Table of contents

Low-frequency bounded whistlers have been launched in a toroidal system to produce a high-density plasma. These waves generate a completely different wave field pattern in the toroidal plasma cavity as compared to helicon waves in thin cylindrical plasma columns. Based on the excitation of toroidal bounded whistlers, which sustain the discharge, a new approach to the problem of current drive is discussed. 1.0–1.5 kA of plasma current is driven with 1.0 kW of input radiofrequency (RF) power. The same RF source sustains steady-state plasma with 10¹² cm⁻³ peak density and 18 eV electron temperature. Along with studies on the dependence of plasma current on wave and plasma parameters, resonant and nonresonant contributions to the total plasma current are estimated.

Ion temperature at the plasma centre has been measured from Doppler broadening of Ti XXI (2.61 Å) and Ar XVII (3.95 Å) x-ray lines using a newly installed crystal spectrometer with CCD detector in ECH, NBI and ICRF plasmas of Large Helical Device (LHD). The ion temperature obtained in a range of 0.6 and 3.5 keV was analysed with electron density and compared with electron temperature. A new parameter range of T_i>T_e was found in low-density (n_e<10¹³ cm⁻³) H-minority ICRF discharges operated in the ion-heating regime (P_i⩾P_e), whereas the ion temperature was roughly equal to the electron temperature in NBI discharges operated in the electron-heating regime (P_i<P_e). The ion temperature in the ICRF and ICRF + NBI discharges was correlated with the input power to bulk ions over the ion density (P_i/n_i). As a result, a good correlation was obtained between them and it also indicated a clear rise of the ion temperature for the increasing P_i in a range of P_i/n_i<5 (MW/10¹³ cm⁻³).

Experiments have been performed to disentangle the individual role of upper and lower triangularity on density buildup of lower single null, type-I ELMy H-mode discharges in JET. Comparison with corresponding data from ASDEX Upgrade allowed us to determine a dimensionless representation of the relation between the main chamber recycling and core density and to widen the triangularity variation in the data base. To incorporate the recycling flux density Γ in a dimensionless form, an effective scrape-off layer density n_e,SOL∝Γ^0.5 is introduced allowing us to parametrize and scale the core density by the density rise factor _e/n_e,SOL. The scaling uses edge-specific definitions of normalized Larmor radius, collisionality and beta. Rewritten in dimensional form, a behaviour is found which is very similar to energy confinement scalings for the ELM-averaged conditions considered here: the density rise factor exhibits an almost linear dependence on plasma current, a weak negative toroidal field dependence as well as power degradation. While a pronounced positive dependence of density buildup on the upper triangularity is observed, no significant correlation with the lower triangularity is found. In particular the dependences on plasma current and upper triangularity emphasize the importance of transport physics for the density buildup.

The scaling of energy transport with dimensionless parameters has been measured in high-temperature plasmas with the goals of guiding theory and predicting energy confinement in future fusion devices. Validation of this approach requires demonstration of similarity in plasmas with identical dimensionless parameters but very different physical parameters. Within measurement uncertainties, the heat diffusivities and global energy confinement exhibit similarity in high-confinement regimes on the DIII-D and JET tokamaks and in low-confinement regimes on the DIII-D and Alcator C-Mod tokamaks.

Fast plasma shutdown without runaway electron generation by gas puffing is investigated in the JT-60U tokamak. Argon-only injection enables a fast shutdown; however, it induces runaway electron generation. Hydrogen-only injection generates much less runaway electrons; however, the shutdown time is considerably longer. Mixed injection of hydrogen and argon achieves a fast plasma shutdown without runaway generation. Argon atoms contribute to radiate the energy of plasma leading to a fast plasma shutdown, whilst hydrogen atoms contribute to increase the electron density for avoiding runaway electron generation.

The dynamical interplay between turbulent transport, density gradients and radial electric fields has been investigated in the plasma boundary of magnetic confined plasmas. Experimental results show that there is a dynamical relationship between gradients and turbulent transport. Fluctuations on gradients, which turns out to have a rather Gaussian probability density function (PDF), and turbulent transport, which has a non-Gaussian PDF, are strongly coupled. Also, the size of turbulent events increases when the plasma deviates from the average gradient. This increase is particularly significant when the gradient increases above its average value. The system relaxes to the most probable state in a time comparable to the correlation time of turbulence. The present results emphasize the importance of the characterization of fluctuations in the terms of PDFs to test critically edge turbulence models.

Transport processes of fast ions in axisymmetric low-aspect-ratio spherical torus (ST) plasmas are investigated, which are induced by non-conservation of the magnetic moment μ. The reason for non-conservation of μ of fast ions in STs is the relatively large adiabaticity parameter ε typically exceeding the value 0.1 (ε = ratio of ion gyroradius to the gradient scale length of the magnetic field). Both analytical and numerical evaluations of the magnitude of non-adiabatic variations of μ are performed. Non-adiabaticity effects are shown to be most significant for fast ions for which the bounce oscillations are in resonance with the gyromotion, i.e. for ions with ω_B−lω_b = 0, where ω_B and ω_b represent the bounce-averaged gyrofrequency and the bounce frequency, respectively, and l is an integer. The critical threshold of the adiabaticity parameter, ε_cr, to be exceeded for the transition to stochastic behaviour of fast ions in axisymmetric STs is inspected. Non-adiabatic variations of μ are shown to lead to collisionless transformation of trapped orbits into circulating ones and vice versa. For the case of strong non-adiabaticity, ε>ε_cr we assess the transport coefficients describing intense collisionless pitch angle diffusion, whereas, in the case of weak non-adiabaticity, ε<ε_cr, the more substantial coefficients of enhanced collisional radial diffusion and convection of fast ions gyrating resonantly with the bounce oscillations are estimated.

