[en] Full text: TCV’s principal goal is to explore and develop the physics basis for ITER exploitation and to aid in the development of DEMO. In the initial design, a combination of X2 and X3 ECH power was planned and installed to provide precision auxiliary heating. With a nominal < 1.5 T toroidal field strength, X2 heating remains limited to electron densities< ∼4 x1019/m³ and X3 to electron densities below ∼10²⁰/m³. Several realms of plasma operation, pertinent to our stated goals of ITER and DEMO relevance, remained, however, only marginally accessible. When ECH power alone heats the plasma, electron-ion collisional equipartition decreases resulting in extremely hot electrons with the ion temperature trailing, on TCV, by a factor that can exceed 30. For all these reasons, a phased upgrade programme is underway on TCV that not only extends the power range of X2 and X3 heating but also introduces direct ion auxiliary heating using state-of-the-art neutral beam injection. In a first stage, (scheduled for final acceptance early in 2016), a 1 MW tangentially launched neutral beam of hydrogen or deuterium is installed and reported in this paper. By ion dominated heating at sufficient densities (> 5 x 10¹⁹/m³) that increases collisional electron heating, efficient X3 power deposition also becomes possible. To harness this effect, part of the future upgrades, envisage some X3 power launched through the lateral launchers where, with sufficient electron temperature (> 2 keV), precision X3 deposition, similar to X2 heating today, becomes available, but at higher electron densities. This paper concentrates on a comparison between the modelled and experimentally observed plasma behaviours and improved access to reactor relevant regimes. Models predict T_i/T_e ratios above 3 in L-mode and slightly less in H-mode with one beam and X2(L-mode) or X3(H-mode). With an eye to the future, with the planned increase in ECH power and the second counter injected beam, ion-electron temperature equalization with β_N> 2.8 is expected. This combination of a large electron heating power density and sufficient ion heating to achieve T_i/T_e∼1 will make TCV plasmas unique. Discharges with a highly nonthermal electron distribution together with those with a strong fast ion component, both relevant to reactor scenarios, become possible. (author)

No abstract available

[en] Full text: The behaviour of fast particles in high temperature plasmas must be understood in view of future fusion devices. Fast particles result as a product of the fusion process and can be generated by neutral beam injection (NBI) or ion cyclotron heating. They heat the background plasma via collisions on electrons and ions and can, in case of an anisotropic velocity space distribution, drive noninductive currents. The corresponding heating and current drive profiles depend strongly on the fast-ion confinement properties which can be affected in presence of plasma instabilities, such as Alfvén eigenmodes or sawtooth crashes. Since a clear experimental quantification and reliable theoretical predictions of the instability-induced fast-ion transport are still missing, detailed studies of the fast-ion distribution function are required. In the TCV tokamak, fast ions can be generated by a newly installed NBI source with 1 MW power, which started operation in January 2016. The beam has a tangential geometry and injects deuterium neutrals with a full energy of 25 keV. The fast-ion D (FIDA) technique is employed to study the corresponding fast-ion distribution function. Toroidal lines of sight that cross a diagnostic neutral beam collect strongly Doppler shifted Balmer alpha radiation from fast ions after charge exchange. Good signal to noise ratios are obtained due to the application of a F/2 spectrometer and due to the presence of very large fast-ion densities in TCV, explained by the high ratio of NBI heating power to plasma volume (∼1 m³). For the interpretation of measured FIDA signals, the TRANSP and FIDASIM codes have been implemented at TCV. TRANSP predicts theoretical fast-ion distribution functions which are used as input for FIDASIM to calculate synthetic FIDA measurements. First comparisons between neoclassical predictions and measured FIDA spectra and profiles will be discussed. In addition, this contribution will present initial results on the effect of the fast-ion population on the loop voltage, internal inductance, normalized and the neutron rate. These quantities change significantly when NBI is turned on and allow one to address the fast-ion transport properties and to validate theoretical models. (author)

