Search

Recent theoretical work predicts intrinsic toroidal rotation in the tokamak edge to depend strongly on the normalized major radial position of the X-point. With this motivation, we conducted a series of Ohmic L-mode shots on the Tokamak a Configuration Variable, moving the X-point from the inboard to the outboard edge of the last closed flux surface in both lower and upper single null configurations. The edge toroidal rotation evolved from strongly co-current for an inboard X-point to either vanishing or counter-current for an outboard X-point, in agreement with the theoretical expectations. The whole rotation profile shifted roughly rigidly with the edge rotation, resulting in variation of the peak core rotation by more than a factor of two. Core rotation reversals had little effect on the edge rotation. Edge rotation was slightly more counter-current for unfavorable than favorable gradB drift discharges.

The TCV tokamak continues to leverage its unique shaping capabilities, flexible heating systems and modern control system to provide a stepladder approach for extrapolations of experimental results from different ma-chines to ITER and DEMO. TCV's configurational flexibility has recently been enhanced by the installation of two heating neutral beams, new dual frequency gyrotrons, removable divertor gas baffles together with upgrades of its diagnostic infrastructure.Plasma regimes with high plasma pressure, a wider range of temperature ratios and significant fast-ion population are now attainable on TCV. A 1.3 MW/28 keV heating beam (NBI-1) has been operated on TCV from 2015 introducing the direct ion auxiliary heating. A second 1MW/50-60 keV high-energy neutral beam (NBI-2) has since been procured, manufactured and installed on TCV in July 2021. NBI-2 injected near anti-co -linearly to NBI-1 is designed to probe plasma physics issues of higher plasma density, fast ion plasma interactions with static and dynamic fields and plasma rotation. The higher NBI-2 energy greatly enhances the operational space for fast ion studies. NBI-2 commissioning is presently ongoing in parallel with TCV experiments using both NBIs.This paper summarises the experience with intensive use and improvements of the NBI-1, together with the status of NBI-2 commissioning.

Edge intrinsic rotation was investigated in Ohmic L-mode discharges on the Tokamak a Configuration Variable, scanning the major radial position of the X point, R-X. Edge rotation decreased linearly with increasing RX, vanishing or becoming countercurrent for an outboard X point, in agreement with theoretical expectations. The core rotation profile shifted fairly rigidly with the edge rotation, changing the central rotation speed by more than a factor of two. Core rotation reversals had little effect on the edge rotation velocity. Edge rotation was modestly more countercurrent in unfavorable than favorable del B shots.

The paper is an overview of the H-mode physics studies on TCV. Historically, ELMy H-modes were obtained with ohmic heating only (q953 are now routinely obtained. The pedestal database developed for EUROfusion is presented and used within the paper.

Within the range of plasma shapes and plasma currents investigated, the electron confinement time, Tau_E increases with density, elongation and negative triangularity (-0.4

Turbulence and Transport Reduction with Negative Triangularity : Correlation ECE Measurements in TCV Due to turbulence, core energy transport in fusion devices such as tokamaks generally exceeds collisional transport by at least an order of magnitude. It is therefore crucial to understand the instabilities driving the turbulent state and to find ways to control them. Plasma shape is one of these fundamental tools. In low collisionality plasmas, such as in a reactor, changing the plasma shape from Dee-shape to inverse Dee-shape (from positive to negative triangularity δ) reduces the energy transport by a factor two: the heat flux necessary to sustain the same profiles and stored energy in a discharge with δ=-0.4 is only half of that at δ=+0.4. This is significant, since it opens the possibility of having Hmode-like confinement time within an L-mode edge; or at least with smaller ELMs. Recent correlation ECE measurements show that this reduction of transport at negative δ is reflected in a reduction by a factor of two of both 1) the amplitude of temperature fluctuations in the broadband frequency range 30-150 kHz, and 2) the fluctuation correlation length, measured at mid-radius (ρv~0.6). In addition, the fluctuations amplitude is reduced with increasing collisionality, consistent with theoretical estimates of the collisionality effect on Trapped Electron Modes (TEM). The correlation ECE results are compared to gyrokinetic code results: 1) global linear gyrokinetic simulations (LORB) have predicted shorter radial TEM wavelength λ⊥ for negative triangularity plasmas, consistent with the shorter radial turbulence correlation length λc observed. 2) At least close to the strongly shaped plasma boundary, local nonlinear gyrokinetic simulations with the GS2 code predict that the TEM induced transport decreases with decreasing triangularity and increasing collisionality, in fair agreement with the experimental observations. 3) Calculations are now extended to global nonlinear simulations (ORB5). This work was supported in part by the Swiss National Science Foundation

In DEMO reactor to produce electricity, a total installed power up to 150 MW is considered. In the heating mix, neutral beam (NB) injectors of high neutral particle energy (0.8-1 MeV) with high wall-plug efficiency (η in the range of 50% to 60%) will be required. Present design considered a NB system delivering in total 50 MW. In the Siphore NB concept [1], one of the promising candidates to meet these challenging requirements, negative ions will be extracted from a long, thin ion source 3 m high and 15 cm wide, accelerated and subsequently photo-neutralized. This requires the development of a new generation of negative ion sources. At the Swiss Plasma Center (SPC), a novel helicon plasma source, based on a resonant network antenna, is currently under study. The source delivers up to 10 kW at 13.56 MHz, and is installed on a linear (1.8 m long, 0.4 m diameter) vacuum vessel, allowing for full plasma characterization. In this work, the principles of operation of resonant antennas as helicon sources will be introduced, and absolute spectroscopic, Langmuir probe and interferometry measurements will be presented to characterize the performance of the source in terms of hydrogen/deuterium dissociation and negative ion production as a function of the input power at low gas pressure (0.3 Pa) and moderate magnetic field (~100 Gauss), as required by Siphore.

