variety of Mach numbers works fairly well; scalings of the intrinsic rotation velocity with normalized gyro-radius or collisionality show no correlation. Whether this suggests the predominant role of MHD phenomena such as ballooning transport over turbulent processes in driving the rotation remains an open question. For an ITER discharge with βN = 2.6, an intrinsic rotation Alfven Mach number of MA ≃ 0.02 may be expected from the above deduced scaling, possibly high enough to stabilize resistive wall modes without external momentum input.

Comparisons between time dependent simulations and experiments in ion cyclotron heated plasmas in the JET tokamak have been made. A time dependent code, PION-T, has been used to simulate the heating. The scenario that has been analysed is minority heating of hydrogen in a deuterium plasma. Two measured quantities have been compared with the calculations, namely the anisotropic plasma energy content and the 2.4 MeV neutron flux from (D,D) reactions. Two versions of the PION-T code have been used, one with zero banana width and one in which a simplified model for taking the finite width of drift orbits into account is employed. The zero banana width calculations yield good agreement between measurements and calculations for relatively low power levels only. However, with the second version of the code, taking the finite width of the drift orbits into account, good agreement can also be obtained for higher power levels. The non-thermal neutron yield caused by second harmonic absorption by the deuterium is simulated and agreement is found provided the hydrogen concentration, for which no reliable measurements are available, is suitably chosen. Finally, by studying the rapid changes in fast ion energy content in connection with sawtooth instabilities, it is found that about 40% of the fast ions are expelled outside the q=1 surface and that prompt losses are negligible

Observations of bulk plasma rotation in radio frequency (RF) heated JET discharges are reported. This study is concentrated on RF heated L-mode plasmas. In particular, the toroidal rotation profiles in plasmas heated by ion cyclotron resonance frequency (ICRF) waves and lower hybrid (LH) waves have been analysed. It is the first time that rotation profiles in JET plasmas with LH waves have been measured in dedicated discharges. It is found that the toroidal plasma rotation in the outer region of the plasmas is in the co-current direction irrespective of the heating scenario. An interesting feature is that the toroidal rotation profile appears to be hollow in many discharges at low plasma current, but a low current in itself does not seem to be a sufficient condition for finding such profiles. Fast ion transport and finite orbit width effects are mechanisms that could explain hollow rotation profiles. This possibility has been investigated by numerical simulations of the torque on the bulk plasma due to fast ICRF accelerated ions. The obtained torque is used in a transport equation for the toroidal momentum density to estimate the effect on the thermal bulk plasma rotation profile.

Results are presented from a series of dedicated experiments carried out on JET in tritium, DT, deuterium and hydrogen plasmas to determine the dependence of the H mode power threshold on the plasma isotopic mass. The Pthr ∝ Aeff-1 scaling is established over the whole isotopic range. This result makes it possible for a fusion reactor with a 50:50 DT mixture to access the H mode regime with about 20% less power than that needed in a DD mixture. Results on the first systematic measurements of the power necessary for the transition of the plasma to the type I ELM regime, which occurs after the transition to H mode, are also in agreement with the Aeff-1 scaling. For a subset of discharges, measurements of Te and Ti at the top of the profile pedestal have been obtained, indicating a weak influence of the isotopic mass on the critical edge temperature thought to be necessary for the H mode transition.

Scenarios of steady-state, fully non-inductive current in Tore Supra are predicted using a package of simulation codes (CRONOS). The plasma equilibrium and transport are consistently calculated with the deposition of power. The achievement of high injected energy discharges up to 1 GJ is shown. Two main scenarios are considered: a low density regime with 90% non-inductive current driven by lower hybrid waves—lower hybrid current drive (LHCD)—and a high density regime combining LHCD and ion cyclotron resonance heating with a bootstrap current fraction up to 25%. The predictive simulations of existing discharges are also reported.

Analysis of MHD activity in pellet enhanced performance (PEP) pulses is used to determine the position of rational surfaces associated with the safety factor q. This gives evidence for negative shear in the central region of the plasma. The plasma equilibrium calculated from the measured q values yields a Shafranov shift in reasonable agreement with the experimental value of about 0.2 m. The corresponding current profile has two large off-axis maxima in agreement with the bootstrap current calculated from the electron temperature and density measurements. A transport simulation shows that the bootstrap current is driven by the steep density gradient, which results from improved confinement in the plasma core where the shear is negative. During the PEP phase (m,n)=(1,1) fast MHD events are correlated with collapses in the neutron rate. The dominant mode preceding these events usually is n=3, whereas the mode following them is dominantly n=2. Toroidal linear MHD stability calculations assuming a non-monotonic q-profile with an off-axis minimum decreasing from above 1 to below 1 describe this sequence of modes (n=3,1,2), but always give a larger growth rate for the n=1 mode than for the n=2 mode. This large growth rate is due to the high central poloidal beta of 1.5 observed in the PEP pulses. Finally, a rotating (m,n)=(1,1) mode is observed as a hot spot with a ballooning character on the low field side. The hot spot has some of the properties of a 'hot' island consistent with the presence of a region of negative shear

The kinetic theory of runaway electron avalanches caused by close Coulomb collisions is extended to account for radial diffusion. This is found to slow down the growth of avalanches. An approximate analytical formula for the growth rate is derived and is verified by a three-dimensional Monte Carlo code constructed for this purpose. As the poloidal magnetic flux that is available to induce an electric field in a tokamak is limited, avalanches can be prevented altogether by sufficiently strong radial diffusion. The requisite magnetic fluctuation level is sensitive to the mode structure and the speed of the disruption. It is estimated to be δB/B∼10−3 for parameters typical of large tokamaks.

The confinement of fast particles is of crucial importance for the success of future burning plasma experiments.. On JET, the confinement of ICRF accelerated fast hydrogen ions with energies exceeding 5 MeV has been measured using the characteristic γ-rays emitted through their inelastic scattering with carbon impurities, 12 C(p,p'γ) 12 C. Recent experiments have shown a significant decrease in this γ-ray emission (by a factor of 2) during so-called tornado mode activity (core-localised TAEs within the q = 1 surface) in sawtoothing plasmas. This is indicative of a significant loss or extensive re-distribution of these (> 5 MeV) particles from the plasma core. In this paper, mechanisms responsible for the radial transport and loss of these fast ions are investigated and identified using the HAGIS code, which describes the interaction of the fast ions and the TAE observed. The calculations show that the overlap of wave-particle resonances in phase-space leads to an enhanced radial transport and loss. On both JET and ASDEX Upgrade, new fast ion loss detectors have been installed to further investigate the loss of such particles. On JET, fast ion loss detectors based around an array of Faraday cups and a scintillator probe have been installed as part of a suite of diagnostic enhancements. On ASDEX Upgrade, a new fast ion loss detector has been mounted on the mid-plane manipulator allowing high resolution measurements in pitch angle, energy and time. This has enabled the direct observation of fast ion losses during various MHD phenomena to be studied in detail. ELM induced fast ion losses have been directly observed along with the enhancement of fast ion losses from specific areas of phase-space in the presence of NTMs and TAEs.