[en] Calculations of the particle flux caused by neutrals originating from the edge are performed for TCV and JET using the one-dimensional kinetic transport code Kn1D. The analysis, confirmed by experimental evidence, shows that for TCV and JET edge fuelling as well as neutral beam fuelling cannot be responsible for the density gradient in the plasma bulk, confirming the presence of inward particle convection. The results corroborate the expectation of a peaked density profile in ITER, despite the lack of bulk particle fuelling

[en] The flux of charge exchange (CX) neutrals measured by neutral particle analysers (NPAs) is the line integral along the view line of the NPA and contains information about the ion energy distribution of the observed plasma. On the Tokamak a Configuration Variable (TCV) a single chord NPA is used to scan the plasma cross section by vertically displacing a reproducible discharge across its fixed line of sight. The ion temperature inferred from the passive CX flux as a function of the distance of the NPA chord to the magnetic axis is used to obtain an ion temperature profile T_i(ρ). To model the neutral source, simulations of neutral particle penetration from the edge and the neutralization processes are reported. In plasmas with thermalized ion populations, the NPA hydrogen or deuterium temperature profiles agree with the carbon ion temperature profile measured by charge exchange recombination spectroscopy. Matching the simulation with synchronous NPA measurements of two plasma species provides absolute profiles of neutral particles densities and the isotopic composition of the plasma, which are required for the transport analysis. With further modelling, the ion temperature profile may be iteratively reconstructed from the CX spectrum without displacing the plasma

[en] This paper focuses on interpreting variations in the NPA measured energy distribution of neutral fluxes from the TCV high density H-mode plasma discharges with strong third harmonic electron cyclotron heating (P_X3>P_Ω). Two quasi-stationary regimes: ELMy H-mode and ELM-free H-mode, routinely and reproducibly obtained in TCV, with a plasma density 5-10x10¹⁹ m^-3, electron temperature 2-3 keV and ion temperature of 0.7-1.0 keV. The ELMy X3-heated H-mode plasma on TCV is significantly perturbed by ELMs, sawteeth activity and modes. In X3-heated plasmas ELMs are characterised by increased amplitudes and lower frequencies than are typical in ohmic H-modes with strong sawteeth synchronised with ELM cycle. The energy losses per ELM can exceed the 15% of the total stored energy and the plasma density and electron temperature profiles were resolved during the ELM cycle. NPA measurements in the presence of ELMs and sawteeth cannot be explained with the classical theory of two-body Coulomb electron-ion collisions alone. Additional effects (such as a modification of the ion temperature radial profile and/or ion redistribution in the coordinate and velocity space due to plasma perturbations) must be considered

[en] The first toroidal rotation measurements in TCV ohmic L-mode plasmas with no external momentum injection are presented. The toroidal velocity profile of the fully stripped carbon species is measured by active Charge eXchange Recombination Spectroscopy with a temporal resolution of typically 90 ms and a spatial resolution of 2.5 cm, about 1/10 of the plasma radius. The observed carbon velocity is of the order of the deuterium diamagnetic drift velocity and up to 1/5 of the deuterium thermal velocity. It is directed opposite to plasma current in the electron diamagnetic toroidal drift direction. The profile reverses when reversing the plasma current. The angular velocity profile is flat, or hollow, inside the sawtooth inversion radius and decreases quasi linearly towards the plasma edge. By vertically shifting the plasma magnetic axis within the TCV vessel the plasma edge velocity profile was measured with high resolution. Such experiments confirm that, close to the limiter, the stationary rotation velocity is close to zero or somewhat positive (co-current directed). This suggests that the angular momentum is not driven from the plasma edge. The maximum carbon velocity scales as v_φ,Max [km s^-1] = -12.5T_i/I_p [eV/kA] for a significant range of densities and values of the edge safety factor. Comparison with neoclassical predictions show that the TCV plasma rotation is mainly driven by radial electric fields, with a negligible contribution from the toroidal electric fields. The neoclassical theory of small toroidal rotation quantitatively and qualitatively disagrees with the experimental observation. An alternative empirical equation for the angular momentum flux, able to reproduce the measured stationary profile outside the inversion radius, is proposed