This contribution reports on the progress towards including drift effects in SOLPS-ITER simulations to assess power distribution and detachment onset in the light of the 2019 TCV divertor upgrade [1]. The installation of in-vessel gas baffles (Figure 1a) is predicted to increase the divertor neutral density by a factor ∼ 5 and therefore facilitates the access to detachment [2]. The conditions for the onset of detachment at each strike point depend on the power entering each divertor leg and thus simulations must correctly include the power distribution between the inner and outer divertor that is greatly affected by scrape-off layer drifts. Surprisingly, the sharing of particles and heat between the targets in reversed field conditions is found to be shifted towards the outer target. This counter-intuitive flowpattern is due to an electric potential well below the X-point that occurs in high density reversed field simulations.

This paper reports on the first demonstration of electron Bernsteinwave heating (EBWH) by double mode conversion from ordinary (O-) to Bernstein (B-) via the extraordinary (X-) mode in an over-dense tokamak plasma, using low field side launch, achieved in the TCV tokamak H-mode, making use of its naturally generated steep density gradient. This technique offers the possibility of overcoming the upper density limit of conventional EC microwave heating. The sensitive dependence of the O-X mode conversion on the microwave launching direction has been verified experimentally. Localized power deposition, consistent with theoretical predictions, has been observed at densities well above the conventional cut-off. Central heating has been achieved, at powers up to two megawatts. This demonstrates the potential of EBW in tokamak H-modes, the intended mode of operation for a reactor such as ITER.

Transport analyses using first principle turbulence codes as well as 1½-D transport codes often study transport properties between the plasma axis and a normalized radius around 0.8. In this region, heat transport shows significant stiffness properties leading to R/LTe values that are relatively independent of auxiliary input power. In this work, we study experimentally in the TCV tokamak, the transport properties of the edge region, close to the last closed flux surface, namely between ρV=0.8 and 1 (ρψ≥0.9). It is shown that electron transport is not stiff in this region and extremely high R/LTe values can be attained even for L-mode confinement. These results bring a new perspective to several "accepted" understandings. In particular a specific study related to the Ip scaling of ohmic and ECH L-mode discharges shows that the strong Ip scaling is in fact related in a large part to this non-stiff edge region. The Te scale length is shown to be proportional to Ip in the edge region and constant (independent of Ip) in the core region. The relation with L-H transition and the I-modes will also be discussed, since it shows that the edge gradient can be continuously increased, by increasing the input power, even in the L-mode phase. It is also proposed that the pedestal width is related to the width over which the transport is non-stiff, and can be studied in detail already in L-mode. The present study also explains the large improved confinemet obtain with negative triangularity and a new model is proposed including non-stiff edge local transport which can recover the experimental observations. This work was supported in part by the Swiss National Science Foundation.

This paper reports simultaneous measurements of intrinsic toroidal and poloidal plasma rotation during the evolution of electron Internal Transport Barriers (eITBs) on TCV. A CXRS diagnostic-beam system provides ion parameter profiles for stationary pre-barrier formation and eITB sustainment phases with a 15mm spatial resolution across the entire barrier width. This special configuration is used to assess the role of Er and E ×B shearing for the eITBs sustainment and formation in TCV. Two eITBs targets were developed either by applying central counter-ECCD with off-axis ECH, giving a central barrier, or off-axis co-ECCD with central ECH/counter-ECCD. The first target, characterized by a central Te barrier, is used to study the evolution of the rotation and Er with respect to the development of MHD modes, ECH power and plasma density. MHD modes cause a toroidal rotation reversal leading to increasingly positive Er. When applying central counter-ECCD, a peaked counter current rotation is sustained in the core with rotation values approximately doubled compared with the ECH phase where both cases have flat Er profiles. This shows experimentally for the first time on TCV that the E ×B shearing does not influence the eITB.

Since pioneering the alternative “snowflake”(SF) divertor, TCV research has concentrated on understanding its usefulness and applicability for a fusion reactor. It was first demonstrated that this divertor configuration is compatible with the high confinement H-mode regime on TCV and other machines. Since the main hope for this configuration is to lower the peak power incident on the divertor, and in particular near the strike points, this paper presents a résumé of experiments on TCV that seek to understand and model the particle and energy flows of a SF divertor configuration. The variety of possible snowflake configurations are presented and compared with, for reference, a similarly shaped (and in use on many machines), single-null configuration. The resulting heat and particle flows to the divertor strike plates are compared to each-other and with the first results from physics based numerical modelling. Most recently, initial experiments with wide density scans and impurity injection, designed to test possible advantages of a SF in increasing the radiated fraction or facilitating divertor detachment are reported.

Within the range of plasma shapes and plasma currents investigated, the electron confinement time, tau_Ee, increases with safety factor, density and negative triangularity similar to the Ohmic heating case. There is little dependence of tau_Ee on the heating location provided power deposition occurs inside the q=1 surface; as power deposition moves out of the inversion surface, tau_Ee decreases. The power-induced energy confinement degradation exponent (tau_Ee~PaP) is as usual: alpha_P ~-0.5. As a general trend, central relaxations decrease in amplitude with increasing qa, P_EC, or negative delta, in a situation where the confinement time increases.