On the Tokamakà Configuration Variable (TCV), electron internal transport barriers (eITBs) can be formed during a gradual evolution from a centrally peaked to a hollow current profile while all external actuators are held constant. The formation occurs rapidly (<τ eE) and locally and, according to ASTRA modelling, is consistent with the appearance of a local minimum in the safety factor (q) profile. The eITB is sustained by non-inductively driven currents (including the off-axis bootstrap current) for many current redistribution times while the current in the tokamak transformer is held constant. The maximum duration is limited by the pulse length of the gyrotrons. The transformer coil can be used as a counter (or co-) current source with negligible accompanying input power. In established eITBs the performance can be enhanced (degraded) by altering solely the central current or q-profile. New experiments show that the same stationary eITB performance can be reached starting from discharges with centrally peaked current. A fine scan in surface voltage shows a smooth increase in performance and no sudden improvement with voltage despite the fact that q min must pass through several low-order rational values.

The TCV tokamak contributes to physics understanding in fusion reactor research by harnessing a wide experimental tool set: in particular flexible shaping and high power electron cyclotron heating. Plasma regimes with high plasma pressure, a wider range of temperature ratios and significant fast-ion population are now attainable with a TCV heating system upgrade. In a first stage, a 1 MW neutral beam was installed (2015) and is reported in this paper. The installation of the NB injector required modifications of the vacuum vessel and considerable work on the machine infrastructure, resulting in a shutdown from late 2013 to mid-2015. TCV is now operating partly as a European Medium-Size Tokamak (MST) facility under the auspices of the EUROfusion consortium. The NBI was intensively operated in the February-July 2016 phase of the MST campaign. Record ion temperatures of 2.0-2.5 keV and toroidal rotation velocities up to 160 km/s were promptly attained in the first few L-mode discharges with NB injection. Ion temperatures up to 3.5 keV were subsequently achieved in ELMy H-mode. The injector produces a focused deuterium neutral beam with 25 keV energy, 1 MW neutral power and 2 s duration. Highlights: • Installation of 1 MW, 25 keV neutral beam, direct ion heating, access to Ti/Te ≥ 1. • Specific low divergence neutral beam injector with tunable power and energy. • Ion temperature of 2.0 keV, toroidal rotation of 160 km/s attained with NB heating.

No abstract

The Tokamak à Configuration Variable (TCV) tokamak (R = 0.88 m, a < 0.25 m, B < 1.54 T) programme is based on flexible plasma shaping and heating for studies of confinement, transport, control and power exhaust. Recent advances in fully sustained off-axis electron cyclotron current drive (ECCD) scenarios have allowed the creation of plasmas with high bootstrap fraction, steady-state reversed central shear and an electron internal transport barrier. High elongation plasmas, κ = 2.5, are produced at low normalized current using far off-axis electron cyclotron heating and ECCD to broaden the current profile. Third harmonic heating is used to heat the plasma centre where the second harmonic is in cut-off. Both second and third harmonic heating are used to heat H-mode plasmas, at the edge and centre, respectively. The ELM frequency is decreased by the additional power. In separate experiments, the ELM frequency can be affected by locking to an external perturbation current in the internal coils of TCV. Spatially resolved current profiles are measured at the inner and outer divertor targets by Langmuir probe arrays during ELMs. The strong, reasonably balanced currents are thought to be thermoelectric in origin.

In noninductively driven discharges, 0.9 MW second harmonic ͑X2͒ off-axis co-electron cyclotron current drive deposition is combined with 0.45 MW X2 central heating to create an electron internal transport barrier ͑eITB͒ in steady plasma conditions resulting in a 1.6-fold increase of the confinement time ( Ee ) over ITER-98L-mode scaling. The eITB is associated with a reversed shear current profile enhanced by a large bootstrap current fraction ͑up to 80%͒ and is sustained for up to 10 current redistribution times. A linear dependence of the confinement improvement on the product of the global shear reversal factor (q 0 /q min ) and the reversed shear volume ( q-min 2 ) is shown. In other discharges heated with X2 the sawteeth are destabilized ͑respectively stabilized͒ when heating just inside ͑respectively outside͒ the qϭ1 surface. Control of the sawteeth may allow the avoidance of neoclassical tearing modes that can be seeded by the sawtooth instability. Results on H-mode and highly elongated plasmas using the newly completed third harmonic ͑X3͒ system and achieving up to 100% absorption are also discussed, along with comparison of experimental results with the TORAY-GA ray tracing code ͓K. Matsuda, IEEE Trans. Plasma Sci. PS-17, 6 ͑1989͒; R. H. Cohen, Phys. Fluids 30, 2442 ͑1987͔͒.