[en] FIDASIM is a code that models signals produced by charge-exchange reactions between neutrals and ions (both fast and thermal) in magnetically confined plasmas. With the ion distribution function as input, the code predicts the efflux to a neutral particle analyzer diagnostic and the photon radiance of Balmer-alpha light to a fast-ion D${}_{\mathrm{\alpha <!--\; ??\; -->}}$ diagnostic, in addition to many other related quantities. A new, parallelized version of the Monte Carlo code FIDASIM has been developed in Fortran90 that is substantially faster than the original interactive data language version. Modified algorithms include more accurate treatments of the time dependent collisional-radiative equations that describe neutral energy levels, of the cloud of ‘halo’ neutrals that surround the injected neutral beam, and of finite Larmor radius effects. Enhanced physics capabilities include modelling ‘passive’ signals from cold edge neutrals, the ability to treat general three-dimensional magnetic confinement configurations, and calculations of diagnostic-specific weight functions that enable tomographic reconstructions of the fast-ion distribution function. Neutral beam attenuation, beam emission, and fast-ion birth profiles are also modelled. The new algorithms have been successfully validated against experimental data and new features have been tested through benchmarks between two independently developed versions of the code. (paper)

[en] Full text: The scaling of toroidal rotation has been studied over a wide range of in plasma and shape parameters on the TCV tokamak using a diagnostic neutral beam that injects a negligible momentum into the plasma. As on other Tokamaks, the toroidal rotation magnitude on TCV, for a given shape and plasma conditions, is observed to scale inversely with the plasma current. More specifically, this describes the maximum toroidal rotation in plasmas with strong sawteeth activity. The toroidal rotation gradient from the plasma edge was similar over a wide range of plasma currents and only diverged inside the sawtooth inversion radius. By extrapolating the maximum core rotation from the rotation profiles outside the sawtooth inversion radii and adding a correction for the observed ion temperature changes, the scaling of the intrinsic toroidal rotation with plasma current is no longer observed. The TCV CXRS diagnostic was configured to measure the toroidal rotation profile with a low spatial resolution (4 points across the radial co-ordinate) and a high temporal resolution (2 ms). Using ECH X2 power deposited close to the sawteeth inversion radius, the sawteeth repetition time was extended to over 12 ms. The measurement used a newly available Real-Time node to generate a sequence of trigger pulses from analysis of the soft X-ray such that each spectroscopic frame was taken at 2, 4, 6, 8, ... ms after each sawtooth. The radial toroidal rotation profile evolution over a sawtooth was determined using coherent resampling over many sawtooth events. Initial results indicate that, at the sawtooth crash, the plasma core accelerates in the co current direction and then the core rotation profile relaxes back to the counter current direction with a time constant ∼ 10 ms. This implies that the scaling of intrinsic rotation with plasma current may be better understood as a peaked rotation profile (or an inwards momentum pinch) in the plasma core region that is flattened by sawtooth activity. This change of viewpoint should be integrated into the theory of momentum generation and transport in tokamaks such as ITER in that these mechanisms are not as universally affected by the plasma current as was at first thought. (author)

[en] Carbon ion velocity profiles are measured in TCV with a charge exchange diagnostic using a negligibly perturbing diagnostic neutral beam. These 'intrinsic' rotation profiles are measured up to the plasma edge in the toroidal and poloidal directions for both limited and diverted plasma configurations in Ohmic plasmas and in the presence of strong second harmonic electron cyclotron heating (ECH). Absolute toroidal velocities are shown to scale with peak ion temperature and inversely with plasma current. The plasma edge rotation is always small in limited configurations but evolves smoothly with the core density for diverted configurations. A strong intrinsic rotation builds up in the plasma core in the counter-current direction for limited configurations but is observed in the co-current direction for diverted plasmas. Unexpectedly, above a given density threshold, the rotation profile reverses to the co-current direction for limited configurations (and surprisingly, in the counter-current direction for diverted configurations). This threshold density is found to depend on plasma current, the presence of ECH and the magnetic topology. Poloidal velocity measurements are used to deduce the radial electric field change across the transition. A strong dependence of the rotation profile on plasma triangularity is reported and possible physics models for these observations are discussed. The origin of the momentum drive, its reversal and its magnitude are not yet clearly understood even for these relatively 'simple' experimental configurations