Properties of the instability of compressional Alfvén eigenmodes (CAEs) in tokamak plasmas are studied in the cold plasma approximation with an emphasis on the instability driven by the energetic minority ion cyclotron resonance heating (ICRH) ions. The goal of the paper is to address a long standing problem of CAE half proton cyclotron harmonic excitation observed in pure deuterium plasma basing on new observations from Joint European Torus. As an example we apply earlier developed theory (Gorelenkov N.N. and Cheng C.Z. 1995 Nucl. Fusion35 1743) to compare two cases: ion cyclotron emission (ICE) driven by charged fusion products and ICRH minority-driven ICE (MICE) (Cottrell G.A. 2000 Phys. Rev. Lett.84 2397) recently observed on JET. Particularly in MICE spectrum, only instabilities with even harmonics of deuterium cyclotron frequency at the low-field-side plasma edge were reported. Odd deuterium-cyclotron frequency harmonics of ICE spectrum with frequencies between the integer cyclotron harmonics of protons can be driven only via the Doppler shifted cyclotron wave–particle resonance of CAEs with fusion products, but are shown to be damped due to the electron Landau damping in experiments on MICE. Excitation of odd harmonics of MICE with high-field-side heating is predicted. Dependence of the MICE instability on the electron temperature is studied and is shown to be strong. Low electron β is required to excite odd harmonics of MICE.

Upgrade of the Tore Supra ergodic divertor (ED) has led to significant progress in ED physics. Pulse durations of 30 s with LHCD have been achieved demonstrating the heat exhaust capability of both the actively cooled technology at hand and of this specific divertor concept. The disruptive limit governed by the stochastization of the outer magnetic surfaces is found to occur for a value of the Chirikov parameter reaching two on the magnetic surface q = 2+(3/12). This experimentally observed robustness allows one to operate at very low safety factor on the separatrix (q∼2). Numerical analysis of ballooning turbulence in a stochastic layer indicates that the decay of the density fluctuations is associated with an increase of the fluctuating electric drift velocity. This results in an enhanced cross-field transport in the vicinity of the target plates. This lowering of confinement appears to be compensated by an intrinsic transport barrier on the electron temperature. The three-dimensional response of the temperature field is computed with a fluid code. The code can recover the intrinsic transport barrier at the separatrix, reported experimentally, together with small amplitude temperature modulations in the divertor volume. Experimental evidence for the three density regimes (linear, high recycling and detachment) is reported. The low critical density values for transitions between these regimes indicate that similar parallel physics governs the axisymmetric and ED, despite the open configuration of the latter. Measurement and understanding of these density regimes provide a means for feedback control of plasma density and an improvement in ion cyclotron radiofrequency heating coupling scenarios. Experimental data also indicated that particle control with the vented target plates is effective. Increase of both impurity control and radiation efficiency are reviewed. Global power balance has been analysed in order to account for non-axisymmetric radiation. These results, taken together, confirm the large radiation capability of the ED.

The natural H-mode density, i.e. the plasma density evolving in an H-mode discharge without active fuelling, reaches Greenwald fractions in JET typically higher than in ASDEX Upgrade. According to general thinking this reflects device-specific differences as regards recycling-induced fuelling and beam fuelling. This paper presents evidence for a different view, namely that at sufficiently low plasma fuelling rates any fuelling rate dependence of the plasma density vanishes and the plasma particle content is completely determined by the plasma itself. It is shown that this limit, which would impose a lower boundary on the H-mode density, is reached in JET and ASDEX Upgrade natural density discharges, and its scaling is determined. Possible overlapping with existing density limit scalings in next-generation tokamaks is discussed with a view to a narrowing or even vanishing H-mode operation window.

Plasma current ramp-up assisted by outer vertical field coils with non-inductively driven current is studied during the ignition access phase in a high aspect ratio tokamak such as ITER-FEAT class tokamak reactor. In a tokamak without the Ohmic transformer, the pl1asma current ramp-up to 8 MA could be achieved by the vertical field with the heating/current drive power of 100 MW. The current ramp-up time can be chosen arbitrarily, as shorter than that expected in the usual, non-inductive current drive operation analyses, and it depends on the fusion power ramp-up time. This makes the reactor operation flexible. Experimental evidence showing the current drive by a vertical field in JT-60U has also been presented with plasma circuit theory.

The energy spectrum of the d+t→α+n neutron emission has been measured in experiments carried out at JET for plasmas of deuterium–tritium subjected to minority ion cyclotron resonance heating (ICRH) tuned to deuterium. The data obtained with the magnetic proton recoil spectrometer were of sufficient quality to distinguish up to three spectral components of neutron emission some of which were time resolved. A new analysis model was used to derive information on the underlying deuteron velocity distributions and their corresponding energy densities in the plasma. This experiment represents the first use of neutron emission spectroscopy for detailed diagnosis of the response of fusion plasmas to the applied ICRH power for different plasma conditions, including the time evolution over the heating pulse duration for individual discharges. In particular, ICRH effects on the plasma, together with the power absorption mechanisms, were studied as a function of the minority ion concentration in the range 9–20%.